Synthesis of 2-Cycloalkenones (Parts of 1,4-Diacyl-1,3-butadiene Systems) and of a Heterocyclic Analogue by Metal-Catalyzed Decomposition of 2-Diazoacylfurans

Furans with side‐chains at C(2) of various lengths terminating in diazomethyl keto groups are shown to undergo Rh2(OAc)4‐catalyzed furan unravelling with the production of 2‐cyclopentenone, 2‐cyclohexenone, and 2‐cycloheptenone to each of whose olefinic C(β) is attached an acrylaldehyde unit. Interposition of a cyclohexane or a methylaminomethylene moiety between the furan and diazoketo functions leads to the formation of a hydroindenone and pyrrolone, respectively. Replacement of the diazomethylketo terminus by an α‐diazoethylketo system or a α‐diazo‐β‐keto‐ester function produces 2‐substituted 2‐cycloalkenones. A furan with a C4, diazo‐methylketo‐terminating side‐chain at C(3) is described to be transformed into a 4‐formylmethylidene‐2‐cyclohe‐xenone.