Mesoionic 1,3-Oxazinium Olates. Rearrangement to Acylketenes and 3-Azabicyclo[3.1.1]heptanetriones Hassan Sheibani, 1 Paul V. Bernhardt, and Curt Wentrup* Chemistry Building, School of Molecular and Microbial Sciences, The University of Queensland, Brisbane, Qld 4072, Australia wentrup@uq.edu.au Received March 6, 2005 Metastable but isolable mesoionic 1,3-oxazinium 4-olates 9d-f undergo ring opening to acylketenes 10 at or near room temperature. The ketenes undergo intramolecular criss-cross [2 + 2] cycloaddition to afford 3-azabicyclo[3.1.1]heptanetriones 12. The structure of 12d was established by X-ray crystallography. Introduction We have recently reported the reversible valence isomerization of mesoionic pyridopyrimidinium olates to the corresponding amidinoylketenes, e.g., 1 f 2, which takes place in solution at or slightly above room temper- ature (eq 1). 2 Although the transient ketenes 2 could not be observed directly, the calculated activation barrier for the ring opening, 1 f 2, was as low as 20 kcal/mol in one case. 2a The formally nonmesoionic pyridopyrimidi- nones 3 undergo reversible ring opening to imidoylketenes 4 under conditions of flash vacuum thermolysis (FVT) (eq 2). 2,3 In addition, five-membered heterocyclic me- soionic compounds can undergo valence isomerization to the ring-opened ketenes, albeit sometimes with more difficulty, e.g., pyrrolopyridinium olates 5 to ketenes 6 4 and sydnones to N-nitrosaminoketenes. 5,6 These ring- opening reactions are of considerable interest as likely examples of pseudopericyclic reactions, 7 taking place via a planar or nearly planar transition state and with very low activation barriers from the ketenes to the cyclized products. 8 We now report the synthesis of 2-vinyl-1,3-oxazinium olates 9, their facile ring opening to ketenes 10, and recyclization to 3-azabicyclo[3.1.1]heptanetriones 12 in an intramolecular [2 + 2] criss-cross cycloaddition reac- tion. Results and Discussion The oxazinium olates 9d-f (see Scheme 1 for key) were obtained as highly colored (red-orange) solids by reac- (1) On leave from the Department of Chemistry, Shahid Bahonar University of Kerman, Kerman 76175, Iran. (2) (a) Andersen, H. G.; Mitschke, U.; Wentrup, C. J. Chem. Soc., Perkin Trans. 2 2001, 602-607. (b) Fiksdahl, A.; Plu ¨ g, C.; Wentrup, C. J. Chem. Soc., Perkin Trans. 2 2000, 1841-1845. (3) Plu ¨ g, C.; Frank, W.; Wentrup, C. J. Chem. Soc., Perkin Trans. 2 1999, 1087-1093. (4) Pyrrolo[1,2-a]pyridinium olates to (2-pyridyl)carbonylketenes: Ye, X.; Andraos, J.; Bibas, H.; Wong, M. W.; Wentrup, C. J. Chem. Soc., Perkin Trans. 2 2000, 401-406. (5) Plu ¨ g, C.; Wallfisch, A.; Andersen, H. G.; Bernhardt, P. V.; Baker, L.-J.; Clark, G. R.; Wong, M. W.; Wentrup, C. J. Chem. Soc., Perkin Trans. 2 2000, 2096-2108. (6) Photochemical ring opening of sydnones to N-nitrosamino- ketenes: Veedu, R. N.; Wentrup, C. Unpublished results, The Uni- versity of Queensland, 2004. (7) (a) Birney, D. M.; Wagenseller, P. E. J. Am. Chem. Soc. 1994, 116, 6262-6270. (b) Birney, D. M. J. Org. Chem. 1996, 61, 243-251. BATCH: jo8a02 USER: dgw69 DIV: @xyv04/data1/CLS_pj/GRP_jo/JOB_i16/DIV_jo050428m DATE: June 6, 2005 10.1021/jo050428m CCC: $30.25 © xxxx American Chemical Society J. Org. Chem. XXXX, XX, A PAGE EST: 2.9 Published on Web 00/00/0000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44