Evidence for a concerted [4+1]-cycloaddition between electron-rich carbenes and electron-deficient dienes

Results from the thermal reactions of deuterated dienes 12−15 provide evidence of the concertedness of the [4 + 1]-cycloaddition between dimethoxycarbene and electron-deficient dienes. Other evidence suggests that the main pathway is a concerted [4 + 1]-cycloaddition rather than a cyclopropanation followed by a vinylcyclopropane rearrangement. Ionic pathways can become competitive when steric or geometrical constraints are present.