5292
P. Patel et al. / Tetrahedron Letters 48 (2007) 5289–5292
30%. The deprotection of 5-hydroxy-eicosatetraenoic
acid (5-HETE) by the procedure described here is an
important result since 5-HETE 3 is the natural substrate
for 5-hydroxy eicosanoid dehydrogenase (5-HEDH)8
and its total synthesis is constantly required. This
deprotection procedure can improve our overall yields
by ꢀ50%.
of the mechanism of the desilylation reaction in the
presence of olefins.
Acknowledgments
This work was supported by the National Institutes of
Health, grant number HL81873 (J.R.); Canadian Insti-
tutes of Health Research, grant number MOP-6254
(W.S.P.); the Heart and Stroke Foundation of Quebec,
and the J.T. Costello Memorial Research Fund. J.R.
also wishes to acknowledge the National Science Foun-
dation for the AMX-360 (CHE-90-13145) and Bruker
400 MHz NMR (CHE-03-42251) instruments.
As mentioned above, the TBDMS group in entries 5, 6
and 7 is resistant to reductive deprotection. We found
it very interesting that in entries such as 1–4 and 12–
16, in which there is a double bond, hydroboration did
not occur. The experimental conditions used in Table 1
are the most commonly used for effecting hydrobora-
tion.9 Since the time scale for the two reactions, silyl
deprotection and hydroboration, are also comparable,
one would have expected some hydroboration to occur
in entries 14 and 16. To ascertain that the hydroboration
process can occur under our reaction conditions we
subjected 29 and 31 (entries 14 and 15) to identical
reductive desilylation procedures. As can be seen, the
silyl deprotection of the TES occurred very efficiently.
Also the hydroboration of 31 proceeded smoothly
and the hydroboration product 32 was isolated in 90%
yield.
References and notes
1. Greene, T. W.; Wuts, P. G. M. In Protective Groups in
Organic Synthesis, 3rd ed.; John Wiley & Sons: New York,
1999; p 999.
2. Kim, S.; Jacobo, S. M.; Chang, C. T.; Bellone, S.; Powell,
W. S.; Rokach, J. Tetrahedron Lett. 2004, 45, 1973–1976.
3. Yeom, C. E.; Kim, Y. J.; Lee, S. Y.; Shin, Y. J.; Kim,
B. M. Tetrahedron 2005, 61, 12227–12237.
4. Hanessian, S.; Lavallee, P. Can. J. Chem. 1975, 53, 2975–
2977.
To probe this point further, we performed the experi-
ment in entry 16. It was designed to allow an equimolar
mixture of a primary olefin 33 and a separate TES deriv-
ative 6 to compete for the catalyst. As can be seen, the
TES group was removed in excellent yield and the pri-
mary olefin 33 was recovered unchanged.
5. Collington, E. W.; Finch, H.; Smith, I. J. Tetrahedron
Lett. 1985, 26, 681–684.
6. Torisawa, Y.; Shibasaki, M.; Ikegami, S. Tetrahedron
Lett. 1979, 20, 1865–1868.
7. Kim, S.; Bellone, S.; Maxey, K.; Powell, W. S.; Lee, G. J.;
Rokach, J. Bioorg. Med. Chem. Lett. 2005, 15, 1873–1876.
8. Powell, W. S.; Rokach, J. Prog. Lipid Res. 2005, 44, 154–
183.
9. Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J. Am. Chem.
Soc. 1992, 114, 6671–6679.
It has been reported10,11 in studies of the Wilkinson-
catalyzed hydroboration of olefins that an intermediate
such as 34 (Scheme 3a) is formed, which then complexes
with an olefin to result in a boronate derivative 37,
which is subsequently oxidized to afford alcohol 38.
10. Burgess, K.; Van der Donk, W. A.; Westcott, S. A.;
Marder, T. B.; Baker, R. T.; Calabrese, J. C. J. Am. Chem.
Soc. 1992, 114, 9350–9359.
11. Evans, D. A.; Fu, G. C.; Anderson, B. A. J. Am. Chem.
Soc. 1992, 114, 6679–6685.
In order to explain the lack of hydroboration, one
assumption can be, using the published hydroboration
complex 34, that the silyl derivative complexes with
the borane rhodium derivative 36 and does so more effi-
ciently than complexation with the olefin. One must
also assume that at the end of the catalysis cycle the cat-
alyst is in a form which cannot be used for hydrobora-
tion. On the assumption that one of the primary
products formed in this reduction is the silane, we
added commercial TBS silane 42 and catechol borane
in dry THF and observed the formation of 40 and 41
(Scheme 3b). We are attempting to clarify some aspects
12. An interesting suggestion by the reviewer that the selective
deprotection in entries 2, 3 and 4 at C-15 may be due to
the allylic nature of the silyl ether is worth considering.
13. Spectroscopic data of substrate 9 in Table 1: 1H NMR
(400 MHz, CDCl3): 0.806 (t, 3H), 1.181–1.267 (m, 5H),
1.341–1.525 (m, 4H), 2.139–2.189 (m, 1H), 2.423–2.683
(m, 2H), 2.70–2.827 (m, 2H), 4.979–4.995 (m, 1H), 4.979–
5.010 (m, 1H), 5.157–5.20 (m, 1H), 5.485–5.611 (m, 2H),
7.193–7.393 (m, 2H), 7.479–7.498 (t, 1H), 7.912–7.932 (d,
2H); 13C NMR (400 MHz, CDCl3): d 9.75, 18.29, 20.71,
27.40, 30.6, 32.95, 33.30, 38.44, 49.70, 67.93, 74.79, 78.99,
124.07, 124.26, 125.35, 129.08, 132.11, 161.82, 172.15.