7026
K. Ohmori et al. / Tetrahedron Letters 43 (2002) 7023–7026
(J=10.8 Hz) and 14ax (J=3.3 Hz). The ratio of 14eq
and 14ax was 31/69. Upon warming, these peaks
became closer with broadening, went through coales-
cence, and further warming (60°C) led to a sharp
doublet (J=5.5 Hz).
7. Handa, Y.; Inanaga, J. Tetrahedron Lett. 1987, 28, 5717.
8. Girard, P.; Namy, J. L.; Kagan, H. B. J. Am. Chem. Soc.
1980, 102, 2693.
9. This combination was used for the intermolecular reac-
tion of two acetals. See: Studer, A.; Curran, D. P. Synlett
1996, 255.
In contrast, the parent alcohol 9 showed greater prefer-
ence of the diequatorial conformer, due to the internal
hydrogen bonding between the hydroxy and benzyloxy
groups.17 The ratio of 9eq and 9ax was about 88/12 in
CDCl3 at −10°C.
10. Roskamp, E. J.; Pedersen, S. F. J. Am. Chem. Soc. 1987,
109, 6551.
11. Manzer, L. E. Inorg. Synth. 1982, 21, 138.
12. Tsunoda, T.; Suzuki, M.; Noyori, R. Tetrahedron Lett.
1980, 21, 1357.
13. Imamoto, T.; Kusumoto, T.; Hatanaka, Y.; Yokoyama,
In summary, semi-pinacol cyclization of aldehyde–ace-
tal biaryl compounds has been described, enabling
access to a useful intermediate for the synthesis of
pradimicin-class antibiotics. Further studies directed
toward the total synthesis are now in progress.
M. Tetrahedron Lett. 1982, 23, 1353.
14. Experimental procedure for cyclization of 8 with SmI2 and
BF3·OEt2: To a mixture of SmI2 (1.7 mL, 0.1 M THF
solution, 0.17 mmol) and BF3·OEt2 (36 mg, 0.25 mmol)
in THF (1.6 mL) was added a solution of 8 (51 mg, 0.074
mmol) in THF (1.7 mL) at −78°C. After stirring for 10
min, the reaction mixture was warmed to 0°C, and stirred
for 20 min. The reaction was stopped by adding 10%
aqueous K2CO3, and the products were extracted with
EtOAc (×3). The combined organic extracts were washed
with brine, dried (Na2SO4), and concentrated in vacuo.
The residue was purified by preparative TLC (benzene/
EtOAc=4/1) to afford 9 (35 mg, 81%) as yellow solid.
Compound 9: IR (KBr): 3479, 2928, 1728, 1585, 1447,
1339, 1286, 1046, 813 cm−1; 1H NMR (400 MHz, C6D6 at
80°C): l 2.32 (s, 3H), 3.35 (s, 3H), 3.45 (s, 3H), 3.47 (s,
3H), 3.66 (s, 3H), 3.81 (s, 3H), 4.43 (d, J=9.1 Hz, 1H),
4.64 (d, J=9.1 Hz), 4.68 (d, J=12.0 Hz), 4.80 (d, J=12.0
Hz), 6.71 (s, 1H), 7.10–7.23 (m, 3H), 7.38–7.40 (m, 3H),
8.24 (s, 1H); 13C NMR (100 MHz, C6D6 at 80°C): l 19.2,
51.6, 57.0, 61.0, 61.4, 73.3, 73.4, 74.1, 82.7, 109.5, 119.89,
119.91, 121.0, 121.7, 124.0, 128.8, 129.5, 132.0, 136.7,
137.1, 138.9, 141.3, 146.7, 154.4, 156.0, 156.3, 168.7. Anal
calcd for C32H31ClO8: C, 66.38; H, 5.40. Found: C, 66.13;
H, 5.34%.
15. For comparison, substrate 13, in which positions of the
acetal and aldehyde moieties were exchanged (cf. 8), was
subjected to the reaction of SmI2 and BF3·OEt2. Forma-
tion of many unidentified products was observed with
substantial recovery of 13 (50%). Thus, the substituent on
the aromatic ring exerts a significant influence on the
activation of the acetal moiety. The electron-withdrawal
by the methoxycarbonyl group in 13 retards activation of
the acetal moiety by Lewis acid.
16. Change of the color of the reaction mixture (deep blue to
bright orange) suggested that Ti(Oi-Pr)4 was reduced by
SmI2. No reaction occurred with this reduced species.
17. Kato, H.; Ohmori, K.; Suzuki, K. Chirality 2000, 12, 548.
18. Kato, H.; Ohmori, K.; Suzuki, K. Tetrahedron Lett.
2000, 41, 6827.
Acknowledgements
We thank Mr. Minoru Tamiya for him experimental
assistance.
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