Towards enantiomeric 2,3-epoxypropylphosphonates

Hydrolysis of diethyl 2,3-epoxypropylphosphonate in the presence of (R,R)-salen–Co(III)-OAc after 19 h afforded a mixture of (S)-epoxide (82% ee) and diethyl (R)-2,3-dihydroxypropylphosphonate (98% ee). Improved enantiomeric excess (93%) of the (S)-epoxide was obtained in the 72 h hydrolytic kinetic resolution experiment. Acid-catalyzed hydrolysis of (S)-epoxide (91% ee) gave (S)-diol (72% ee) due to low C-3 regioselectivity of the reaction.     

Asymmetric hydrogenation of (E)-α,β-bis(N-acylamino)acrylates catalyzed by a rhodium complex with trans-chelating chiral diphosphine ligand

The asymmetric hydrogenation of (E)-α,β-bis(N-acylamino)acrylates was promoted by a rhodium complex bearing a trans-chelating chiral diphosphine ligand (R,R)-(S,S)-PrTRAP, providing the corresponding optically active (2S,3R)-2,3-bis(N-acylamino)carboxylates with 79–82% ee. The 2,3-bis(N-acylamino)carboxylates isolated were readily hydrolyzed under acid to afford (2S,3R)-2,3-diaminocarboxylic acids in 95% yield without epimerization.     

Diastereoselective cycloaddition of chiral 1-acryloyl-2-imidazolidinone and o-quinodimethane generated by reduction of 1,2-bis(bromomethyl)benzene with zinc

The reduction of 1,2-bis(bromomethyl)benzene with zinc powder followed by cycloaddition with the chiral dienophile (4R,5S)-1-acryloyl-3,4-dimethyl-5-phenyl-2-imidazolidinone in the presence of BF₃·Et₂O under ultrasound irradiation gave the corresponding Diels–Alder cycloadduct in high yield (90%) and high diastereoselectivity (R:S = 87:13).     

Asymmetric synthesis of 3-substituted 8-hydroxy-3,4-dihydroisocoumarins from (S)-4-isopropyl-2-(2-methoxy-6-methylphenyl)oxazoline

Reaction of the laterally lithiated (S)-4-isopropyl-2-(2-methoxy-6-methylphenyl)oxazoline with p-tolualdehyde gave an inseparable mixture of the addition products in low diastereoselectivity. However, the (S,S)-product cyclized to the corresponding 3,4-dihydroisocoumarin faster than the (S,R)-product on silica gel, which allowed to be produced both enantiomers of 8-methoxy-3-(p-tolyl)-3,4-dihydroisocoumarin in moderate to good optical purity [S-enantiomer: 75% ee; R-enantiomer: 96% ee]. This procedure was applied to the short-step synthesis of optically active 3,4-dihydroisocoumarin natural products such as (R)-8-hydroxy-3-(1-tridecyl)-3,4-dihydroisocoumarin and (R)-phyllodulcin.     

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.     

Highly diastereoselective alkylation of vicinal dianions of chiral succinic acid derivatives: a new general strategy to (R)-β-arylmethyl-γ-butyrolactones

The vicinal dianions derived from chiral succinic acid derivatives, 1,4-bis[(4R,5S)-3,4-dimethyl-2-oxo-5-phenylimidazolidin-1-yl]butane-1,4-dione and 1,4-bis[(4S,5R)-3,4-dimethyl-2-oxo-5-phenylimidazolidin-1-yl]butane-1,4-dione react with arylmethyl bromides with high diastereo- and regio-selectivity to provide the corresponding chiral α-arylmethylated succinic acid derivatives; the (R)-products are converted into (R)-β-arylmethyl-γ-butyrolactones and (R)-α-arylmethyl-γ-butyrolactones.     

Asymmetric synthesis of 3,4-dihydroxyglutamic acids via enantioselective reduction of cyclic meso-imide

Stereoselective synthesis of (3S,4S)- and (3R,4R)-series of 3,4-dihydroxyglutamic acids was investigated. The key reaction in this synthesis is asymmetric reduction of meso-imide derived from meso-tartaric acid. Lewis acid-promoted cyanation of the obtained optically active lactam via the acyliminium intermediate followed by standard deprotection procedure afforded the desired 3,4-dihydroxyglutamic acids.     

Asymmetric palladium-catalyzed annulation of benzene-1,2-diol and racemic secondary propargylic carbonates bearing two different substituents

The palladium-catalyzed cyclization of benzene-1,2-diol with various racemic secondary propargyl carbonates having no acetylenic hydrogen in the presence of (R)-Binap as the chiral ligand afforded the two regioisomers of the corresponding 2,3-dihydro-1,4-dioxin derivatives in quite good yields, and also in enantioselectivities going from 40 to 97%. The cyclization of benzene-1,2-diol with methyl (R)-1-methyl-3-phenylpro-2-yn-1-yl carbonate in the presence of dppb as the achiral ligand afforded 2-benzylidene-3-methyl-2,3-dihydro-1,4-benzodioxine as the major product with 15% ee. The use of (R)-Binap as the chiral ligand afforded the (+) cyclized compound in 45% ee, when the (−) enantiomer was obtained with 77% ee in the presence of (S)-Binap. All the results suggest that in this case the enantioselective step is the diastereoselective protonation of the palladium-carbene intermediates.     

Silicon-Directed Regio- and Enantioselective Synthesis of α-Hydroxy-ketones Preliminary Communication

α-Silylated ketones (S)? 2 (ee ≥ 98%), easily available through silylation or silylation/alkylation from ketones 1 using the (?)-(S)-1-amino-2-(methoxymethyl)pyrrolidine (SAMP)?/(+)-(R)-1-amino-2-(methoxymethyl)pyrrolidine (RAMP)-hydrazone method, are oxidized to give α-hydroxy-ketones (R)? 5 of high enantiomeric purity (ee ≥ 98%) and in good overall yields (51-70%). The key step of the procedure is the silicon-directed diastereoselective oxidation of the corresponding silyl enol ethers of (S)? 2 , with m-chloroperbenzoic acid or 3-phenyl-2-(phenylsulfonyl)oxaziridine, followed by flash chromatography and desilylation.     

Enzymatic desymmetrization of meso cis-2,6- and cis,cis-2,4,6-substituted piperidines. Chemoenzymatic synthesis of (5S,9S)-(+)-indolizidine 209D

The stereoselective acylation of meso piperidines 3a,b by vinyl acetate (solvent and acyl donor) in the presence of Candida antarctica lipase gave the corresponding (2S,6R) and (2S,4R,6R) monoesters 2a,b in high enantiomeric purity. (5S,9S)-(+)-Indolizidine 209D was prepared in eight steps from (2S,6R)-2a.     

Highly enantioselective hydrosilylation of simple ketones catalyzed by rhodium complexes of P-chiral diphosphine ligands bearing tert-butylmethylphosphino groups

P-Chiral diphosphine ligands, (S,S)-1,2-bis(tert-butylmethylphosphino)ethane [(S,S)-t-Bu-BisP¹], (R,R)-bis(tert-butylmethylphosphino)methane [(R,R)-t-Bu-MiniPHOS], and (R,R)-2,3-bis(tert-butylmethylphosphino)quinoxaline [(R,R)-QuinoxP¹], were applied to the rhodium-catalyzed enantioselective hydrosilylation of simple ketones. The corresponding secondary alcohols were obtained in high yields with good to excellent enantiomeric excesses of up to 99%.     

Resolution of C2-symmetric 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid and 2,3-diphenylsuccinic acid using (S)-proline

Racemic 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid is resolved to obtain the corresponding (S,S)-isomer in 96±2% ee and the (R,R)-isomer in 97±2% ee through complexation with (S)-proline in methanol. The racemic 2,3-diphenylsuccinic acid has been resolved to obtain the (S,S)-isomer in 93% ee using (S)-proline in methanol.     

Asymmetric synthesis of 3-substituted 3,4-dihydroisocoumarins via stereoselective addition of laterally lithiated chiral 2-(o-tolyl)oxazolines to aldehydes followed by diastereomer-selective lactonization

Lateral lithiation of (S)-4-isopropyl-2-(o-tolyl)oxazoline in diethyl ether followed by the reaction with aldehydes in the presence of TMEDA produced the addition products with stereoselectivities up to 84% de. Utilization of TMEDA as a ligand is essential for the good selectivity. Rationale for the stereoselectivity is proposed based on ab initio calculation of the lateral lithio species. The major (S,S)-products lactonized faster than the minor (S,R)-products to the corresponding 3,4-dihydroisocoumarins under acidic conditions. Thus, (3S)-3,4-dihydroisocoumarins were obtained in good optical purities up to 97% ee by sequential application of these matched stereoselective reactions.     

Enantioselective hydrogenation of functionalized olefins catalyzed by Rh-chiral bidentatephosphite complexes

A novel catalytic system for the hydrogenation of dimethyl itaconate has been developed by using rhodium–diphosphite complexes. These chiral diphosphite ligands were derived from glucopyranoside, d-mannitol derivatives, and binaphthyl or H₈-binaphthyl phosphochloridites. The ligands based on the methyl 3,6-anhydro-α-d-glucopyranoside backbone and (R)- and (S)-binaphthol and/or (R)- and (S)-2,2′-dihydroxy-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthol gave almost complete conversion of the dimethyl itaconate and both enantiomers of dimethyl 2-methylsuccinate with excellent enantioselectivities. The stereochemically matched combination of methyl 3,6-anhydro-α-d-glucopyranoside and H₈-(S)-binaphthyl in ligand 2,4-bis{[(S)-1,1′-H₈-binaphthyl-2,2′-diyl]-phosphite} methyl 3,6-anhydro-α-d-glucopyranoside was essential to afford dimethyl 2-methylsuccinate with up to 98% ee. The sense of the enantioselectivity of products was predominantly determined by the configuration of the biaryl moieties of the ligands. An initial screening of [Rh(cod)₂]BF₄ with these ligands in the hydrogenation of (E)-2-(3-butoxy-4-methoxybenzylidene)-3-methylbutanoic acid was carried out. Good enantioselectivity (75% ee) and low yield for (R)-2-(3-butoxy-4-methoxybenzyl)-3-methylbutanoic acid were obtained.     

Lipase-catalyzed alcoholysis of diol dibenzoates: selective enzymatic access to the 2-benzoyl ester of 1,2-propanediol and preparation of the enantiomerically pure (R)-1-O-benzoyl-2-methylpropane-1,3-diol

Enzymatic debenzoylation of 1,2-propanediol dibenzoate with 1-octanol has been studied in organic solvent using lipases from different sources. In general a slow, highly regioselective alcoholysis in diisopropyl ether affords exclusively a monoester benzoylated at the secondary hydroxy group although the reaction proceeds with low enantioselectivity. In the presence of Pseudomonas cepacia lipase absorbed onto celite, a faster reaction allows the preparation of the 2-benzoyl ester of (R)-1,2-propanediol (82% ee) and the enantiomerically pure (R)-1-O-benzoyl-2-methylpropane-1,3-diol (>98% ee).     

Asymmetric induction in the conjugate addition of thioacetic acid to methacrylamides with chiral auxiliaries

The conjugate addition of thioacetic acid to methacrylamides with chiral C₂-symmetric trans-2,5-disubstituted pyrrolidines afforded the addition products in excellent stereoselectivities (>99% de) and good yields (80–90%). The high selectivity was attributed mainly to the steric effect of the chiral auxiliaries. The cyclic nature of the chiral auxiliaries seemed also important for both the stereoselectivity and the reaction rate. Acidic hydrolysis of the adduct containing (2R,5R)-bis(methoxymethyl)pyrrolidine gave (S)-3-mercapto-2-methylpropanoic acid, a key intermediate for captopril, in 98% ee and 96% yield. The chiral auxiliary was recovered in the demethylated form of N-Boc-(2R,3R)-bis(hydroxymethyl)pyrrolidine in 90% yield.     

Reversed diastereoselectivity in the ring-opening reaction of (S)-TFPO with a chiral aminoacetonitrile Schiff base

Diastereoselective ring opening of (S)-2,3-epoxy-1,1,1-trifluoropropane [(S)-TFPO: 75% ee] with an aminoacetonitrile Schiff base bearing the (R,R,R)-hydroxypinanone chiral auxiliary [(R)-Schiff base] are described. The reaction of (S)-TFPO with the (R)-Schiff base selectively gave one diastereomer out of the four possible, while that with the (S)-Schiff base gave a complex mixture of all four possible diastereomers. The stereochemistry of the reaction products was greatly dependent on the base used for abstracting a proton from the (R)-Schiff base; utilization of LDA and KO^tBu resulted in production of (R,R,R,S,S)- and (R,R,R,R,S)-isomers as the major diastereomers, respectively.     

Synthesis of chiral epoxyalkynes

The synthesis of novel chiral propargylic epoxides ((R)-1-t-butyldimethylsilyl-3,4-epoxy-1-butyne, (3S,4S)-3,4-epoxy-1-octyne, (3R,4S)-1-t-butyldimethylsilyl-3,4-epoxy-1-pentyne) has been developed starting from the readily available tartaric acid derivative, (S,S)-(+)-2,3-O-isopropylidene-L-threitol.     

Chemoenzymatic synthesis of glycosylated enantiomerically pure 4-pentene 1,2- and 1,3-diol derivatives

1-Dimethylthexylsiloxy-2-chloroacetoxy-4-pentene 2 and 1-dimethylthexylsiloxy-3-chloroacetoxy-4-pentene 3 were saponified with Pseudomonas lipase to give (R)-1-dimethylthexylsiloxy-4-pentene-2-ol (ee=99%) and (S)-2 (ee=99%) and (S)-1-dimethylthexylsiloxy-4-pentene-3-ol (ee=99%) and (R)-3 (ee=98%), respectively. All enantiomers were chemically transformed into the corresponding enatiomerically pure 2-benzoyloxy-4-pentene-1-ols 8 and 3-benzoyloxy-4-pentene-1-ols 14, respectively. Mannosylation of (R)-8 and (S)-14 with 2,3,4,6-tetra-O-benzoyl-a-d-mannopyranosyl trichloroacetimidate afforded the corresponding mannopyranosides.     

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.