Intramolecular Butenolide Allene Photocycloadditions and Ensuing Retro-Ene Reactions of Some Photoadducts

This report describes intramolecular [2+2] photocycloadditions of several butenolides with an allenylmethyl substituent at the 5-position. These photosubstrates were prepared from 2-(silyloxy)furans through silver-mediated reactions with allenylmethyl bromides. Two cases were studied where the lactone was replaced by an N-Boc-lactam. Irradiations of the photosubstrates were carried out with 300 nm UV-light in acetonitrile/acetone mixtures. Crossed cycloadducts were produced in good yields via cycloaddition of the heterocyclic double bond with the allene internal double bond. The products were gamma-lactones (gamma-lactams) containing the strained 5-methylenebicyclo[2.1.1]hexane skeleton. In some cases the products were unstable due to remarkably facile retro-ene reactions (1,5-hydrogen shifts) leading to unique alpha,beta-unsaturated gamma-lactone derivatives. These reactions occurred only with well-oriented C-H bonds so that strain-releasing four-membered ring opening could take place. This retro-ene reaction was studied in detail for more highly substituted derivatives in order to establish its relevance in connection with the instability of the natural terpenoid solanoeclepin A, the hatching agent of potato cyst nematodes. To this end, the methylenecyclobutane moiety in one of the cycloadducts was converted into a bicyclo[2.1.1]hexan-5-one (cyclobutanone) substructure.