The photochemical rearrangement of several 3heteroaryl substituted cyclopropenes has been studied. The rearrangements are derived from the '~t--'lt* singlet state of the cyclopropene. Ring opening occurs to give a vinylcarbene intermediate which undergoes a subsequent electrocycliration. The transient intermediate so produced can undergo either a 1,3-or a 1,5-sigmatropic hydrogen shift to give the observed products. One strong piece of evidence for carbene intervention is the observation of a 1,3-butadiene derivative as one of the photoproducts obtained from the photolysis of a 3-pyrrolyl substituted cyclopropene. The 1,3-diene is thought to be derived by insertion of the vinylcarbene into the neighbouring methyl group. The regioselectivity of the rearrangement can be accounted for in terms of competitive 1,3and 1,5-sigrnatropic hydrogen migrations.

Durch Kondensation des Cyclopropenylsalzes (I) mit den lithiierten Heteroaromaten (II), (VI), (XI), (XV) "und (XX) nach Breslow werden die entsprechenden Cyclopropenylderivate (III), (VII), (XII), (XVI), (XVII) und (XXI) erhalten.