Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D487/00—Heterocyclic compounds containing nitrogen atoms as the only ring hetero atoms in the condensed system, not provided for by groups C07D451/00 - C07D477/00
  • C07D487/02—Heterocyclic compounds containing nitrogen atoms as the only ring hetero atoms in the condensed system, not provided for by groups C07D451/00 - C07D477/00 in which the condensed system contains two hetero rings
  • C07D487/04—Ortho-condensed systems

Definitions

• the present invention relates to a process for the efficient preparation of enantiomerically enriched beta amino acid derivatives which are useful in the asymmetric synthesis of biologically active molecules.
• the process comprises an enantioselective hydrogenation of a prochiral beta-amino acrylic acid derivative substrate in the presence of an ammonium salt and a transition metal precursor complexed with a chiral ferrocenyl diphosphine ligand.
• the present invention provides an efficient process for the preparation of an enantiomerically enriched beta amino acid derivative of structural formula I:
• Z is OR2, SR2, or NR2R3;
• Rl is Cl-8 alkyl, aryl, heteroaryl, aryl-Ci-2 alkyl, or heteroaryl-Ci-2 alkyl;
• R2 and R3 are each independently hydrogen, Ci_8 alkyl, aryl, or aryl-Ci-2 alkyl; or R2 and R ⁇ together with the nitrogen atom to which they are attached form a 4- to 7-membered heterocyclic ring system optionally containing an additional heteroatom selected from O, S, and NCl _4 alkyl, said heterocyclic ring system being optionally fused with a 5- to 6-membered saturated or aromatic carbocyclic ring system or a 5- to 6-membered saturated or aromatic heterocyclic ring system containing one to two heteroatoms selected from O, S, and NC 1.4 alkyl, said fused ring system being unsubstituted or substituted with one to two substituents independently selected from hydroxy, amino, fluoro, C 1.4 alkyl, Ci -4 alkoxy, and trifluoromethyl.
• the process of the present invention relates to a method for the preparation of chiral beta amino acid derivatives of structural formula I in an efficient enantioselective fashion via transition metal- catalyzed asymmetric hydrogenation of a prochiral enamine of structural formula II:
• amino group is unprotected, in the presence of an ammonium salt and a transition metal precursor complexed to a chiral ferrocenyl diphosphine ligand.
• the requirement for amine protection introduces two additional chemical steps into the sequence, namely protection and deprotection, and the synthesis of the protected substrate may also be difficult.
• the process of the present invention circumvents the need for protecting the primary amino group in the substrate for the asymmetric hydrogenation reaction and proceeds with excellent reactivity and enantioselectivity.
• the present invention is concerned with a process for the preparation of enantiomerically enriched beta amino acid derivatives of structural formula I.
• the process utilizes an asymmetric hydrogenation of a prochiral beta amino acrylic acid derivative, wherein the primary amino group is unprotected, in the presence of an ammonium salt and a transition metal precursor complexed with a chiral ferrocenyl diphosphine ligand.
• the process of the present invention is applicable to the preparation of beta amino acid derivatives on a pilot plant or industrial scale.
• the beta amino acids are useful to prepare a wide variety of biologically active molecules.
• R is C 1-8 alkyl, aryl, heteroaryl, aryl-Ci-2 alkyl, or heteroaryl-Ci-2 alkyl;
• R2 and R ⁇ are each independently hydrogen, Ci_8 alkyl, aryl, or aryl-Cl-2 alkyl; or R2 and R3 together with the nitrogen atom to which they are attached form a 4- to 7-membered heterocyclic ring system optionally containing an additional heteroatom selected from O, S, NH, and NCi_4 alkyl, said heterocyclic ring being unsubstituted or substituted with one to three substituents independently selected from oxo, hydroxy, halogen, Cl .4 alkoxy, and C 1.4 alkyl wherein alkyl and alkoxy are unsubstituted or substituted with one to five fluorines; and said heterocyclic ring system being optionally fused with a 5- to 6-membered saturated or aromatic carbocyclic ring system or a 5- to 6-membered saturated or aromatic heterocyclic ring system containing one to two heteroatoms selected from O, S, and NC ⁇ -4 alkyl, said fused ring system being unsubsti
• the process of the present invention comprises the step of hydrogenating a prochiral enamine of structural formula II:
• R4 is C 1.4 alkyl or aryl
• R 5 and R.6 are each independently Cl -6 alkyl, C5.12 cycloalkyl, or aryl
• R7 is C i_4 alkyl or unsubstituted phenyl.
• the catalytic complex of the transition metal precursor and the chiral ferrocenyl diphosphine ligand may be either (a) generated in situ by the sequential or contemporaneous addition of the transition metal species and the chiral ferrocenyl diphosphine ligand to the reaction mixture or (b) pre-formed with or without isolation and then added to the reaction mixture.
• a pre-formed catalytic complex is represented by the formula:
• X represents a non-coordinating anion, such as trifluoromethanesulfonate, tetrafluoroborate, and hexafluorophosphate
• L is a neutral ligand such as an olefin (or chelating di-olefin such as 1,5- cyclooctadiene or norbornadiene) or a solvent molecule (such as MeOH and TFE).
• olefin or chelating di-olefin such as 1,5- cyclooctadiene or norbornadiene
• solvent molecule such as MeOH and TFE
• the ligands of structural formula III are known in the art as Josiphos ligands and are commercially available from Solvias AG, Basel, Switzerland.
• the carbon stereogenic center marked with an ** has the (i?)-configuration as depicted in formula IV:
• R4 is Ci_2 alkyl
• R.5 and R.6 are C1.4 alkyl
• R7 is unsubstituted phenyl.
• R4 is methyl
• R ⁇ and R ⁇ are r-butyl
• R7 is unsubstituted phenyl.
• the latter ligand is known in the art as ⁇ -butyl Josiphos.
• Commercially available forms of the i-butyl Josiphos ligand are the S,R and R,S enantiomeric forms.
• R,S-t-buty ⁇ Josiphos is ⁇ (i?)-l-[( ⁇ S)-(diphenylphosphino) ferrocenyl] ⁇ ethyl-di-ter ⁇ butylphosphine of formula V below:
• the ferrocenyl diphosphine ligands of formula III have two centers of asymmetry, and the process of the present invention is intended to encompass the use of single enantiomers, individual diastereomers, and mixtures of diastereomers thereof.
• the present invention is meant to comprehend the use of all such isomeric forms of the ligands of structural formula III for the asymmetric hydrogenation of a compound of formula II.
• the facial enantioselectivity of the hydrogenation reaction will depend on the particular stereoisomer of the ligand that is employed in the reaction. It is possible to control the configuration at the newly formed stereogenic center in a compound of formula I marked with an * by the judicious choice of the chirality of the ferrocenyl diphosphine ligand of formula III.
• Rl is ben2yl wherein the phenyl group of benzyl is unsubstituted or substituted one to three substituents selected from the group consisting of fluorine, trifluoromethyl, and trifluoromethoxy.
• Z is OK.2 or NR2R3. in a class of this embodiment, NR2R3 is a heterocycle of the structural formula VI:
• R ⁇ is hydrogen or C 1.4 alkyl which is unsubstituted or substituted with one to five fluorines.
• Z is OR2.
• Rl is 6- methoxy-pyridin-3-yl and Z is C 1.4 alkoxy.
• Z is methoxy or ethoxy.
• Suitable organic solvents include lower alkanols, such as methanol, ethanol, isopropyl alcohol, hexafluoroisopropyl alcohol, phenol, 2,2,2-trifluoroethanol (TFE), and mixtures thereof; tetrahydrofuran; methyl t-bvtiyl ether; and mixtures thereof.
• the asymmetric hydrogenation reaction is also carried out in the presence of about 0.01 to about 10 mol % (relative to the prochiral enamine substrate of formula II) of an ammonium salt.
• the ammonium salt is an ammonium halide salt selected from the group consisting of ammonium chloride, ammonium bromide, and ammonium iodide.
• the ammonium halide salt is ammonium chloride.
• the ammonium salt is an ammonium carboxylate salt such as ammonium acetate and ammonium formate.
• the ratio of the ammonium salt to prochiral enamine substrate is about 0.05 to about 5 mol %.
• the reaction temperature for the reaction may be in the range of about 10 0 C to about 90
• a preferred temperature range for the reaction is about 45 0 C to about 65 0 C.
• the hydrogenation reaction can be performed at a hydrogen pressure range of about 20 psig to about 1500 psig.
• a preferred hydrogen pressure range is about 80 psig to about 200 psig.
• the transition metal precursor is [M(monoolefin)2Cl]2, [M(diolefm)Cl]2, [M(monoolefm)2acetylacetonate], [M(diolefm)acetylacetonate], [M(monoolefin)4]X, or [M(diolefin)2]X wherein X is a non-coordinating anion selected from the group consisting of methanesulfonate, trifluoromethanesulfonate (Tf), tetrafluoroborate (BF4), hexafluorophosphate (PFg), and hexafluoroantimonate (SbF ⁇ ), and M is rhodium (Rh) or iridium (Ir).
• Transition metal precursors where M is ruthenium (Ru) are [M(arene)Cl2]2, [M(diolefin)Cl2]n, or [M(diolef ⁇ n)(D3-2-methyl-l-propenyl)2].
• the transition metal precursor is [Rh(cod)Cl]2, [Rh(norbornadiene)Cl]2, [Rh(cod)2]X, or [Rh(norbornadiene)2]X.
• the transition metal precursor is [Rh(cod)Cl]2-
• the ratio of transition metal precursor to substrate is about 0.01 to about 10 mol %.
• a preferred ratio of the transition metal precursor to substrate is about 0.05 mol % to about 0.4 mol %.
• the beta amino acrylic acid derivative substrates of formula II for the asymmetric hydrogenation contain an olefmic double bond, and unless specified otherwise, are meant to include both E and Z geometric isomers or mixtures thereof as starting materials.
• the squiggly bond in the substrate of structural formula II signifies either the Z ox E geometric isomer or a mixture thereof.
• the geometric configuration of the double bond in the beta amino acrylic acid derivative substrate for the asymmetric hydrogenation reaction is the Z-configuration as depicted in formula VTI:
• Sources of ammonia "NH3" include ammonium acetate, ammonium hydroxide, ammonium formate, ammonium lactate, ammonium citrate dibasic, ammonium carbonate, ammonium carbamate, and ammonium benzoate.
• the source of ammonia is ammonium acetate.
• the beta-keto esters can be prepared as described by D.W. Brooks et al., Angew. Chem. Int. Ed. Engl., 18: 72 (1979).
• the beta amino acrylamides can be prepared from the corresponding esters via amide exchange as described in Org. Syn. Collect., Vol. 3, p. 108.
• Another embodiment of the present invention concerns a process for the preparation of a compound of structural formula 1:
• Ar is phenyl which is unsubstituted or substituted with one to five substituents independently selected from the group consisting of fluorine, trifluoromethyl, and trifluoromethoxy; and R8 is hydrogen or Ci_4 alkyl unsubstituted or substituted with one to five fluorines; comprising the steps of:
• R4 is C 1-4 alkyl or aryl
• R5 and R.6 are each independently Ci-6 alkyl, C5.42 cycloalkyl, or aryl; and R7 is C 1-4 alkyl or uns ⁇ bstituted phenyl.
• Ar is 2,5-difluorophenyl or 2,4,5 -trifluorophenyl.
• R ⁇ is trifluoromethyl.
• the rhodium metal precursor is chloro(l,5- cyclooctadiene)rhodium(I) dimer ⁇ [Rh(cod)Cl]2 ⁇ .
• R4 is methyl
• R5 and R6 are both /-butyl
• R? is unsubstituted phenyl.
• the rhodium metal precursor is chloro(l ,5- cyclooctadiene)rhodium(I) dimer.
• R4 is methyl
• R ⁇ and R6 are both f-butyl
• R7 is unsubstituted phenyl
• Ar is 2,5-difluorophenyl or 2,4,5-trifluorophenyl
• ⁇ is trifluoromethyl
• the rhodium metal precursor is chloro(l,5-cyclooctadiene)rhodium(I) dimer.
• the ammonium salt is ammonium chloride.
• both chemical transformations are carried out in the same reaction vessel in the presence of an ammonium salt.
• the ammonium salt is selected from the group consisting of ammonium acetate, ammonium hydroxide, ammonium formate, ammonium lactate, ammonium citrate dibasic, ammonium carbonate, ammonium carbamate, and ammonium benzoate.
• the ammonium salt is ammonium formate.
• the compound of structural formula 1 is obtained with an enantiomeric excess of greater than 90%. In a class of this embodiment the compound of structural formula 1 is obtained with an enantiomeric excess of greater than 95%.
• % enantiomeric excess (abbreviated “ee”) shall mean the % major enantiomer less the % minor enantiomer. Thus, a 70% enantiomeric excess corresponds to formation of 85% of one enantiomer and 15% of the other.
• enantiomeric excess is synonymous with the term “optical purity.”
• the process of the present invention provides compounds of structural formula I with high optical purity, typically in excess of 70% ee.
• compounds of formula I are obtained with an optical purity in excess of 80% ee.
• compounds of formula I are obtained with an optical purity in excess of 90% ee.
• compounds of formula I are obtained with an optical purity in excess of 95% ee.
• enantioselective shall mean a reaction in which one enantiomer is produced (or destroyed) more rapidly than the other, resulting in the predominance of the favored enantiomer in the mixture of products.
• alkyl groups specified above are intended to include those alkyl groups of the designated length in either a straight or branched configuration.
• exemplary of such alkyl groups are methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tertiary butyl, pentyl, isopentyl, hexyl, isohexyl, and the like.
• the alkyl groups are unsubstituted or substituted with one to three groups independently selected from the group consisting of halogen, hydroxy, carboxy, aminocarbonyl, amino, C 1-4 alkoxy, and C 1-4 alkylthio.
• cycloalkyl is intended to mean cyclic rings of alkanes of five to twelve total carbon atoms, or any number within this range (i.e., cyclopentyl, cyclohexyl, cycloheptyl, etc).
• halogen is intended to include the halogen atoms fluorine, chlorine, bromine, and iodine.
• aryl includes phenyl and naphthyl.
• Aryl is unsubstituted or substituted with one to five substituents independently selected from fluoro, hydroxy, trifluoromethyl, amino, C 1-4 alkyl, and C i .4 alkoxy.
• arene refers to benzene, naphthalene, and o-, m-, or/7-isopropyltoluene ( ⁇ , m, or/7-cymene).
• olefin refers to a acyclic or cyclic hydrocarbon containing one or more double bonds including aromatic cyclic hydrocarbons.
• the term includes, but is not limited to, 1,5- cyclooctadiene and norbornadiene ("nbd").
• heteroaryl means a 5- or 6-membered aromatic heterocycle that contains at least one ring heteroatom selected from O, S and N. Heteroaryls also include heteroaryls fused to other kinds of rings, such as aryls, cycloalkyls and heterocycles that are not aromatic.
• heteroaryl groups include, but are not limited to, pyrrolyl, isoxazolyl, isothiazolyl, pyrazolyl, pyridinyl, oxazolyl, 1,2,4-oxadiazolyl, 1,3,4-oxadiazolyl, thiadiazolyl, thiazolyl, imidazolyl, triazolyl, tetrazolyl, furyl, triazinyl, thienyl, pyrimidinyl, pyrazinyl, benzisoxazolyl, benzoxazolyl, benzothiazolyl, benzothiadiazolyl, dihydrobenzofuranyl, indolinyl, pyridazinyl, indazolyl, isoindolyl, dihydrobenzothienyl, indolizinyl, cinnolinyl, phthalazinyl, quinazolinyl, naphthyridinyl,
• Hydrazine (20.1 g, 35 wt% in water, 0.22 mol) was mixed with 310 mL of acetonitrile. 31.5 g of ethyl trifluoroacetate (0.22 mol) was added over 60 min. The internal temperature was increased to 25 0 C from 14 0 C. The resulting solution was aged at 22 - 25 0 C for 60 min. The solution was cooled to 7 0 C. 17.9 g of 50 wt% aqueous NaOH (0.22 mol) and 25.3 g of chloroacetyl chloride (0.22 mol) were added simultaneously over 130 min at a temperature below 16 0 C.
• Step B Preparation of 5-(trifluoromethyl)-2-(chlorornethyiy 1 ,3 ,4-oxadiazole
• Step D Preparation of 3-ftrifluoromethyl)-5,6,7,8-tetrahydro[l .2,4]triazolo[4,3- ⁇ ]pyrazine.
• hydrochloride salt (1-4) A suspension of amidine J ⁇ 3 (27.3 g, 0.13 mol) in 110 mL of methanol was warmed to
• Step A Preparation of 4-oxo-4-[3-(trifluoromethyl)-5.6-dihvdro[1.2,41triazolor4,3- ⁇ 1 ⁇ yrazin-
• 2,4,5-Trifluorophenylacetic acid (2-1) 150 g, 0.789 mol
• Meldrum's acid 125 g, 0.868 mol
• 4-(dimethylamino)pyridine (DMAP) 7.7 g, 0063 mol
• DMAP 4-(dimethylamino)pyridine
• Pivaloyl chloride (107 mL, 0.868 mol) was added dropwise over 1 to 2 h while maintaining the temperature between 0 and 5 0 C.
• the reaction mixture was aged at 5 0 C for 1 h.
• Triazole hydrochloride L4 (180 g, 0.789 mol) was added in one portion at 40-50 0 C.
• the reaction solution was aged at 70 0 C for several h.
• 5% Aqueous sodium hydrogencarbonate solution (625 mL) was then added dropwise at 20 - 45 0 C.
• the batch was seeded and aged at 20 - 30 0 C for 1-2 h. Then an additional 525 mL of 5% aqueous sodium hydrogencarbonate solution was added dropwise over 2-3 h.
• the slurry was cooled to 0 — 5 0 C and aged 1 h before filtering the solid.
• the wet cake was displacement-washed with 20% aqueous DMAc (300 mL), followed by an additional two batches of 20% aqueous DMAc (400 mL), and finally water (400 mL).
• the cake was suction-dried at room temperature.
• Step B Preparation of (2Z)-4-oxo-4-r3-rtrifluoromethyl)-5.6-dihvdro[l,2.41triazolo
• Step C Preparation of r2RV4-oxo-4-f3-( ' trifluoromethyl)-5.6-dihvdrori,2,41triazolor4.3- a1pyrazin-7(8H)-yl]-l -(2 A5-trifluorophenv0butan-2-amine (2-5) Into a 250 ml flask were charged chloro(l,5-cyclooctadiene)rhodium(I) dimer
• the optical purity was further enhanced in the following manner.
• the methanol solution from the hydrogenation reaction (18 g in 180 mL MeOH) was concentrated and switched to methyl t- butyl ether (MTBE) (45 mL).
• MTBE methyl t- butyl ether
• aqueous ⁇ 3PO4 solution 0.5 M, 95 mL
• 3NNaOH 35 mL
• the MTBE solution was concentrated and solvent switched to hot toluene (180 mL, about 75 0 C).
• the hot toluene solution was then allowed to cool to 0 0 C slowly (5 - 10 h).
• Solvent B 20/80 vol% Water/Methanol 10 mM TRIS p ⁇ 9 Gradient: 0 min 55% A : 45% B 8 min 24% A : 76% B
• HPLC high-performance liquid chromatographic

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