Cyclization of cross-conjugated trienes: the vinylogous Nazarov reaction.

Cross-conjugated trienoates can be readily prepared from 1,4-pentadien-3-one precursors via a 2-step process: 1,2-addition of lithiated ethynyl ethyl ether followed by rearrangement of the resulting propargylic alcohol with catalytic VO(acac)(2). Treatment of the trienoates with stoichometric (TiCl(4)) or catalytic (Sc(OTf)(3)) Lewis acid provided conjugated alkylidenecyclopentenes via an apparent vinylogous Nazarov cyclization.