Diastereoselectivity in the addition and cycloaddition reactions of a chiral ester of 2H-azirine-3-carboxylic acid

Abstract

The Azirine ester 2 bearing Oppolzer’s N,N-diethyl-(1R)-isobornyl-10-sulfonamide chiral auxilliary shows moderate diastereoselectivity in its Diels-Alder reaction with cyclo-pentadiene whereas the addition of thiophenol is highly diastereoselective; X-ray crystal structures of the aziridine esters 6 and 7 are reported.

References

1. M. J. Alves and T. L. Gilchrist, Tetrahedron Lett., 1998, 39, 7579.
2. W. Oppolzer, C. Chapuis and M. J. Kelly, Helv. Chim. Acta, 1983, 66, 2358; W. Oppolzer, Tetrahedron, 1987, 43, 1969.
3. W. Oppolzer, C. Chapuis and G. Bernardinelli, Tetrahedron Lett., 1984, 25, 5885.
4. Crystal data for 6. C₂₂H₃₄N₂O₄S, M= 422.64, a= 7.4889(12), b= 8.9188(12), c= 16.948(2)Å, U= 1107.4(3)ų, T= 293(2) K, Z= 2, µ(Mo-Kα)= 0.135 mm^–1; 5572 reflections measured, 2891 unique (R_int= 0.0861). The final wR₂ was 0.1289 (all data). CCDC reference number 207/327. See http://www.rsc.org/suppdata/p1/1999/1399 for crystallographic Wles in .cif format.
5. M. J. Alves, T. L. Gilchrist and J. H. Sousa, J. Chem. Soc., Perkin Trans. 1, 1999, 1305.
6. Crystal data for 7. C₂₃H₃₄N₂O₄S₂, M= 466.71, a= 10.5115(12), b= 11.468(3), c= 20.141(3)Å,U= 2427.9(7)ų, T= 213(2) K, Z= 4, µ(Mo-Kα)= 0.206 mm^–1; 13051 reflections measured, 3150 unique (R_int= 0.0643). The Wnal wR₂ was 0.0812 (all data).
7. Calculations were performed by Dr D. L. Cooper(University of Liverpool). RHF calculations with a standard 6-31G^** basis were carried out in which the carbonyl carbon was forced coplanar with the heavy atoms of the ring and all other parameters were fully optimised for a given dihedral angle. The difference in energy of conformations with dihedral angle 0° and 180° is 3 kJ mol^–1 and for conformations with dihedral angle 0° and 90° is 8 kJ mol^–1.
8. Preliminary results indicate that both diastereoisomers are also formed with the corresponding azirine bearing a more bulky N,N-dicyclohexylsulfonamidomethyl side chain.
9. A referee has questioned this explanation and has suggested that reaction might occur more selectively through a higher energy s-cis conformation of the ester, in which the rotamer 2c would be preferred since it avoids interactions between the methylene group of the azirine and the methine of the auxiliary. This is feasible, although the proportion of the s-cis conformer of an ester is usually small at room temp. (K. B. Wiberg and K. E. Laidig, J. Am. Chem. Soc., 1987, 109, 5935). We thank the referee for this suggestion (graphic not included).
10. D. Tanner, Angew. Chem., Int. Ed. Engl., 1994, 33, 599; H. M. I. Osborn and J. Sweeney, Tetrahedron: Asymmetry, 1997, 8, 1693.