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 Rh 2 (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 C 4 , diazo‐methylketo‐terminating side‐chain at C(3) is described to be transformed into a 4‐formylmethylidene‐2‐cyclohe‐xenone.