Selective Formation of (E)-α-Ethynyl-α,β-Unsaturated Esters

Significance

The authors report a highly selective synthesis of (E)-α-ethynyl-α,β-unsaturated esters. The DABCO-catalyzed elimination of acetic acid from substituted allenyl acetates proceeds in high yields under the exclusive formation of the E-configured products. The proposed mechanism involves the formation of a vinylammonium salt, which was also detected by NMR spectroscopy resulting from the S_N2′-type displacement of acetate by the catalyst. The different steric interactions in the two possible reactive conformers are thought to account for the high stereoselectivity of this process.