2‐Vinylcyclobutanones by Cycloaddition of Vinylketenes to Simple Olefins

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Abstract
Selected [2+2]‐cycloadditions of three alkylvinylketenes 2 to one mono‐ and seven dialkyl‐olefins 3 yielded eleven 2‐alkyl‐2‐vinylcyclobutanones 4 (Tables 1 and 2). Three methods were compared, all involving in situ generation of the ketenes 2 by HCl‐elimination from α,β‐unsaturated acid chlorides 1; the most effective employed a large excess of olefin 3 and a high reaction temperature. The [2+2]‐cycloadditions were fully regio‐ and stereoselective with respect to the olefin 3, but less so with respect to the ketene 2, so that ‐ where possible ‐ two stereoisomers of 4 resulted, namely A and B, whose configurations were determined from their 1H‐NMR, spectra, mechanistic considerations and, in one case, 4f, by chemical correlation with a previously known cycloadduct 8.