Diastereoselective SmI₂-mediated cascade radical cyclisations of methylenecyclopropane derivatives—a synthesis of paeonilactone B

Abstract

The SmI₂-mediated cascade reaction of methylenecyclopropyl ketone 9 proceeds with high diastereoselectivity, which is critically dependent on the presence of HMPA, and provides a short route to paeonilactone B.

References

1. M. Malacria, Chem. Rev., 1996, 96, 289.
2. (a) For a review on SmI₂-initiated tandem reactions, see: G. A. Molander and C. R. Harris, Tetrahedron, 1998, 54, 3321 Surprisingly few cascade radical process initiated by SmI₂ have been reportedTwo notable examples are: (b) T. L. Fevig, R. L. Elliott and D. P. Curran, J. Am. Chem. Soc., 1988, 110, 5064; (c) R. A. Batey, J. D. Harling and W. B. Motherwell, Tetrahedron, 1996, 52, 11 421.
3. T. Hayashi, T. Shinbo, M. Shimizu, M. Arisawa, N. Morita, M. Kimura, S. Matsuda and T. Kikuchi, Tetrahedron Lett., 1985, 26, 3699.
4. For previous syntheses of paeonilactones see: M. Rönn, P. G. Andersson and J.-E. Bäckval, Acta Chem. Scand., 1998, 52, 524; S. Hatakeyama, M. Kawamura, Y. Mukugi and H. Irie, Tetrahedron Lett., 1995, 36, 267 and references cited therein.
5. For previous studies on cyclisations of methylenecyclopropylalkyl radicals see: C. Destabel, J. D. Kilburn and J. Knight, Tetrahedron, 1994, 38, 11 267; M. Santagostino and J. D. Kilburn, Tetrahedron Lett., 1995, 36, 1365 and references cited therein.
6. T. Oishi, M. Nagai and Y. Ban, Tetrahedron Lett., 1968, 491.
7. All compounds were characterised by ¹H and ¹³C NMR and IR spectroscopy, and by HRMS or microanalysis.
8. We thank Dr M. Webster, University of Southampton, for carrying out the X-ray crystallographic analysis. Details will be published elsewhere.
9. G. A. Molander and J. A. McKie, J. Org. Chem., 1995, 60, 872.
10. For a study on the effects of HMPA and DMPU as additives in SmI₂-mediated cyclisations of unactivated olefinic ketones, see: G. A. Molander and J. A. McKie, J. Org. Chem., 1992, 57, 3132 See also ref. 2b.
11. Electronic repulsion between the ketyl oxygen functionality and the alkene π-system is generally accepted as being a major factor determining stereoselectivity in SmI₂-mediated cyclisations of unactivated olefinic ketones: see refs. 10 and 2(b).
12. C. H. Heathcock, S. D. Young, J. P. Hagen, R. Pilli and U. Badertscher, J. Org. Chem., 1985, 50, 2095.
13. T. J. Brocksom, R. B. dos Santos, N. A. Varanda and U. Brocksom, Synth. Commun., 1988, 18, 1403.
14. D. J. Cram and C. A. Kingsbury, J. Am. Chem. Soc., 1960, 82, 1810.
15. D. Boschelli, T. Takemasa, Y. Nishitani and S. Masamune, Tetrahedron Lett., 1985, 26, 5239.