The combined carbometalation–zinc homologation–allylation reaction of the resulting stereodefined 3,3-disubstituted allylmetal species with ketones allow the preparation of allylic vicinal diol derivatives in good yields with excellent diastereomeric ratios from commercially available alkynes. Two adjacent quaternary centers are formed with the concomitant formation of three new carbon–carbon bonds in a single-pot operation in an acyclic system. The bulky substituent of the ketone occupies a pseudo-axial position in the Zimmerman–Traxler transition state.