On the Enantioselectivity of a 2-Aza-divinylcyclopropane Rearrangement

The optically active cyclopropanecarbonyl azide 6 (ee 90%) was synthesized from the known cyclopropanecarboxylate 4. Upon heating in refluxing benzene, 6 rearranged to the dihydroazepinone 9 (ee 65%) via the isocyanate 7 and the enamide 8. In contrast, the dimethyl‐substituted cyclopropanecarbonyl azide 20 rearranged to the isocyanate 21, but decomposed at higher temperatures rather than to undergo Cope rearrangement to 22. The results are consistent with a mechanism involving a boat‐like endo‐transition state 11 for the rearrangement, which may not be reached with the dimethyl derivative 21 for steric reasons. The origin of change in enantiomer composition, upon going from 6 to 9, is at present unknown.