WO2003000662A2

WO2003000662A2 - Chiral preparation of a diastereomeric compound

Info

Publication number: WO2003000662A2
Application number: PCT/US2002/019881
Authority: WO
Inventors: Stephen W. Horgan; Timothy A. Ayers; Timothy James-Norman Watson
Original assignee: Aventis Pharmaceuticals Inc.
Priority date: 2001-06-22
Filing date: 2002-06-24
Publication date: 2003-01-03
Also published as: WO2003000662A3; AU2002315415A1; WO2003000662A8

Abstract

This invention is directed to steps of a process for the preparing Compound A as shown in Scheme I. The invention is also directed to intermediates, compounds i-viii, used in Scheme I.

Description

CHIRAL PREPARATION OF A DIASTEREOMERIC COMPOUND

Field of the Invention

This invention relates to a stereospecific process for the preparation of β-[[(25)-2-[4,4- dimethyl-3-[[4-[[[(2-methylphenyl)amino]-carbonyl]amino]phenyl]methyl]-2,5-dioxo-l- imidazolidmyl]-4-methyl-l-oxopentyl]amino]-(βS)-phenylpropanoic acid (hereinafter referred to as 'Compound A). Compund A is a VLA-4 antagonist that possesses excellent anti- inflammatory,

Compound A immunosuppressive and anti-allergic activity. This invention is also directed to intermediates useful in the process for preparing Compound A.

Background of the Invention

The preparation of Compound A is described in Example 17 in German Patent Application DE19751251 and US patent No. 6,331,552.. The steps in Scheme A below represent the process for preparing Compound A. That process discloses the stereospecific introduction of one chiral center in the molecule in the condensation reaction using (3S)-amino-3-phenylpropanoic acid. US patent No. 6,331,552 describes that one arrives at individual diastereomers, such as compound 6 to yield compound A, by resolving of the compound 6 or intermediates leading thereto, e.g., compounds 3-5, bearing the 2-methylpropyl moiety used in Scheme A. An alternate stereospecific method for preparing Compound A with a high degree of diastereomeric purity would be useful. Scheme A

Summary of the Invention

This invention is directed to steps of a process for the preparing Compound A as shown in Scheme I. The invention is also directed to intermediates, compounds i-viii, used in Scheme I. Scheme I

Detailed Description of the Invention

As used above, and throughout the description of the invention, the following abbreviations, unless otherwise indicated, shall be understood to have the following meanings:-

Designation Reagent or Fragment

BOC or Boc tert-butyl carbamate BOP benzotriazol- 1 -yl-oxy tris (dimethylamino)phosphonium hexaflurophosphate t-Bu tert-butyl

DCC dicyclocarbodiimide

DCM dichloromethane (CH₂C1₂)

DIC 1,3-diisopropylcarbodiimide

DIPEA diisopropylethylamine

DMAP 4-(N,N-dimethylamino)pyridine

DMF dimethylformamide

DMSO dimethylsulfoxide

EA elemental analysis

EDCI l-ethyl-3-(3-dimethylaminopropyl) carbodiimide HCI

Et ethyl

Et₂O diethyl ether

EtOH ethanol

EtOAc ethyl acetate

FMOC 9-fluorenylmethoxycarbonyl

HOAt 1 -hydroxy-7-azabensotriazole

HOBT 1 -hydroxybenztriazole

HOSu N-hydroxysuccinamide

Me methyl

TFA trifluoroacetic acid

Tf₂O triflate anhydride

THF tetrahydrofuran

THP tetrahydropyran Definitions of Terms

As used above, and throughout the description of the invention, the following terms, unless otherwise indicated, shall be understood to have the following meanings. "Alkyl" means an aliphatic hydrocarbon group which may be straight or branched having about 1 to about 20 carbon atoms in the chain. Preferred alkyl groups have 1 to about 12 carbon atoms in the chain. Branched means that one or more lower alkyl groups such as methyl, ethyl or propyl are attached to a linear alkyl chain. "Lower alkyl" means about 1 to about 4 carbon atoms in the chain which may be straight or branched. The alkyl is optionally substituted with one or more halo or C, ₆ alkoxy which may be the same or different.

"Carboxylic acid protecting group" and "acid protecting group" mean an easily removable group which is known in the art to protect a carboxylic acid (-CO₂H) group against undesirable reaction during synthetic procedures and to be selectively removable. The use of carboxylic acid protecting groups is well known in the art and many such protecting groups are known, CF, for example, T.H. Greene and P.G.M. Wuts, Protective Groups in Organic Synthesis, 2nd edition, John Wiley & Sons, New York (1991), incorporated herein by reference. Examples of carboxylic acid protecting groups include esters such as methoxymethyl, methylthiomethyl, tetrahydropyranyl, benzyloxymethyl, substituted and unsubstituted phenacyl, 2,2,2-trichloroethyl, tert-butyl, cinnamyl, substituted and unsubstituted benzyl, trimethylsilyl, allyl. Preferred carboxylic acid protecting groups are tert-butyl and benzyl.

"Coupling agent" means a compound that reacts with the hydroxyl moiety of a carboxy moiety thereby rendering it susceptible to nucleophilic attack. Exemplary coupling agents include DIC, EDCI, DCC, and the like.

"Hydrogenation labile acid protecting group" means an acid protecting group as defined herein which is readily removed by hydrogenation while remaining relatively stable to other reagents. A preferred hydrogenation labile acid protecting group is benzyl. "Patient" includes both human and other mammals.

Preferred Embodiments

Preparation of Compounds of the Invention

Compounds of the invention may be prepared by the application or adaptation of known methods, by which is meant methods used heretofore or described in the literature, for example those described by R.C. Larock in Comprehensive Organic Transformations, VCH publishers (1989). We have found that the use of dichloromethane as solvent in Stage 5 (formation of the urea) results in isolation of the (S,S)-diastereomer due to an unexpected solubility difference between this and the (R,5)-diastereomer. As a consequence, product which is isolated initially had high de (>96%) and furthermore the chiral purity could be further improved by slurrying it in hot dichloromethane. Consequently, the present process is particularly suitable for the large scale synthesis of CompoundA.

According to a first aspect, the present invention provides multiple steps leading for preparing Compound A: The first step in Scheme I is directed to the reacting of the 2-methyl-N-[(4-nitrophenyl)- methyl]alanine methyl ester (i) with alkali metal (M) cyanate such as sodium cyanate or potassium cyanate, more preferably sodium cyanate, in the presence of organic acid such as lower .₄ alkanoic acid, preferably acetic acid at a temperature from about 20°C to about 80°C to yield the corresponding hydantoin (ii). The second step in Scheme I is directed to reacting compound ii with an alkali metal (M) containing strong base such as alkali metal hydroxide, preferably potassium hydroxide, in an appropriate organic solvent such as an lower Cι_₆ alcohol, preferably ethanol, at a temperature form about 0°C to about 50°C, peferably at about 50°C, to yield the alkali salt of the hydantoin (iii)

The third step in Scheme I is directed to alkylating compound iii with the protected triflate of the following formula to provide compound iv that is converted according to the fourth step in

Scheme I without isolation into the compound v. This alkylation is achieved with stereochemical inversion and a high stereospecificity, >95% enantiomeric excess. The alkylation is effected in a suitable aprotic solvent, such as l-methyl-2-pyrrolidinone, at a temperature from about -15°C to about ambient temperature. The subsequent deprotection is carried out under appropriate deprotection conditions, that depends on the nature of the protecting agent, i.e., whether it is removable (labile) under acid, base, or hydrogenation conditions, and other reactive moieties in the compound undergoing deprotection, i.e., a deprotecting agent is chosen to carry out the deprotection without effecting the other reactive moieties unless a concomitant reaction is desired. For example deprotection can be effected by treating the reaction mixture with an aqueous acid such as sulfuric acid, at a temperature from about ambient temperature to about 90°C, preferably at about 90°C. The deprotecting is carried out using an appropriate deprotecting agent A particular acid protecting agent is C ₈ lower alkyl; more particular methyl. A particular deprotecting agent is an inorganic base such as an alkali hydroxide; more particular NaOH. Particular deprotection conditions encompass carrying out the deprotection in an alcoholic solvent such as MeOH or EtOH at about room temperature.

The fifth step in Scheme I is directed to condensing (coupling) compound v with (βS)- phenylalanine ethyl ester hydrochloride to yield compound vi. The condensation reaction is effected under coupling conditions such as in the presence of coupling agents such as DIC or DCC and HOAt, 0-(benzotriazol-l-yl)-N,iV,N',N -tetramethyluronium tetrafluoroborate in an inert solvent, such as DCM or DMF and at a temperature from about -10°C to about ambient temperature. PyBop and DIPEA are appropriate additives for facilitating the reaction.

The sixth step in Scheme I is directed to deprotecting compound vi under appropriate deprotection conditions, that depends on the nature of the protecting agent, i.e., whether it is removable (labile) under acid, base, or hydrogenation conditions, and other reactive moieties in the compound undergoing deprotection, i.e., a deprotecting agent is chosen to carry out the deprotection without effecting the other reactive moieties unless a concomitant reaction is desired. The deprotecting is carried out using an appropriate deprotecting agent A particular acid protecting agent is -g lower alkyl; more particular methyl. A particular deprotecting agent is an inorganic base such as an alkali hydroxide; more particular NaOH. Particular deprotection conditions encompass carrying out the deprotection in an alcoholic solvent such as MeOH or EtOH at about room temperature. For example, for acidic deprotection of an ester such as a lower .₄ alkyl ester the reaction mixture is treated with an mineral acid such as sulfuric acid, at a temperature from about ambient temperature to about 90°C, preferably at about 60°C. Base hydrolysis using, for example, an alkali metal hydroxide, such as lithium hydroxide or potassium hydroxide, in a suitable solvent such as an alcohol (e.g. ethanol), is carried out at a temperature of about ambient temperature. The diasteromeric purity of compound vii can also be enhanced by recrystallization. The seventh step in Scheme I is directed to reduction of the nitro group in compound vii to yield the corresponding amino compound viii. The reduction is effected using standard hydrogenation conditions such as hydrogenation in the presence of a suitable catalyst, such as palladium on carbon, under elevated pressure (e.g., about 50 psi) in an autoclave, at a temperature below 25°C and in a suitable solvent, such as a mixture of methanol and ethanol.

The eighth step in Scheme I is directed to reacting compound viii with o-tolyl isocyanate to provide Compound A. Formation of compound A is effected in an inert solvent, preferably DCM, and at a temperature of about 4°C. Carrying out the reaction in DCM enhances the optical purity of compound A.

Compound i is prepared according as shown in Scheme 2 where 2-aminoisobutyric acid

Scheme 2

is esterified using standard reaction conditions, such as reaction with a Cι_-4 lower alcohol such as methanol in the presence of thionyl chloride, at an elevated temperature (e.g. up to reflux temperature) to yield the corresponding Cι_ lower alkyl (Alk^a) 2-aminoisobutyrate. N-alkylation of the Cι_₄ lower alkyl (Alk^a) 2-aminoisobutyrate with a 4-nitrobenzylhalide, such as 4- nitrobenzylbromide, in the presence of a suitable base, such as an alkali metal carbonate, e.g. potassium carbonate, and in a suitable solvent such as acetonitrile at a temperature at about 60°C provides the 2-methyl-N[(4-nitrophenyl)methyl]alanine methyl ester (i).

The triflate compound used in the third step in Scheme I is prepared as shown in Scheme 3.

Scheme 3

Reaction of D-leucine with sodium nitrite in the presence of sulfuric acid at a temperature from about 0°C to about ambient temperature provides 2-hydroxy-2-methylpentanoic acid. Esterification of 2-hydroxy-2-methylpentanoic acid using standard reaction conditions, such as reaction with methanol in the presence of hydrochloric acid, at a temperature from about 10°C to about ambient temperature provides the corresponding ester. Reaction of the ester with Tf₂O at a temperature from about -5°C to about +5°C and in an inert solvent, such as tert-butyl methyl ether, and in the presence of a base such as N,N-diisopropylethylamine, provides the triflate (II).

It is to be understood that each of the Steps in Scheme I alone or in sequential combinations of two or more represent further aspects of the present invention. Stage 5 represents a particular embodiment of the present invention.

The invention can be better understood by reference to the following Examples, which are presented for illustration and not intended to limit the scope of the invention.

Example 1

The following Examples illustrate the invention, but are not intended to limit the invention in its scope in any way.

Nuclear magnetic resonance spectra (^XH NMR) are recorded on a Varian XL-300 spectrometer at 300 MHz. In the nuclear magnetic resonance spectra (NMR) the chemical shifts (δ) are expressed in ppm relative to tetramethylsilane. Abbreviations have the following significances: s = singlet; d = doublet; t = triplet; m = multiplet; q = quartet; dd = doublet of doublets; ddd = doublet of double doublets.

Mass spectra (MS) are recorded on a Finnigan Mat TSQ 700 spectrometer.

High Pressure Liquid Chromatography (HPLC) conditions for determination of retention times (R"p) using a Waters HPLC system (including a 600E system controller, a photodiode array detector, and a 746 data module) with a Millenium 32 operating system are as follows:- Method A: Waters Symmetry C18 column (3.9 x 150 mm), detection wavelength = 260 nm, flow rate = 1 mL/minute, 60% acetonitrile/40% water (0.1% trifluoroacetic acid);

Method B: CHIRALPAK AD column (4.6 x 250 mm), detection wavelength = 240 nm, flow rate = 1 mL/minute, 60% heptane/40% isopropyl alcohol (0.1 % trifluoroacetic acid). EXAMPLE 1 β-rr(25)-2-r4.4-dimethyl-3-rr4-rrr(2-methylphenyl)aminol-carbonyllaminolphenyllmethvn-2.5- dioxo-l-imidazolidinyll-4-methyl-l-oxopentyllaminol-(β5)-phenylpropanoic acid. Compound A

(A) Methyl 2-aminoisobutyrate hvdrochloride

A 30 gallon reactor is charged with methanol (80 L) and 2-aminoisobutyric acid (2.50 kg, 24.244 mol). Thionyl chloride (5105 g, 42.910 mol) is added over 0.65 hours at 19-39°C. The reaction mixture is heated to and held at reflux (67°C) for 4 hours, then cooled to 30°C and then concentrated at 35-50°C/50 torr to give an off-white solid. The solid is filtered off, then washed three times with acetone (2 L), then air dried at ambient temperature for 24 hours and then dried at 40°C/50 torr to give the title compound (3.31 kg, 89% yield) as a white solid, m.p. 187-188°C (with decomposition). [Elemental Analysis:- C, 38.92; H, 7.85; N, 9.03%. Calculated for

C5H_UN02.HC1:- C, 39.10; H, 7.87; N, 9.12%]. MS: 118 (M+H)+

(B) 2-Methyl-N-r(4-nitrophenyl)methyll-alanine. methyl ester, monohydrochloride

A 30 gallon glass-lined reactor is charged with a mixture of methyl 2-aminoisobutyrate hydrochloride [3.3 kg, 21.5 mol, prepared as described in (A) above], and acetonitrile (20 L). Powdered potassium carbonate (7.16 kg, 51.8 mol) is added over 5 minutes at 25°C and after stirring for 30 minutes this mixture is treated with a slurry of 4-nitrobenzylbromide (5.88 kg, 27.2 mol) in acetonitrile (40.5 L) in portions over 30 minutes. The reaction mixture is heated to and held at 60°C for 2 hours, then cooled to 20°C and then filtered. The solids are washed with acetonitrile (20 L). The combined filtrate and washings are evaporated at 40°C/50 torr to give an oil. This oil is dissolved in ethyl acetate (15 L) and the solution is washed with water (8.5 L), then with saturated sodium chloride solution (4 L) and then dried over magnesium sulfate. The drying agent is filtered off and washed three times with ethyl acetate (2 L). The combined filtrate and washings are cooled and kept below 20°C whilst bubbling hydrogen chloride through the solution. The stirred mixture is cooled to and maintained at 0°C for 3 hours then filtered. The filtered solid is washed three times with ethyl acetate (5 L) and then dried at 40°C/50 torr to give the title compound (5.475 kg, 88.2% yield) as a white solid, m.p. 220-221°C (with decomposition). [Elemental Analysis:- C, 50.07; H, 5.98; N, 9.68%. Calculated for C₁₂H₁₆N₂O₄.HCl:- C, 49.92;

H, 5.93; N, 9.70%]. MS: 253 (M+H)+

(C) 5,5-Dimethyl-l-l(4-nitrophenyl)methyll-2,4-imidazolidinedione A 22 L round bottom flask fitted with a stirrer and heating mantle is charged with a mixture of 2-methyl- V-[(4-nitrophenyl)methyl]-alanine, methyl ester, monohydrochloride [3.069 kg, 10.63 mol, prepared as described in (B) above] and acetic acid (11.7 L). The stirred heterogeneous mixture is treated with sodium cyanate (1.47 kg, 22.6 mol) over a period of 5 minutes at 20°C. An exothermic reaction occurred (the reaction temperature rose to 54°C) and a steady gas evolution took place for 30 minutes. The reaction mixture is stirred at 80°C for 45 minutes, then allowed to cool to 60°C and then charged into a 20 gallon glass-lined reactor containing water (58 L). This mixture is stirred for 30 minutes and then filtered. The filtered solid is washed three times with water (4L) and then dried at 40°C in a circulating air oven to give crude 5,5-dimethyl-l-[(4- nitrophenyl)methyl]-2,4-imidazolidinedione (2.774 kg, 99.1% yield).

A total of 9.564 kg of crude 5,5-dimethyl-l-[(4-nitrophenyl)methyl]-2,4-imidazolidinedione (batches of similar quality based on NMR) is charged to a 20 gallon glass-lined reactor with absolute ethanol (38 L). The heterogeneous mixture is stirred and heated at 80°C for 1 hour then transferred to a 15 gallon stainless steel container and after the solution phase had cooled to 40°C the solid is collected by filtration. This material is washed twice with ethanol (5 L) and then air dried to give the title compound (8.352 kg, 87.3% recovery) as a white solid, m.p. 199-200°C. [Elemental Analysis:- C, 53.59, 53.68; H, 4.94, 4.93; N, 15.58, 15.58%. Calculated For

^C12^H13^N3°4 (^{M W}- 263.25):- C, 54.75; H, 4.98; N, 15.97%]. MS: 298 (M+Cl)^"; 262 (M-H)^".

(D) 5.5-Dimethyl-l-[(4-nitrophenyl)methyl1-2.4-imidazolidinedione, potassium salt

A 50 L flask fitted with a stirrer, thermometer and dropping funnel is charged with a mixture of 5,5-dimethyl-l-[(4-nitrophenyl)methyl]-2,4-imidazolidinedione [4.0 kg, 15.19 mol, prepared as described in (C) above], tetrahydrofuran (25 L) and absolute ethanol (4.16 L). The stirred mixture is heated to 50°C and then treated with ethanolic potassium hydroxide solution [6.68 L, 2M, prepared from 1.064 kg of 85% potassium hydroxide solution and ethanol (6.95L)] over 3 hours whilst maintaining the reaction temperature at 49-52°C. The reaction mixture is stirred while cooling to 35°C, then stood for 18 hours at ambient temperature. The resulting solid is filtered off, then washed well with tetrahydrofuran and then air dried at ambient temperature to give the title compound (3.099 kg, 63.9 % yield) as a white solid, m.p. 213-216°C (with decomposition).

[Elemental Analysis:- C, 43.87, 43.70; H, 4.27, 4.24; N, 12.69, 12.59; H₂0, 5.03%. Calculated for C₁₂H₁₂KN₃O₄-5.03% H₂O:- C, 45.42; H, 4.38; N, 13.25%]. 5,5-Dimethyl-l-[(4-nitrophenyl)methyl]-2,4-imidazolidinedione (1.404 kg, 35.1% recovery) suitable for recycling is recovered from the reaction mixture filtrate by:- (i) treating with acetic acid (1 L); then (ii) stirring this mixture for 15 minutes; then (iii) evaporating the mixture at 35°C/7 torr; then (iv) treating the residue with water (4 L); then (v) stirring this aqueous mixture at ambient temperature for 15 minutes; then (vi) filtering the mixture and washing the solid three times with water (2 L); and then air drying the solid at ambient temperature.

(E) (2fl)-2-hydroxy-4-methvI-pentanoic acid

A 30 gallon glass-lined reactor is charged with sulfuric acid (2.28 kg, 23.2 mol) in water (46.5 L). After cooling to 25°C D-leucine (4.0 kg, 30.5 mol) is added. This mixture is stirred, cooled to < 0°C and then treated with a solution of sodium nitrite (3.76 kg, 54.5 mol) in water (6.69 L) over 5 hours whilst maintaining the reaction temperature at 0°C. After stirring overnight the reaction mixture is warmed to 20°C then extracted with t-butyl methyl ether (30L and the two lots of 10L). The combined extracts are dried over magnesium sulfate then concentrated at 30°C/50 torr to give a yellow solid (3.82 kg), which is recrystallized from boiling heptane [15 L, after cooling the hot solution to 0°C the solid is filtered off, then washed twice with cold (0°C) heptane (2.5 L) and then dried at 25°C/50 torr] to give crude (2R)-2-hydroxy-4-methyl-pentanoic acid (2.76 kg, 69% yield). This material is recrystallized from a mixture of heptane (26.2 L) and ethyl acetate (1.38 L) to give the title compound (2.525 kg, 63% yield) as a white solid, m.p. 73-75°C. [Elemental analysis:- C, 54.51%; H, 9.09%. Calculated for C₆H₁₂O₃:- C, 54.53%; H, 9.15%].

MS: 131 (M-H)".

(F) (2R)-2-Hydroxy-4-methyl-pentanoic acid, methyl ester

A solution of (2R)-2-hydroxy-4-methyl-pentanoic acid [4.84 kg, 36.6 mol, prepared as described in (E) above] in methanol (11.6 L), at 10°C, is treated with a solution of hydrochloric acid in methanol (11.6 L, 0.5 M) which had been previously cooled to -10°C. This mixture is stirred at ambient temperature for 2 hours and treated with trimethyl orthoformate (750 mL). The reaction mixture is stirred for 1 hour at ambient temperature, then concentrated at 25°C/50 ton. The residue is treated three times with tertiary-butyl methyl ether (5 L) followed by evaporation 25°C/50 torr to help remove the excess hydrochloric acid. The residue is maintained at 25°C/50 torr for 60 hours to give the title compound (4.88 kg, 91.2% yield) as a colorless liquid. 1H NMR [300 MHz, CDCI3]: 60.92 (m, 7 H); 1.55 (m, 2H); 1.82 (septet, 1H); 3.73 (s, 3H); 3.82 (broad s, 1H); 4.20 (t, 1 H). (G) (2R)-4-Methyl-2-[f(trifluoromethyl)sulfonyl1oxy1-pentanoic acid, methyl ester

A solution of (2Λ)-2-hydroxy-4-methyl-pentanoic acid, methyl ester [4.85 kg, 33.2 mol, prepared as described in (F) above] in tert/αry-butyl methyl ether (88 L), at - 5°C, is treated with diisopropylethylamine (7.05 L, 40.4 mol) in one portion. After stirring for 30 minutes this mixture is treated with triflic anhydride (11.0 kg, 39.0 mol) over 3 hours whilst keeping the internal temperature at -5 to + 5°C. The reaction mixture is stirred for an additional 18 hours then filtered and the filter cake is washed with terti ry-butyl methyl ether (10 L). The combined filtrate and washings are washed with saturated sodium bicarbonate solution (40 L), then dried over magnesium sulfate. The drying agent is filtered off and washed with tertiαry-butyl methyl ether (12 L). The combined filtrate and washings are concentrated at 25°C/30 torr to give the title compound (8.4 kg,

86.0% yield) as a brown liquid. H NMR [300 MHz, CDC1₃]: δ 1.0 (m, 7 H); 1.79 (m, 2 H); 3.88 (s, 3 H); 5.18 (m, 1 H).

(H) (ocS)-4,4-Dimethyl-α-(2-methylpropyl)-3-r(4-nitrophenyl)methyll-2,5-dioxo-l- imidazolidineacetic acid

A solution of 5,5-dimethyl-l-[(4-nitrophenyl)methyl]-2,4-imidazolidinedione, potassium salt [2.15 kg, 7.14 mol, prepared as described in (D) above] in l-methyl-2-pyrrolidinone (7.5 L), at - 15 °C, is treated with (2R)-4-methyl-2-[[(trifluoromethyl)sulfonyl]oxy]-pentanoic acid, methyl ester (2.50 kg , 8.98 mol, prepared as described in (G) above]) over a 45 minute period, while keeping the internal temperature below -5°C. The mixture is stirred at -5°C for 20 minutes, then warmed to ambient temperature over 2 hours [the reaction is monitored using HPLC (Condition A): R- of 5,5- dimethyl-l-[(4-nitrophenyl)methyl]-2,4-imidazolidinedione, potassium salt = 1.5, R of (27?)-4- methyl-2-[[(trifluoromethyl)sulfonyl]oxy]-pentanoic acid, methyl ester = 5.6] and then added in one portion to a mixture of water (4.0 L) and sulfuric acid (4.0 L) in a 50 L flask. This mixture is stirred and heated to, and maintained at, 90°C for 18 hours under a continuous nitrogen flow, which helped remove methanol from the system. The reaction mixture is cooled to room temperature and then extracted twice with tertiαrv-butyl methyl ether (25 L). The combined extracts are washed with sodium hydroxide solution (13.0 L, 1.2 M). The pH of the aqueous washing is adjusted to 1.0 by addition of concentrated hydrochloric acid (1.42 L) at 20°C and then it is extracted with tertiary- butyl methyl ether (10 L). The organic extract is washed with water (5 L) and then dried over magnesium sulfate (1 kg). The drying agent is filtered off and washed with tert αry-butyl methyl ether (2 L). The combined filtrate and washings are evaporated at 30°C/50 torr to give the title compound (2.77 kg, 103% yield, 87.6% pure (HPLC Method A: R_τ = 8.8 minutes). 96.82% ee

(HPLC Method B: Rf of (S)-isomer = 11.5 minutes, R of (R)-isomer = 13.4 minutes)] as a light yellow colored foam. 1H NMR [300 MHz, (CD₃)₂SO]: δ 0.88 (d of d, 6 H); 1.26 (s, 6 H); 1.42 (m, 1 H); 1.79 (m, 1 H); 2.16 (t of t, 1 H); 4.58 (m, 1 H); 4.68 (s, 2H); 7.58 (d, 2 H); 8.11 (d, 2 H).

(I) (βS)-β-[r(25)-2-[4,4-Dimethyl-3-r(4-nitrophenyl)methvn-2,5-dioxo-l-imidazolidinvn-4-methyl- 1-oxopentynaminol- phenylpropanoic acid, ethyl ester

A solution of (ocS)-4,4-dimethyl-α-(2-methylpropyl)-3-[(4-nitrophenyl)methyl]-2,5-dioxo-l- imidazolidineacetic acid (5.55 kg, 14.7 mol, prepared as described in (H) above]) and 0-(benzotriazol-l-yl)-/V, V,N'/V -tetramethyluronium tetrafluoroborate (6.20 kg, 19.3 mol) in dichloromethane (60 L) is charged to a 30 gallon glass-lined reactor. The stirred solution is cooled to - 5°C and then treated with diisopropylethylamine (6.6 L, 37.8 mol) whilst keeping the internal temperature below 0°C. This mixture is stirred at -10°C for 30 minutes and then treated in one portion with a solution of (βS)-phenylalanine ethyl ester hydrochloride (4.10 kg, 17.9 mol) dissolved in l-methyl-2-pyrrolidinone (10 L) [an exotherm from -10°C to -3°C occurred]. The reaction mixture is warmed and stirred at ambient temperature for 2.0 hours [the reaction is monitored using HPLC Method A : Rj of (αS)-4,4-dimethyl-α-(2-methylpropyl)-3-[(4- nitrophenyl)methyl]-2,5-dioxo-l-imidazolidineacetic acid methyl ester = 2.8, Rj of (αS)~4,4- dimethyl-α-(2-methylpropyl)-3-[(4-nitrophenyl)methyl]-2,5-dioxo-l-imidazolidineacetic acid = 7.3] and then concentrated at 25°C/30 torr. The residual brown oil is dissolved in f-butyl methyl ether (50 L) and the solution, is washed with water (25 L), then with hydrochloric acid (48 L, IM), then with sodium hydroxide solution (44 L, IM), then with saturated sodium chloride solution (30 L) and then dried over magnesium sulfate (2 kg). The drying agent is filtered off and washed with t- butyl methyl ether (10L). The combined filtrate and washings are evaporated at 25°C/50 torr to give the title compound [8.17 kg, 100 % yield, 97.4% pure (HPLC, Method A), 97% ee (HPLC, Method B)] as a light yellow viscous oil, 1H NMR [300 MHz, (CD₃)₂SO]: δ 0.88 (d of d, 6 H); 1.10 (t, 3 H); 1.22 (s, 3 H); 1.29 (s, 3 H); 1.42 (m, 1 H); 1.76 (t of t, 1 H); 2.23 (t of t, 1 H); 2.80 (d, 2 H); 4.0 (m, 2 H); 4.56 (m, 1 H); 4.66 (d, 1 H); 5.26 (q, 1 H); 7.24 (m, 1 H); 7.33 (m, 4 H); 7.59 (d, 2 H); 8.21 (d, 2 H); 8.49 (d, 1 H).

Alternative Step I for preparing (βS)-β-[[(25)-2-[4,4-dimethyl-3-[(4-nitrophenyl)methyl]-2,5-dioxo- l-imidazolidinyl]-4-methyl-l-oxopentyl]amino]phenylpropanoic, ethyl ester:

Nitro acid v (181 g, 0.48 mol) and TBTU (200 g, 0.624 mol) are dissolved in 2.2 L of CH₂C1 , charged to a 5 L 3-necked flask, and the stirred solution is cooled to - 5 °C. DIEA (217 mL, 1.25 mol) is added dropwise while keeping the internal temperature < 0 °C. The reaction mixture is stirred at -10 °C for 30 min, followed by the addition of a solution of methyl (S)-3- amino-3-phenylpropanoate (132 g, 0.576 mol) dissolved in NMP (300 mL) in one portion [exotherm from -10 °C to +2 °C]. The reaction mixture is warmed and stirred at ambient temperature for 2 h. The reaction is monitored using HPLC (Condition A) : t_R of v= 2.8, t_R of vi = 7.3 . The reaction mixture is concentrated at 25 °C/30 torr to a brown oil. This oil is dissolved in diethyl ether (3 L) and washed with water (5 L), 1 M HC1 (2 L), 1 M NaOH (2 L), water (2 L) and the ether layer is dried over MgSO₄. The drying agent is filtered off and the filtrate is concentrated at 25 °C/30 torr to give 257 g of the nitro ester 20, 96 % yield, 98 % pure, 94.4 % ee .

(J) β-l[(25)-2-[4,4-Dimethyl-3-[(4-nitrophenyl)methyl1-2,5-dioxo-l-imidazolidinyl]-4-methyl-l- oxopentyll aminol -( βιS)-phenylpropanoic acid

A solution of (β5)-β-[[(2S)-2-[4,4-dimethyl-3-[(4-nitrophenyl)methyl]-2,5-dioxo-l- imidazolidinyl]-4-methyl-l-oxopentyl]amino]phenylpropanoic acid, ethyl ester [4.11 kg, 7.44 mol, prepared as described in (I) above] in ethanol (16.6 L) is charged to a 50 L flask. The stirred solution is cooled to 5°C and then treated with a solution of 10% potassium hydroxide in water (8.285 kg) over 1 hour. The reaction temperature at the end of the addition is 9.2°C. [The reaction is monitored by thin layer chromatography on silica (Merck silica gel plates) with ethyl acetate eluant, Rp of ester = 0.84, Rp of acid = 0.0]. The reaction mixed is treated with concentrated hydrochloric acid (1.24 L) over 20 minutes whilst maintaining the temperature at 9°C to 10°C then stirred at 10°C for a further 30 minutes resulting in the formation of a precipitate. The stirred mixture is cooled to 5°C, then treated with water (8.285 L), then is cooled to and stirred at 0°C for 1 hour. The precipitate is filtered off, washed three times with aqueous ethanol (4 L, 1:3, v/v), and then air dried at ambient temperature to give the title compound [2.528 kg, 64.8% yield, 96.9% de

(HPLC Method B: RT = 24.56 minutes)] as a light yellow solid, m.p. 150-152°C. 1H NMR [300 MHz, (CD₃)₂SO]: δ 0.88 (d of d, 6 H); 1.22 (s, 3 H); 1.29 (s, 3 H); 1.42 (m, 1 H); 1.78 (t of t, 1 H);

2.22 (t of t, 1 H); 2.70 (d, 2 H); 4.56 (m, 1 H); 4.66 (d, 1 H); 5.20 (q, 1 H); 7.24 (m, 1 H); 7.33 (m, 4 H); 7.59 (d, 2 H); 8.21 (d, 2 H); 8.42 (d, 1 H).

Alternative Step J for preparing β-ff(25)-2-r4,4-dimethyl-3-[(4-nitrophenyl)methyll-2,5-dioxo-l- iιradazolidinyl1-4-methyl-l-oxopentyl]amino]-(βS)-phenyrpropanoic acid:

To nitro ester vi (257 g, 0.465 mol) dissolved in THF (2 L) is added 10 % LiOH in water (1 L) and the mixture is stirred for 3 h at ambient temperature , at which time the reaction is complete. The reaction is monitored by HPLC (condition A): t_R of vi = 7.3, f_R of vii = 3.4. The mixture is concentrated at 25 °C/30 torr and then partitioned between water (10 L) and dichloromethane (4 L). While stirring, the pH is adjusted to 3 using 12 M HC1. The mixture is vigorously stirred and then the phases are separated. The aqueous phase is extracted a second time with dichloromethane (1 L) and the combined organic extracts are dried over Na₂SO₄. The drying agent is filtered off and the filtrate is concentrated at 25 °C/30 torr to give 292 g of the crude nitro acid vii. This is combined with another 129 g of vii and recrystallized from 2 L of 1: 1 EtOH/ H₂O to give 360 g. A second crystallization from 1.5 L of 2:1 EtOH/ H₂O gave 258 g of pure nitro acid vii in a combined yield of 72 %, 98.8 % pure by HPLC, 96.7 % ee .

(K) (β5)-β-rr(2S)-2-r4.4-Dimethyl-3-rr4-rir(2-methylphenvnaminol- carbonyllamino1phenynmethvn-2,5-dioxo-l-imidazolidinyl1-4-methyl-l-oxopentyl]amino]- phenylpropanoic acid

A solution of (βS)-β-[[(2S)-2-[4,4-dimethyl-3-[(4-nitrophenyl)methyl]-2,5-dioxo-l-imidazolidinyl]- 4-methyl-l-oxopentyl]amino]phenylpropanoic acid [1.20 kg, 2.29 mol, prepared as described in (J) above] in a mixture of methanol (6.5 L) and ethanol (6.5 L) is hydrogenated at 50 psi in a 5 gal autoclave in the presence of 10 % Pd/C/50%H₂O (62.5 g) for 2.0 hours. [Note: the autoclave is pre- cooled to 14°C and the reaction temperature is kept below 25°C]. The reaction is followed by HPLC (Method A) until complete when the reaction mixture is filtered through Celite filter aid. The filter pad is washed with ethanol (2 L), and the combined filtrate plus washings are concentrated at 25°C/10 torr to give (βS)-β-rf(25)-2-r4,4-dimethyl-3-|(4-aminophenyl)methyll-2,5- dioxo- 1 -imidazolidinyl] -4-methyl- 1 -oxopentyll aminolphenylpropanoic acid (1.33 kg) as a light yellow foam. An additional 1.105 kg of compound vii is prepared in the same manner. The 4.384 mol of (βS)-β-[[(25)-2-[4,4-dimethyl-3-[(4-aminophenyl)methyl]-2,5-dioxo-l- imidazolidinyl]-4-methyl-l-oxopentyl]amino]-phenylpropanoic acid (VII) is dissolved in dichloromethane (15 L) and the solution is dried over magnesium sulfate (1 kg). The drying agent is filtered off and washed with dichloromethane (5.6 L). The combined filtrate and washings are charged to a 50 L flask fitted with a stirrer, thermometer and continuous nitrogen purge. The solution is stirred, cooled to 4.3°C and then treated with o-tolyl isocyanate (627.2 g, 584mL, 4.71 mol) in a slow stream over 6 minutes. The reaction temperature rose to 5°C during the addition. The reaction mixture is stirred for 1 hour (during which time a heavy precipitate formed) then diluted with dichloromethane (4 L). This mixture is stirred for 15 minutes, then cooled at 5°C for 18 hours, then filtered. The filtered solid is washed with dichloromethane (2 L) and then dried at 25°C/30 torr to give the title compound [2.280 kg, 82.8% yield, 96.9% de (HPLC Method B)] as a white solid. Further purification of (βS)-β-[[(2S)-2-[4,4-Dimethyl-3-[[4-[[[(2-methylphenyl)amino]- carbonyl] amino]pheny 1] methyl]-2,5-dioxo- 1 -imidazolidiny 1] -4-methyl- 1 -oxopentyl]amino] - phenylpropanoic acid :-

(β5)-β-[[(2S)-2-[4,4-Dimethyl-3-[[4-[[[(2-methylphenyl)amino]-carbonyl]amino]phenyl]methyl]- 2,5-dioxo-l-imidazolidinyl]-4-methyl-l-oxopentyl]amino]-phenylpropanoic acid (6.85 kg, 96.9% de) and dichloromethane (70 L) are charged to a 30 gallon glass-lined reactor. The stirred suspension is heated to and held at 36°C for 4 hours, then cooled to 20°C over 2 hours. The mixture is discharged from the reactor into a 30 gallon portable stainless steel vessel, then cooled at 5°C for 18 hours and then filtered using a Hastelloy Nutsche filter, washing the filter cake with cold, (5°C), dichloromethane (10 L). The filtered solid is dried at 22°C/100 torr for 18 hours, screened through 200 mesh stainless steel, then re-dried at 22°C/100 torr for 5 days to give (βS)-β- [[(25)-2-[4,4-dimethyl-3-rr4-rfr(2-methylphenyl)aminol-carbonyl1aminolphenyll-methyll-2,5- dioxo- 1 -imidazolidinyll -4-methyl - 1 -oxopentyll aminol -phenylpropanoic acid [6.30 kg, enantiomeric purity: 99.36% (S,S)-isomer, 0.49% (R.S)-isomer, 0.15% (5,R)-isomer] as a white powder, m.p. 120- 123°C . Specific rotation -37.4° (Na D-line, lOmg/mL, methanol, 25° C). [Elemental Analysis:- C, 66.66; H, 6.48; N, 10.98; H₂O, 0.72%. Calculated for C₃5H₄₁N₅O₆ 0.72% H₂O:- C, 66.97; H,

6.58; N, 11.16%]. MS: 628 (M+H)+, 645 (M+NH₃+H)+, 626 (M-H)^".

Claims

What is claimed is:

1. A method of preparing a compound of formula ii

comprising reacting a compound of formula i

wherein ALK^a is a Cι_ lower alkyl, with M(-OCN), wherein M is an alkali metal to yield the compound of formula ii.

2. The method according to claim 1 wherein ALK^a is methyl.

3. The method according to claim 1 wherein M is potassium.

4. A method for preparing a compound of formula iii

(iii) wherein M is an alkali metal, comprising reacting a compound of formula ii

with a strong alkali metal base to yield the compound of formula iii.

5. The method according to claim 4 wherein M is potassium.

6. The method according to claim 4 wherein the strong alkali metal base is potassium hydroxide.

7. A method for preparing a compound of formula v

comprising alkylating a compound of formula iii

(iii)

wherein M is an alkali metal, with a compound of formula

wherein PG is an acid protecting group, and deprotecting the product of the alkylation to yield the compound of formula v.

8. The method according to claim 7 wherein M is potassium.

9. The method according to claim 7 wherein PG is methyl.

10. A method for preparing a compound of formula vi

wherein PG is an acid protecting group, comprising condensing a compound v

with a compound of formula

wherein PG is an acid protecting group, to yield the compound of formula vi.

11. The method according to claim 10 wherein PG is ethyl.

12. A method for preparing a compound of formula vii

comprising deprotecting a compound of formula vi

wherein PG is an acid protecting group, to yield the compound of formula vii.

13. The method according to claim 12 wherein PG is ethyl.

14. A method for preparing a compound of formula viii

comprising hydrogenating a compound of formula vii

to yield the compound of formula viii.

15. A method for preparing a compound of formula A

comprising reacting a compound of formula viii

with a compound of formula

to yield the compound of formula A.

16. A compound of formula ii

17. A compound of formula iii

(iii) wherein M is an alkali metal.

18. The compound of claim 17 wherein M is potassium.

19. A compound of formula v

20. A compound of formula vi

wherein PG is an acid protecting group.

21. The compound of claim 17 wherein PG is ethyl.

22. A compound of formula vii

23. A compound of formula viii