The Important Role of the Byproduct Triphenylphosphine Oxide in the Magnesium(II)-Catalyzed Enantioselective Reaction of Hemiacetals and Phosphorus Ylides.

By employing a simple in situ generated magnesium catalyst, a direct asymmetric reaction between hemiacetals and phosphorus ylides was achieved through a tandem Wittig-oxa-Michael reaction sequence. Enantioenriched chromans, isochromans, and tetrahydropyrans were obtained in good chemical yields, and (-)-erythrococcamide B was synthesized in enantioenriched form. The byproduct triphenylphosphine oxide was identified as a necessary additive for this process.