Pergamon Tetrahedron Letters 40 (1999) 4453--4456 TETRAHEDRON LETTERS Highly Stereoselective Intramolecular Epoxidation in Unsaturated Oxaziridines Alan Armstrong* and Alistair G. Draffan School of Chemistry, University of Nottingham, Nottingham, NG7 2RD, UK Received 6 April 1999; accepted 19 April 1999 Abstract: Highly stereoselective intramolecular epoxidation is observed in unsaturated oxaziridinesv/a oxaziridinium salts. The observed selectivities are consistent with a spiro transition state for the intramolecular epoxidation. © 1999 ElsevierScienceLtd. All rights reserved. The efficient asymmetric epoxidation of alkenes remains a significant challenge in organic synthesis. Notable recent progress has been achieved with chiral dioxiranes derived from ketones) -4 We were attracted5 to the related area of oxaziridinium epoxidation pioneered by Hanquet and Lusinchi (Scheme 1).6 Since these early studies, chiral iminium salts have been used as catalysts in asymmetric epoxidation, yielding epoxides in moderate to good enantioselectivity. 6c'6e'7 Further progress in this area, particularly from the viewpoint of rational design of chiral catalysts, would undoubtedly be aided by knowledge of the transition state stereoelectronics. Two extreme geometries can be envisaged (Figure 1): the planar mode, where the three atoms of the oxaziridine ring and the two carbons of the olefin lie in the same plane; and the spiro mode, where the plane defined by the oxaziridine ring is perpendicular to the plane defined by the developing epoxide ring. For epoxidation by chiral dioxiranes, the observed sense of enantioselectivity provides good evidence that a spiro mode is preferred, in accord with calculations. L2'8 However, there is currently no literature evidence on the transition state geometry involved in epoxidation by oxaziridinium salts. BF4- + + Uc-.. Ph Planar Spiro Scheme I Figure I We have been interested in intramolecular epoxidation as a means of controlling regio- and stereochemistry, and recently reported that treatment of unsaturated oxaziridines with MeOTf results in intramolecular epoxidation, presumably via oxaziridinium salts.9 We wished to investigate the effect of oxaziridine ring stereochemistry on this process and here report the first examples of highly stereoselective intramolecular epoxidation in such systems. We reasoned that oxidation of an imine derived from an enantiomerically pure chiral primary amine and an unsaturated aldehyde could yield separable diastereomeric oxaziridines, allowing the oxygen transfer process to be studied in each. The first substrate to be studied was imine la prepared from (S)-ct- methylbenzylamine and the corresponding unsaturated aldehyde 1° (Scheme 2). Oxidation with buffered Oxone ® 0040-4039/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PIL" S0040-4039(99)00771-6