The Catalytic Enantioselective Synthesis of Optically Active Epoxides and Tetrahydrofurans.Asymmetric Epoxidation, the Desymmetrization of meso-Tetrahydrofurans, and Enantiospecific Ring-Enlargement

Recent developments in metallosalen-catalyzed asymmetric oxidation, namely, the epoxidation of simple olefins and the desymmetrization of meso-tetrahydrofurans by using second-generation metallosalen complexes are discussed. As an alternative approach, the enantiospecific ring-enlargement of oxetanes for preparing optically active tetrahydrofurans is also described.Oxygen heterocycles such as epoxides and tetrahydrofurans occur in a wide variety of natural products where their configurations often play an important role in defining biological activity. Furthermore, these oxygen heterocycles can serve as versatile chiral building blocks for the enantiospecific synthesis of various compounds. Obviously, methods for the stereocontrolled synthesis of epoxides and tetrahydrofurans will be of great value for organic synthesis. Accordingly, during the last two decades the development of new methodologies for the stereoselective synthesis of this class of compounds from readily available starting materials has been a major objective in organic synthesis as attested by numerous publications. As a result, many efficient catalysts and reagents have been devised and high enantioselectivities achieved for synthesizing oxygen heterocycles. Examples are chiral titanium-tartrate (1,2), metalloporphyrins (3), (salen)manganese complexes (4,5,6), ketones (via dioxiranes) (7), and iminium salts (via oxaziridinium salts), for achieving epoxidation with high enantioface-selectivity. Moreover, the desymmetrization of meso-heterocycles has been effected with (salen)manganese complexes (4). In the present review, we describe some recent developments in metallosalen-catalyzed asymmetric epoxidation and the desymmetrization of meso-tetrahydrofurans and related compounds. In addition, the enantiospecific ring-enlargement of oxetanes to tetrahydrofurans is briefly described.