A Practical Route to 2,3-Di-/1,2,3-Trisubstituted Indolizines from α-EWG Ketene S,S-Acetals and Their Application in Bis(1-indolizinyl)methane Synthesis.

An easy synthesis of 2,3-di-/1,2,3-trisubstituted indo-l-izines has been developed via a formal [3+2] annulation of α-EWG ketene S, S-acetals with 2-pyridine-/2-quinolinecarbaldehyde. The disubstituted products are formed via an intramolecular aza-Michael- addition and subsequent elimination of acetic acid, followed by desulfenylation- assisted by acetic acid, whereas the trisubstituted products are obtained via a similar conjugate addition followed by elimination of alkanethiol. This strategy has been applied to the synthesis of bis(1-indolizinyl)methanes by the condensation of a 2,3-disubstituted indolizine with aldehydes/ketones in the presence of a catalytic amount of BF 3 ·OEt 2 .