Custom synthesis of substituted butenolides, dihydropyranones, and α-E-alkylidenelactones via alkenylalumination

Alkenylalumination of aldehydes with [α-(ethoxycarbonyl)-β-(t-butyldimethylsilyloxyalkyl)alkenyl]diisobutylaluminum provides the corresponding α-Z-alkylidene-β′-hydroxy esters, which upon protection and treatment with trifluoroacetic acid lactonizes to furnish α-acetoxyalkyl butenolides (n = 1) and dihydropyranones (n = 2) in 40% overall yield. Conjugate addition to these lactenones and concurrent acetate elimination constitute a general and stereospecific synthesis of the corresponding α-E-alkylidene lactones.