Extending Pummerer reaction chemistry. Asymmetric synthesis of spirocyclic oxindoles via chiral indole-2-sulfoxides.

The conversion of (Ss)-3-(omega-allylsilane and silyl enol ether)indole-2-sulfoxides to spirocyclic indolenines and then to oxindoles proceeds, in favorable cases, with moderate levels of chirality transfer from sulfur to C3 of the indole core. A mechanistic model, which features either an Sn2'-like additive Pummerer sequence or a tight ion pair generated by an Sn1-like vinylogous Pummerer transform, is proposed to rationalize the sense of asymmetric induction.