1,3-Dipolar character of six-membered aromatic rings. Part XVIII. Adducts from 3-oxido-1-phenylpyridinium and their quaternisation and conversion into tropone derivatives

Abstract

3-Oxido-1-phenylpyridinium forms adducts with diethyl maleate, diethyl fumarate, styrene, p-substituted styrenes, and phenylacetylene. The concerted character of the cycloaddition is demonstrated by the preservation of the addend stereochemistry in the adducts. The adducts are quaternised under vigorous conditions: the methylation occurs with equatorial approach. Attempted Hofmann degradation of the quaternary salts frequently leads to demethylation, but the methyl acrylate adduct affords an isolable dihydrotropone which has been converted into the corresponding tropolone.