Diastereoselectivity control in the zirconocene-mediated ring contraction of 4-vinylfuranosides to enantiopure multiply functionalized cyclobutanes.

[reaction: see text] The ring contraction induced by treatment of 4-vinylfuranosides with zirconocene in the presence of boron trifluoride etherate can be rendered highly diastereoselective by proper adjustment of substituent stereochemistry. The two competing transition states that are seemingly involved appear to be usefully sensitive to nonbonded steric interactions.