Synthesis of pipecolic acid derivatives via Aza-Diels-Alder reactions of thio-substituted 1,3-butadienes with iminium salts

The titled compounds 1a-b were readily prepared by thermal desulfonylation of thio-substituted-3-sulfolenes 2a-b. The Aza-Diels-Alder reactions of 1a-b with iminium salts derived from alkyl or aryl amine hydrochlorides and ethyl glyoxalate gave the pipecolic acid derivatives 3-4 in good yields with complete control of regiochemistry.