New 1,3-dihydroazepin-2-one derivatives by [3,3]-sigmatropic rearrangement of suitably substituted 2-alkenylcyclopropyl isocyanates

Abstract

The 2-siloxysubstituted 2-alkenylcyclopropanecarboxylic acids 10–14 were converted into the corresponding carbonyl azides by treatment with DPPA (diphenyl phosphorazidate) and triethylamine. On heating to 80 °C these intermediates smoothly furnished azepinone derivatives 19–25 in moderate to good overall yields, which are the result of a sequence of Curtius reaction to cyclopropylisocyanates, [3,3]-sigmatropic rearrangement, and a final proton shift. The primary products may undergo desilylation (to afford azepin-2,5-diones such as 23) or double bond migration (to compound 25). Cyclopropanecarboxylic acids cis-32, cis-34, and cis-35, which bear no 2-siloxy group, similarly provided azepinone derivatives 36–38 in good yields. No influence of the 2-siloxy substituent on the reaction course and rates could be observed in these qualitative studies, which was confirmed by DFT calculations with model compounds 39–42 showing similar reaction barriers for rearrangements of cyclopropylisocyanates with or without a 2-hydroxy group.Key words: vinyl cyclopropanes, carbonylazides, [3.3]-sigmatropic rearrangements, azepinones.