Treatment of 5-vinylpyranosides with Et2Zn and catalytic Pd (0), in the presence of ZnCl2, results in the formation of 5-membered carbocyclic products. This carbohydrate ring- contraction features an intramolecular allylation of a ring-opened carbohydrate aldehyde by an in situ-generated nucleophilic allylzinc species. The stereoselectivity about vinyl and free hydroxyl groups at the newly created stereogenic centers varies from low to moderate ...