Synthesis of (R)-(+)-methyl 3-amino-3-(5-hydroxy-2-pyridinyl)propanoate,an analog of l-azatyrosine

A concise asymmetric synthesis of (R)-(+)-methyl 3-amino-3-(5-hydroxy-2-pyridinyl)propanoate 2, a β-amino acid analog of l-azatyrosine 1 starting from the commercially available (−)-Andersen reagent (−)-3 in good overall yield is described.     

(S)-(−)-α-Methylbenzylamine as an efficient chiral auxiliary in enantiodivergent synthesis of both enantiomers of N-acetylcalycotomine

(S)-(−)-α-Methylbenzylamine 2 was used as a chiral auxiliary in the enantiodivergent synthesis of simple isoquinoline alkaloids. The prochiral imine moiety in compound 4 was reduced with different reagents, giving diastereomeric amines 5a or 5b, which subsequently were transformed to either (S)-(−)-N-acetylcalycotomine 6 or (R)-(+)-N-acetylcalycotomine ent-6 in good enantiomeric excess. ¹⁹F NMR of its Mosher's acid ester was used to establish the purities of final compounds.     

Asymmetric hydrogenation reactions mediated by a new class of bicyclic bisphosphinites

The bicyclic alcohol (−)-4 was prepared from (−)-bicyclo[3.2.0]hept-2-en-6-one (−)-1 in 50% yield. The diol (−)-4 was coupled to selected chlorophosphines 6–12 to produce a series of bisphosphinites 13–19 in 89–95% yield. From these bisphosphinites were prepared the rhodium complexes 20–26 which were characterised by ³¹P NMR and used in situ for the asymmetric hydrogenation of α-enamides 27–29. Complexes 21, 23–25 proved to be the superior catalysts for the production of (R)-N-acetylphenylalanine (91, 84, 90 and 87.5% ee) from 27 and (S)-N-acetylalanine methyl ester (70, 72, 68 and 71% ee) from 28.     

A straightforward synthesis of both enantiomers of allo-norcoronamic acids and allo-coronamic acids,by asymmetric Strecker reaction from alkylcyclopropanone acetals

Methylcyclopropanone hemiacetal (2S)-3a underwent the asymmetric Strecker reaction induced by a chiral amine to provide a useful synthesis of enantiomerically pure (1R,2S)-(+)-allo-norcoronamic acid 1 in good yield and high enantiomeric excess. From racemic alkyl hemiacetal (±)-3, the same methodology also constituted a useful way to prepare both (+)-1 and (−)-1 and (+)-allo-coronamic acid 2 and its antipode (−)-2 with good yield and high enantiomeric excess.     

2-Methoxy-2-(1-naphthyl)propionic acid,a powerful chiral auxiliary for enantioresolution of alcohols and determination of their absolute configurations by the 1H NMR anisotropy method

Racemic 2-methoxy-2-(1-naphthyl)propionic acid (1, MαNP acid) was enantioresolved as its esters derived from various chiral alcohols. For example, a diastereomeric mixture of esters prepared from (±)-1 and (1R,3R,4S)-(−)-menthol was easily separated by HPLC on silica gel yielding esters (−)-2a and (−)-2b, the separation factor α=1.83 being unusually large. The ¹H NMR chemical shift differences, Δδ=δ(R)–δ(S), between diastereomers 2a and 2b, are much larger than those of conventional chiral auxiliaries, e.g. Mosher’s MTPA and Trost’s MPA acids. This acid 1 is therefore very powerful for determining the absolute configuration of chiral alcohols by the ¹H NMR anisotropy method. Solvolysis of the separated esters yielded enantiopure acids (S)-(+)-1 and (R)-(−)-1, which are useful for enantioresolution of racemic alcohols.     

Study on enantiomerically pure 2-substituted N,N-dialkyl-1-naphthamides: resolution,absolute stereochemistry,and application to desymmetrization of cyclic meso anhydrides

The axially chiral 2-substituted N,N-diisopropyl-1-naphthamides 1 and 2 were resolved by HPLC over a chiral stationary phase to provide enantiomerically pure atropisomers. The absolute stereochemistry of (−)-syn-1 was determined by X-ray crystallographic analysis of the corresponding (1S)-camphanic acid ester derivative. Desymmetrization of cyclic meso anhydrides 5a and 5b using (−)-syn-1 gave a single diastereomer in good yield.     

Synthetic studies on pterin glycosides: the first synthesis of 2′-O-(α-d-glucopyranosyl)biopterin

l-Rhamnose was led, in a 14-step-sequence, to N²-(N,N-dimethylaminomethylene)-1′-O-(4-methoxybenzyl)-3-[2-(4-nitrophenyl)ethyl]biopterin (23), an appropriately protected precursor for 2′-O-glycosylation, while 4,6-di-O-acetyl-2,3-di-O-(4-methoxybenzyl)-α-d-glucopyranosyl bromide (32), a novel glycosyl donor, was efficiently prepared from d-glucose in 8 steps. The first synthesis of 2′-O-(α-d-glucopyranosyl)biopterin (2a) was achieved by treatment of the key intermediate 23 with 32 in the presence of silver triflate and tetramethylurea, followed by successive removal of the protecting groups.     

Improved synthetic methods of CP-060S,a novel cardioprotective drug

Two synthetic methods of CP-060S, (−)-2-(3,5-di-tert-butyl-4-hydroxyphenyl)-3-[3-[N-methyl-N-[2-[3,4-(methylenedioxy)phenoxy]ethyl]amino]propyl]-1,3-thiazolidin-4-one (−)-1, have been developed. The first method involved preparative HPLC resolution of bromo-intermediate 4. The second one involved resolution of 1 by crystallization of the corresponding diastereomeric salt.     

Convenient access to both enantiomers of new azido- and aminoinositols via a chemoenzymatic route

1,2-diacetylconduritol E, (±)-1, through complementary use of Mucor miehei (Lipozyme^® IM) and Candida cylindracea lipases, affords (1S)-1,2-diacetylconduritol E, (+)-1, (1R)-1,2-diacetylconduritol E, (−)-1, (1S)-1,2,4-triacetylconduritol E, (+)-2, (1R)-1,2,4-triacetylconduritol E, (−)-2, with high enantiomeric excesses and chemical yields. Following two different methods, diester (+)-1 has been transformed into azidoinositol (−)-4 to give 1L-4-amino-4-deoxy-chiro-inositol, whereas triester (−)-2 furnished the azidoinositol (+)-13, easily converted into 1L-4-amino-4-deoxy-myo-inositol.     

Synthesis of (S)-(−)- and (R)-(+)-O-methylbharatamine using a diastereoselective Pomeranz–Fritsch–Bobbitt methodology

Laterally lithiated (S)-(−)- and (R)-(+)-o-toluamides 6 with a chiral auxiliary derived from (S)- and (R)-phenylalaninol, respectively, were used as the building blocks and chirality inductors in the asymmetric modification of the Pomeranz–Fritsch–Bobbitt synthesis of isoquinoline alkaloids. Their addition to imine 2 proceeded with partial cyclization, giving isoquinolones (+)-7 and (−)-7 along with acyclic products, (−)-8 and (+)-8, respectively. LAH-reduction of (+)-7 and (−)-7, followed by cyclization, afforded both enantiomers of the alkaloid, (S)-(−)- and (R)-(+)-O-methylbharatamine 5, in 32% and 40% overall yield and with 88% and 73% ees, respectively.     

A chiral 1,4-oxazin-2-one: asymmetric synthesis versus resolution,structure, conformation and VCD absolute configuration

1,4-Oxazin-2-one 3 is obtained from 2-pinanone in 4 steps and 78% overall yield. Enantiopure (e.e. >99%) (R)-(+)-3 and (S)-(−)-3 were obtained through chiral supercritical fluid chromatography (using a semi preparative Chiralpak AS column) with almost quantitative recovery of material. The structure and the boat-conformation of the lactone ring have been determined by NMR and the absolute configuration determined by VCD.     

Enzyme-mediated preparation of enantioenriched forms of trans- and cis-p-menthan-1,8-dien-5-ol

The synthesis of the trans-p-menthan-5-ol (±)-1 was carried out by Diels–Alder cycloaddition of 3-keto-1-butenyl-acetate 3 with isoprene followed by Wittig methylenation. PS lipase resolution of the alcohol afforded acetate (−)-5 with 98% ee, which was hydrolysed to give (−)-1. Alternatively, enzymatic hydrolysis of a keto ester followed by Wittig methylenation and hydrolysis afforded the same alcohol with an ee of 86%. The cis-p-menthan-5-ol (−)-2 was obtained by Swern oxidation of (−)-1, followed by diastereoselective reduction with L-Selectride without lost of enantiomeric excess.     

Kinetic resolution of 1,2-dihydroxy-3-ferrocenylpropane by sequential lipase-catalysed esterification

One-pot sequential esterification of 1,2-dihydroxy-3-ferrocenylpropane 1, catalysed by lipase from Pseudomonas cepacia, allowed kinetic resolution of the racemate, affording (−)-(R)-1-acetoxy-2-hydroxy-3-ferrocenyl-propane (−)-2, and (+)-(S)-1,2-diacetoxy-3-ferrocenylpropane (+)-3, in high chemical yield and enantiomeric excess.     

Novel stereoselective syntheses of (2E,4E)-4-(4,4-dimethylpent-2-ynylidene)-N1,N5-dimethyl-N1,N5-bis(naphthalen-1-ylmethyl)pent-2-ene-1,5-diamine

We report two different synthetic approaches for the stereoselective synthesis of (2E,4E)-4-(4,4-dimethylpent-2-ynylidene)-N¹,N⁵-dimethyl-N¹,N⁵-bis(naphthalen-1-ylmethyl)pent-2-ene-1,5-diamine 1, an important impurity-reference standard for the chemical characterization of terbinafine. The diamine 1 was obtained in only six steps with a good overall yield starting from commercially available glutaconic acid.     

Microbial reduction of 2-(6-m-methoxyphenyl-3-oxohexyl)-2,4-dimethylcyclopenta-1,3-dione with Schizosaccharomyces pombe (NRRL Y-164)

A mixture of cis- and trans-2-(6-m-methoxyphenyl-3-oxohexyl)-2,4-dimethylcyclopenta-1,3-dione (±)-10 was synthesized and incubated with Schizosaccharomyces pombe (NRRL Y-164) to give (+)-11, (+)-12, (−)-13, and (−)-14 in 19, 13, 22, and 16% yields, respectively. Chromic acid oxidation of these microbiologically reduced products gave (−)-10a, (+)-10b, (+)-10a, and (−)-10b, respectively.     

A concise synthesis of (S)-(−)-3-(2-carboxy-4-pyrrolyl)-alanine

A convergent synthesis of (S)-(−)-3-(2-carboxy-4-pyrrolyl)-alanine (CPA) 1, a non-proteinogenic amino acid is described starting from a commercially available dimethyl l-aspartate 2 in good overall yield.     

A concise synthesis of (+)-cassiol

A synthesis of the anti-ulcerogenic compound (+)-cassiol 1b with 43% overall yield has been achieved. This short and efficient synthesis features the one-pot Julia olefination reaction of lactol (S)-2 with sulfone 3b through the key intermediate (−)-4b.     

Total syntheses of macrosphelides (+)-A, (−)-A and (+)-E

Total syntheses of (+)-macrosphelide A 1 (18.5% overall yield in 11 steps) and (+)-macrosphelide E 2 (23.9% overall yield in 11 steps) have been achieved via the chemoenzymatic reaction product (4R,5S)-4-benzyloxy-5-hydroxy-2(E)-hexenoate 4. The enantiomer (−)-A (1) (14.2% overall yield in 11 steps) of (+)-1 was also synthesized from the chemoenzymatic reaction product (4S,5R)-4-benzyloxy-5-hydroxy-2(E)-hexenoate 4.     

A chemoenzymatic approach to the synthesis of the stereoisomers of a β-adrenergic receptor antagonist

The four stereoisomers of Δ²-isoxazoline 2, a β-adrenergic receptor antagonist structurally related to Falintolol 1, were prepared by an enzyme-catalyzed kinetic resolution of the unsaturated secondary alcohol (±)-7 followed by its cycloaddition to pyruvonitrile oxide. Through this strategy, diastereomeric aminoalcohols (+)-2a/(−)-2b and (−)-2a/(+)-2b were obtained in 99 and 92% enantiomeric excess, respectively. The absolute configuration to the target compounds was assigned via chemical correlation to the enantiomers of epoxides 4a and 4b, whose stereochemistry had been previously established.     

Enzymatic resolution of (±)-conduritol-B,a key intermediate for the synthesis of glycosidase inhibitors

Lipases from porcine pancreas, Candida cylindracea and Mucor miehei (adsorbed on support, Lipozyme^® IM) catalysed in t-butylmethylether the alcoholysis of rac-conduritol-B peracetate, (±)-1, by n-butanol to give enantiopure (2S,3S)-diacetoxy-(1R,4R)-dihydroxycyclohex-5-ene, (−)-3, and (1S,2R,3R,4S)-tetraacetoxy-cyclohex-5-ene, (+)-1. The enantioforms (+)- and (−)-conduritol-B, obtained after chemical hydrolysis of (−)-3 and (+)-1, respectively, may be employed to prepare both the enantiomers of conduritol-B epoxide and cyclophellitol, powerful inhibitors of glycosidases.