Carbocation Lewis acid TrBF 4 -catalyzed 1,2hydride migration of α-alkyldiazoacetates themselves or in situ-generated cross-coupling adducts of aldehydes and αalkyldiazoacetates has been developed, affording (Z)-α,βunsaturated esters and α-branched β-ketocarbonyls, respectively, in good yields and with high regioselectivities.

Herein we report Lewis acid HfCl4-catalyzed [4 + 2] cycloaddition between β,γ-unsaturated α-keto esters and various symmetric or unsymmetric alkynes, giving the desired polysubstituted 4H-pyrans in up to 98% yield and with excellent regioselectivity (>99:1) via a vinyl carbocation under mild conditions.