C. Ramesh et al. / Tetrahedron Letters 44 (2003) 1465–1467
1467
the protection of the amine group. However, with
prenyl esters of N-Boc protected amino acids, the rate
of deprotection of prenyl esters while keeping the N-
Boc group intact was slow, and the yields of the
corresponding acids were poor. If a large amount of the
reagent, NaHSO ·SiO was used, the rate of deprotec-
tion of the esters was increased but both the prenyl
ester and N-Boc groups were found to be cleaved. The
present procedure did not cause epimerization of chiral
centers neighboring the prenyl esters, the regenerated
parent acids showing no change in the values of their
optical rotations, indicating complete retention of
configuration.
8786; (c) Nishizawa, M.; Garcia, D. M.; Minagawa, R.;
Nogachi, Y.; Imagawa, H.; Yamada, H.; Watanabe, R.;
Yoo, Y. C.; Azuma, I. Synlett 1996, 452–454; (d) Chavan,
S. P.; Zubiadha, P. K.; Dantale, S. W.; Keshavaraj, A.;
Ramaswamy, A. V.; Ravindranathan, T. Tetrahedron Lett.
1996, 37, 237–240; (e) Gajare, A. S.; Shaikh, N. S.; Bonde,
B. K.; Deshpande, V. H. J. Chem. Soc., Perkin Trans. 1
2000, 639–640; (f) Yadav, J. S.; Reddy, B. V. S.; Rao, C.
V.; Chand, P. K.; Prasad, A. R. Synlett 2002, 137–139.
4. Ager, D. J.; Prakash, I.; Schaad, D. R. Chem. Rev. 1996,
96, 835–875.
4
2
5. Breton, G. W. J. Org. Chem. 1997, 62, 8952–8954.
7
6. Experimental procedure: Prenyl ester (1 mmol) and
NaHSO ·SiO (200 mg) were taken in CH Cl (10 ml). The
4
2
2
2
In conclusion, we have developed a mild and easy
process for selective deprotection of prenyl esters of
carboxylic acids using NaHSO ·SiO under slightly
mixture was stirred at room temperature. After completion
of the reaction (TLC) the catalyst was filtered off and
washed with EtOAc (2×5 ml). The filtrate and washings
were combined and the solvents were removed under
vacuum. The residue was purified by column chromatogra-
phy over silica gel using mixtures of hexane and EtOAc as
eluents to yield the parent carboxylic acid. The spectral
4
2
acidic conditions. The process is remarkable for high
yields of the parent acids and high chemoselectivity
without causing epimerization of the neighboring chiral
centers and isomerization of double bonds. The opera-
tional simplicity of the procedure is also attractive. The
catalyst can be prepared easily with readily available
inexpensive reagents and is heterogenous and non-haz-
ardous. To our knowledge, this is the first report of an
efficient general method for selective deprotection of
prenyl esters at room temperature by using a safe,
simple catalyst.
data of some of the compounds are given below:
1
Compound 1e: H NMR (200 MHz, CDCl ): l 7.98 (2H,
3
d, J=8.0 Hz), 6.84 (2H, d, J=8.0 Hz), 6.02 (1H, m), 5.40
(1H, m), 5.38 (1H, d, J=15.0 Hz), 5.26 (1H, d, J=10.0
Hz), 4.76 (2H, d, J=7.0 Hz), 4.59 (2H, d, J=5.5 Hz), 1.78
+
(3H, s); LSIMS: m/z 247 (M +1).
1
Compound 2e: H NMR (200 MHz, CDCl ): l 8.05 (2H,
3
d, J=8.0 Hz), 6.92 (2H, d, J=8.0 Hz), 6.05 (1H, m), 5.40
(
1H, d, J=15.0 Hz), 5.31 (1H, d, J=10.0 Hz), 4.56 (2H, d,
+
Acknowledgements
J=5.5 Hz): LSIMS: m/z 179 (M +1).
Compound 1k: H NMR (200 MHz, CDCl ): l 7.38–6.97
1
3
(
10H, m), 5.22 (1H, m), 5.12 (1H, d, J=7.0 Hz), 5.04 (2H,
The authors thank UGC and CSIR, New Delhi for
financial assistance.
s), 4.58 (2H, d, J=7.0 Hz), 4.45 (1H, m), 3.04 (2H, d,
J=7.0 Hz), 1.78 (3H, s) 1.72 (3H, s); LSIMS: m/z 368
+
(
M +1).
1
Compound 2k: H NMR (200 MHz, CDCl ): l 7.40–7.02
3
References
(10H, m), 5.18 (1H, d, J=7.0 Hz), 5.06 (2H, s), 4.64 (1H,
+
m), 3.20–3.01 (2H, m); LSIMS: m/z 300 (M +1).
1
1
. Srinivas, K. V. N. S.; Mahender, I.; Das, B. Green Chem.
002, submitted.
. (a) Kocienski, P. J. Protecting Groups; Thieme: Stuttgart,
994; (b) Greene, T. W.; Wuts, P. G. M. Protecting Groups
in OrganicSynthesis; 3rd ed.; John Wiley and Sons: New
York, 1999.
. (a) Cossy, J.; Albouy, S.; Scheloske, M.; Pardo, D. G.
Tetrahedron Lett. 1994, 35, 1539–1540; (b) Lemaire-
Audoire, S.; Savignac, M.; Blart, E.; Pourcelot, G.; Genet,
J. P.; Bernard, J. M. Tetrahedron Lett. 1994, 35, 8783–
Compound 1m: H NMR (200 MHz, CDCl ): l 7.40–7.18
3
2
(5H, m), 5.42 (1H, brs), 5.28 (1H, m), 5.02 (2H, s), 4.59
2
(2H, d, J=7.0 Hz), 1.77 (3H, s), 1.73 (3H, s), 1.54 (6H, s),
+
1
LSIMS: m/z 306 (M +1).
1
Compound 2m: H NMR (200 MHz, CDCl
): l 7.38–7.24
3
(5H, m), 5.41 (1H, brs), 5.04 (2H, s), 1.58 (6H, s); LSIMS:
+
3
m/z 238 (M +1).
7. (a) Boden, E. P.; Keck, G. E. J. Org. Chem. 1985, 50,
2934–2935; (b) Bochnitschek, S. F.; Waldman, H.; Kunz,
H. J. Org. Chem. 1989, 54, 751–756.