Reactions of 1,2-dehydropyrrolidin-5-one with 1,3-dienes. Synthesis of dl-gephyrotoxin 223AB

Evidence is presented that the regiospecific and often stereospecific pseudo-Diels–Alder reactions of 1,3-dienes with N-acyl immonium salts, derived from 5-ethoxy-2-pyrrolidinone, are stepwise reactions. 6,7-Dehydroindolizidinones, unsaturated lactam esters, and new indene and furan derivatives are described. A synthesis of dl-gephyrotoxin 223AB (indolizidine 223AB) from trans-1,3-heptadiene was achieved.