The sulfinyl group as a remote chiral auxiliary in stereoselective conjugate additions of alkyl groups to α-methylidene carbonyl compounds initiated by Et_3B/O_2

摘要

Reactions of α-[2-(p-tolylsulfinyl)phenyl] α-methylidene carbonyl compounds 1 and 2 with alkyl radicals generated from Et _3B/O_2 and RI give, after protonation, β-alkyl derivatives with a high degree of control of the configuration at the α carbon. In the case of aldehyde 2, when further combined with allylation of the carbonyl group, a one-pot radical-addition/protonation/allylation sequence provides a highly stereoselective synthesis of compounds bearing two adjacent chiral centres. The stereochemical course of the reaction is controlled by the sulfinyl group acting as a remote chiral auxiliary, and this group can be easily removed with tBuLi. The sulfinyl group can be an efficient remote chiral auxiliary in the stereoselective radical addition to α-methylidene carbonyl compounds. Radicals were generated from alkyl iodides with Et _3B/O_2. The stereocontrol comes from the formation of chelated complexes of the Lewis acid with the carbonyl and sulfinyl oxygen atoms. This process gives access to enantiopure compounds with chiral benzylic centres.