E. Minta et al. / Tetrahedron Letters 46 (2005) 1795–1797
1797
O
O
O
1) BOP,
THF, R.T.,10 min.
Bn2N
TMSONa
Bn2N
Bn2N
OtBu
OMe
OtBu
ONa
OtBu
OH
2) NaBH4,
THF, R.T., 16 h
THF 0°C à R.T., 30 min
O
O
73-75%
Scheme 3.
4. Lundquist, J. T., IV;Pelletier, J. C. Org. Lett. 2002, 4,
3219–3221.
5. Fmoc Solid Phase Peptide Synthesis;Oxford University
Press: Oxford, 2000.
6. Laganis, E. D.;Chenard, B. L. Tetrahedron Lett. 1984, 25,
5831–5834.
With Fmoc-Xxx(OMe)-OH, whatsoever the nature of
the resin, dibenzofulvene appeared in HPLC before
cleavage and has been identified by H NMR after iso-
lation by silicagel column chromatography. This can
be explained by the fact that TMSONa or TMSOK
are too basic to preserve Fmoc protecting group. The
methyl ester was still present on the lateral chain of
Glu or Asp showing that the kinetics of the Fmoc re-
moval were much faster than the methyl ester
hydrolysis.
1
7. Standard methyl ester removal procedure: the protected
anchored aminoacid was stirred in a reactor with 1.5 equiv
of alkali silanolate in different solvents. The kinetic was
evaluated after acidic cleavage from a few beads (condi-
tions for Barlos: 1% TFA in DCM + 1% scavenger TIPS ;
for Wang: 50%TFA in DCM + 1% scavenger TIPS;for
Rink-amide 95%TFA + 1% scavenger TIPS) and HPLC
analysis at different time (30 min, 1, 2, 4, 20, 24 h, on
nucleosil C18 reverse phase Symmetry ShieldTM column,
0–100% CH3CN). The rate of hydrolysis was calculated by
comparaison of Xxx(OMe)-OH and Xxx(OH)-OH peaks
area by HPLC, after LC–MS identification by electro-
spray on a micromass ESI Platform II. The reaction was
stopped at 24 h. Then the Xxx(OMe)-OH and Xxx(OH)-
OH were cleaved from the resin.
To exclude the hypothesis that the different reaction
rates were linked to the fact that the experiments were
carried out on solid support, we performed silanolate-
mediated saponifications in homogenous conditions,
obtaining the same results.
Microwave irradiations8,9 were used for hydrolysis acti-
vation on solid phase but without any success: by-prod-
ucts and support degradations were observed after 5 min
at 600 W. As described in9 only preloaded Wang resin
resisted to microwave irradiations.
8. Allen, D.;Callaghan, O.;Cordier, F. L.;Dobson, D. R.;
Harris, J. R.;Hotten, T. M.;Owton, W. M.;Rathmell, R.
E.;Wood, V. A. Tetrahedron Lett. 2004, 45, 9645–
9647.
9. Collins, J. M.;Collins, M. J.;Steorts, R. C. Peptides 2004,
Standard hydrolysis procedure in the microwave oven:
same conditions as in Ref. 7 but placed in a microwave
vessel and taken up in different solvents with 1.5 equiv
silanolates (DCM, DMF, THF). The vessel was sealed and
heated to 180 ꢁC for 5 min at 600 W in a CEM Explorer
microwave oven.
Because of the interest of this method, it has been ap-
plied on the selective deprotection of the side chain carb-
oxylic function of Bn2-Glu(OMe)-OtBu, followed by a
one pot reduction using the BOP/NaBH4 system.10 In
this way, the anhydrous sodium salt obtained in the first
step was directly reduced in alcohol (Scheme 3). The
overall yield was 73–75% and the tButyl ester was not
affected by the whole procedure.11
10. McGeary, R. P. Tetrahedron Lett. 1998, 39, 3319–3322.
11. Bn2-Glu(OMe)-OtBu (1 g, 2.52 mmol, 1 equiv) was dis-
solved in anhydrous THF (15 mL). Commercially avail-
able TMSONa solution in DCM (1 M, 2.8 mL, 2.8 mmol,
1.1 equiv) was added and the mixture was stirred at rt
during 16 h. Bop reagent (1.45 g, 3.27 mmol, 1.3 equiv)
was added at 0 ꢁC. The suspension was stirred 20 min at
rt, become clear brown and NaBH4 was slowly added
(190 mg, 5.03 mmol, 2 equiv) at 0 ꢁC. The yellow mixture
was stirred 40 min at rt. The mixture was evaporated
under reduced pressure and white residue was extracted by
ethyl acetate and distilled water (20 mL). The aqueous
layer was extracted again with ethyl acetate (2 · 20 mL).
The organic layers were washed with brine (20 mL) dried
over Na2SO4 and the solvent was eliminated by evapora-
tion under reduced pressure to give a colourless oil. This
oil was purified by chromatography on silicagel with
petroleum ether/diethylether (6/4) (v/v) as solvent system.
Overall yields: 73%. 1H NMR, DMSO d 6, d, ppm: 7.2–7.4
The use of silanolates was a potent system for selective
removal of methyl esters on the lateral chain of Glu or
Asp when their NH2 were protected by Z or Bn2 group,
on solid support and homogenous conditions. Further
studies involving a Boc or a phthalimido group as N-
protection are under way in our laboratory.
For SPPS applications the last Glu or Asp derivatives
were protected by Z group and after cleavage of the
Mtt group by 1% TFA, the cyclisation smoothly oc-
curred on solid support and will be reported in due
course.
References and notes
(10H, m, Harom), 3.85 and 3.53 (2*2H, 2*d, Hbenzyl
,
0
JH–H benzyl = 14 Hz), 3.3 (2H, m, Hd, Jd-OH = 5.1 Hz), 3.07
(1H, t, Ha, Ja–b = 7.6 Hz), 1.67 (2H, m, Hb), 1.32 and 1.52
(2H, m, Hc). 13C NMR d 6, d, ppm: 171,39 (C@O), 139.42
(Carom 1), 128.18 and 127.96 (Carom m and Carom o), 126.89
(Carom p), 80.28 (Cquat tBu), 64.88 (Ca), 62.86 (Cd), 53.72
(CH2benzyl), 29.17 (Cc), 28.19 (CH3tBu), 25.25 (Cb).
1. Alcaro, M. C.;Sabatino, G.;Uziel, G.;Chelli, M.;
Ginanneschi, M.;Rovero, P.;Papini, A. M. J. Pept. Sci.
2004, 10, 218–228.
2. Kochegarov, A. A. Expert Opin. Ther. Patents 2002, 12,
243–287.
3. Adams, M. E. Toxicon 2004, 43, 509–525.
Minta, Ewelina
