1,3-Dipolar character of six-membered aromatic rings. Part 51. Cycloadditions of 1-(β-benzoylvinyl)-3-oxidopyridiniums and subsequent transformations

Abstract

β-p-Chloro-, β-p-bromo-, and β-2-chloro-5-nitrobenzoyl-vinyl chloride react with 3-hydroxypyridine to give quaternary salts which with base give the corresponding betaines. These betaines undergo thermal dimerisation and cycloadditions with mono- and di-enes at the 2,6- and 2,4-positions, respectively. The site-, regio-, and stereo-selectivity of these cycloadditions are discussed with reference to MO predictions. The β-aroylvinyl substituents in the adducts can be hydrolytically removed.