The reaction is completely regioselective with respect to the alkyne. The larger substituents always occupy the position adjacent to the carbonyl group in the product. However, the use of an unsymmetrical alkene usually leads to a mixture of regioisomers (Scheme 6).

Scheme 6

Examples:

Ref : M. E. Krafft, J. A. Wright, L. V. R. Bonaga, Tetrahedron Lett. 2003 , 44 , 3417.

Ref : P. Magnus, M. J. Slater, L. M. Principe, J. Org. Chem. 1989 , 54 , 5148.