Molecular design by cycloaddition reactions. Part IX. Further investigation of the cycloaddition reactions of pyridinium allylides [1-(1-pyridinio)prop-2-enides] to give indolizines

Abstract

Pyridinium allylides, readily obtainable from 1-(2,3-bismethoxycarbonylallyl)pyridinium salts [(3)–(5) and (7)] in the presence of alkali, cyclised intramolecularly to give 3-unsubstituted indolizine derivatives [(10)–(14)]. However, similar treatment of the 4-methylpyridinium allylide (6) gave only tetramethyl benzene-1,2,4,5-tetracarb-oxylate (15). In contrast, reactions of the 1-(3,3-bisethoxycarbonylallyl)pyridinium salts (8) and (9) with ethyl-propiolate in the presence of alkali gave the 3-vinylindolizine derivatives (16) and (17), respectively. The mechanisms for formation of the indolizine derivatives are discussed.