Lewis Acid Catalyzed Ene Reactions of α,β-Unsaturated N-Acyloxazolidinones

Abstract

Chiral α,β-unsaturated N-acyloxazolidinones undergo a variety of Lewis acid catalyzed addition reactions to alkenes with modest to excellent asymmetric induction. The nature of the reaction is a function of alkene structure and reaction conditions. N-Crotonyloxazolidinone 1b undergoes Me2AlCl-catalyzed ene reactions with methylenecyclopentane and allyltrimethylsilane to give 6 (67% de) and 18 (33% de), respectively, as the major products. (Trimethylsilyl)cyclopentane 20 is formed stereospecifically as one of eight possible isomers as a minor product. Isobutylene undergoes an enantiospecific Me2AlCl-catalyzed hetero-Diels-Alder reaction with 1b to afford, after hydrolysis, lactone 10. 2-Ethyl-1-butene undergoes ene reaction slowly with 1b at -30 °C with modest asymmetric induction. The ene adducts 6 and 11 undergo Me2AlCl-catalyzed Friedel-Crafts acylation at 0 °C to give cyclohexenones 16 and 13. N-Acryloyloxazolidinones 21 and 26 undergo Me2AlCl-catalyzed ene reactions with 1,1-di- and trisubstituted alkenes to give ene adducts in excellent yield. However, asymmetric induction with ethylidenecyclohexane is poor, since ene reaction can occur by exo or endo transition states and by abstraction of syn or anti hydrogens. Ene reaction of 26 with (E)-3,4,4-trimethyl-2-pentene proceeds selectively through the exo transition state to give 28 with 80% de. © 1991, American Chemical Society. All rights reserved.