The intramolecular [4+2]-cycloadditions of di- and trisubstituted nitroalkenes with unactivated olefins are described. The cycloadditions proceed readily at low temperatures in the presence of SnCl4. The reactions are shown to be stereospecific in the preservation of dienophile configuration in the product. The configuration of the heterodiene controls the stereochemistry of the ring fusion but the selectivity is high only with trisubstituted nitroalkenes. The rate of cycloaddition of a cis dienophile is ca. 20-fold faster than a trans dienophile. The implications for mechanism and transition structure are discussed.
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Acknowledgements. We gratefully acknowledge the financial support provided by the National Instituteso f Health (PHS GM-30938). M.S.D. and Y.-CM. acknowledge the University of Illinois for Graduate Fellowships. C.J.C thanks the National Science Foundation for a GraduateF ellowship (1984-1987). S.E.D. also acknowledgess upportf orm the NSF (PresidentialY oung InvestigatorA ward (19851990) and the Alfred P. Sloan Foundation (198%1 989).