Stereoselective synthesis of 2-deoxy-α-d-arabino-hexopyranosides of triterpene alcohols

2-Deoxy-2-iodo--d-mannopyranosides of triterpene alcohols of the oleanane series were synthesized inca. 60% yields by glycosylation with readily accessibled-glucal triacetate in the presence ofN-iodosuccinimide as the promotor. Their deiodination by catalytic hydrogenation followed by deacetylation yielded 2-deoxy--d-arabino-hexopyranosides of glycyrrhetic acid derivatives and allobetulin.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2340–2346, September, 1996.     

Facile synthesis of enantioenriched Cγ-tetrasubstituted α-amino acid derivatives via an asymmetric nucleophilic addition/protonation cascade

An asymmetric nucleophilic addition/protonation reaction of 3-substituted oxindoles and ethyl 2-phthalimidoacrylate has been described. This strategy can give direct access to C(γ)-tetrasubstituted α-amino acid derivatives bearing 1,3-nonadjacent stereocenters with up to 98% yield, 94:6 dr, and >99% ee. Dual activation is proposed in the transition state, and the opposite enantiomers can be obtained simply by changing cinchonidine-derived catalyst to the cinchonine analogue.     

Asymmetric synthesis of α,β-substituted γ-amino acids via conjugate addition

The first conjugate addition reaction of organocuprates to N-enoyl oxazolidinone where a N-protected γ-nitrogen atom and an α-methyl group are present into α, β-unsaturated system is described. This reaction gave anti-products in moderate yields and high diastereomeric ratios. The anti-products have two contiguous stereogenic centers, one formed by the conjugate addition reaction and the other by a diastereoselective protonation reaction. The removal of chiral oxazolidinone moiety and N-deprotection of amino group furnished chiral α, β-disubstituted γ-amino acids.     

A one-pot synthesis of α-formyl-α-allylacetates via nucleophilic catalysis

A step-economical one-pot nucleophilic catalysis/thermal Claisen-rearrangement protocol for the direct synthesis of α-formyl-α-allylacetates from allylic alcohols and activated alkynes has been developed. The product α-formyl-α-allylacetates were further reacted in situ to give either protected enol ethers or β-hydroxy-4-pentenoates.     

Stereoselective synthesis of ambiphilic alkenes via regioselective methylation of α-trifluoromethanesulfonyl carbonyl compounds with trimethylsilyldiazomethane

α-Trifluoromethanesulfonyl esters, ketones and amides are C-H acids capable of reacting with trimethylsilyldiazomethane to afford the corresponding ambiphilic alkenes. While esters were found to be non-selective, ketones were highly regioselective for O-methylation and displayed variable E/Z stereoselectivity. Amides were observed to be both highly regio- and stereoselective, affording O-methylation with exclusive formation of the Z-alkene.     

Stereoselective reduction of N-phthaloyl α-amino ketones: an expeditious synthesis of statine

A highly diastereoselective synthesis of a protected statine derivative via syn-selective LiAlH(OBu-t)₃ reduction of a leucine derived N-phthaloyl α-amino ketone is described.     

Stereoselective Pd-catalyzed synthesis of quaternary α-D-C-mannosyl-(S)-amino acids

In this paper, we report the stereoselective synthesis of α-D-C-mannosyl-(S)-amino acids exploiting, as a key step, an allylic alkylation of glycal-derived π-allyl Pd(II) intermediates, prepared by oxidative addition of Pd(0) species to 2,3-unsaturated pyranosides (pseudoglycals). The reaction of 4,6-di-O-acetyl α-pseudoglucal carbonate 10a with racemic alanine-, valine-, and phenylalanine-derived azlactones gave the corresponding (4S)-4-α-D-C-mannosyl-2-phenyloxazol-5(4H)-ones as the major diastereoisomers in high yields. The final α-D-C-mannosyl-(S)-amino acids were obtained in a few steps comprising highly diastereoselective dihydroxylation of the glucal derivative double bond followed by the one-pot hydrolysis of the benzamido and acetate protecting groups. Main features of this method are the conciseness of the synthetic sequence, the high diastereoselection of the allylic alkylation step, the use of racemic α-amino acids as starting material, and the good overall yields.     

Diastereoselective synthesis of aliphatic α,α-difluoro-β3-amino esters via a sonocatalyzed Reformatsky reaction

(R)-2-Phenylglycine ethyl ester was found to be a cheap and effective auxiliary for the preparation of aliphatic α,α-difluoro-β(3)-amino esters via a Reformatsky reaction performed under sonication conditions. The products were obtained in good to high yield and ≥96:4 dr, thus providing a new stereoselective route to this under-represented class of compounds. A facile one-pot removal of the phenylglycine moiety and concomitant Boc protection subsequently afforded the corresponding Boc-protected β(3)-amino esters in excellent yield.     

Asymmetric synthesis of β-amino alcohols by the transfer hydrogenation of α-keto imines

The asymmetric transfer hydrogenation of representative aryl and benzofuranyl 2-tert-butylaminoethanones with formic acid–triethylamine, catalyzed by RhCl[(R,R)-TsDPEN](C₅Me₅), produced the corresponding β-tert-butylamino alcohols in 97–99% ee. A short asymmetric synthesis of (R)-bufuralol, a potent β-adrenergic receptor antagonist, is described. This approach to β-amino alcohols from ketones circumvents the halogenation–reduction–amination sequence.     

Direct asymmetric synthesis of α-deuterated α-amino acid derivatives from the parent α-amino acids via memory of chirality

A method for asymmeyric α-deuteration of α-amino acid derivatives has been developed by a memory of chirality (MOC) strategy. The α-deuterated α-amino acid derivatives with 88–93% D were obtained in 60–98% ee in retention of the configuration (7 examples). The characterisrtic feature of the present procedure is that asymmeric induction was achieved by employing the parent amino acid derivatives as a sole source of chirality without the use of any external chiral souces.     

Stereoselective synthesis of the dolastatin units by organotrifluoroborates additions to α-amino aldehydes

Dolastatin units were synthesized from the 1,2-addition reactions of potassium allyl or crotyltrifluoroborate salts to aldehyde derivatives from natural amino acids. The reactions were carried out in presence of a phase-transfer catalyst in a biphasic medium at room temperature and excellent yields (>89-93%) and stereoselective (>90:10 to 98:2) were obtained. The dolastatin units 8 and 14a-b were obtained after three steps in good overall yields (50-62%).     

Transition-metal-free synthesis of multisubstituted N-arylindoles via reaction of arynes and α-amino ketones

A simple transition-metal-free protocol for the synthesis of indoles has been developed using aryne cycloaddition. The in situ-generated arynes couple with α-amino ketones through a one-step N-arylation–nucleophilic addition process under mild conditions and efficiently produce multisubstituted N-arylindoles.     

Stereoselective synthesis of α-hydroxy-β-amino acid derivatives from β-hydroxy-γ,δ-unsaturated sulfilimine

The first report on the use of N-sulfinyl benzylcarbamate for the preparation of N-Cbz sulfilimine from the corresponding sulfoxide is reported. The sulfilimine moiety is utilized as an intramolecular nucleophile for the regio- and stereoselective heterofunctionalization of an alkene to furnish a bromo carbamate which is used as a key advanced intermediate in the synthesis of representative α-hydroxy-β-amino acid derivatives.     

Diastereoselective synthesis of trans-trifluoromethyl-β-lactams and α-alkyl-β-trifluoromethyl-β-amino esters

The diastereoselective synthesis of β-lactams was examined from N-tosyl-1-chloro-2,2,2-trifluoroethylamine 3 and various nonactivated aliphatic acid chlorides in the presence of a Brønsted base. The mild reaction conditions allowed to get trifluoromethyl-β-lactams in good yields with high trans-diastereo selectivity. In addition, we also demonstrated that ring-opening of β-lactams easily provided α-alkyl-β-trifluoromethyl-β-amino esters.