Diels-Alder Reactions of Siloxyallylidenecyclopropanes: Facile Syntheses of Spiro[2.5]Octan-3-Ones

Abstract

Although 1,1-disubstituted dienes are known to be quite unreactive in Diels-Alderreactions, allylidenecyclopropanes due to the strain inherent in their methylenecyclopropane moiety have been found to react smoothly with activated enes.^1,2b Moreover, it has been shown that the spirocyclopropane group in the cycloadducts can be reduced to a gem-dimethyl group.^la While such cycloadditions have been done with unsubstituted allylidenecyclopropane,¹ and recently also a 3-heterosubstituted allylidenecyclopropane has been synthesized,² a 2-heterosubstituted analog has not been reported to our knowledge. The 2-trialkylsiloxyallylidenecyclopropanes 2 can easily be prepared by silylenolization³ from the readily available cyclopropylideneketones l.⁴