was always isolated in 25–35% yields along with 2. Although there are a few differences in yields between
the previous procedures10 and ours, probably because of different reaction conditions, it appears that these
results support the sulfur dioxide mechanism for the deoxygenations of pyridine N-oxides with
alkanesulfonyl chlorides and Et3N (Scheme 2). Thus, the reason why our deoxygenations do not experience
chlorinations of the pyridine nucleus may be due to the sulfur dioxide mechanism different from the
conventional pathway shown in Scheme 1.
This mild deoxygenation procedure of pyridine N-oxides using the inexpensive and accessible MsCl and
Et3N is apparently more favorable than that using commercially unavailable sulfur dioxide-triethylamine
complex10a and that using intractable gaseous sulfur dioxide.10b The more detailed reaction mechanism is
under investigation.
ACKNOWLEDGMENTS
H. K. thanks the Suntory Institute for Bioorganic Research for the SUNBOR Scholarship. This work was
partially supported by the Saneyoshi Scholarship Foundation and a Grant-in Aid for Encouragement of
Young Scientists from the Japan Society for the Promotion of Science.
REFERENCES AND NOTES
1.
2.
3.
R. T. Morrison and R. N. Boyd, ‘Organic Chemistry, 6th ed.,’ Prentice-Hall, Inc., Englewood
Cliffs, NJ, 1992, Chap. 6, pp. 233–235.
T. W. Greene and P. G. M. Wuts, ‘Protective Groups in Organic Synthesis, 2nd ed.,’ John Wiley &
Sons, Inc., New York, 1991, pp. 117–118, 168–170, and 379–385.
Y. Morimoto and C. Yokoe, Tetrahedron Lett., 1997, 38, 8981; Y. Morimoto, C. Yokoe, H.
Kurihara, and T. Kinoshita, Tetrahedron, 1998, 54, 12197.
4.
5.
Y. Morimoto, H. Kurihara, C. Yokoe, and T. Kinoshita, Chem. Lett., 1998, 829.
R. A. Abramovitch and J. G. Saha, Adv. Heterocycl. Chem., 1966, 6, 229; M. R. Grimmett, Adv.
Heterocycl. Chem., 1993, 58, 271. For examples of the deoxygenation with chlorine-containing
reagents without chlorination of the pyridine nucleus, see the following: A. Katritzky and C. W.
Rees, ‘Comprehensive Heterocyclic Chemistry,’ Pergamon Press, Oxford, 1984, Vol. 2, p. 261.
J. A. Joule and G. F. Smith, ‘Heterocyclic Chemistry, 2nd ed.,’ Van Nostrand Reinhold,
Wokingham, 1978, pp. 71–74.
6.
7.
For reviews, see the following: G. Opitz, Angew. Chem., Int. Ed. Engl., 1967, 6, 107; N. H.
Fischer, Synthesis, 1970, 393; J. F. King, Acc. Chem. Res., 1975, 8, 10.
8.
9.
J. F. King and D. R. K. Harding, Can. J. Chem., 1976, 54, 2652; J. Nakayama, M. Tanuma, Y.
Honda, and M. Hoshino, Tetrahedron Lett., 1984, 25, 4553.
J. S. Grossert and M. M. Bharadwaj, J. Chem. Soc., Chem. Commun., 1974, 144; J. F. King, E.
A. Luinstra, and D. R. K. Harding, J. Chem. Soc., Chem. Commun., 1972, 1313.
10. (a) G. A. Olah, M. Arvanaghi, and Y. D. Vankar, Synthesis, 1980, 660. (b) F. A. Daniher and B. E.
Hackley Jr., J. Org. Chem., 1966, 31, 4267.