Rearrangement of propargylic esters:: Metal-based stereospecific synthesis of (E)- and (Z)-Knoevenagel derivatives

Alkoxy-substituted propargylic esters undergo regioselective 1,3-acyloxy migration in the presence of Pt(II) and Cu(I) catalysts, allowing the preparation of alpha-ylidene-beta-keto and -malonate esters. The reaction proved to also be stereodivergent as the Pt(II) and the Cu(I) catalysts perform the isomerization with complementary Z/E selectivity. Moreover, alkynyl-conjugated Knoevenagel products are produced from (bisalkynyl)methyl acetates. In such a case, the reaction is chemoselective as the 1,3-acetyl migration takes place through the alkoxyalkyne group in preference over the phenylalkyne group. The resulting (E)-alkynylenone unit suffers metal-catalyzed cyclization into the furyl ring, generating a copper(I) carbene species.