8598
In accord with this, treatment of 20 with 2,4,6-tris(trifluoromethyl)thiophenol 22 as catalyst21
and 3 as initiator in refluxing octane resulted in complete (]98%) conversion to the trans-acet-
onide 21, which could be deprotected (Amberlyst-15, excess MeOH, reflux) to give the free
dl-diol 19. The choice of thiol catalyst for epimerisation of 20 proved to be critical and no
significant conversion to 21 was observed when, under otherwise identical conditions, the thiol
22 was replaced by TBST or tert-dodecanethiol. With thiophenol itself, conversion of 20 to 21
took place only to the extent of 75%. For epimerisation to take place efficiently both steps A and
B of Scheme 1 must be sufficiently rapid to maintain the propagation cycle. These results
indicate that the SꢀH bonds in TBST and tert-dodecanethiol are too strong for this requirement
to be met for the intermediate benzylic radical (step B is too slow), while the SꢀH bond in
thiophenol is a little too weak for step A to occur efficiently, even though the abstraction is from
a benzylic CH group.
Acknowledgements
We are grateful to Dr. Abil Aliev for assistance with the NMR experiments to determine the
structure of 14 and we acknowledge financial support from the EPSRC.
References
1. Dang, H.-S.; Roberts, B. P. Tetrahedron Lett. 1999, 40, 4271.
2. Herman, A.; Becker, B.; Wojnowski, W. Z. Anorg. Allg. Chem. 1979, 450, 178.
3. The role of the collidine (2,4,6-trimethylpyridine) is probably to act as a scavenger of acid resulting from
reactions between the initiator and the thiol.1
4. Roberts, B. P. Chem. Soc. Rev. 1999, 28, 25.
5. Calculations were carried out using PCMODEL version 7 (Serena Software, Bloomington, Indiana 47402-3076,
USA); differences in MMX energies are reported.
6. Obtained from Peroxid-Chemie and handled as a 50% w/w solution in involatile aliphatic hydrocarbons (also
available from Aldrich). The half-life of this peroxide is ca. 1 h at 125°C.
7. This equilibrium composition corresponds to a free energy difference of 9.8 kJ mol−1 at 126°C (the bp of octane),
significantly larger than the calculated MMX energy difference between the cis and trans isomers. Using the
MMFF94 force field, available in PCMODEL, the calculated energy difference increases to 9.7 kJ mol−1
.
8. Helm, R. F.; Ralph, J.; Anderson, L. J. Org. Chem. 1991, 56, 7015.
9. Compound 12: NMR (500 MHz for 1H, CDCl3 solvent, J in Hz); lH 1.44 (3H, s, Me), 1.45 (3H, s, Me), 3.28 (1H,
dd, J=12.0 and 7.4, H-5A), 3.33 (1H, dd, J=9.3 and 7.4, H-2), 3.47 (3H, s, OMe), 3.52 (3H, s, OMe), 3.55–3.60
(2H, m, H-3 and H-5B), 4.10 (1H, dd, J=12.0 and 4.7, H-4), 4.53 (1H, d, J=7.4, H-1); lC 26.6, 26.8, 56.4, 57.8,
65.2, 76.7, 77.9, 80.2, 102.6 and 111.7.
1
10. The stereochemistry of 14 was deduced from COSY and NOESY H NMR experiments.
11. Rabow, L. E.; Stubbe, J.; Kozarich, J. W. J. Am. Chem. Soc. 1990, 112, 3196.
12. The structure of 16 was confirmed by independent synthesis from methyl 2,3-O-isopropylidene-b-D-
ribopyranoside13 by deoxygenation at the 4-position via reaction of the corresponding xanthate ROC(ꢁS)SMe
with triphenylsilane in dioxane at 60°C in the presence of di-tert-butyl hyponitrite initiator (see Cole, S. J.;
Kirwan, N. J.; Roberts, B. P.; Willis, C. R. J. Chem. Soc., Perkin Trans. 1 1991, 103). Bp 48–50°C/0.05 mmHg,
1
[h]2D2=−106.1 (c 4.8, CHCl3). NMR (500 MHz for H, CDCl3 solvent, J in Hz); lH 1.36 (3H, s, Me), 1.52 (3H,
s, Me), 1.89 (1H, d[q], J=14.6 and 4.0, H-4A), 2.01 (1H, dd[t], J=14.6, 9.8 and 4.9, H-4B), 3.46 (3H, s, OMe),
3.66 (1H, ddd, J=11.5, 9.8 and 3.7, H-5A), 3.77 (1H, d[t], J=11.5 and 4.7, H-5B), 3.86 (1H, [t], J=5.1, H-2),
4.38 (1H, [q], J=4.8, H-3) and 4.46 (1H, d, J=4.7, H-1); lC 25.9, 27.3, 27.9, 56.2, 58.9, 71.5, 74.7, 101.8 and
109.0. The use of [multiplet] indicates an apparent multiplet with line spacing corresponding to an average