C O M M U N I C A T I O N S
Scheme 2. Radical Cyclization
or dialkyl substituents gave high selectivity because of the genera-
tion of stable benzyl or tertiary alkyl radicals B. In the case of
mono alkyl-substituted MCPs 1b and 1c, the mixtures of regioi-
someric products were obtained due to the small difference in the
stability between primary and secondary radicals (see Table 2,
entries 2 and 3). Finally, homoallyl radical B was hydrogenated
by Bu2SnIH to give vinyltin 2 with regeneration of a tin radical.
applied to a one-pot coupling reaction without isolation or change
of solvent. Moreover, a radical cyclization initiated from the
reduction of MCPs was accomplished. Mechanistic consideration
and reactivity of more substituted MCPs are under investigation.
Acknowledgment. This research has been supported by JSPS
Research Fellowships for Young Scientists, by a Grant-in-Aid for
Scientific Research on Priority Areas (459 and 460) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan, and by the JFE 21st Century Foundation and Nihonshouken
Foundation.
Scheme 1. Plausible Mechanism
Supporting Information Available: Experimental details and
characterization data. This material is available free of charge via the
References
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(4) We have been developing synthetic use of Bu2SnIH. (a) Shibata, I.; and
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This highly selective metalation to the central carbon is a
characteristic feature of Bu2SnIH. In contrast, when MCP 1d was
treated with Bu3SnH in the presence of Et3B, cyclopropylmethyltin
6d was obtained in 85% yield without ring-opening products such
as 2 or 3, in which the addition to terminal olefin carbon is
exclusive.9 In addition, Pd(Ph3P)4-catalyzed hydrostannation pro-
duced only homoallyltin 7d in 79% yield,2a and the reaction
promoted by Lewis acid such as AlCl3 or B(C6F5)3 resulted in a
complicated mixture.10 Although a credible reason why the tin
radical Bu2ISn• selectively adds to the internal carbon of MCPs 1
is not clear yet and the mechanism is only tentative, a interaction
between the π-bond and/or σ-bond of MCPs and the tin center of
Bu2SnIH, whose Lewis acidity is increased by an iodine substituent,
might affect this regioselectivity.
(6) The most substituted proximal bond cleavage of “terminally substituted
MCPs” have been reported by some researchers. The first report:
Singleton, D. A.; Church, K. M. J. Org. Chem. 1990, 55, 4780. Reaction
by tin radical: Huval, C. C.; Singleton, D. A. J. Org. Chem. 1994, 59,
2020.
(7) Reference 4b and Fugami, K.; Ohnuma, S.; Kameyama, M.; Saotome,
T.; Kosugi, M. Synlett 1999, 63.
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54, 4603. (d) Hollis, R.; Hughes, L.; Bowry, V. W.; Ingold, K. U. J. Org.
Chem. 1992, 57, 4284. (e) Newcomb, M.; Choi, S. Y. Tetrahedron Lett.
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Soc. 1998, 120, 10379.
In the next stage, we applied this radical cleavage to a trans-
formation from the MCP 1h bearing allylic ether moiety into cy-
clized vinyltin 2h (Scheme 2).11 After the reaction at room tem-
perature for 20 h, a subsequent coupling reaction with iodobenzene
was performed to produce cyclic ether 4h in moderate yield. In
this reaction the generated cyclopropylmethyl radical C quickly
isomerizes to radical D, which cyclizes to intermediate E. The
following hydrogenation and coupling gave the desired product 4h.
In conclusion, the hydrostannation of MCPs using Bu2SnIH to
produce R-substituted vinyltins was presented, which showed
unprecedented regioselectivity. The generated vinyltins could be
(9) Similar regioselectivity was reported in the radical reaction of “terminally
unsubstituted MCPs 1′′ with carbon radical and thiol. (a) Gilchrist, T. L.;
Rees, C. W. J. Chem. Soc. C 1968, 776. (b) Kozhushkov, S. I.; Brandl,
M.; Meijere, A. Eur. J. Org. Chem. 1998, 1535.
(10) Reference of conditions: Gevorgyan, V.; Liu, J. -X.; Yamamoto, Y. J.
Org. Chem. 1997, 62, 2963.
(11) Kilburn’s group reported radical cyclizations initiated by intramolecular
addition of carbon radical to MCPs moiety. Selected examples: (a)
Santagostino, M.; Kilburn, J. D. Tetrahedron Lett. 1995, 36, 1365. (b)
Pike, K. G.; Destabel, C.; Anson, M.; Kilburn, J. D. Tetrahedron Lett.
1998, 39, 5877. (c) Underwood, J. J.; Hollingworth, G. J.; Horton, P. N.;
Hursthouse, M. B.; Kilburn, J. D. Tetrahedron Lett. 2004, 45, 2223.
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