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Stereoselective ring expansion of vinyl oxiranes: mechanistic insights and natural product total synthesis.

The central part of our research program is the development of broadly applicable atom-efficient synthetic methods to generate structural complexity. We recently reported a new copper-catalyzed ring expansion of vinyl oxiranes using commercially available, structurally simple, and air-stable copper(II) catalysts. These studies demonstrated the success of this reaction for a broad range of substrates. Missing from these early studies were experiments focused on assessing the stereoselective potential of the ring expansion. The studies presented herein are aimed at addressing this point to learn if the oxirane C O bond, which is broken during the ring expansion, could be stereoselectively transferred to the olefin terminus. Moreover, an added benefit to the proposed studies would be critical mechanistic insights that could shed additional light on the exact mechanism of this unique catalytic ring-expansion reaction. If vinyl oxiranes can indeed be stereoselectively ring expanded using copper catalysts then the practical benefits of this new reaction would be significantly expanded. Also this transformation would rival existing methods to stereoselectively access 2,5-dihydrofurans in terms of efficiency and scope. Our studies have focused on disubstituted vinyl oxirane substrates (Scheme 1), which depending on their stereochemistry and olefin geometry could serve as precursors for accessing either a 2,5-cisor a 2,5-trans-substituted 2,5dihydrofuran product. In the case of a stereoselective ring expansion, vinyl oxiranes 3 and 4 would be expected to afford the cis product 1. whereas 5 and 6 should afford the trans product 2. When these substrates are ranked with respect to steric crowding during formation of the new C O bond, vinyl oxiranes 3 and 5 emerge as more suitable precursors for accessing the 2,5-cisand 2,5-trans-dihydrofuran products, respectively. Therefore we synthesized vinyl oxiranes 7 and 8, which differ only in the oxirane stereochemistry (transor cissubstituted oxiranes). Ring expansion of these two substrates afforded the symmetrically substituted cisand trans-dihydrofurans 9 and 10 (Table 1). Under the standard reaction conditions used in our previous paper (Table 1, entries 2 and