Insight into the unusual reactions of stabilized phosphorus ylides with lactols. A specific intramolecular hydroxyl group effect leads to high z-selectivity.

In reactions with stabilized ylides, γ-lactols of ribo configuration afford 90% Z-olefins whereas γ-lactols of allo configuration afford predominantly E-olefins. Lactols of arabino configuration give intermediate results. The unusual results in the ribo series completely disappear on protection of the γ-hydroxyl group.