MW-enhanced high-speed deprotection of Boc group using p-TsOH and concommitant formation of N-Me-amino acid benzyl ester p-TsOH salts

Article abstract of DOI:10.1081/SCC-200063953

A high-speed, complete deprotection of Boc group from Boc amino acids and protected peptide esters employing p-TsOH in toluene under microwave irradiation is found to be complete in 30s. The deprotection can be carried out in methanol and acetonitrile also. Under the present conditions, C-peptide benzyl esters and O-benzyl ethers have been found to be stable. This has permitted us to carry out the synthesis of [Leu]enkephalin employing the Boc/Bzl-group strategy. Further more, it has been found that both N^α-Fmoc and N ^α-Z groups are completely stable. The present conditions can be extended for the concomitant removal of the Boc group and the formation of C-benzyl amino acid esters as well. This has been utilized for the synthesis of N-Me amino acid benzyl esters starting from Boc-N-Me amino acids in a single step. Copyright Taylor & Francis, Inc.

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MW‐Enhanced High‐Speed
Deprotection of Boc
Group Using p‐TsOH and
Concommitant Formation of
N‐Me‐Amino Acid Benzyl Ester
p‐TsOH Salts
Vommina V. Suresh Babu a , Basanagoud S. Patil a &
Ganga‐Ramu Vasanthakumar a
a Department of Studies in Chemistry, Central
College Campus, Bangalore University, Bangalore,
India
Published online: 15 Aug 2006.
To cite this article: Vommina V. Suresh Babu , Basanagoud S. Patil & Ganga‐Ramu
Vasanthakumar (2005): MW‐Enhanced High‐Speed Deprotection of Boc Group Using
p‐TsOH and Concommitant Formation of N‐Me‐Amino Acid Benzyl Ester p‐TsOH Salts,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 35:13, 1795-1802
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Synthetic Communicationsw, 35: 1795–1802, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200063953
MW-Enhanced High-Speed Deprotection of
Boc Group Using p-TsOH and
Concommitant Formation of N-Me-Amino
Acid Benzyl Ester p-TsOH Salts
Vommina V. Suresh Babu, Basanagoud S. Patil, and
Ganga-Ramu Vasanthakumar
Department of Studies in Chemistry, Bangalore University,
Bangalore, India
Abstract: A high-speed, complete deprotection of Boc group from Boc amino acids
and protected peptide esters employing p-TsOH in toluene under microwave irradiation
is found to be complete in 30 s. The deprotection can be carried out in methanol and
acetonitrile also. Under the present conditions, C-peptide benzyl esters and O-benzyl
ethers have been found to be stable. This has permitted us to carry out the synthesis
of [Leu]enkephalin employing the Boc/Bzl-group strategy. Further more, it has been
found that both Na-Fmoc and Na-Z groups are completely stable. The present
conditions can be extended for the concomitant removal of the Boc group and the
formation of C-benzyl amino acid esters as well. This has been utilized for the
synthesis of N-Me amino acid benzyl esters starting from Boc-N-Me amino acids in
a single step.
Keywords: Boc group, deprotection, microwave irradiation, N-Me amino acid
benzyl esters
INTRODUCTION
For the chemical synthesis of peptides and small proteins in solution, the tert-
butoxycarbonyl (Boc) group is the preferred urethane-type-protecting group
Received in India February 18, 2005
Address correspondence to Vommina V. Suresh Babu, Department of Studies in
Chemistry, Central College Campus, Bangalore University, (Dr. B.R. Ambedkar
Veedhi), Bangalore 560 001, India. E-mail: hariccb@rediffmail.com
1795

1796
V. V. Suresh Babu, B. S. Patil, and G.-R. Vasanthakumar
for Na-amino protection of amino acids over an equally popular benzyloxycar-
bonyl (Z) group. This is primarily because it is cleaved by relatively mild
acids. Consequently, it is one of the most commonly employed amino-protect-
ing groups in solution-phase peptide chemistry.[1] There is a necessity for the
protection of several functional groups present in the side chains of the amino
acids. The stability of such side chain–protecting groups during the
elongation of the peptide chain and their removal at the end of the synthesis
are the key factors in the selection of protecting groups. This has led to the
use of combinations of Z/tBu, Boc/Bzl, and Fmoc/tBu strategy.[2–4]
In general, the Boc group is deprotected by using HCl/AcOH, dioxane
EtoAc, anhydrous liquid HF, HBr/AcOH, 10% H2SO4/dioxane, 98%
[6]
HCO2H, and so for.[5] The use of Lewis acid BF3 Et2O in AcOH and
.
several silicon reagents such as SiCl4-phenol,[7] TMSCl-phenol,[8] TMSI,[9]
TMSOTf-TFA,[10] and TMSOTf-2,6-lutidine[11] have been explored.
Boc-group deprotection was performed by using AlCl3 doped with neutral
alumina under microwave (MW) irradiation for 1–2.5 min (with 76–93%
yield)[12] also. Removal of the Boc group using the strong acid TFA either
neat or in combination with CH2Cl2 (50%) is commonly preferred and more
convenient than deprotection with mineral acids or 98% HCO2H because,
after deprotection, excess of trifluoroacetic acid (TFA) can be easily and com-
pletely removed (bp of TFA is 71–728C). The selective cleavage of the Boc
group using an organic acid p-toluenesulfonic acid (p-TsOH) in ether/
ethanol, 1,4-dioxane, AcOH-CH2Cl2, or acetonitrile requires several
hours.[13–17] Over a period of time, the microwave irradiation technique
found several applications in organic synthesis[18–20] In the present communi-
cation, we describe the utility for the rapid removal of the Boc group in
peptide synthesis using p-TsOH.
Initial experiments in the present studies centered on the deprotection of
the Boc group from amino acids under MW irradiation. (The microwave
reaction was carried out in a LG MS 194A microwave oven producing
microwave radiation with a frequency of 2450 MHz. The microwave oven
is 1200-W oven and the reaction was specifically carried out at 60% of the
total power output, which would correspond to an average power of
720 W.) In a typical procedure, a solution of Boc-amino acid in toluene and
p-TsOH was exposed to MW irradiation (Scheme 1). The deprotection,
as monitored by TLC, was found to be complete in 30 s. Further more, depro-
tection was confirmed by recording IR spectra (revealed by the absence
of a urethane carbonyl stretching vibrational frequency in the region
Scheme 1. Deprotection of Boc group under MW irradiation.

Deprotection of Boc Group
1797
1690–1705 cm21). After cooling, in most of the cases, the resulting amino
acid p-TsOH salt separated out as a solid. Filtration, washing with toluene,
and, if necessary, recrystallization using suitable solvent resulted in
the isolation of the product (Table 1). The deprotection has been found to
be complete within 30 s when Boc derivatives were dissolved in acetonitrile
and methanol also. Because of their low bp, it was necessary to use a
greater quantity of solvent. In addition, the use of toluene led to the separation
of the amino acid ester salt as a solid, which can be easily isolated by filtration.
The studies are then extended to demonstrate its utility for the removal of
the Boc group in peptide synthesis. It has been found that the use of two
equivalents of p-TsOH in toluene is sufficient to achieve the complete depro-
tection under MW irradiation within 30 s (Table 2). The deprotection of the
Boc group from Boc-peptide esters when carried out using acetonitrile and
MeOH was also found to be smooth and complete. As reported, the deprotec-
tion of the Boc group using p-TsOH in acetonitrile at ambient temperature
requires about 3 to 4 h.[13,14] The deprotection carried out by refluxing
the toluene using an oil bath (around 105–1108C) took about 2 h for the
complete removal of the Boc group. Thus, employing the present conditions,
the removal of the Boc group from several dipeptides as well as all the four
Boc-protected intermediates in the synthesis of pentapeptide [Leu]enkephalin
have been carried out (Scheme 2). The final pentapeptide was obtained in
63% yield [Yield 63%; mp 158–598C; found: C, 60.48; H, 6.83; N, 12.40.
C28H37N5O7 calcd. for C, 60.53; H, 6.71; N, 12.60%; [a]2D5 222.9 (c ¼ 1,
1
DMF); H NMR (d, CDCl3): 0.83–0.9 (6H, m), 1.14 (2H, t), 1.6 (lH, m),
2.65 (4H, m), 2.8–3.2 (4H, m), 3.4 (2H, br) 3.6–4.0 (3H, m), 5.7–6.1
Table 1. Physical characteristics for the amino acid derivatives
Time
(s)
Mp
(8C)
Yield
(%)
Entry Protected amino acid
Product
.
1
2
2
2
3
4
5
6
7
8
9
Boc-Ala-OH
Boc-Val-OH
TsOH H-Ala-OH
30
30
30
30
45
30
—
—
—
—
—
170
96
95
98
94
96
92
—
—
—
—
—
.
TsOH H-Val-OH
66–68
114–16
156–57
204–05
114–16
—
.
Boc-Phe-OH
Boc-Cys(Bzl)-OH
Boc-Trp-OH
TsOH H-Phe-OH
.
TsOH H-Cys(Bzl)-OH
.
TsOH H-Trp-OH
.
Boc-Glu(OBzl)-OH
Fmoc-Lys(Z)-OH
Fmoc-Ser(Bzl)-OH
TsOH H-Glu(OBzl)-OH
No reactiona
No reactiona
137–38
120–30
132–33
—
Fmoc-Asp(OBzl)-OH No reactiona
Fmoc-Arg(Pmc)-OH
Z-NMe-Leu-OH
No reactiona
No reactiona
aMW irradiation for 15 min in 15 cycles with 720 W of power has resulted in no
change of the reactants and the starting compounds have been isolated completely.
No decomposition was noticed.

1798
V. V. Suresh Babu, B. S. Patil, and G.-R. Vasanthakumar
Table 2. List of peptides deportected using p-TsOH under MW irradiation
Time
(s)
Mp
(8C)
Yield
(%)
Entry
Protected peptide
Producta
1
2
3
4
5
6
7
Boc-His-Pro-OMe
Boc-Ile-Gly-OMe
Boc-Phe-Ile-OMe
Boc-Pro-Phe-OMe
Boc-Val-Tyr-OEt
Boc-Ser-Gly-OBzl
Boc-Arg(NO2)-
Ala-OBzl
H-His-Pro-OMe
H-Ile-Gly-OMe
H-Phe-Ile-OMe
H-Pro-Phe-OMe
H-Val-Tyr-OEt
H-Ser-Gly-OBzl
H-Arg(NO2)-
Ala-OBzl
30
30
30
30
30
30
30
145
182–83
102
89
92
96
88
86
90
82
177–78
113–15
170
170–71
8
9
Boc-Gly-Leu-
Val-OMe
H-Gly-Leu-
Val-OMe
30
30
115–17
50–60
88
8
Boc-Val-Tyr-
Pro-OBzl
H-Val-Tyr-
Pro-OBzl
10
11
Boc-Phe-Leu-OBzl
Boc-Gly-Phe-
Leu-OBzl
H-Phe-Leu-OBzl
H-Gly-Phe-
Leu-OBzl
30
30
174–76
168–70
84
78
12
Boc-Gly-Gly-Phe-
Leu-OBzl
H-Gly-Gly-Phe-
Leu-OBzl
30
154–56
76
1
aMass and H NMR spectra are satisfactory.
(4H, m), 7.2–7.6 (9H, m), and 8.2 (lH, s). Mass: (M þ H)þ556.28.]. Its purity,
1
as analyzed by HPLC, was satisfactory. The H NMR and mass spectra also
confirmed the same.
During this work, a systematic study regarding the stability of various
protecting groups normally employed for carboxyl protection of amino
acids and side chains of several bifunctional amino acids has been carried
out. It is found that amino acid benzyl, methyl, and ethyl esters are completely
stable. The exposure of amino acid benzyl ester [Fmoc-Asp(OBzl)-OH] or
peptide benzyl ester (H-Phe-Leu-OBzl) to MW irradiation for 15 min in 15
cycles with 720 W of power has resulted in no change of the reactants
(as analyzed by TLC as well as by IR). Further more, it has been found that
Scheme 2. Synthesis of Leu-enkephalin; (i) coupling employing HBTU using stan-
dard conditions, (ii) deprotection of Boc group using p-TsOH under MW irradiation,
and (iii) CTH using 98% HCOOH/Pd-C/MeOH as solvent.

Deprotection of Boc Group
1799
Na-Fmoc and Z groups, Asp(OBzl), Ser(Bzl), and Arg(Pmc) have been found
to be completely stable (Table 1).
Our group is half-way to the solution synthesis of cyclosporine O and its
analogues in few gram quantities by employing Boc chemistry with the
segment condensation approach. For this work, we needed to make large quan-
tities of N-Me-Leu-OBzl and N-Me-Val-OBzl. In general, the preparation of
N-Me amino acid benzyl esters has been carried out in two steps. In the first
step, Na-protected (Boc, Z, or Fmoc) N- Me amino acids have been
converted to methyl or benzyl esters. Then, Na-protecting group was
removed to obtain N-Me amino acid esters.[21–25] Consequently, we made
initially Boc-N-Me-Leu-OBzl employing Boc-N-Me-Leu-OH, Cs2CO3, and
benzyl bromide following the reported procedure.[26] After the deprotection
of Boc group using TFA from the resulting Boc-N-Me-Leu-OBzl, N-Me-
Leu-OBzl was obtained in 47% yield. Consequently, we attempted to
deblock the Boc group using p-TsOH under MW irradiation. The deprotection
was found to be clean and complete. Based on these results as well as our
recently published method for the synthesis of amino acid benzyl ester p-
TsOH salts,[27] we reasoned that both the deprotection of the Boc group and
simultaneous formation of the benzyl ester can be accomplished in a single
step (Scheme 3). It is found that the use of two equivalents of p-TsOH along
with BOC-N-Me-Leu-OH and benzyl alcohol in toluene and exposure of the
mixture to MW irradiation for 30–45 s has resulted in the direct formation of
N-Me amino acid benzyl ester p-TsOH salt in one pot. Under these conditions,
we have prepared few other benzyl esters required for our synthesis (Table 3).
In conclusion, these studies demonstrate that the Boc group can be depro-
tected very rapidly in solution synthesis of peptides. The use of two equivalents
of p-TsOH is sufficient for the complete removal of the Boc group from
peptides in 30 s. On the other hand, deprotection of the Boc group at ambient
temperature needs several hours. After the deprotection, the resulting peptide
ester p-TsOH salts can be easily converted to the corresponding amino free
amino acid ester and employed for the further extension of the chain. In
“green chemistry,” which aims to reduce the use of toxic chemicals, the use
of p-TsOH in place of TFA (regularly used in peptide synthesis) may find
wide acceptability. The utility of the present procedure for the removal of
the Boc group in solid-phase synthesis of [Leu]enkephalinamide is in progress.
Further, the synthesis of N-Me amino acid benzyl ester is usually accom-
plished in a two-step process involving the conversion of Boc or Fmoc-N-Me
Scheme 3. Concomitant deprotection of Boc group and formation of N-Me amino
acid benzylester TsOH salt.

1800
V. V. Suresh Babu, B. S. Patil, and G.-R. Vasanthakumar
Table 3. p-TsOH-included MW-irradiated concomitant deprotection of the Boc
group and formation of N-Me amino acid benzyl esters
Boc-N-Me
amino acid
Time
(s)
Mp
(8C)
Yield
(%)
[a]2D0 (c ¼ 1,
Entry
1
Producta
CHCl3)24
Boc-N-Me- H-N-Me-
Leu-OH Leu-OBzl
Boc-N-Me-D- H-N-Me-D-
30
30
30
30
gum
—
96
95
92
93
26.98 (26.90,
c ¼ 1, CHCl3)
þ6.42 (þ6.06,
c ¼ 0.90, CHCl3)
25.13 (25.20,
c ¼ 1.18, CHCl3)
þ5.28 (þ5.10,
c ¼ 1, CHCl3)
2
3
4
Leu-OH
Boc-N-Me-
Val-OH
Leu-OBzl
H-N-Me-
Val-OBzl
—
Boc-N-Me-D- H-N-Me-D-
Val-OH Val-OBzl
—
aThe IR analysis confirms the presence of benzyl ester and the absence of the peak
corresponds to urethane carbonyl vibrational stretching. Spectral data is given in the
experimental section.
amino acid to the corresponding benzyl ester and then the removal of protec-
ting group. It is now demonstrated that the synthesis of N-Me amino acid
benzyl esters in a single operation rapidly and with high yields under MW
irradiation technique can be accomplished.
EXPERIMENTAL
Melting points were determined by the capillary method and are uncorrected.
IR spectra were recorded on a Nicolet model impact 400D FT-IR spectrometer
(KBr pellets, 3 cm21 resolution). 1H and 13C NMR spectra were recorded on a
Bruker AMX 400-MHz spectrometer. Mass spectra were recorded on PE-
SCIEX 150 EX LCMS. Specific rotations were recorded on a Rudolf
Research Autopol IV automatic polarimeter. The TLC analysis was carried
out on precoated silica-gel plates using solvent systems (a) ethyl acetate–
petroleum ether (35:65 v/v) and (b) chloroform–methanol–acetic acid
(40:2:1 v/v). All solvents were freshly distilled prior to use.
General Procedure for Boc-Group Deprotection
A mixture of Boc amino acid or peptide ester (1 mmol) and p-TsOH (2 mmol)
in toluene (5 mL) in a beaker was exposed to microwave irradiation
until the completion of the deprotection. On cooling to rt, the amino acid
p-TsOH salt has been separated as a solid, which was filtered and washed
with toluene (2 mL).
In the case of peptides, after the deprotection of the Boc group, the
product was isolated as follows: the resulting peptide ester p-TsOH salt was

Deprotection of Boc Group
1801
dissolved in water (10 mL) and the organic layer was separated. The aqueous
layer was neutralized using 10% Na2CO3 and extracted with ethyl acetate
(3 ꢀ 10 mL). The combined organic layer was given a water wash
(1 ꢀ 10 mL), dried over anhydrous Na2SO4, and evaporated under reduced
pressure to obtain the amino free peptide ester.
General Procedure for the Preparation of N-Me Amino Acid Benzyl
Ester Starting from Boc-N-Me Amino Acid
A mixture of Boc-N-Me amino acid (1 mmol), p-TsOH (2 mmol), benzyl
alcohol (5 mmol), and toluene (5 mL) was exposed to microwave irradiation.
After completion of the reaction, the resulting mass was cooled and 10%
Na2CO3 (10 mL) was added to neutralize the p-TsOH salt and extracted
with ethyl acetate (3 ꢀ 10 mL). The combined organic layer was given
water wash (1 ꢀ 10 mL), dried over anhydrous Na2SO4, and evaporated
under reduced pressure to obtain the free N-Me amino acid benzyl ester.
N-Me-Val-OBzl: IR 1735 cm21; 1H NMR (d, CDCl3) 0.91–0.95 (6H, m), 1.8
(1H, m), 1.5 (1H, br. s), 2.35 (3H, s), 3.4 (1H, m), 5.15 (2H, d), 7.2–7.4 (5H,
m); 13C NMR (d CDCl3) 18.7, 18.9, 29.2, 34.6, 42.2, 61.6, 66.2, 128.2, 128.5,
135.5, 175.8; ES MS m/z observed 236.3.
N-Me-Leu-OBzl: IR 1738 cm21; 1H NMR (d, CDCl3) 0.85–0.93 (6H, m), 1.45
(2H, m), 1.55 (1H, br. s), 1.65 (1H, m), 2.35 (3H, s), 3.25 (1H, t), 5.15 (2H, d),
7.28–7.38 (5H, m); 13C NMR (d CDCl3) 22.3, 22.5, 24.8, 34.5, 42.5, 61.78,
66.19, 128.1, 128.4, 135.8, 175.2; ES MS m/z observed 250.4.
The spectral data of N-Me-D-Val-OBzl and N-Me-D-Leu-OBzl are
satisfactory.
ACKNOWLEDGMENTS
Authors are grateful to the Council of Scientific and Industrial Research,
Government of India, for financial assistance. We thank Fred Naider, City
University of New York, New York, for useful discussions and Sophisticated
Instruments Facility, Indian Institute of Science (I. I. Sc.), Bangalore for NMR
data. B.S.P thanks Council of Scientific and Industrial Research (CSIR),
Government of India, for a Senior Research Fellowship.
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