Stereoselective Additions of Nucleophilic Alkenes to Chiral Thionium Ions

Article abstract of DOI:10.1021/jo00311a013

A convenient one-pot process has been developed for conversion of an aldehyde to an arylthionium ion (e.g., Schemes IV and V), which can be trapped by a nucleophilic alkene.The stereochemistry of the reactions of such chiral and prochiral arylthionium ions with achiral and prochiral nucleophilic alkenes has been studied.The major adducts are those predicted by qualitative application of the Cram-Felkin rule.Quantitatively, however, the nature of the thionium aryl group has a marked effect.For example, whereas 12 reacts with the phenylthionium ion derived from 2-phenyl propanol to give keto sulfides 13a and 14a in a ratio of 4:1, the corresponding reaction of the mesitylthionium ion affords the analogous keto sulfides 24a and 25a in a ratio of > 98:2.In reactions between prochiral thionium ions and prochiral enol silanes, good simple (anti) relative stereochemistry is observed, especially with enol silane 35 (Scheme VII, Table IV).Mesitylthionium ions of α-chiral aldehydes react with prochiral enol silanes to give one of the four possible products in predominance (Scheme VIII, Table V).Again, however, enol silane 35 is found to be a superior reagent, giving 41 in 97percent stereoisomeric purity.The α-methyl-β-arylthio ketones produced in these thionium ion reactions can be transformed by a straightforward process, which includes desulfurization, into chain compounds having anti 1,3-dimethyl branches (e.g., Scheme IX).An iterative application of this scheme can be used to prepare deoxypolypropionate structures, as shown in Scheme X.