Calcium(II) Catalyzed Cycloisomerization of cis-6-Hydroxy/(Acyloxy)hex-2-en-4-ynals to 2-Acyl- and 2-(Acyloxyalkenyl)furans

A cycloisomerization reaction of cis-6-hydroxy and cis-6-acyloxyhex-2-en-4-ynals to 2-acylfurans and 2-(1-acyloxyalkenyl)furans under calcium(II) catalysis is reported. This strategy allows for the efficient synthesis of 3,4-fused bicyclic 2-acylfurans, multi-substituted 2-acylfurans and structurally novel 2-(acyloxyalkenyl)furan derivatives which are substructures of many bioactive natural products. Very broad and divergent substrate scope with respect to propargylic alcohols and linkers has been achieved. The transformation fulfils some of the green chemistry principles in terms of a) use of non-toxic, biodegradable calcium, b) 100% atom economy, and c) catalytic process and mild reaction conditions (RT).