Radical promoted cyclisations of trichloroacetamides with silyl enol ethers and enol acetates: the role of the hydride reagent [tris(trimethylsilyl)silane vs. tributylstannane]

Abstract

Reactions between 1-(carbamoyl)dichloromethyl radicals and electron-rich alkenes acting as radical acceptors are reported for the first time. The intramolecular reaction of trichloroacetamides with silyl enol ethers gives ketones using (TMS)₃SiH as the mediator, and alcohols when using Bu₃SnH. The reaction with enol acetates gives acetates using either of the above hydride reagents. These radical processes have been applied to the synthesis of 2-azabicyclo[3.3.1]nonanes.

References

1. For recent examples in natural product synthesis, see: (a) J. Quirante, C. Escolano, A. Merino and J. Bonjoch, J. Org. Chem., 1998, 63, 968; (b) L. Boiteau, J. Boivin, A. Liard, B. Quiclet-Sire and S. Z. Zard, Angew. Chem., Int. Ed., 1998, 37, 1128; (c) M. Ikeda, M. Hamada, T. Yamashita, F. Ikegami, T. Sato and H. Ishibashi, Synlett, 1998, 1246; (d) M. Ikeda, T. Sato and H. Ishibashi, Rev. Heteroatom Chem., 1998, 18, 169.
2. For some starting materials leading to a radical centre at the β-position to a nitrogen atom (3-azacarbon radicals) using the hydride method, see inter alia: S. J. Danishefsky and J. S. Panek, J. Am. Chem. Soc., 1987, 109, 917; Y. Watanabe, Y. Ueno, C. Tanaka, M. Okawara and T. Endo, Tetrahedron Lett., 1987, 28, 3953; G. Stork and R. Mah, Heterocycles, 1989, 28, 723; H. Ishibashi, T. S. So, K. Okochi, T. Sato, N. Nakamura, H. Nakatani and M. Ikeda, J. Org. Chem., 1991, 56, 95; P. F. Keusenkothen and M. B. Smith, Tetrahedron, 1992, 48, 2977; M. Ishizaki, K. Kurihara, E. Tanazawa and O. Hoshino, J. Chem. Soc., Perkin Trans. 1, 1993, 101; E. W. Della and A. M. Knill, J. Org. Chem., 1996, 61, 7529; A. F. Parsons and R. M. Pettifer, Tetrahedron Lett., 1997, 38, 5907; V. Gupta, M. Besev and L. Engman, Tetrahedron Lett., 1998, 39, 2429.
3. For examples of the use of (alkoxy)methoxyalkenes as radical acceptors of 3-azaradicals, see S. Knapp and F. S. Gibson, J. Org. Chem., 1992, 57, 4802.
4. (a) For this intermolecular process mediated by a tin hydride, see: B. Giese, H. Horler and M. Leising, Chem. Ber., 1986, 119, 444; P. Renaud, Tetrahedron Lett., 1990, 31, 4601; (b) for addition of acyl radicals to silyl enol ethers, see: D. L. Boger and R. J. Mathvink, J. Org. Chem., 1992, 57, 1429.
5. For procedures to promote additions of electrophilic radicals to silyl enol ethers other than the hydride method, see: (a) an oxidative process, E. Baciocchi, A. Casu and R. Ruzziconi, Synlett, 1990, 679; Y. Kohno and K. Narasaka, Chem. Lett., 1993, 1689; (b) the atom transfer method, K. Miura, Y. Takeyama, K. Oshima and K. Utimoto, Bull. Chem. Soc. Jpn., 1991, 64, 1542; (c) thiol-catalysed addition of aldehydes, H.-S. Dang and B. P. Roberts, Chem. Commun., 1996, 2201; (d) photo-irradation, M. Mitani and H. Sakata, Chem. Commun., 1998, 1877.
6. It has been noted that nucleophilic alkyl radicals do not react with silyl enol ethers: H. Urabe and I. Kuwajima, Tetrahedron Lett., 1986, 27, 1355.
7. For a review on radical cyclisation reactions, see: B. Giese, B. Kopping, T. Göbel, J. Dickhaut, G. Thoma, K. J. Kulicke and F. Frach, Org. React., 1996, 48, 301.
8. For references on (a) carbocyclic series, see ref. 6; (b) 1-oxa-2-silacyclohexanes, see R. D. Walkup, R. R. Kane and N. U. Obeyesekere, Tetrahedron Lett., 1990, 31, 1531 for related heterocyclic compounds: J. H. Hutchinson, T. S. Daynard and J. W. Gillard, Tetrahedron Lett., 1991, 32, 573; A. G. Myers, D. Y. Gin and D. H. Rogers, J. Am. Chem. Soc., 1993, 115, 2036.
9. (a) For cyclisations of alkyl radicals with enol ethers, see: A. L. J. Beckwith and D. H. Roberts, J. Am. Chem. Soc., 1986, 108, 5893; T. V. RajanBabu, T. Fukunaga and G. S. Reddy, J. Am. Chem. Soc., 1989, 111, 1759; J.-C. Lopez and B. Fraser-Reid, J. Am. Chem. Soc., 1989, 111, 3450; J. Marco-Contelles and B. Sánchez, J. Org. Chem., 1993, 58, 4293; C. Imboden, T. Bourquard, O. Corminboeuf, P. Renaud, K. Schenk and M. Zahouily, Tetrahedron Lett., 1999, 40, 495; (b) for cyclisations with aryl radicals, see: S. Atarashi, J.-K. Choi, D.-C. Ha, D. J. Hart, D. Kuzmich, C.-S. Lee, S. Ramesh and S. C. Wu, J. Am. Chem. Soc., 1997, 119, 6226; (c) for cyclisations with acyl radicals, see ref. 4b.
10. (a) For cyclisations of stabilized radicals with enol acetates, see: F. Barth and C. O-Yang, Tetrahedron Lett., 1991, 32, 5873; (b) for cyclisations from aryl radicals, see: S. A. Ahmad-Junan and D. A. Whiting, J. Chem. Soc., Chem. Commun., 1988, 1160.
11. For hydride reagent promoted radical cyclisations of trichloroacetamides with alkenes with an electron-withdrawing substituent, see: (a) Y. Hirai, A. Hagiwara, T. Terada and T. Yamazaki, Chem. Lett., 1987, 2417; (b) A. F. Parsons and R. J. K. Taylor, J. Chem. Soc., Perkin Trans. 1, 1994, 1945; (c) K. Goodall and A. F. Parsons, Tetrahedron, 1996, 52, 6739; (d) J. Quirante, C. Escolano, M. Massot and J. Bonjoch, Tetrahedron, 1997, 53, 1391.
12. For hydride reagent promoted radical cyclisations of trichloroacetamides with non-activated alkenes, see: (a) H. Nagashima, N. Ozaki, M. Ishii, K. Seki, M. Washiyama and K. Itoh, J. Org. Chem., 1993, 58, 464; (b) J. Quirante, C. Escolano, F. Diaba and J. Bonjoch, Heterocycles, 1999, 50, in the press.
13. For hydride reagent promoted radical cyclisations of N-vinylic trichloroacetamides, see: H. Ishibashi, M. Higuchi, M. Ohba and M. Ikeda, Tetrahedron Lett., 1998, 39, 75; M. Ikeda, S. Ohtani, T. Yamamoto, T. Sato and H. Ishibashi, J. Chem. Soc., Perkin Trans 1, 1998, 1763.
14. For a preliminary report of part of this work, see: J. Quirante, C. Escolano, L. Costejà and J. Bonjoch, Tetrahedron Lett., 1997, 38, 6901.
15. (a) Y.-W. Guo, A. Madaio, G. Scognamiglio and E. Trivellone, Tetrahedron, 1996, 52, 8341; (b) R. Downham, F. W. Ng and L. E. Overman, J. Org. Chem., 1998, 63, 8096.
16. (a) F. Kong, R. J. Andersen and T. M. Allen, J. Am. Chem. Soc., 1994, 116, 6007; (b) N. Matzanke, R. J. Gregg and S. M. Weinreb, J. Org. Chem., 1997, 62, 1920.
17. K. Sakamoto, E. Tsujii, F. Abe, T. Nakanishi, M. Yamashita, N. Shigematsu, S. Izumi and M. Okuhara, J. Antibiot., 1996, 49, 37.
18. R. D. Miller and D. R. McKean, Synthesis, 1979, 730.
19. For a related process catalysed by a ruthenium(II) phosphine complex, see: N. Kamigata, K. Udodaira and T. Shimizu, J. Chem. Soc., Perkin Trans. 1, 1997, 783.
20. The great strength of the Si–Cl bond in Me₃SiCl (112 kcal mol^–1) can drive the process.
21. C. Chatgilialoglu, J. Dickhaut and B. Giese, J. Org. Chem., 1991, 56, 6399.
22. Attempts to reduce ketone 3 were also unsuccessful when working with TBTH–AIBN, alone, with tributylchlorostannane or with trimethylchlorosilane in the reaction medium.
23. When the ratio of TBTH is low (see Table 1) a significant percentatge of ketones 4 and 5 are isolated, suggesting that a competitive mechanism, similar to that depicted in Scheme 2, is also operating.
24. (a) The direct formation of alcohols in reactions promoted by TBTH and silyl enol ethers has not previously been noted, see refs. 4 and 8; (b) for a simple reduction of α-silyloxyl radicals by TBTH, see: S.-Y. Chang, W.-T. Jiaang, C.-D. Cherng, K.-H. Tang, C.-H. Huang and Y.-M. Tsai, J. Org. Chem., 1997, 62, 9089.
25. D. P. Curran, N. A. Porte and B. Giese, Stereochemistry of Radical Reactions, VCH, Weinheim, 1996; W. Damm, B. Giese, J. Hartung, T. Hasskerl, K. N. Houk, O. Hüter and H. Zipse, J. Am. Chem. Soc., 1992, 114, 4067.
26. J. Burfeindt, M. Patz, M. Müller and H. Mayr, J. Am. Chem. Soc., 1998, 120, 3629 and refs. therein.
27. D. P. Curran, Synlett, 1991, 63.