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.     

Chemoenzymatic preparation of (2S,3S)- and (2R,3R)-2,3-butanediols and their esters from mixtures of d,l- and meso-diols

An efficient method of preparing the pure enantiomers of 2,3-butanediol from commercially available mixtures of the d,l- and meso-isomers was developed. It furnished (2S,3S)-2,3-butanediol with >99% e.e. and a >99.5/0.5 diastereomeric ratio and (2R,3R)-2,3-butanediol in 95% e.e. and >95/<5 diastereomeric ratio.     

A novel asymmetric synthesis of oseltamivir phosphate (Tamiflu) from (−)-shikimic acid

Oseltamivir phosphate 1 was synthesized starting from a readily available acetonide, that is, ethyl (3R,4S,5R)-3,4-O-isopropylidene shikimate 2, through a new route via 11 steps and in 44% overall yield. The synthesis described in this article is characterized by two particular steps: the highly regioselective and stereoselective facile nucleophilic replacement of an OMs by an N₃ group at the C-3 position of ethyl (3R,4S,5R)-3,4-O-bismethanesulfonyl-5-O-benzoyl shikimate 5, and the mild ring-opening of an aziridine with 3-pentanol at the C-1 position of ethyl (1S,5R,6S)-7-acetyl-5-benzoyloxy-7-azabicyclo[4,1,0]hept-2-ene-3-carboxylate 8.     

Concise,efficient and highly selective asymmetric synthesis of (+)-(3S,4R)-cisapride

A concise asymmetric synthesis of the gastroprokinetic agent (+)-(3S,4R)-cisapride {(+)-(3S,4R)-N(1)-[3′-(4″-fluorophenoxy)propyl]-3-methoxy-4-(2″′-methoxy-4″′-amino-5″′-chlorobenzamido)piperidine} from commercially available starting materials has been developed. The key step of this synthesis employs the diastereoselective conjugate addition of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide to tert-butyl 5-[N-3′-(4″-fluorophenoxy)propyl-N-allylamino]pent-2-enoate and in situ enolate oxidation with (?)-camphorsulfonyloxaziridine to set the (3S,4R)-configuration found within the piperidine ring of the product. This synthesis proceeds in 9 steps from commercially available 1-(4′-fluorophenoxy)-3-bromopropane with an overall yield of 19%.     

Polyhydroxylated pyrrolizidines. Part 6: A new and concise stereoselective synthesis of (+)-casuarine and its 6,7-diepi isomer, from DMDP

A new synthesis for (+)-casuarine (1) and its 6,7-diepi isomer (15) in a stereocontrolled manner, is reported herein. An appropriately protected polyhydroxylated pyrrolidine, such as (2R,3R,4R,5R)-3,4-dibenzyloxy-2′-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine (3, protected DMDP), easily available from d-fructose, was chosen as the chiral starting material. Compounds 1 and 15 were obtained from 3, in seven steps, in a 23.2 and 20.5% overall yields, respectively.     

Cyclooctatetraene made easy

Cyclooctatetraene, C₈H₈, has been made readily available from 1,5-cyclooctadiene in 65% yield without the need of using hazardous or toxic reagents by the straightforward oxidation of the intermediate [Li(tmeda)]₂C₈H₈ (3, tmeda=N,N,N′,N′-tetramethylethylenediamine) with di-tert-butylperoxide.     

Asymmetric total syntheses of (+)-2,5-dideoxy-2,5-imino-d-glucitol [(+)-DGDP] and (−)-1-deoxymannojirimycin [(−)-DMJ] via an extended chiral 1,3-oxazine

The asymmetric total syntheses of (+)-2,5-dideoxy-2,5-imino-d-glucitol [(+)-DGDP] 1 and (?)-1-deoxymannojirimycin [(?)-DMJ] 2 were achieved using an extended chiral 1,3-oxazine. The key synthetic strategies included extension of the chirality of anti,syn-oxazine 3 using diastereoselective dihydroxylation, and piperidine and pyrrolidine ring formation. Starting from readily available anti,syn-oxazine 3, (+)-DGDP 1 was synthesized in 5 steps with 31.6% overall yield and (?)-DMJ 2 was synthesized in 4 steps with 60.6% overall yield.     

An efficient approach to the synthesis of enantiomerically pure trans-1-amino-2-(hydroxymethyl)cyclopropanephosphonic acids

The synthesis of (1R,2S)- and (1S,2R)-1-amino-2-(hydroxymethyl)cyclopropanephosphonic acids was accomplished using different strategies. The (1R,2S)-stereoisomer could be efficiently obtained by the cyclopropanation of ethyl diethoxyphosphorylacetate with the cyclic sulfate derived from (S)-3-benzyloxy-1,2-propandiol as a key step. The (1S,2R)-stereoisomer was synthesized from a readily available (1S,5R)-3-oxabicyclo[3.1.0]hexan-2-on-1-phosphonate.     

Theoretical study of LiK and LiK+ in adiabatic representation

The potential energy curves have been calculated for the electronic states of the molecule LiK within the range 3 to 300 a.u., of the internuclear distance R. Using an ab initio method, through a semiempirical spin-orbit pseudo-potential for the Li (1s ²) and K (1s ²2s ²2p ⁶3s ²3p ⁶) cores and core valence correlation correction added to the electrostatic Hamiltonian with Gaussian basis sets for both atoms. The core valence effects including core-polarization and core-valence correlation are taken into account by using an l-dependent core-polarization potential. The molecular orbitals have been derived from self-consistent field (SCF) calculation. The spectroscopic constants, dipole moments and vibrational levels of the lowest electronic states of the LiK molecule dissociating into K (4s, 4p, 5s, 3d, and 5p) + Li (2s, 2p, 3s, and 3p) in ^{1, 3}Σ, ^{1, 3}Π, and ^{1, 3}Δ symmetries. Adiabatic results are also reported for ²Σ, ²Π, and ²Δ electronic states of the molecular ion LiK⁺ dissociating into Li (2s, 2p, 3s, and 3p) + K⁺ and Li⁺ + K (4s, 4p, 5s, 3d, and 5p). The comparison of the present results with those available in the literature shows a very good agreement in spectroscopic constants of some lowest states of the LiK and LiK⁺ molecules, especially with the available theoretical works. The existence of numerous avoided crossing between electronic states of ²Σ and ²Π symmetries is related to the charge transfer process between the two ionic systems Li⁺K and LiK⁺.     

Enantioselective synthesis of α-hydroxy-β-amino acids from α-amino acids mediated by sulfoxides

The synthesis of the optically pure (2S,3S)- and (2R,3S)-3-amino-2-hydroxybutanoic acids from commercially available (S)-alanine derivatives is reported. The key step of the synthetic sequence is the conversion of γ-amino sulfoxides into γ-amino alcohols by treatment with TFAA and sym-collidine. The efficiency of this non-oxidative Pummerer reaction (NOPR) is dependent on the stereochemistry of the starting sulfoxide.     

An efficient enantioselective synthesis of florfenicol via asymmetric aziridination

An efficient enantioselective synthesis of florfenicol is accomplished in 44.7% overall yield from commercially available p-(methylsulfonyl)benzaldehyde. Key features of this synthesis are the asymmetric aziridination reaction mediated by the Wulff’s catalyst in situ derived from (R)-VANOL and diastereoselectively ring-opening of (2S,3S)-fluoroaziridine 13.     

The synthesis of chiral 1,2,4-benzothiadiazine derivatives

Novel enantiomerically pure benzothiadiazines were prepared using the commercially available enantiopure (1R,2S,5R)-(−)-menthol and 1,7-dichloro-3-trifluoromethyl-1λ⁴-benzo[1,2,4]thiadiazine.     

An efficient and stereodivergent synthesis of threo- and erythro-β-methylphenylalanine. Resolution of each racemic pair by semipreparative HPLC

threo and erythro diastereoisomers of the constrained amino acid (βMe)Phe can be obtained separately on a multigram scale through a three-step synthesis from the corresponding Z and E isomers of 2-phenyl-4(α-phenylethylidene)-5(4H)-oxazolone. The 5(4H)-oxazolones are readily available from acetophenone and hippuric acid. The four enantiomerically pure isomers of β-methylphenylalanine, (2R,3R)-(βMe)Phe, (2S,3S)-(βMe)Phe, (2R,3S)-(βMe)Phe and (2S,3R)-(βMe)Phe, have been prepared by HPLC resolution of the racemic precursors methyl threo (or erythro)-2-benzamide-3-phenylbutanoates.     

A concise and stereoselective chemoenzymatic synthesis of Sitophilate,the male-produced aggregation pheromone of Sitophilus granarius (L.)

(2S,3R)-Sitophilate, the male-produced aggregation pheromone of the granary weevil Sitophilus granarius (L.) was prepared stereoselectively using a novel chemoenzymatic approach in 50% overall yield. The synthetic design was based on an enantioselective fungal reduction of ethyl 2-methyl-3-oxopentanoate with a strain of Aureobasidium pullulans (CCM H1), followed by a Mitsunobu inversion at C3. The last step in the synthetic sequence was a lipase-mediated transesterification using the commercially available Candida antarctica B lipase (CaL B, Novozym 435) using microwave irradiation under solvent-free conditions.     

Synthesis of (R)- and (S)-4-hydroxyisophorone by ruthenium-catalyzed asymmetric transfer hydrogenation of ketoisophorone

The first synthesis of (R)- and (S)-4-hydroxyisophorone by catalytic transfer hydrogenation of ketoisophorone is reported. Ruthenium catalysts containing commercially available chiral amino alcohols afforded 4-hydroxyisophorone in up to 97% selectivity and 97% ee. (R)- or (S)-4-Hydroxyisophorones with >99% ee were isolated by crystallization. The catalyst precursors [RuCl₂((S,R)-ADPE)(η⁶-p-cymene)] ((S,R)-ADPE=(1S,2R)-amino-1,2-diphenylethanol-N) and (R_Ru)-[RuCl((S,R)-ADPE⁻¹)(η⁶-p-cymene)] (ADPE⁻¹=amino-1,2-diphenylethanolato-N,O) were isolated for the first time and the X-ray crystal structure of the latter determined.     

Stereoselective additions to the exocyclic CC bond of some α-alkylidene-(+)-camphor derivatives

Stereoselective additions to the exocyclic CC double bond of some (1R,3E,4S)-3-alkylidene-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones and (1R,4E,5S)-4-alkylidene-1,8,8-trimethyl-2-oxabicyclo[3.2.1]octan-3-ones were studied. All additions took place predominantly from the less hindered endo-face of the methylidene compounds to give the corresponding exo-adducts as the major isomers. Thus, catalytic hydrogenations afforded the α-alkylated (1R,3R,4R)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones and (1R,4R,5R)-1,8,8-trimethyl-2-oxabicyclo[3.2.1]octan-3-ones in 28–100% de. Similarly, 1,3-dipolar cycloadditions of 2,4,6-trisubstituted benzonitrile oxides gave the corresponding spiro cycloadducts in 66–100% de. The structures were determined by 2D NMR techniques, NOESY spectroscopy and X–ray diffraction.     

New synthetic strategy for preparation of the anticoagulant drug Rivaroxaban via an asymmetric Henry reaction

A new synthetic approach towards the anticoagulant drug (S)-Rivaroxaban was described. This reaction sequence involved six steps overall, starting from commercially available and inexpensive N-(4-aminophenyl)morpholin-3-one. The stereogenic centre was introduced by an asymmetric Henry reaction catalysed by the complex of copper(II) acetate and (2R,5S)-2-(pyridine-2-yl)imidazolidine-4-one with 87% ee. The individual reaction steps proceeded with high yields and did not require any unusual or expensive reagents.     

Transformation of (+)-thiomicamine into the p-methylthio analogue of (+)-5-epi-cytoxazone

(2S,3S)-(+)-Thiomicamine 3, a commercially available aminodiol, was transformed into (4R,5S)-5-hydroxymethyl-4-(p-methylthiophenyl)-2-oxazolidinone 10, a compound related to cytoxazone-type biologically active natural products. The synthetic strategy of the highly regio- and stereoselective synthesis was based upon the reversal of the position of the hydroxyl and amine functionalities in 3, accomplished via azidolysis of the key intermediate, epoxide 6.     

Efficient synthesis of (2R,3S)-2-amino-3-(benzyloxy)-4,4,4-trifluorobutanoic acid (4,4,4-trifluoro-OBn-d-allothreonine)

An efficient synthesis of the non-proteinogenic amino acid (2R,3S)-4,4,4-trifluoro(OBn)-threonine is described. Starting with commercially available (S)-Garner’s aldehyde, the desired amino acid was prepared as its hydrochloride salt in five steps and an overall yield of 33% (59% based on recovered starting material). The utility of this unusual amino acid was demonstrated by its elaboration into a potent and selective androgen.     

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.