Preparation and reductive transformations of vinylogous sulfonamides (beta-sulfonyl enamines), and application to the synthesis of indolizidines

Abstract

Condensation between the methiodide salts of 1-alkylpyrrolidine-2-thiones and ethyl [(4-methylphenyl)sulfonyl]acetate or 1-[(4-methylphenyl)sulfonyl]propan-2-one afforded several 2-[[(4-methylphenyl)sulfonyl]methylene]pyrrolidines in good yield. These beta-sulfonyl enamines are sufficiently nucleophilic for cyclisation with internal electrophiles to give sulfone-substituted indolizines, potentially useful scaffolds for alkaloid synthesis. The carbon-carbon double bond in vinylogous sulfonamides was reduced stereoselectively either by catalytic hydrogenation or by treatment with sodium borohydride to yield beta-sulfonyl amines. The sulfone group in beta-acyl-beta-sulfonyl enamines could be removed by hydrogenolysis with sodium amalgam in THF-methanol to give enaminones.