Construction of the A ring of halichomycin via a RCM strategy

Attempts to the synthesis of the A ring of halichomycin via ring-closing metathesis (RCM) reaction were investigated. When triene 5 was used as the precursor of cyclization, only unexpected byproduct aldehyde 17 was obtained. When diene 6 was used as the precursor of cyclization, the desired product 20 was obtained in reasonable yield. This work demonstrated that both modification of the substrate and the RCM reaction conditions are important for obtaining the desired 11-membered macrocycle in reasonable yield.