Tandem Cyclizations Involving Carbene as an Intermediate: Photochemical Reactions of Substituted 1,2-Diketones Conjugated with Ene-Yne

We have investigated photoreactions of a novel system in which a carbene generator and a carbene trap are contained in the same molecule for studying tandem cyclizations involving carbene. The photocyclization of 1,2-diketones conjugated with ene-yne to (2-furyl)carbene was employed as a carbene-generating system. Upon photoirradiation of 1,2-diketones possessing biphenyl and 2-acetyl-1-cyclopentenyl systems as a carbene trapping unit in aprotic solvents, we observed tandem cyclizations via a carbene intermediate to produce fluorenylfuran and bifuran derivative with nearly quantitative yield, respectively. The solvent dependency for the tandem cyclization clearly indicated that 1,2-diketone is equilibrated with (2-furyl)carbene under the steady state photoirradiation conditions. The results reported here indicated that such compounds containing a carbene generator and a trap within a molecule are extremely useful for studying the sequential carbene addition reaction.