4,5-Didehydro-7-silyloxymethyl-2-oxepanone and formal total syntheses of Hagen’s gland lactones and trans-kumausynes

A concise and enantiospecific route to the 2,6-dioxabicyclo[3.3.0]octan-3-one ring system from commercially available (R)-(+)- and (S)-(−)-glycidols is described. The key features involve ring closing metathesis to construct the 7-substituted-4,5-dehydro-2-oxepanone and its base-catalyzed single-step rearrangement into the 2,6-dioxabicyclo[3.3.0]octan-3-one skeleton. Using this strategy, formal total syntheses of (7R)-cis-Hagen’s gland lactones and (+)- and (−)-trans-kumausynes have been achieved.