Intramolecular [2 + 2] cycloadditions of alkoxyketenes and alkoxyketeniminium salts

Abstract

(Alkenyloxy) ketenes, prepared from the acid chloride by treatment with triethylamine in benzene at reflux, undergo facile intramolecular [2 + 2] cycloaddition to give polycyclic cyclobutanones. Electronic effects of the alkyl substituents on the double bond control the regiochemistry of the cycloaddition. Alkenes in which the internal carbon is more highly substituted react to give bicyclo[3.2.0]heptanones or bicyclo[4.2.0]octanones. Alkenes in which the terminal carbon is more highly substituted react to give bicyclo[3.1.1]heptanones or bicyclo-[4.1.1]octanones. Mono- and 1, 2-disubstituted alkenes are not nucleophilic enough to react. (Alkenyloxy)-keteniminium salts, prepared from the dimethylamide by treatment with triflic anhydride and collidine in benzene at reflux, undergo facile intramolecular [2 + 2] cycloaddition to mono- and 1, 1- and 1, 2-disubstituted alkenes to give, after hydrolysis, polycyclic cyclobutanones. Other classes of alkenes give Friedel-Crafts products. The 25 cases examined indicate the scope and limitations of intramolecular [2 + 2] cycloadditions of ketenes to alkenes as a synthetic method. Baeyer-Villiger oxidation of the cycloadducts gives furofuranones of a type closely related to the furofurans present in insect antifeedants and aflatoxins. © 1985, American Chemical Society. All rights reserved.