An enyne metathesis/(4+2)-dimerization route to (±)-differolide

[GRAPHICS] A concise total synthesis of (+/-)-differolide (1) has been achieved. 2-Vinylbutenolide (2) was prepared by enyne metathesis of allyl propynoate (3) using the Grubbs initiator 4, This reaction was examined by H-1 NMR spectroscopy, which led to the hypothesis that low concentration of ruthenium species and high concentration of enyne substrate would be advantageous. Accordingly, slow addition of 4 to solutions of enyne 3 was found to be beneficial. Spontaneous dimerization of 2 gave (+/-) differolide (1) and an isomer.