Zinc bromide as catalyst for the stereoselective construction of quaternary carbon: improved synthesis of diastereomerically enriched spirocyclic diols

Zinc bromide (ZnBr2) has proved to be a facile and efficient catalyst for the stereoselective semipinacol rearrangement of alpha -hydroxy epoxides at room temperature. Of note are the presence in the product of two adjacent chiral carbon centers, particularly the creation of a stereoselective quaternary center, and the efficient synthesis of beta -hydroxy ketones, including some with naturally occurring spiroalkane skeletons. As an example of its application, important diastereomerically enriched spirocyclic diol ligands have been synthesized conveniently via this rearrangement followed by reduction of the spirocyclic beta -hydroxy ketones obtained with appropriate hydride reagents.