A stereoselective formal synthesis of leucascandrolide A

A stereoselective formal synthesis of leucascandrolide A was accomplished through the tandem and organocatalytic oxa-Michael reactions, which were promoted by the gem-disubstituent effect, in conjunction with the dithiane coupling reaction.     

Highly stereoselective hydroxy-directed Diels-Alder reaction

The successful stereocontrol of the Diels-Alder reaction of semicyclic dienes possessing a secondary and tertiary allylic magnesium alkoxide alcohol functionality and activated dienophiles such as methyl acrylate, methacrolein, acrolein, and N-phenylmaleimide is described.     

Highly stereoselective benzylation of N-sulfinylketimines

The benzylation of N-sulfinyl ketimines with 2-(p-tolylsulfinyl)ethylbenzene and LDA afford t-alkylamines in good yields. The configuration at each one of the new chiral centers simultaneously created in this reaction is controlled by the configuration of the sulfinyl groups at the nucleophile and electrophile, respectively. Thus, the reactions of the (S)-sulfoxide 6 with the N-(S)-sulfinylketimines 3 only yield the anti diastereoisomers 18, whereas the syn diastereoisomers 19 are exclusively formed in reactions of (S)-6 with N-(R)-sulfinylketimines 3. After a two-step desulfinylation process ((i) TFA, (ii) Ra-Ni), this reaction provides a procedure for synthesizing any epimer of alpha,alpha-dibranched beta-alkylarylamines in optically pure form by choosing the configuration of the starting materials. A similar behavior is observed for carbanions derived from the O-protected 2-(p-tolylsulfinyl) benzyl alcohol 7 thus allowing the synthesis of the optically pure anti- and syn-1,2-amino alcohols containing a chiral quaternary carbon adjacent to the nitrogen.     

Highly stereoselective synthesis of anti-N-protected-alpha-amino epoxides

A simple and efficient method for the synthesis of anti-N-protected amino epoxides from carbamate-protected amino acids is described. The two key steps are the monobromination of a beta-ketoester and chelation-controlled reduction of a bromomethyl ketone intermediate. Good overall yields, high diastereoselectivity, and excellent functional group compatibility are characteristic.     

Highly stereoselective biocatalytic reduction of alpha-halo ketones

The use of recombinant carbonyl reductase biocatalysts for the reduction of alpha-halo ketone intermediates to their corresponding alpha-halo alcohols has been investigated. The alpha-halo alcohol is obtained in good yield from the corresponding ketone in a stereoselective manner.     

Highly stereoselective synthesis of novel cyclobutane-fused azanucleosides

Stereoselective syntheses of several 3-azabicyclo[3.2.0]heptane nucleoside analogues have been efficiently completed starting from a homochiral a,b-unsaturated-g-lactam. The target compounds were prepared by condensation of a common bicyclic acetate intermediate with pyrimidine and purine bases under modified Vorbrüggen conditions. The anti-HIV activity of the newly synthesized azanucleosides has been evaluated.     

Highly efficient and stereoselective synthesis of beta-glycolipids

beta-Galactosylceramide and glycolipid analogues were prepared in high yield and with complete chemo and stereoselectivity by reaction of alpha-iodo glycosides with stannyl ceramides, formed in situ. TBAI was used to activate both the iodogalactose and the stannyl ether.     

Highly stereoselective addition of stannylcuprates to alkynones

The addition of stannylcuprate reagents such as (Bu3Sn)(PhS)CuLi to alkynones has been found to proceed in high yield and with excellent stereoselectivity for the Z isomer of the product (>95%). The behavior of the stannylcuprates is thus very different from that of their "carbocuprate" counterparts such as Me2CuLi or Me2Cu(CN)Li2 which are nonstereoselective. Furthermore, in contrast to the reactions of (R3Sn)(PhS)CuLi with the corresponding alkynoates, the presence of a proton source in the reaction medium has no effect on the stereoselectivity of the reaction of alkynones.     

Highly chemoselective and stereoselective synthesis of z-enol silanes

Three-component nickel-catalyzed couplings of enals, alkynes, and silanes have been developed as a new entry to enol silanes. The enol silane and a trisubstituted alkene are both formed with >98:2 stereoselectivity, and the reaction tolerates a broad range of functionality including aldehydes, ketones, esters, free hydroxyls, and basic secondary amines. A mechanistic pathway involving the formation of a metallacycle that possesses an eta1 nickel O-enolate motif explains the high level of stereoselection.     

Highly stereoselective addition of silylphosphines to chiral aldehydes

The reaction between diphenyl(trimethylsilyl)phosphine and chiral aldehydes proceeds with high stereoselectivity to give diastereomerically pure tertiary α-trimethylsiloxyalkylphosphines. The diastereomeric purity of the addition products was 90-100%. The products were purified via the formation of borane-phosphine complexes. The reaction of bis(trimethylsilyl)phenylphosphine with acetonide (R)-glyceraldehyde provides tertiary bis(glyceryl)phosphines.     

Highly stereoselective asymmetric 6pi-azaelectrocyclization utilizing the novel 7-alkyl substituted cis-1-amino-2-indanols: formal synthesis of 20-epiuleine

Highly stereoselective asymmetric 6pi-azaelectrocyclization was achieved with generality, based on the reaction between the (E)-3-carbonyl-2,4,6-trienal compounds and the (-)-7-alkyl-cis-1-amino-2-indanol derivatives as the effective novel chiral amines. Furthermore, the chiral auxiliaries of the cyclized products obtained were efficiently removed by the novel manganese dioxide oxidation under remarkably mild conditions, and the method was successfully applied to the formal synthesis of optically active 20-epiuleine.     

Highly stereoselective C-allylation of an enantiopure alpha-sulfinyl thioacetamide

The alkylation of the lithium enolate of enantiopure alpha-cyclohexylsulfinyl thioacetamide 1 with allyl bromides 5 possessing an electron-withdrawing group at the vinylic position does not occur at the sulfur center - as expected in the sulfur series - but at the carbon center through conjugate addition followed by bromide elimination. The modest to excellent 1,2-asymmetric induction achieved by the alkylsulfinyl group (dr up to 100:0) is explained by an electronic model.     

Highly stereoselective ring expansion of enantiopure α-hydroxyalkyl azetidines

Stereodefined α-hydroxyalkyl azetidines, prepared in a few steps from enantiopure β-amino alcohols, are chlorinated or transformed into methanesulfonyloxymethyl derivatives in good yields. Heating of these compounds in chloroform or dimethylformamide induces a stereospecific ring enlargement to give 3-chloro or 3-methanesulfonyloxy pyrrolidines. The ease of this rearrangement depends on the nature of the migrating group (Cl⁻ or MsO⁻), of the class of the starting alcohol (primary or secondary) and of the relative stereochemistry of the starting material.     

Highly stereoselective oxidative esterification of aldehydes with beta-dicarbonyl compounds

Copper-catalyzed oxidative esterification of aldehydes with beta-dicarbonyl compounds was developed using tert-butyl hydroperoxide as an oxidant. In general, the enol esters were synthesized in good yields (up to 87%) and high stereoselectivity under the optimized reaction conditions.     

Highly stereoselective synthesis of 1,3-aminoalcohols via Mannich reactions

Diastereoselective synthesis of β-amino ketones by a one-pot Mannich reaction and their subsequent reduction afforded sterically congested enantiomerically pure 1,3-aminoalcohols in high diastereoselectivity: dr up to >98:<2 over two steps. The absolute configurations of the newly created stereogenic centers were assigned by NMR spectroscopy and chemical correlation.