October 1998
SYNLETT
1095
This new rearrangement is not restricted to pyran ring systems. We have
extended the method to also encompass furan ring systems. Thus the
methyl ketone 24 was formed from the lactone by the reduction/
alkylation and Grignard addition route, in a similar manner to the pyran
systems described above. Following the same protocol as before, 24 is
converted to predominantly the exo silyl enol ether, and subjected to the
standard rearrangement conditions to give the two diastereoisomers of
primary alcohol 25 in a 1:1 ratio and in 56% combined yield (Scheme
References and Notes
(1) Levy, D.; Tang, C. The Chemistry of C-Glycosides, Pergamon,
1995, and references cited therein.
(2) See also Buffet, M. B.; Dixon, D. J.; Edwards, G. L.; Ley, S. V.;
Tate, E. W. Synlett, 1997, 1055.
(3) For some related examples see: i) Reetz, M. T.; Mallerstarke, H.
Ann. Chem., 1983, 10, 1726; ii) Toshima, K.; Miyamoto, N.;
Matsuo, G.; Nakata, M.; Matsumura, S. J. Chem. Soc. Chem.
Commun., 1996, 1379; iii) Uenishi, J.; Sohma, A.; Yonemitsu, O.
Chem. Lett., 1996, 595; iv) Craig, D.; Tierney, J. P.; Williamson,
C. Tetrahedron Lett., 1997, 38, 4153; and references cited therein.
7
9).
(4) Weinreb, S. M.; Nahm, S. Tetrahedron Lett., 1981, 22, 3815.
1
(5) Selected spectroscopic data for 9: H NMR (600MHz, CDCl ):
3
7.89-7.94 (m, 2H, Ph), 7.44-7.64 (m, 3H, Ph), 4.94 (dd, J = 6.7,
2.7 Hz, 1H, CHOH), 3.98-4.05 (m, 1H, OCHCHOH), 3.74-3.79
(m, 2H, OH and CH CH(CH )O), 0.79-1.89 (m, 19H, CH and
2
2
3
Scheme 9
13
8xCH ); C NMR (150MHz, CDCl ): 199.9 (C=O), 134.5,
2
3
133.5, 128.7, 128.5 (Ph-C), 76.3 (CHOH), 73.3 (CH CH(CH )O),
2
2
We believe that the methodology described above represents
a
70.9 (OCHCHOH), 31.6, 30.3, 29.1, 28.3, 27.0, 25.5, 22.7, 18.4
significant extension of our original concept of anomeric oxygen to
carbon rearrangements, being the first reported example of using
anomerically linked silyl enol ethers to effect C-glycosidation.
Furthermore, the formation of the carbon-carbon bonds at the anomeric
position occurs with concurrent stereocontrol at one or more stereogenic
centres.
(8xCH ), 14.0 (CH ).
2
3
(6) Re-exposure of
a
single diastereoisomer to identical
rearrangement conditions shows no evidence of equilibration to
the other diastereoisomer over an extended period, indicating that
the rearrangement proceeds under kinetic control.
(7) The minor side products observed in this reaction were tentatively
assigned as the rearrangement products of the endo enol ether.
Acknowledgements. We thank the EPSRC (to EWT and DJD) and the
Novartis Research Fellowship (to SVL) and Pfizer inc., Groton, U.S.A.
for further financial support.