Aprotic Conjugate Addition of Allyllithium Reagents Bearing Polar Groups to Cyclic Enones. 2. 2-Alkyl-, 2, 3-Dialkyl-, and 1, 3-Dialkylallyl Systems

As an extension of the work carried out on the conjugate addition reactions of lithiated 3-alkylallylic sulfoxides, phosphine oxides, and phosphonates to cyclic enones, the effects of placing methyl groups at C2, at C2 and C3, and at C1 and C3 of the allyl system and of placing the allyl system within a five-membered ring are examined. From the lithiated 2-methallyl and (E)-2-methylbut-2-enyl (“tiglyl”) sulfoxides, mixtures of diastereomeric (E)- and (Z)-vinylic sulfoxides resulting from conjugate addition to cyclopentenone are obtained. The proportion of Z diastereomers formed increases with the reaction temperature. In contrast, lithiated (Z)-2-methylbut-2-enyl (“angelyl”) sulfoxides and the tiglyl and angelyl phosphine oxides undergo highly diastereoselective conjugate addition to give (E)-vinylic products. Lithiated l,3-dimethylbut-2-enyl sulfoxides undergo stereoconvergent reactions in that the starting sulfoxides, as mixtures of diastereomers, are converted into vinylic sulfoxides, which are obtained as single diastereomers. The individual diastereomers of tert-butyl cyclopentenyl sulfoxide upon lithiation undergo conjugate addition with cyclopentenone to give the same vinylic sulfoxide. A sulfenate ester also results from carbonyl addition. The structures of the diastereomers have been established by high-field ¹H NMR spectroscopy, by chemical correlation, and in two cases by X-ray crystallographic studies. The destabilizing influence of the methyl groups on the normal “trans-fused chair-chair”-like extended transition state causing access to “cis-fused boat-boat”-like, “cis-fused chair-chair”-like, and “trans-fused boat-chair”-like transition states involving planar lithiated reagents provides a rationale for the results. The temperature dependence of some of these reactions, and simple quenching experiments in which the individual lithiated diastereomers of the cyclopentenyl sulfoxide are converted into a single diastereomer, provide evidence for planar lithiated sulfoxides.