WO2005100378A1

WO2005100378A1 - Process for the preparation of a peptide, and compounds comprising a thioester carboxyl-activating group for use therein

Info

Publication number: WO2005100378A1
Application number: PCT/GB2005/001398
Authority: WO
Inventors: David John Evans
Original assignee: Avecia Limited
Priority date: 2004-04-13
Filing date: 2005-04-11
Publication date: 2005-10-27
Also published as: GB0408156D0

Abstract

There is provided a process for the solid-phase synthesis of a polymer which comprises reacting an amine, secondary amine or hydroxyl group of a first monomer with a thioester on a second monomer wherein at least one the monomers is immobilised on a solid-phase support. There is also provide novel activated amino acids of Formula (1) wherein: R is an amino acid side chain a protected amino acid side chain or a peptide chain; X is an amino protecting group; and Z is an optionally substituted hydrocarbyl, and certain supported activated amino acids of Formula (5) wherein: SUPPORT is a solid support; L is a linking group; R is an amino acid side chain; a protected amino acid side chain or a peptide chain; and X3is an amino protecting group or one or more amino acids linked by amide bonds where the terminal residue bears an amino protecting group for use in said process.

Description

PROCESS FOR THE PREPARATION OF A PEPTIDE , AND COMPOXMDS COMPRISING A THIOESTER CARBOXYL-ACTIVATING GROUP FOR USE THEREIN The present invention relates to a process for the synthesis of a polymer, particularly a polymer comprising amide bonds and especially a peptide, and to free and supported compounds comprising a thioester carboxyl-activating group. The reagent 1-hydroxybenzotriazole (HOBt) is routinely used in the coupling step in both solution and solid-phase peptide synthesis. For example, in the carbodiimide technique the activated carboxyl comprises a HOBt active ester and in the uranium technique HOBt acts as an auxiliary nucleophile to compliment active agents such as TBTU (2-[1H-benzotriazole-1-yl]-1, 1, 3, 3-tetramethyluronium tetrafluoroborate) and HBTU (2-[1 H-benzotriazole-1 -yl]-1 , 1, 3, 3-tetramethyluronium hexaflourophosphate). HOBt is the reagent of choice in industry for both research and bulk manufacturing since as well as being effective it also provides minimal racemisation of chiral sensitive amino acids during amide bond formation. However, the reagent is currently under temporary classification as a 'Self Reactive Solid' and subject to transport restrictions. If re-categorised as an explosive risk the use of this reagent in industrial large-scale operations could become unfeasible. Thus an alternative reagent to HOBt is required for large-scale solid-phase peptide synthesis. Many reagents are known which are able to act as coupling agents in peptide synthesis (for a review see Albericio and Carpino, Methods in Enzymology, 1997, 289, 104-126). However, few of these agents demonstrate the practicality and activity of HOBt when used in the solid-phase technique. Any alternative reagent used in place of HOBt in peptide synthesis must demonstrate, when used in a solid-phase reaction; similar or reduced amino acid racemisation potential; similar or improved amide bond formation kinetics; and a similar or reduced potential for side reactions. For large-scale industrial processes a replacement for HOBt should also be inexpensive, stable and have an acceptable SHE (Safety, Health and Environmental) profile. Solution phase amide bond formation via thioesters has been known for over 50 years (Wieland et al., Angew. Chem., 1951, 63, 6, 146-147). A number of these reagents are regarded as highly reactive and have been used for coupling to poorly nucleophilic/electron deficient entities or in circumstances where steric hindrance has a detrimental effect, for example when using O-esters. Lloyd and Young (J. Chem. Soc. [C], 1971 , 2890-2896) demonstrated the synthesis of protected dipeptides in solution via 2-pyridyl thioesters. Kurosu (Tet. Letters, 2000, 41, 591-594) also demonstrated solution phase amide bond formation via dimethylaluminium amides and thioesters. Davis (Tet. Letters, 1994, 35, 27, 4865-4868), also in solution phase, used pentafluorothiophenol to form a cholic acid thioester which was reacted with an amine to form an amide bond. Ueda employed a comparable solution phase method to generate polyamides using 2- benzothiazolyl dithioesters (M. Ueda et al, J. Polymer ScL, 1978, 16, 475-482). In another similar approach amides were synthesised using 3-(benzothiazol-2-yl-thio)-1 ,2- benzisothiazole 1,1 -dioxide with amines in a solution phase media over prolonged periods of up to 6 hrs (M. Ueda et al, Synthesis: Communications, 1982, 11, 933-935). In addition, Lee has reported the use of S-acyl derivatives of benzothiazole-2-thiol as a convenient route to alkyl/aryl amides and carbamates within solution phase applications (J. H. Lee et al, Bull. Korean Chem. Soc, 1997, 18, 4, 442-444). The application of thioesters of 2-mercaptobenzothiazole (2-MBT) has also been demonstrated in a number of examples facilitating the synthesis of antimicrobial agents by solution phase methods (P. K. Khare et al, J. Indian Chem. Soc, 1996, 73, 11 , 627-628; M. A. Abd-Alla et al, Collect. Czech. Chem. Comm., 1992, 7, 1547-1552). Furthermore, these 2-MBT thioesters have also been utilised as intermediates for amide bond formation to generate β-Lactams / Cephalosporins (J. C. Rodriguez et al, Farmaco, 2003, 58, 5, 363-369; H. Tsubouchi et al, Tetrahedron: Assymetry, 1994, 5, 3, 441-452; G. Cainelli et al, Tetrahedron, 1995, 51, 17, 5067-5072; E. Defoβa et al, Leibigs Ann., 1996, 11, 1743- 1749; J. Fetter et al, 1997, 4, 118-119; B. Kammermeier et al, Org. Process Res. Dev., 1997, 1 , 2, 121-123; H. Park et al; J. Antibiot, 1994, 47, 5, 606-608), oxaisocephems (H. Ishikawa et al, Biorg. Med. Chem. Letters, 1994, 4, 9, 1147-1152), β-lactones (M. N. Rao et al, J. Chem. Soc, Chem. Comm., 1991, 15, 1007-1008) and isophthalic disaccharides (S. Valverde et al, Synlett, 200, 1 , 22-26) in solution. However, this chemistry has not been used in solid-phase (peptide) synthesis. This is due to a number of reasons. Firstly, Lloyd and Young demonstrated that 2-pyridyl thioester derivatives facilitate rapid amide bond formation only in non-polar media. Solid- phase peptide synthesis is routinely carried out by two procedures termed the Boc (tert- butyloxycarbonyl) (Merrifield) technique and the Fmoc (9-fluorenylmethoxycarbonyl) (Sheppard) technique. These methodologies commonly employ a polar acylation media such as N,N-dimethylformamide (DMF) and/or N-methylpyrrolidone (NMP) (see Chan and White, Fmoc Solid-Phase Peptide Synthesis - A Practical Approach, 2000, Oxford University Press, P. 11-13). Although it is feasible to perform a solid-phase peptide synthesis using the Merrifield or Sheppard protocols in a less polar medium, such as for example dichloromethane (DCM), under these conditions there is a higher potential for sensitive amino acids to undergo chiral inversion (racemisation). In addition, in a solid- phase peptide synthesis the solvation of the peptide-resin is a key aspect of the synthesis. Without adequate solvation the growing peptide-resin complex quickly collapses and reduces the accessibility of the reactive sites to the acylation reagents. Adequate solvation of the peptide-resin requires the addition of polar solvents such as dimethylsulfoxide (DMSO), hexafluoroisopropanol (HFIP), NMP, trifluoroethanol (TFE), etc. (see Chan and White, Fmoc Solid-Phase Peptide Synthesis - A Practical Approach, 2000, Oxford University Press, P. 118-119 and references therein). Also, the majority of α-nitrogen Fmoc and Boc protected amino acid derivatives commonly used in solid-phase peptide syntheses are poorly soluble in a less-polar medium such as DCM; for example Fmoc-Cys[Trt]-OH is virtually insoluble in DCM (throughout this specification standard three letter abbreviations for amino acids will be used). This solubility issue may be overcome by the addition of a tertiary base to the less- polar medium. However, the addition of such additives again facilitates base-catalysed racemisation. Thus again the use of a high polarity solvent is favoured. As with the 2-pyridyl thioester derivatives the utilisation of 2-MBT thioesters has also favoured the use of the less polar DCM or toluene medias (Defoβa et al, Ishikawa et al, Park ef al, Rodriguez et al, Fetter et al, Cainelli et al, Roa et al, Tsubouchi et al and Valverde ef al). Also, importantly, the generation of reactive 2-MBT thioester amino acid intermediates proceeds through an unfavoured preparatory route. Synthesis of the reactive 2-MBT thioesters is achieved through the use of highly reactive acid chlorides (Ueda et al, 1978 & Lee et al). If this acid chloride methodology was used to generate amino acid 2-MBT amino acid thioester derivatives it could result in unacceptable chiral inversion due to over- activation of the carboxylic acid component of the amino acid (M. Bodanszky, Principles Of Peptide Synthesis, Springer-Verlag, Berlin, 1984, P.10-12). In addition, the acid chloride methodology could not be used with amino acid derivatives that contain acid- labile protecting groups, thus seriously restricting applicability to general peptide synthesis (i.e. all Nα-Boc protected derivatives, all ferf-butyl & trityl based side-chain protecting groups employed in Fmoc amino acid synthesis, etc., L. Carpino et al, Ace Chem. Res., 1996, 29, 268-274). Also, the synthesis of amino acid chlorides is generally achieved by refluxing the amino acid with excess SOCI₂ a process that would again lead to racemisation of chirally sensitive amino acids. Some specific protected amino acid chlorides (i.e. Fmoc-Tyr[tBu]-CI) have been successfully manufactured and utilised in a peptide synthesis. However, when using these reagents a suitable tertiary amine must be included as an additive to neutralise the HCI liberated during the aminolysis reaction. The base additive is essential to avoid incidental loss of acid labile protection. Unfortunately, the addition of tertiary amine promotes the formation of the less reactive corresponding 5(4H)-oxazolone which tends to undergo chiral inversion. Thus the benefits of enhanced reactivity using an amino acid chloride must be addressed against an increase in racemisation. Thus, the preparation of the necessary amino acid chloride intermediates and use of such derivatives (to generate the required 2-MBT thioesters) is not appropriate for peptide synthesis (J. Jones, In The Peptides; E. Gross & J. Meienhofer, Eds., Academic Press: New York, 1979, Vol. 1, P. 65). Based on the above, thioester facilitated amide bond formation would seem to a skilled person to be just another solution phase method that was not suitable for use in the solid-phase. We have however surprisingly found that 'activated'-thioesters were able to act as a direct substitute for HOBt in the favoured Merrifield and Sheppard solid-phase protocols. We have also surprisingly found that these same protocols may be applied when an 'activated'-thioester is to be reacted with a hydroxyl. The present invention provides a process for the solid-phase synthesis of a polymer that comprises reacting an amine, secondary amine or hydroxyl group of a first monomer with a thioester on a second monomer wherein at least one of the monomers is immobilised on a solid-phase support. The monomers may comprise a single unit; such as, for example, a monosaccharide or amino acid; or may be a condensed polymer of such units. In addition to the first and second monomers the polymer may comprise one or more additional monomers which may or may not comprise either amine, secondary amine or hydroxyl groups, or thioester groups or both. The additional monomers may either be immobilised or free in solution. Preferably the present invention provides a process for the solid-phase synthesis of a polymer comprising amide bonds which comprises reacting an amine or secondary amine of a first monomer with a thioester on a second monomer wherein at least one of the monomers is immobilised on a solid-phase support. Preferably in the present invention the polymer comprising amide bonds comprises a peptide. Thus, the present invention preferably provides a process for the solid-phase synthesis of a peptide and so the first and second monomers comprise amino acids or amino acid residues. Any suitable solid-phase support may be used in the process of the present invention and the support may be chosen from, but not limited to, microporous or macroporous resin, controlled pore glass (CPG), polypropylene, cellulose, silica, or any support suitable for peptide elongation via solid phase means. Preferably the support is of the type commonly used, and well known to a person skilled in the art, in solid-phase chemistry, for example, solid-phase peptide, oligonucleotide, peptide nucleic acid (PNA), oligosaccharide or chemical (combinatorial) synthesis. The first monomer preferably comprises an amino acid or peptide bound to a solid support with a free α-amino group. The second monomer preferably comprises an α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide and especially an α-nitrogen protected α-carbon-thioester activated amino acid. Preferably the present invention comprises a process for the solid-phase synthesis of a peptide where an α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide is reacted with the free α- amino of an amino acid or peptide wherein at least one of the monomers is immobilised on a solid-phase support. More preferably the present invention comprises a process for the solid-phase synthesis of a peptide where an α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide is reacted with the free α-amino of an amino acid or peptide immobilised on a solid support. The present invention preferably comprises a process for the solid-phase synthesis of a peptide which comprises attaching an α-nitrogen protected amino acid or peptide to a solid support, removing the α-nitrogen protecting group and assembling a peptide chain on said α-nitrogen by sequentially reacting with α-nitrogen protected α-carbon-thioester activated amino acids or α-nitrogen protected α-carbon-thioester activated peptides. In these preferred embodiments the assembled peptide is then preferably cleaved from the solid support and optionally isolated and/or purified. A preferred process for the solid-phase synthesis of a peptide comprises the steps of: (a) attaching an α-nitrogen protected first amino acid or peptide to a solid support; (b) deprotecting the α-nitrogen of the attached first amino acid or peptide by removing the α-nitrogen protecting group under conditions such that the attached first amino acid or peptide remains connected to the solid support and coupling an α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide to the unprotected α-nitrogen of the attached amino acid or peptide to yield an attached peptide with a protected α-nitrogen; (c) deprotecting the α-nitrogen of the attached peptide by removing the α-nitrogen protecting group under conditions such that the attached peptide remains connected to the solid support and coupling an additional α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide to the unprotected α-nitrogen of the attached peptide and repeating until the desired peptide with a protected N- terminal amino group is assembled on the solid support; (d) removing the N-terminal protecting group from the protected N-terminal amino group, cleaving the link between the peptide and the solid support so that the peptide is released from the solid support and optionally removing any side chain protecting groups. The peptide released in step (d) is preferably isolated and then purified. The α-nitrogen protected amino acid or peptide used in step (a) may be any amino acid, peptide or analogue thereof able to be linked to the solid support via functional side chain or C-α-carboxyl groups and provide the C-terminal of the desired peptide. The solid support in step (a) may be any support known in the art that is suitable for use in solid-phase peptide synthesis. The support may be any of those described above. Preferably the solid support is based on a polystyrene or polydimethylacrylamide polymer. More preferably the support is a copolymer of styrene with about 0.5 to 2% divinyl benzene as a cross-linking agent or a polydimethylacrylamide polymer comprising N,N-dimethy!acrylamide, N,N-bisacryloylethylenediamine and acryloylsarcosine methyl ester monomers. Details of these preferred supports and other suitable supports may be found in Chan and White, Fmoc Solid-Phase Peptide Synthesis, Oxford University Press, 2000 which is incorporated herein by reference. In step (a) the α-nitrogen protected first amino acid or peptide is usually attached to the solid support via a linker group. Any suitable linker group known in the art may be used. Examples of suitable linkers may be found in Chan and White, Fmoc Solid-Phase Peptide Synthesis, Oxford University Press, 2000, on pages 15 to 19 inclusive and page 20, which pages are incorporated herein by reference. Preferably the linker group comprises a trityl moiety, hydroxymethyl moiety, aminomethyl or xanthyl based moiety, more preferably a trityl moiety especially a 2- chlorotrityl moiety or suitably substituted variant thereof. A particularly preferred solid support is 2-chlorotrityl chloride polystyrene. Step (a) may be carried out using methods which are well known in the art and are described in many standard texts on the subject such as Atherton and Sheppard, Solid- Phase Peptide Synthesis - A Practical Approach, IRL Press at Oxford University Press, 1989 and Chan and White, Fmoc Solid-Phase Peptide Synthesis, Oxford University Press, 2000, which are incorporated herein by reference. Depending on the nature of the amino acid or peptide to be attached in step (a), and the linker it is to be bound to, the α-carboxyl in the α-nitrogen protected amino acid or peptide may need to be activated. The α-nitrogen protecting group may be any suitable group known in the art such as, for example, 9-fluorenylmethoxycarbonyl (Fmoc); te/t-butyloxycarbonyl (Boc); substituted sulfonylethyl carbamates such as the N-nitrophenyl sulfonylethoxy carbonyl (Nsc) group; 2-trimethylsilylethoxycarbonyl (Teoc); 1-methyl-1-(4- biphenylyl)ethoxycarbonyl (Bpoc); allyloxycarbonyl (Alloc); benzyloxycarbonyl (Cbz); amide-protecting groups, such as formyl, acetyl, trihaloacetyl, benzoyl, and nitrophenylacetyl; sulfonamide-protecting groups, such as 2-nitrobenzenesulfonyl; and imine- and cyclic imide-protecting groups, such as phthalimido and dithiasuccinoyl. Preferably the α-nitrogen protecting group is a urethane based protecting group. More preferably the α-nitrogen protecting group is a base-labile protecting group such as Fmoc or Nsc, especially Fmoc. When the resin is 2-chlorotrityl chloride polystyrene and an Fmoc α-nitrogen protecting group is used, step (a) would typically comprise suspending an α-nitrogen- Fmoc protected amino acid in a suitable solvent such as N,N-dimethylformamide, dichloromethane or a mixture thereof and then adding, with mixing, N,N- diisopropylethylamine (DIPEA). This α-nitrogen-Fmoc protected amino acid solution may then be added to the resin and allowed to react before collecting the resin and washing with a suitable solvent, such as N,N-dimethylformamide. The resin may then be optionally capped with a solution of methanol/DIPEA to ensure removal of any remaining reactive functional chloride followed by washing with N,N-dimethylformamide. The thioester moiety in the α-nitrogen protected α-carbon-thioester activated amino acid or peptide may be any thioester which is able to react with the free amino of the resin bound amino acid or peptide. The α-nitrogen protected α-carbon-thioester activated amino acid may be derived from any naturally occurring or synthetic amino acid and the amino acid side chain may comprise a protecting group. Preferably the α-nitrogen protected α-carbon-thioester activated amino acid is of Formula (1)

Formula (1) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; X is an amino protecting group; and Z is an optionally substituted hydrocarbyl. Hydrocarbyl groups that may be represented by Z independently include alkyl, alkenyl, aryl and heterocyclic groups, and any combination thereof, such as aralkyl and alkaryl, for example benzyl groups. Alkyl groups that may be represented by Z include linear and branched alkyl groups comprising up to 20 carbon atoms, particularly from 1 to 7 carbon atoms and preferably from 1 to 4 carbon atoms. When the alkyl groups are branched, the groups often comprising up to 10 branch chain carbon atoms, preferably up to 4 branch chain atoms. In certain embodiments, the alkyl group may be cyclic, commonly comprising from 3 to 10 carbon atoms in the largest ring and optionally featuring one or more bridging rings. Examples of alkyl groups that may be represented by Z include methyl, ethyl, propyl, 2-propyl, butyl, 2-butyl, t-butyl and cyclohexyl groups. Alkenyl groups that may be represented by Z include C₂.₂₀, and preferably C₂.₆ alkenyl groups. One or more carbon - carbon double bonds may be present. The alkenyl group may carry one or more substituents, particularly phenyl substituents. Examples of alkenyl groups include vinyl, styryl and indenyl groups. Aryl groups that may be represented by Z may contain 1 ring or 2 or more fused rings which may include cycloalkyl, aryl or heterocyclic rings. Examples of aryl groups that may be represented by Z include phenyl, tolyl, fluorophenyl, pentafluorophenyl, chlorophenyl, pentacholorphenyl, bromophenyl, trifluoromethylphenyl, nitrophenyl, anisyl, naphthyl and ferrocenyl groups. Heterocyclic groups that may be represented by Z independently include aromatic, saturated and partially unsaturated ring systems and may constitute 1 ring or 2 or more fused rings which may include cycloalkyl, aryl or heterocyclic rings. The heterocyclic group will contain at least one heterocyclic ring, the largest of which will commonly comprise from 3 to 7 ring atoms in which at least one atom is carbon and at least one atom is any of N, 0, S or P. Examples of heterocyclic groups that may be represented by Z include pyridyl, pyrimidyl, pyrrolyl, thiophenyl, furanyl, indolyl, quinolyl, isoquinolyl, benzthiazoyl, imidazoyl and triazoyl groups. When any of Z is a substituted hydrocarbyl group, the substituent(s) should be such so as not to adversely affect the ability of the thioester moiety to react with a free amine to yield an amide. Optional substituents include halogen, cyano, nitro, hydroxy, amino, thiol, acyl, hydrocarbyl, heterocyclyl, hydrocarbyloxy, mono or di- hydrocarbylamino, hydrocarbylthio, esters, carbamates, carbonates, amides, sulphonyl and sulphonamido groups wherein the hydrocarbyl groups are as defined for Z above. One or more substituents may be present. Examples of Z groups having more than one substituent present include -CF₃ and -C₆F₅. Preferably Z is an optionally substituted alkyl, optionally substituted aryl or optionally substituted heteroaryl group. Preferably when Z is an aryl or hetroaryl group the aryl or hetroaryl group is ortho or para substituted, or both, with electron withdrawing functional groups, such as carboxy, phosphato, and sulpho groups and esters thereof, nitro, cyano, halo, -SO₂F, -COR^a, -CONR^aR^b, sulphoxide, sulphone, and -SO₂NR^aR^b wherein R^a and R^b are each independently H, optionally substituted aryl, especially phenyl, or optionally substituted alkyl (especially C^-alkyl) or, in the case of -CONR^aR^b and -SO₂NR^aR^b, R^a and R^b may also together with the nitrogen atom to which they are attached represent an aliphatic or aromatic ring system. Preferably the α-nitrogen protected α-carbon-thioester activated amino acid of

Formula (1) is an L-amino acid. X, the amino protecting group in Formula (1), is preferably a urethane based protecting group, especially an acid-stable base-labile protecting group. More preferably, X, the α-nitrogen protecting group, is Fmoc or Nsc, especially Fmoc. Thus a preferred compound of Formula (1) is of Formula (2);

Formula (2) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; X² is Fmoc or Nsc; and Z is an optionally substituted hydrocarbyl group. The optionally substituted hydrocarbyl group, Z, may be any group that allows the thioester moiety to react with a free amine to yield an amide. Preferably Z is an optionally substituted alkyl, optionally substituted aryl or optionally substituted heteroaryl group. More preferably the aryl or hetroaryl is ortho or para substituted, or both, with electron withdrawing functional groups In one embodiment it is particularly preferred that Z comprises optionally substituted phenyl group especially phenyl, nitrophenyl, pentachlorophenyl and pentafluorophenyl groups; or an optionally substituted heteroaromatic group with a nitrogen ortho to the bridging sulfur such as a 2-pyridyl or 2-benzothiazolyl group. Most preferably Z is pentafluorophenyl group. A first particularly preferred α-nitrogen protected α-carbon-thioester activated amino acid is of Formula (3)

Formula (3) wherein R is an amino acid side chain, a protected amino acid side chain or a peptide chain. A second preferred α-nitrogen protected α-carbon-thioester activated amino acid is of Formula (4)

Formula (4) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; Y is S, O, NR²; R¹ is an optional substituent; R² is optionally substituted C^ alkyl; and n is 0 to 4. Y is preferably S. R¹ is preferably an electron withdrawing substituent. Preferably n is 0. Compounds of Formulae (1) to (4) may be prepared by, for example, reacting an Fmoc α-nitrogen-protected amino acid with a thiol, such as pentafluorothiophenol, in the presence of 1 ,3-dicyclohexylcarbodiimide (DCC) in dichloromethane for 1 hour at 0-5°C and then 18 hours at room temperature. When the α-nitrogen protected α-carbon-thioester activated amino acid monomer is immobilised on a solid support it is preferably of Formula (5):

Formula (5) wherein: SUPPORT is a solid support; L is a linking group; R is an amino acid side chain; a protected amino acid side chain or a peptide chain; and X³ is an amino protecting group or one or more amino acids linked by amide bonds where the terminal residue bears an amino protecting group. Preferably the α-nitrogen protected α-carbon-thioester activated amino acid of Formula (6) is an L-amino acid. The protecting group X³ may be any amino protecting group known to one skilled in the art. Preferably X³ is a urethane based protecting group, especially an acid-stable base-labile protecting group such as Fmoc or Nsc, particularly Fmoc. Thus, a preferred compound of Formula (5) is of Formula (6).

Formula (6) wherein: SUPPORT is a solid support; L is a linking group; X⁴ is Fmoc or Nsc; and R is an amino acid side chain; a protected amino acid side chain or a peptide chain. The linker group, L is any group that allows the thioester moiety to react with a free primary or secondary amino group to yield an amide, or a free hydroxyl group to yield an ester. Preferably L is an optionally substituted hydrocarbyl. More preferably L is optionally substituted alkyl, optionally substituted aryl or optionally substituted heteroaryl.

More preferably the aryl or hetroaryl is ortho or para substituted or both with electron withdrawing functional groups. The linker group, L, may comprise two components. The first component is bound to the thiol group in the thioester and its nature is governed by the requirement that the thioester moiety should be able to react with a free primary or secondary amino group to yield an amide, or a free hydroxyl group to yield an ester. The second, optional, component, L¹, is required to bind the first component to the solid support and may be selected from the linker groups as described and preferred for linkers in step (a) above. The second component may also be chosen so as to enhance the activity of the first component. In one embodiment it is particularly preferred that L comprises optionally substituted phenyl especially phenyl, nitrophenyl, tetrachlorophenyl and tetrafluorophenyl; or an optionally substituted heteroaromatic group with a nitrogen ortho to the bridging sulfur such as 2-pyridyl or 2-benzothiazolyl. Preferably L comprises 2-mercaptobenzothiazolyl or tetrafluorothiophenyl. A first particularly preferred α-nitrogen protected α-carbon-thioester activated amino acid immobilised to a solid support is of Formula (7)

R Fmoc w H SUPPORT

Formula (7) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; L¹ is an optional linking group; and SUPPORT is a solid support. A second particularly preferred α-nitrogen protected α-carbon-thioester activated amino acid immobilised to a solid support is of Formula (8)

Formula (8) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; Y is S, O or N-R² R¹ is an optional substituent; R² is optionally substituted C^alkyl; L¹ is an optional linking group; SUPPORT is a solid support; and n is 0 to 4. Y is preferably S R¹ is preferably an electron withdrawing substituent. Preferably n is 0. Preferred solid supports for the compounds of Formulae (5) to (8) are as described and preferred above. Compounds of Formula (5) may be prepared by, for example, reacting a α- nitrogen protected amino acid with a solid support bearing a thiol derivative in the presence of diisopropylcarbodiimide (DIC) in N, N-dimethylformamide for 1 hour at 0-5°C and then for 1 hour at room temperature. The α-nitrogen protected amino acid may either be purchased or prepared using methods that are well known to one skilled in the art. The solid support bearing a thiol derivative may be prepared by, for example, reacting a S-protected derivative of a thiol, such as 2-mercapto-benzothiazole-6- carboxylic acid (obtainable from Oakwood), with a suitable resin, such as aminomethyl polystyrene resin (AM-PS, available from Novabiochem), in the presence of TBTU and DIPEA in DMF for 1 hour at room temperature. The S-protecting group is then removed by suitable means leaving 2-mercapto-benzothiazole-6-carboxamide attached to the solid support. One suitable S-protected 2-mercapto-benzothiazole-6-carboxylic acid derivative is the symmetrical disulfide that may be prepared by oxidation in H₂O/CH₃CN (basified to pH

8.5-10 with ammonia). This disulfide may also be prepared by reacting 2-mercapto- benzothiazole-6-carboxylic acid with Aldrithiol-2 (obtainable from Sigma-Aldrich) (2,2'- dipyridyl disulfide) in H₂O/CH₃CN. In another approach 2-mercapto-benzothiazole-6-carboxylic acid may be selectively protected as an S-trityl derivative by reaction with triphenyl methanol. Removal of S-protection from the support bound resin may be achieved via reduction with dithiothreitol (DTT) (15 equivalents) in the presence of a catalytic amount of diisopropylethylamine (0.4 equivalents) in DMF at 40°C overnight. S-Trityl based protection may be removed by acidolysis to afford a free thiol. Preparation of a Fmoc α-nitrogen protected amino acid derivatised 2-mercapto- benzothiazole-6-carboxyamide support can be achieved as followed. A sample of 2-mercapto-benzothiazole-6-carboxamide AM-PS is pre-swollen in DMF for 1hr and cooled to 0-5°C. Solid Fmoc-Phe-OH (5 equivalents with respect to resin loading) is then added. The suspension is agitated to dissolve the Fmoc-amino acid. Once dissolved a single charge of diisopropylcarbodiimide (DIC, 5 equivalents) and optionally a catalytic amount of DMAP (0.1eq) in DMF is added. The resultant suspension is then agitated for 1hr at 0-5°C and then 1hr at room temperature. The resin is then washed with DMF, DCM and finally ethyl ether. The Fmoc-Phe-2-mercaptobenzothiazole thioester resin is then dried to constant weight 'in-vacuo'. The resin substitution level (loading) of the Fmoc- amino acid may be calculated using the Fmoc-UV analysis test. Coupling of the amino acid to the solid support in step (a) is preferably by the α- carbon. However coupling may also be through the side chain of the first amino acid as described in WO03/093301 or WO03/093302 both of which are incorporated herein in their entirety. Coupling in the present invention, for example in step (b) and (c), may be carried out with a preformed α-nitrogen protected α-carbon-thioester activated amino acid or alternately the desired α-nitrogen protected α-carbon-thioester activated amino acid may be formed in situ by reacting an α-nitrogen protected amino acid with a thiol under conditions in which a thioester may form. For example, compounds of Formula (3) may be formed by mixing a α-nitrogen Fmoc protected amino acid with pentafluorothiophenol in N, N-dimethylformamide in the presence of diisopropylcarbodiimide (DIC). In the present invention, for example in steps (b) and (c), protecting groups may be used to protect susceptible side chains which could otherwise be modified in the coupling and deprotection cycles. Examples of amino acids with susceptible side chains are Cys, Arg, Ser, Tyr, Thr, Lys, Orn, Asp, Glu, Trp, His, Asn, Gin, Har, Dpr, Dab, Dht, Gla, Hser, Hyp and Pen. Alternatively, a post solid-phase synthesis chemical modification of the peptide may be carried out to yield a desired side chain. For example following the process of US 5,318,899 homoarginine can be prepared by guanidation of a lysine residue. In steps (c) and (d) the α-nitrogen protecting group on the N-terminal amino group of the desired peptide may be the same of different from the α-nitrogen protecting groups used to protect amino acids linked in the growing chain. The α-nitrogen protecting group on the N-terminal amino group may be any group known in the art but preferably is Fmoc, Nsc or Boc More preferably the α-nitrogen protecting group on the N-terminal residue of the desired peptide is Boc since this allows the N-terminal protecting group to be conveniently removed in step (d) at the same time as the peptide is liberated from the solid support and the side chain protecting groups are removed. Conditions for cleaving the peptide from the solid support in step (d) would be well known to one skilled in the art. In step (d) any side chain protecting groups may be removed either before or after the link between the solid support and the peptide has been cleaved. However, it is preferred that the peptide is cleaved from the solid support and that the N-terminal protecting group and any side chain protecting groups are removed in a single process. Typically this may be achieved by treating the peptide resin with trifluoroacetic acid in the presence of suitable scavengers such as, for example, triisopropylsilane, phenol, ethanedithiol and/or water. In step (d) during the cleavage it is possible to generate α-carbon-modified peptide analogues such as peptide amides. For example when an 4-hydroxy-methylbenzoic acid (HMBA) agent has been employed as a linker removal of the side chain and the N-terminal protecting group and cleavage of the peptide moiety from the solid support is typically performed as a 2 step process. In the first step all acid labile protection is removed by treating the peptide-resin with a mixture of trifluoroacetic acid (TFA) and suitable scavengers. This resin is then filtered and washed with TFA, DCM, 10% v/v DIPEA in DCM then dichloromethane to remove deprotection adducts. This resin is then subject to ammonolysis to remove the peptide from the solid support as a peptide amide. Alternative deprotection-cleavage processes are known for HMBA derivatised peptides can yield a variety of α-carbon-modified peptides. These methods and procedures are detailed in the Novabiochem Catalogue 2003/4, P. 3.17-3.18 which reference is included herein in its entirety. Isolation and purification of the peptide may be carried out using standard procedures and techniques that would be well known to one skilled in the art. These methods include precipitation of the peptide in a solvent that will not affect its integrity, such as diisopropylether, followed by preparative reverse phase HPLC, ion-exchange chromatography and/or salt exchange. The isolated peptide may be subjected to further processing, such as, for example ligation, PEGylation, conjugation, etc When the thioester on the second monomer is immobilised on a solid support it is preferably as described in the third aspect of the invention. The process of the present invention may be used to prepare any peptide containing polymer such as a peptide acid, a peptide amide, a peptide hydrazide, peptide alcohol, a peptide aldehyde, a peptide ester, a protected peptide, a branched peptide, a cyclic peptide, a nucleopeptide, a glycopeptide, a phosphopeptide, a PEGylated peptide, a lipopeptide, peptide libraries, peptide prodrugs and conjugates thereof. A second aspect of the invention provides an α-nitrogen protected α- carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide wherein the α-nitrogen protecting group is other than Boc. The α-nitrogen protecting group is preferably 9-fluorenylmethoxycarbonyl (Fmoc); substituted sulfonylethyl carbamates such as the N-nitrophenyl sulfonylethoxy (Nsc) group; 2-trimethylsilylethoxycarbonyl (Teoc); 1-methyl-1-(4-biphenylyl)ethoxycarbonyl (Bpoc); allyloxycarbonyl (Alloc); benzyloxycarbonyl (Cbz); amide-protecting groups, such as formyl, acetyl, trihaloacetyl, benzoyl, and nitrophenylacetyl; sulfonamide-protecting groups, such as 2-nitrobenzenesulfonyl; and imine- and cyclic imide-protecting groups, such as phthalimido and dithiasuccinoyl. Preferably the α-nitrogen protecting group is a urethane based protecting group, more preferably an acid-stable base-labile group, particularly Fmoc or Nsc, especially Fmoc. The α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide may be derived from any naturally occurring or synthetic amino acid and the amino acid side chain(s) may comprise a protecting group. Preferably the α-nitrogen protected α-carbon-thioester activated amino acid is of Formula (9);

Formula (9) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; X⁵ is a base-labile amino protecting group; and Z is an optionally substituted hydrocarbyl group. The α-nitrogen protected Cα-thioester activated amino acid of Formula (9) is preferably an L-amino acid. A preferred compound of Formula (9) is of Formula (10);

Formula (10) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; X⁶ is Fmoc or Nsc; and Z is an optionally substituted hydrocarbyl group. The optionally substituted hydrocarbyl group, Z, may be any group that allows the thioester moiety to react with a amine to yield an amide. Preferably Z is an optionally substituted alkyl, optionally substituted aryl or optionally substituted heteroaryl group. In a preferred embodiment the aryl or hetroaryl is ortho or para substituted or both with electron withdrawing functional groups. In one particularly preferred embodiment Z comprises an optionally substituted phenyl group especially a phenyl, nitrophenyl, pentachlorophenyl and pentafluorophenyl group; or an optionally substituted heteroaromatic group with a nitrogen ortho to the bridging sulfur such as a 2-pyridyl or 2-benzothiazolyl group. Most preferably Z is a pentafluorophenyl group. A particularly preferred α-nitrogen protected α-carbon-thioester activated amino acid is of Formula (11 )

Formula (11) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; and X⁶ is Fmoc or Nsc. A second preferred α-nitrogen protected α-carbon-thioester activated amino acid is of Formula (12)

Formula (12) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; X⁶ is Fmoc or Nsc; Y is S, O, NR²; R¹ is an optional substituent; R² is optionally substituted C^alkyl; and n is 0 to 4. Y is preferably S. R¹ is preferably an electron withdrawing substituent. Preferably n is 0. In the compounds of Formulae (9) to (12) X is preferably Fmoc. Compounds of Formulae (9) to (12) may be prepared by, for example, reacting an Fmoc α-nitrogen-protected amino acid with a thiol, such as pentafluorothiophenol, in the presence of 1 ,3-dicyclohexylcarbodiimide (DCC) in dichloromethane for 1 hours at 0-5°C and then 18 hours at room temperature. According to a third aspect of the invention there is provided an α-nitrogen protected α-carbon-thioester activated amino acid monomer immobilised on a solid support. Preferably the α-nitrogen protected α-carbon-thioester activated amino acid monomer immobilised on a solid support is a compound of Formula (5):

Formula (5) wherein: SUPPORT is a solid support; L is a linking group; R is an amino acid side chain; a protected amino acid side chain or a peptide chain and X³ is an amino protecting group or one or more amino acids linked by amide bonds where the terminal residue bears an amino protecting group. Preferably the α-nitrogen protected α-carbon-thioester activated amino acid of

Formula (6) is an L-amino acid. The protecting group X³ may be any amino protecting group known to one skilled in the art. Preferably X³ is a base-labile protecting group such as Fmoc or Nsc, especially Fmoc. Thus a preferred compound of Formula (5) is of Formula (6).

Formula (6) wherein: SUPPORT is a solid support; L is a linking group; X⁴ is Fmoc or Nsc; and R is an amino acid side chain; a protected amino acid side chain or a peptide chain. The linker group, L. may be any group that allows the thioester moiety to react with a free primary or secondary amino group to yield an amide, or a free hydroxyl group to yield an ester. Preferably L is a hydrocarbyl. More preferably L is optionally substituted alkyl, optionally substituted aryl or optionally substituted heteroaryl. More preferably the aryl or hetroaryl is ortho or para substituted or both with electron withdrawing functional groups. The linker group, L, may comprise two components. The first component is bound to the thiol group in the thioester and its nature is governed by the requirement that the thioester moiety should be able to react with a free primary or secondary amino group to yield an amide, or a free hydroxyl group to yield an ester. The second component, L¹, is required to bind the first component to the solid support and may be selected from the linker groups as described and preferred for step (a) above. In one embodiment it is particularly preferred that L comprises an optionally substituted phenyl group, especially a phenyl, nitrophenyl, tetrachlorophenyl and tetrafluorophenyl group; or an optionally substituted heteroaromatic group with a nitrogen ortho to the bridging sulfur such as 2-pyridyl or 2-bezothiazolyl group. Preferably L comprises 2-mercaptobenzothiazolyl or tetrafluorothiophenyl. A first particularly preferred α-nitrogen protected α-carbon-thioester activated amino acid immobilised to a solid support is of Formula (7)

Formula (8) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; Y is S, O or N-R² R¹ is an optional substituent; R² is optionally substituted C^alkyl; L¹ is an optional linking group; SUPPORT is a solid support; and n is 0 to 4. Y is preferably S. R¹ is preferably an electron withdrawing substituent. Preferably n is 0. Compounds of Formulae (5) to (8) may be prepared as described in the first aspect of the invention. Preferred solid supports are as described in the first aspect of the invention. The invention is now illustrated, but not limited, by the following Examples

Example 1

Preparation of Fmoc Amino Acid Pentafluorothiophenvl Active Esters

Synthesis of Fmoc-Phe-S-Pentafluoroohenol

Abbreviations DCM Dichloromethane DCCI 1 ,3-Dicyclohexylcarbodiimide DIC Diisopropylcarbodiimide DMF N,N-Dimethylformamide Fmoc 9-Fluorenylmethoxycarbonyl HOBt 1 -Hydroxybenzotriazole HS-Pfp Pentafluorothiophenol TBTU 2-[1 H-benzotriazole-1-yl]-1 ,1 ,3,3-tetramethyluronium tetrafluoroborate TIPS Triisopropylsilane TFA Trifluoroacetic acid TLC Thin Layer Chromatography

Fmoc-Phe-OH (2.50g) was suspended in 35 ml HPLC grade DCM in a 250ml round bottomed flask. The suspension was cooled, with stirring, to 0-5°C in an ice bath over a 10 min period. HS-Pfp (1.72ml) was added to the suspension. A aliquot of DCCI (1.771g) was dissolved separately in DCM (20ml) and then added slowly to the cooled, reaction mixture over a 1 -minute period. A clear yellow solution was apparent after 1 minute. This reaction mixture was stirred at 0-5°C for 1 hour and then allowed to reach room temperature when stirring was continued for 18hr in total. At the end of this time the suspension was filtered through a Whatman 1 paper to remove the unwanted urea and the pale yellow filtrate was evaporated to dryness to afford a waxy semi-solid. This residue was reconstituted in DCM (10-12ml) and passed down a 20 x 3cm silica column (Kieselgel 60, 0.063-0.200mm, Merck) with DCM as the mobile phase. 20ml Fractions were collected. Elution of the product from the flash column was monitored by TLC (UV, 254nm). The desired fractions were pooled and evaporated to dryness to afford a foam (experimental yield 84%, 3.1 Og). RP-HPLC analysis of the product indicated a single component of purity greater than 95% (retention time 29.1 min). Mass Spectoscopy and proton NMR confirmed the identity of the product.

RP-HPLC Conditions:

Column Vydac Peptide & Protein C18, 250 x 4.6mm Buffer A 0.1 % v/v TFA in H₂O Buffer B 0.1% v/v TFA in acetonitrile Gradient 5 to 95% Buffer B in Buffer A over 30 minutes Flow rate 1 ml/min^"1 Detection 214nm

Example 2 Use of SH-Pfp in an 'In-Situ' Solid-phase Peptide Synthesis Activation Protocols Solid-phase Synthesis of [Leu⁵] enkephalin (H-Tyr-Glv-Glv-Phe-Leu)

H-Leu-O-2-Chlorotrityl PS resin (2-CITrt PS, 1.00g, 1.12 mmole/g, 1.12mmole) from Novabiochem was pre-swollen in DMF for 1 hr. The resin was then filtered and subjected to a Kaiser resin test for free primary amine. The result indicated a strong positive (blue/black liquor). HS-Pfp (0.448ml) was added to Fmoc-Phe-OH (0.651 g) in DMF (3-4ml). Upon addition of the HS-Pfp an intense yellow colour was noted. After mixing for 1 minute the mixture was cooled to 0-5°C in an ice bath. DIC (0.349ml) was then added, which caused the mixture to turn a deeper yellow colour, and the mixture was agitated for a further 1 minute before being added to the resin in a dry vessel. After about 2 minutes the deep yellow colour subsided. The coupling reaction was monitored using the Kaiser test and allowed to go completion, which took about 30 minutes. The Fmoc protecting group was removed by treating the resin twice with 20% piperidine in DMF (2 x 10ml). In the first treatment the resin and piperidine/DMF mixture were agitated gently for 3 minutes before removing the piperidine/DMF by filtration. In the second treatment the resin and piperidine/DMF mixture were agitated gently for 7 minutes before removing the piperidine/DMF by filtration. Fmoc deprotection gave a brightly yellow coloured solution. The resin was then washed 10 times (10 x 10-15ml) in DMF and the yellow colour was removed after the third wash. The process described above was then repeated coupling Fmoc-Gly-OH to the resin bound Phe, followed by a second Fmoc-Gly-OH and then Fmoc-Tyr(tBu) — OH. Monitoring of the solid phase assembly was assisted by the use of the Kaiser resin test for free primary amine. Upon completion of the desired sequence the N-terminal Fmoc protecting group was removed by treating the resin twice with 20% piperidine in DMF (2 x 10ml). In the first treatment the resin and piperidine/DMF mixture were agitated gently for 3 minutes before removing the piperidine/DMF by filtration. In the second treatment the resin and piperidine/DMF mixture were agitated gently for 7 minutes before removing the piperidine/DMF by filtration. Fmoc deprotection gave a brightly yellow coloured solution. The resin was then washed 10 times (10 x 10-15ml) in DMF and the yellow colour was removed after the third wash. The process afforded the desired resin-bound protected peptide.

H-Tyr(tBu)-Gly-Gly-Phe-Leu-O-2-CI Trt PS

Following completion of assembly of the peptide the peptidyl-resin was washed 5 times with DCM (5 x 10-15ml) and then 5 times with methanol (5 x 10-15ml) and air-dried before being transferred to a 50ml round bottomed flask. A pre-mixed solution of TFA / TIPS / H₂O (95%v/v: 2.5:2.5, 15ml) was added to the dry resin (1.38g), causing an immediate dark ruby red colour, and the resultant suspension was agitated for 1 hour at room temperature (24°C). The resin was then filtered through a glass sinter and washed with TFA (5 x twice resin bed volume). The filtrates were combined and evaporated to near dryness to yield a pale yellow oil. This crude peptide product was precipitated from diethyl ether to yield a white solid which was collected by filtration and dried 'in-vacuo' to constant weight. A sample of the material was analysed by RP-HPLC, using the conditions described in Example 1 , a single peak (retention time 12.6 min) corresponding to a standard of the [Leu⁵] enkephalin previously synthesised by standard TBTU means on an identical support. A co-injection confirmed the identity of the crude product. Thus, this example demonstrates that the reagent SH-Pfp may be used in place of HOBt as an 'in-situ' activation reagent in a standard active ester activation procedure to afford [Leu⁵] enkephalin in a similar purity to that assembled by standard means. By using only 1.5 equivalents of Fmoc amino acid in each of the amino acid coupling steps in the assembly we have shown that formation of the S-Pfp ester is favoured over the corresponding symmetrical anhydride (as would be expected from the analogous HOBt active ester method). Non-formation of the S-Pfp ester would yield only 0.75 equivalents of the corresponding symmetrical anhydride reagent. This would prohibit the coupling reaction from reaching completion once the symmetrical anhydride is exhausted in the coupling step.

Claims

1. A process for the solid-phase synthesis of a polymer which comprises reacting an amine, secondary amine or hydroxyl group of a first monomer with a thioester on a second monomer wherein at least one of the monomers is immobilised on a solid-phase support.

2. A process according to claim 1 for the solid-phase synthesis of a polymer comprising amide bonds which comprises reacting an amine or secondary amine of a first monomer with a thioester on a second monomer wherein at least one of the monomers is immobilised on a solid-phase support.

3. A process according to either claim 1 or claim 2 for the solid-phase synthesis of a peptide.

4. A process according to claim 3 for the solid-phase synthesis of a peptide where an α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide is reacted with the free α-amino of an amino acid or peptide wherein at least one of the monomers is immobilised on a solid-phase support.

5. A process according to either claim 3 or claim 4 for the solid-phase synthesis of a peptide which comprises attaching an α-nitrogen protected amino acid or peptide to a solid support, removing the α-nitrogen protecting group and assembling a peptide chain on said α-nitrogen by sequentially reacting with α-nitrogen protected α-carbon-thioester activated amino acids or α-nitrogen protected α-carbon-thioester activated peptides.

6. A process according to any one of claims 3 to 5 for the solid-phase synthesis of a peptide which comprises the steps of: (a) attaching an α-nitrogen protected, first amino acid or peptide to a solid support; (b) deprotecting the α-nitrogen of the attached first amino acid or peptide by removing the α-nitrogen protecting group under conditions such that the attached first amino acid or peptide remains connected to the solid support and coupling an α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide to the unprotected α-nitrogen of the attached amino acid or peptide to yield an attached peptide with a protected α-nitrogen; (c) deprotecting the α-nitrogen of the attached peptide by removing the α-nitrogen protecting group under conditions such that the attached peptide remains connected to the solid support and coupling an additional α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide to the unprotected α-nitrogen of the attached peptide and repeating until the desired peptide with a protected N- terminal amino group is assembled on the solid support; (d) removing the N-terminal protecting group from the protected N-terminal amino group, cleaving the link between the peptide and the solid support so that the peptide is released from the solid support and optionally removing any side chain protecting groups.

7. A process according to claim 6 where the peptide released in step (d) is preferably isolated and then purified.

8. A process according to either claim 6 or claim 7 wherein the α-nitrogen protected first amino acid or peptide is attached to the solid support via a linker group.

9. A process according to any one of claims 6 to 8 wherein the linker group comprises a trityl moiety.

10. A process according to any one of the preceding claims wherein the α-nitrogen protecting group is a base-labile protecting group.

11. A process according to claim 10 wherein the α-nitrogen protecting group is Fmoc or Nsc.

12. A process according to either claim 10 or claim 11 wherein the α-nitrogen protecting group is Fmoc.

13. A process according to any one of claims 4 to 12 wherein the α-nitrogen protected α-carbon-thioester activated amino acid is of Formula (3)

Formula (3) wherein R is an amino acid side chain, a protected amino acid side chain or a peptide chain.

14. A process according to any one of claims 4 to 12 wherein the α-nitrogen protected α-carbon-thioester activated amino acid is of Formula (4)

Formula (4) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; Y is S, O, NR²; R¹ is an optional substituent; R² is optionally substituted C^ alkyl; and n is 0 to 4.

15. A process according to any one of claims 4 to 12 wherein the α-nitrogen protected α-carbon-thioester activated amino acid is of Formula (5):

Formula (5) wherein: SUPPORT is a solid support; L is a linking group; R is an amino acid side chain; a protected amino acid side chain or a peptide chain and X³ is an amino protecting group or one or more amino acids linked by amide bonds where the terminal residue bears an amino protecting group.

16. A process according to claim 15 where the α-nitrogen protected α- carbon-thioester activated amino acid is of Formula (7)

Formula (7) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; L¹ is an optional linking group; and SUPPORT is a solid support.

17. A process according to claim 15 where the α-nitrogen protected α- carbon-thioester activated amino acid is of Formula (8)

^Ll- SUPPORT

Formula (8) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; Y is S, O or N-R² R¹ is an optional substituent; R² is optionally substituted C^alkyl; L¹ is an optional linking group; SUPPORT is a solid support; and n is 0 to 4.

18. A α-nitrogen protected α-carbon-thioester activated amino acid or α-nitrogen protected α-carbon-thioester activated peptide wherein the α-nitrogen protecting group is other than Boc.

19. A α-nitrogen protected α-carbon-thioester activated amino acid according to claim

18 of Formula (11)

Formula (11) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; and X⁶ is Fmoc or Nsc.

20. A α-nitrogen protected α-carbon-thioester activated amino acid according to claim 18 of Formula (12)

Formula (12) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; X⁶ is Fmoc or Nsc; Y is S, O, NR²; R¹ is an optional substituent; R² is optionally substituted C₁-₄alkyl; and n is 0 to 4.

21. A α-nitrogen protected α-carbon-thioester activated amino acid monomer immobilised on a solid support.

22. A α-nitrogen protected α-carbon-thioester activated amino acid monomer immobilised on a solid support according to claim 21 of Formula (7)

R .. .. - -¹-HSUPPORT Fmoc '- .if..V

23. A α-nitrogen protected α-carbon-thioester activated amino acid monomer immobilised on a solid support according to claim 21 of Formula (8)

Formula (8) wherein: R is an amino acid side chain, a protected amino acid side chain or a peptide chain; Y is S, O or N-R² R is an optional substituent; R² is optionally substituted d-₄alkyl; L¹ is an optional linking group; SUPPORT is a solid support; and n is 0 to 4.