Synthesis of diarylheptanoids, (5S)-5-acetoxy-1,7-bis(4-hydroxy-3-methoxyphenyl)-3-heptanone and (3S,5S)-3,5-diacetoxy-1,7-bis(4-hydroxy-3-methoxyphenyl)heptane

The total syntheses of the first examples of diarylheptanoid natural products (5S)-5-acetoxy-1,7-bis(4-hydroxy-3-methoxyphenyl)-3-heptanone 1, and (3S,5S)-3,5-diacetoxy-1,7-bis(4-hydroxy-3-methoxyphenyl)heptane 2 isolated from the rhizomes of Zingiber officinale were accomplished using Sharpless epoxidation and cross-metathesis reactions as the key steps.     

Chemoenzymatic synthesis of (3R,4S)- and (3S,4R)-3-methoxy-4-methylaminopyrrolidine

An efficient and a convenient enantioselective synthesis of (3R,4S)-3-methoxy-4-methylaminopyrrolidine has been carried out by a lipase-mediated resolution protocol. This method describes the preparation of (±)-1-Cbz-cis-3-azido-4-hydroxypyrrolidine starting from commercially available diallylamine followed by ring-closing metathesis (RCM) via S_N2 displacement reactions. Pseudomonas cepacia lipase immobilized on diatomaceous earth (Amano PS-D) provides (3R,4S)-11 and (3S,4R)-12 in an excellent enantiomeric excess.     

Highly stereoselective transformation of (1S,3S)-cis-1,3-disubstituted tetrahydro-β-carbolines into (1S,3R)-trans-1,3-disubstituted tetrahydro-β-carbolines: an improved asymmetric synthesis of tadalafil from l-tryptophan

An efficient and general method for the highly stereoselective transformation of (1S,3S)-cis-1,3-disubstituted 1,2,3,4-tetrahydro-β-carbolines (THBCs) into (1S,3R)-trans-1,3-disubstituted THBCs is described. The method contains the following three steps: the enantiomerically pure (1S,3S)-cis-1,3-disubstituted THBCs 1 were first converted into (1S,3S)-cis-1,2,3-trisubstituted THBCs 2 by N-1-naphthylmethylation/benzylation; (1S,3S)-cis-1,2,3-trisubstituted THBCs2 were then converted into (1S,3R)-trans-1,2,3-trisubstituted THBCs 3 in high yields and with high stereoselectivities via a base-catalyzed epimerization at C-3; (1S,3R)-trans-1,2,3-trisubstituted THBCs 3 were subsequently converted into (1S,3R)-trans-1,3-disubstituted THBCs 4 after reductive removal of the 1-naphthylmethyl/benzyl group. In addition, as an application of this method, an improved and highly stereoselective synthesis of the PDE5 inhibitor tadalafil (Cialis®) starting from natural and less expensive l-tryptophan was developed.     

A practical enantioselective synthesis of (S)-3-hydroxytetradecanoic acid

(S)-3-Hydroxytetradecanoic acid 1 has been synthesized in an overall yield of 27% from (S)-epichlorohydrin 2 as follows: (1) regio and chemoselective epoxide opening of 2 with a Grignard reagent under the catalysis by Cu(I) followed by consecutive epoxide formation; (2) regioselective epoxide opening of (S)-1,2-epoxytridecane 4 with cyanide anion under pH controlled conditions followed by consecutive nitrile hydrolysis with alkaline H₂O₂ gave crude 1; (3) its purification via the N,N-dicyclohexylammonium salt 6. The method thus devised is practical and scalable for the industrial production of 1.     

[MoFe3S4]3+ and [MoFe3S4]2+ cubane clusters containing a pentamethylcyclopentadienyl molybdenum moiety

A series of organometallic molybdenum/iron/sulfur clusters of the general formula [Cp¹MoFe₃S₄L_n]^m (Cp¹ = η⁵-C₅Me₅; L = S^tBu, SPh, Cl, I, n = 3, m = 1−; L_n = I₂(P^tBu₃), m = 0; L = 2,6-diisopropylphenylisocyanide (ArNC), n = 7, m = 1+) have been synthesized. A cubane cluster (PPh₄)[Cp¹MoFe₃S₄(S^tBu)₃] (2) was isolated from a self-assembly reaction of Cp¹Mo(S^tBu)₃ (1), FeCl₃, LiS^tBu, and S₈ followed by cation exchange with PPh₄Br in CH₃CN, while an analogous cluster (PPh₄)[Cp¹MoFe₃S₄(SPh)₃] (3) was obtained from the Cp¹MoCl₄/FeCl₃/LiSPh/PPh₄Br reaction system or from a ligand substitution reaction of 2 with PhSH. Treatment of 2 with benzoyl chloride gave rise to (PPh₄)[Cp¹MoFe₃S₄Cl₃] (4), which was in turn converted to (PPh₄)[Cp¹MoFe₃S₄I₃] (5) by the reaction with NaI. A neutral cubane cluster Cp¹MoFe₃S₄I₂(P^tBu₃) (6) was generated upon treating 5 with P^tBu₃. Although reduction of 4 by cobaltocene under the presence of ArNC resulted in a disproportionation of the cubane core to give Fe₄S₄(ArNC)₉Cl (7), a similar reduction reaction of 5 produced [Cp¹MoFe₃S₄(ArNC)₇]I (8), where the MoFe₃S₄ core was retained. The crystal structures of 4–6, and 8 were determined by the X-ray analysis.     

New chiral phosphine-phosphonites derived from (2R,3R)-dimethyl tartrate, (S)-binaphthol and (1R,2S)-ephedrine

The reaction of 2-lithiophenyldiphenylphosphine with phosphorus trichloride afforded the new unsymmetric phosphine, dichloro(2-diphenylphosphinophenyl)phosphine (4). Condensation of 4 with (a) (2R,3R)-dimethyl tartrate or (b) (S)-binaphthol in the presence of triethylamine gave new chiral phosphine-phosphonite ligands, (2R,3R)-[2-(2′-(diphenylphosphino)phenyl)-4,5-bis(carbomethoxy)-1,3,2-dioxaphospholane] ((2R,3R)-5) and (S)-[2-(diphenylphosphino)benzene][1,1′-binaphthalen-2,2′-diyl]phosphonite] ((S)-6). The analogous reaction of 4 with (1R,2S)-ephedrine using N-methylmorpholine as the base, gave [2-(2′-(diphenylphosphino)phenyl)-3,4-dimethyl-5-phenyl-1,3,2-oxazaphospholidine] (7) as a 95:5 mixture of diastereoisomers.     

Porcine pancreatic lipase mediated regio- and stereoselective hydrolysis: chemoenzymatic synthesis of (2S,3S)-2-amino-3,4-dihydroxybutyric acid

Porcine pancreatic lipase was used in the chemoenzymatic hydrolysis of 2-azido-3-hydroxy-4-methylcarbonyloxybutyl acetate. The reaction occurred with high regio- and stereoselectivity to give enantiomerically pure (2S,3R)-3-azido-2,4-dihydroxy butyl acetate 5 (e.e. >99%) which was easily converted to (2S,3S)-2-amino-3,4-dihydroxybutyric acid 1, an important synthetic intermediate in the synthesis of β-lactam antibiotics and phytosiderophores.     

Stereospecific synthesis and absolute configuration of the (2S,3S,4S)-isomer of 2-methyl-2-(carboxycyclopropyl)glycine (MCCG)

The conformationally restricted metabotropic glutamate receptor antagonist (2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine 1 (MCCG) has been synthesized in a stereoselective manner (>99% ee) with the (2S,3S,4S) absolute configuration of this molecule being confirmed by X-ray crystallographic analysis. Subsequent physico–chemical studies were undertaken and the data are at odds with those of the commercially available product.     

Synthesis of the (1S,2S)- and (1R,2S)-stereoisomers of the respective E- and Z-isomers of ethyl 4-[(2-hydroxycyclohexyl)methyl]phenoxy-3-methyl-2-butenoate using yeast whole cell bioreduction of the parent ketones

Saccharomyces cerevisiae, strain DBM 2115, was successfully employed in the reduction of the separated Z- and E-isomers of ethyl 4-[(2-oxocyclohexyl)methyl]phenoxy-3-methyl-2-butenoates 1 and 2, in order to prepare the (1S,2S)- and (1R,2S)-enantiomers of the corresponding ethyl 4-[(2-hydroxycyclohexyl)methyl]phenoxy-3-methyl-2-butenoates 3–6. The products were obtained with the required absolute configuration: (1S,2S)-3 (ee = 98%; yield 48%), (1R,2S)-4 (ee = >99%; yield 45%), (1S,2S)-5 (ee = 98.5%; yield 47%), and (1R,2S)-6 (ee = >99%; chemical yield 44%).     

Chemoenzymatic synthesis of both enantiomers of 3-hydroxy- and 3-amino-3-phenylpropanoic acid

Ethyl (S)-3-hydroxy-3-phenylpropionate (S)-2 was obtained by the asymmetric reduction of ethyl 3-phenyl-3-oxopropionate 1 with the yeast Saccharomyces cerevisiae (ATCC 9080). The kinetic resolution of racemic ethyl 2-acetoxy-3-phenyl-propionate rac-3 with the same microorganism, gave after hydrolysis ethyl (R)- and (S)-3-hydroxy-3-phenylpropionates (R)-2 and (S)-2 which were converted by a straightforward series of reactions to the enantiomers of 3-amino-3-phenyl-propionic acids (S)-6 and (R)-6. The asymmetric reduction and hydrolytic kinetic resolution were also tested with several other whole cell systems under a variety of conditions.     

Dielectrically controlled resolution (DCR) of 3-aminopiperidine via diastereomeric salt formation with N-tosyl-(S)-phenylalanine

A useful key intermediate for the dipeptidyl peptidase-4 (DPP-4) inhibitor, 3-aminopiperidine 1, was successfully resolved with an enantiomerically pure resolving agent, N-tosyl-(S)-phenylalanine 2, to give both stereoisomers (R)-1 and (S)-1 as a less-soluble diastereomeric salt with (S)-2, via a dielectrically controlled resolution (DCR) phenomenon.     

Synthesis of enantiomerically pure diethyl (R)- and (S)-2-hydroxy-3-(1,2,3-triazol-1-yl)propylphosphonates

The synthesis and ee determination of diethyl 3-azido-2-hydroxypropylphosphonates from 2,3-epoxypropylphosphonates have been optimised. Enantiomerically enriched diethyl (R)- and (S)-2-hydroxy-3-(1,2,3-triazol-1-yl)propylphosphonates (R)-3a–j and (S)-3a–h as well as (S)-3j were synthesised from diethyl (R)- and (S)-2,3-epoxypropylphosphonates in a reaction sequence including azidolysis followed by 1,3-dipolar cycloaddition with selected alkynes.     

Stereodivergent approach to both syn- and anti-isomers of γ-amino-β-hydroxy acids: (3S,4S)- and (3R,4S)-AHPPA derivatives

A stereodivergent approach employing an N-hydroxymethyl group has been utilized to produce both diastereomeric derivatives of (3S,4S)-AHPPA 3 and (3R,4S)-AHPPA 4, via an intramolecular conjugate addition and an intramolecular epoxidation, respectively. The selectivity of the intramolecular conjugate addition was more than 10:1 while that of the intramolecular epoxidation was more than 1:20.     

Reduced collagen cross links: the first synthesis of all the possible (2S,2′S)-stereoisomers of 5-hydroxylysinonorleucine and of 5,5′-dihydroxylysinonorleucine in enantiomerically pure form

The paper reports the first enantioselective synthesis of all the possible collagen reduced cross links as described: (2S,2′S,5R)- and (2S,2′S,5S)-5-hydroxylysinonorleucine 3a and 3b, (2S,2′S,5R,5′R)-5,5′-dihydroxylysinonorleucine 4a, (2S,2′S,5R,5′S)-5,5′-dihydroxylysinonorleucine 4b and (2S,2′S,5S,5′S)-5,5′-dihydroxylysinonorleucine 4c. The Williams’ glycine template methodology was used both for the introduction of a stereogenic at the α-position and for an easy protection of the amino acidic functionalities during the synthesis of the dimeric amino acids.     

Preparation of enantiomerically pure (1S,2S)-1-aminocyclopropanephosphonic acid from methylcyclopropanone acetal via spirophosphonate intermediates

An easy and efficient one-pot reaction from readily available methylcyclopropanone acetal (2S)-4b gave the spirophosphonates 8a–b with excellent diastereoselectivity. These phosphonates, after catalytic hydrogenolysis and hydrolysis, furnished the enantiomerically pure (1S,2S)-1-amino-2-methylcyclopropanephosphonic acid 3b (analogue of (1R,2S)-allo-norcoronamic acid).     

Asymmetric synthesis of syn-aryl-(2S,3R)-2-chloro-3-hydroxy esters via an engineered ketoreductase-catalyzed dynamic reductive kinetic resolution

We report here a generic, green synthesis of 17 valuable syn-aryl-(2S,3R)-2–chloro-3–hydroxy esters(syn-(2S,3R)-1) in 73%-99% isolated yields along with 6.1:1–83:1 dr and 31%～>99% ee, through dynamic reductive kinetic resolution of racemic aryl α–chloro β-keto esters(2) catalyzed by an engineered ketoreductase which was obtained via ep PCR-based directed evolution. The hectogram scale synthesis of syn-(2S,3R)-1b at a substrate concentration of 120 g/L showcased the application potential of th...     

SN2 reaction of 2-substituted 3-piperidinol mesylate with retention of configuration: application to the asymmetric synthesis of (2R,3S)-CP-99,994

Starting from protected (S)-3-hydroxyglutarimide 2a, the asymmetric synthesis of (2R,3S)-CP-99,994 8 was achieved. The crucial steps were a neighboring group participation leading to pyrrolidino-aziridinium intermediate 25 and the subsequent regioselective ring-opening reaction. In the case where neighboring group participation was not involved, only the eliminated product 15 was obtained.     

A contribution to the asymmetric synthesis of isoquinolines: Concise stereoselective approach to (3S,4S)-6,7-dimethoxy-4-hydroxy-3-phenyl-1,2,3,4-tetrahydroisoquinoline

A highly efficient stereoselective synthesis of (3S,4S)-6,7-dimethoxy-4-hydroxy-3-phenyl-1,2,3,4-tetrahydroisoquinoline 8 (e.e.=96%) starting from enantiomerically pure imine 3 is reported.     

Total synthesis of (4R,5S,6E,14S)- and (4R,5S,6E,14R)-cystothiazoles F

Total synthesis of (4R,5S,6E,14S)- and (4R,5S,6E,14R)-cystothiazoles F 3 was achieved from the chiral bithiazole-type primary alcohols [(S)- and (R)-4-ethoxycarbonyl-2′-(1-hydroxymethylethyl)-2,4′-bithiazoles 8], which were obtained based on the enzymatic resolution of racemic alcohol 8 and its acetate 9. From a direct comparison by means of chiral HPLC between natural cystothiazole F 3 and synthetic compounds [(4R,5S,6E,14S)- and (4R,5S,6E,14R)-cystothiazoles 3], natural cystothiazole F 3 was found to be a 33:67 diastereomeric mixture [(4R,5S,6E,14S)-3:(4R,5S,6E,14R)-3 = 33:67].     

Stereospecific synthesis of differentially protected (2S,4S)-2,4-diaminoglutaric acid suitable for incorporation into peptides

The first synthesis of differentially protected (2S,4S)-2,4-diaminoglutaric acids 2 and 3 suitable for incorporation into peptides has been accomplished in a completely stereospecific manner in seven steps (overall yield 25–28%) from tert-butyl (2S,4S)-4-azido-N-tert-butoxycarbonylprolinate 5.