Preparation of MBHA resin by benzotriazole-mediated amidoalkylation

Article abstract of DOI:10.1016/j.tetlet.2008.06.110

MBHA (4-methylbenzhydrylamine) resin is widely used as a solid support for the synthesis of carboxamides or peptide C-terminal amides. Herein, we report a new method for synthesizing MBHA resin by benzotriazole-mediated amidoalkylation. MBHA resin was efficiently prepared with N-[(benzotriazol-1-yl)(p-tolyl)methyl]formamide or N-[formamido(p-tolyl)methyl]formamide, and it showed excellent properties as a solid support.

Full text of DOI:10.1016/j.tetlet.2008.06.110

Tetrahedron Letters 49 (2008) 5380–5382
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Preparation of MBHA resin by benzotriazole-mediated amidoalkylation
Tae-Kyung Lee a, Jeong-Hyun Choi a, Jang-Woong Byun b, Yoon-Sik Lee a,
*
a School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea
b BeadTech Inc., Institute for Chemical Processes, Seoul National University, Seoul 151-744, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
MBHA (4-methylbenzhydrylamine) resin is widely used as a solid support for the synthesis of carboxam-
ides or peptide C-terminal amides. Herein, we report a new method for synthesizing MBHA resin by ben-
zotriazole-mediated amidoalkylation. MBHA resin was efficiently prepared with N-[(benzotriazol-1-
yl)(p-tolyl)methyl]formamide or N-[formamido(p-tolyl)methyl]formamide, and it showed excellent
properties as a solid support.
Received 1 May 2008
Revised 21 June 2008
Accepted 26 June 2008
Available online 1 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
MBHA resin
Amidoalkylation
1-Benzotriazole
Solid-phase peptide synthesis
Peptide C-terminal amides are widely employed as peptide
drugs, mainly due to their increased biological activity and resis-
tance to biological degradation.1 Such peptide amides are usually
synthesized by solid-phase methods, in which amino acids are cou-
pled to the amino-functionalized polymeric supports, and the
desired peptides with the carboxamide group at C-terminal are
recovered from the supports after elongation. Since benzhydryl-
amine (BHA) resins were first introduced for the synthesis of pep-
tide amides, a variety of the amino-functionalized handles or
supports have been developed and adjusted for optimal cleavage
conditions. Mild acid-sensitive linker systems, such as Rink amide
linker,2 xanthenyl linker,3 PAL handle4 and dibenzocyclohepta-1,4-
dienylamine linker,5 have been utilized in solid-phase peptide syn-
thesis (SPPS) using the Fmoc/t-Bu strategy. Nonetheless, BHA link-
ers remain important in the synthesis of peptide amides using the
Boc/Bzl strategy due to their simple structures and relatively facile
preparation. Among BHA resins, the 4-methyl derivative improved
the efficiency of the synthesis of model peptide carboxamides.6
Initially, in order to introduce BHA structures onto polystyrene
(PS) resin beads, three synthetic methods were employed: benzoyl
group reduction followed by bromination and ammonolysis,7
oxime group reduction,7 and direct benzoyl group reduction by
Leuckart reduction.8 However, these methods have problems such
as crosslinking or formation of secondary amine groups. Further-
more, the Leuckart reduction was too sensitive to the reaction
their preparation. Recently, BHA structures were also constructed
by reducing the imine group obtained from lithiated PS resin and
substituted benzonitriles.11
On the other hand,
a-amidoalkylation has long been adopted to
introduce a-
a
-aminoalkyl groups into aromatic compounds.12 N-(
Functionalized alkyl)carboxylic amides or carbamates are usually
employed under acidic conditions, where a variety of functional
groups, such as hydroxyl, halogen, carboxylate, alkoxide, carbox-
ylic amide and carbamate, can be used as the a-functional group.
However, the reaction conditions were too harsh, and strong acid
or high temperature was mandatory for significant yield of amid-
oalkylated products, except for the highly activated aromatic
systems.
Meanwhile, Katritzky’s group has studied on the benzotriazole-
mediated reaction extensively and revealed that benzotriazole
could act as a leaving group in the electrophilic aromatic substitu-
tion.13 In particular, benzotriazole-functionalized amides were
more readily prepared than other
a
a
-functionalized amides, and
-amidoalkylation of activated
-amidoalkylation of unacti-
vated aromatic compounds has not been described until now. In
this Letter, we report the benzotriazole-mediated -amidoalkyl-
were successfully employed in the
aromatic compounds.14 However, the
a
a
ation of unactivated aromatic compounds and its application to
the preparation of MBHA resin.
Our new linker system, N-[(benzotriazol-1-yl)(p-tolyl)methyl]-
formamide (1), consists of an alkyl group, a carboxylic amide and
1-benzotriazole. Formamide was selected as an amide group be-
cause the formyl group can be removed easily after the aromatic
substitution reaction. Considering the structure of MBHA resin,
the p-tolyl group was employed as an alkyl group. Thus, the benzo-
triazolyl linker (1) was prepared from p-tolualdehyde, formamide
conditions. Alternatively, N-(a-chlorobenzyl)phthalimides were
used to incorporate BHA groups onto PS resin.9,10 Despite the facile
amino-functionalization, tedious reaction steps were required for
* Corresponding author. Tel.: +82 2 880 7080; fax: +82 2 876 9625.
E-mail address: yslee@snu.ac.kr (Y.-S. Lee).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.06.110

T.-K. Lee et al. / Tetrahedron Letters 49 (2008) 5380–5382
5381
H
N
O
H
O
H
H
N
H
N
O
b
O
H
a
+
+
N
H
NH2
N
N
N
N
2
1
H
N
O
H
O
H
H
N
d
c
H2N
+
N
N
N
1% DVB-PS
4
3
1
Scheme 1. Synthesis of benzotriazolyl linker (1) and MBHA resin (4). Reagents and conditions: (a) toluene, reflux, 24 h, 14% yield; (b) AlCl3, benzene, DCE, reflux, 72 h, 12%
yield; (c) AlCl3, DCE, reflux, 72 h; (d) 35% HCl, EtOH, reflux, 12 h.
and 1-benzotriazole.15 The benzotriazolyl linker (1) was then
reacted with benzene in the presence of aluminium chloride to
obtain N-[phenyl(p-tolyl)methyl]formamide (2) (Scheme 1).16
Despite the long reaction time (72 h) and the low yield (12%), this
triazole can replace one of two formamides in the presence of alu-
minium chloride and the resulting benzotriazolyl linker (1) can be
incorporated into the
a-amidoalkylation. Accordingly, catalytic
amounts of 1-benzotriazole were used in the
a-amidoalkylation
reaction demonstrated the possibility of
a
-amidoalkylation of
of benzene with N-[formamido(p-tolyl)methyl]formamide (5),
and N-[phenyl(p-tolyl)methyl]formamide (2) was obtained in
13% yield (Scheme 2). Similarly, the benzotriazole-catalyzed ami-
doalkylation of 1% DVB-PS resin afforded [formamido(p-tolyl)-
methyl] PS resin (3). After hydrolysis of the resin (3), high-
loading capacity MBHA resin (1.03 mmol/g resin, confirmed by
Fmoc-titration after coupling with Fmoc-Gly-OH) was prepared
(Scheme 2).
MBHA resins obtained by benzotriazole-mediated reactions,
such as the benzotriazolyl linker method and the benzotriazole-
catalyzed method, not only possess sufficiently high-loading levels
for SPPS, but also exhibit clean surface morphology and good
swelling properties (see Supplementary data). On the other hand,
in order to evaluate the performance of the MBHA resin in peptide
synthesis, a model peptide, Leu-enkephalin (YGGFL-NH2), was syn-
thesized on Fmoc-Leu-NH-MBHA resin (0.47 mmol/g resin) using
Boc/Bzl chemistry after removing the Fmoc-group with 20% piper-
idine in NMP. After cleavage with TFMSA, the crude product was
analyzed by HPLC. In the HPLC chromatogram, one peak (88%
purity) was detected, which was found to be Leu-enkephalin by
MALDI-TOF (Fig. 1).
unactivated aromatic compounds such as benzene. Moreover, since
ꢀ10% functionalization of phenyl rings in PS resin corresponds to a
loading capacity of about 1.0 mmol/g resin and this loading level
is sufficiently high for SPPS, the relatively low yield (12%) in the
a-amidoalkylation is not considered to pose any problems for the
synthesis of MBHA resin.
In order to prepare MBHA resin by this method, the benzotriaz-
olyl linker (1) was similarly reacted with 1% DVB-crosslinked PS
resin to yield [formamido(p-tolyl)methyl] PS resin (3).17 The exis-
tence of formyl groups in the resin (3) was confirmed by FT-IR
(amide carbonyl band at 1684 cmꢁ1) and by the nitrogen contents
in the elementary analysis (see Supplementary data). MBHA resin
(4) was obtained by hydrolyzing the resin (3), and its loading
capacity was determined to be 0.97 mmol/g resin by Fmoc titration
after coupling with Fmoc-Gly-OH (Scheme 1).
Although the benzotriazolyl linker (1) afforded MBHA resin of
high-loading capacity, a more efficient route was required owing
to its low yield (14%). N,N0-Alkylidene bis(amide) is an alternative
because it can be obtained in a one-step reaction.12 For such a
linker, N-[formamido(p-tolyl)methyl]formamide (5) was synthes-
ized from p-tolualdehyde and formamide in 63% yield (Scheme
2). Bis(formamide) linker (5) was easily obtained by filtering the
reaction mixture without any further purification steps.
In summary, benzotriazole-mediated amidoalkylation was
accomplished on the unactivated aromatic compounds. Benzo-
triazolyl linker (1), or bis(formamide) linker (5) combined with
catalytic benzotriazole, was reacted with benzene to give the for-
mamidoalkylated product. Using this method, MBHA resin was
successfully synthesized after reaction with 1% DVB-PS resin, and
While bis(formamide) linker (5) is unreactive in electrophilic
aromatic substitution, its reactivity can be improved by adding
1-benzotriazole. In the benzotriazole-catalyzed reaction, 1-benzo-
H
N
O
H
H
N
O
H
a
b
O
H
O
2
+
H
NH2
N
H
H
O
5
2
O
H
N
H
N
O
d
H2N
c
H
H
H
+
N
H
O
1% DVB-PS
4
3
5
Scheme 2. Synthesis of bis(formamide) linker (5) and MBHA resin (4). Reagents and conditions: (a) cat. H2SO4, toluene, reflux, 18 h, 63% yield; (b) 1-benzotriazole (0.2 equiv),
AlCl3, benzene, DCE, reflux, 72 h, 13% yield; (c) 1-benzotriazole (0.2 equiv), AlCl3, DCE, reflux, 72 h; (d) 35% HCl, EtOH, reflux, 12 h.

5382
T.-K. Lee et al. / Tetrahedron Letters 49 (2008) 5380–5382
References and notes
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44–48.
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12. Zaugg, H. E. Synthesis 1984, 85–110.
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14. Katritzky, A. R.; Pernak, J.; Fan, W. Q. Synthesis 1991, 868–870.
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4439–4443.
Figure 1. HPLC chromatogram of crude YGGFL-NH2 synthesized on MBHA resin
(MALDI-TOF: calcd 577.3 for YGGFL-NH2 [M+Na]+, found 577.3).
16. Preparation of (2). To a suspension of the benzotriazolyl linker (1) (1.0 g,
0.8 equiv) in benzene (25 mL) and DCE (25 mL) was added aluminium chloride
(1.0 g, 1.6 equiv). The reaction mixture was heated to reflux for 72 h under
nitrogen atmosphere. The reaction mixture was quenched by adding 3 N HCl
(20 mL) after cooling to 0 °C, and transferred to a separatory funnel. The
organic phase was separated, and washed with 3 N HCl (30 mL ꢂ 2) and H2O
(30 mL). The organic layer was dried over anhydrous magnesium sulfate, and
concentrated under reduced pressure. The crude product was purified by
column chromatography using n-Hex/EtOAc (2:1) as eluent (Rf = 0.17), N-
[phenyl(p-tolyl)methyl]formamide (2) was obtained as white solid (0.10 g, 12%
yield).
17. Preparation of (3). To a suspension of PS resin (5.0 g, 1.0 equiv, 1% DVB-
crosslinked, 100–200 mesh) in DCE (80 mL) were added benzotriazolyl linker
(1) (7.7 g, 0.8 equiv) and aluminium chloride (7.7 g, 1.6 equiv). The reaction
mixture was heated to reflux for 72 h under nitrogen atmosphere. The reaction
mixture was quenched by adding 3 N HCl (20 mL)/THF (10 mL) after cooling to
0 °C, and the resin was collected by filtration. The resin was washed with THF
(75 mL)/3 N HCl (25 mL) (ꢂ2), THF (75 mL)/water (25 mL) (ꢂ2), THF
(100 mL ꢂ 2), DCM (100 mL ꢂ 2) and MeOH (100 mL ꢂ 2), and dried in vacuo
to yield [formamido(p-tolyl)methyl] PS resin (3). FT-IR: amide carbonyl band at
1684 cmꢁ1, N analysis: 1.695%, the loading level 1.21 mmol/g resin.
it had good properties as a solid support for SPPS. Considering the
facile availability of bis(formamide) linker (5), the benzotriazole-
catalyzed version is believed to be a simple, beneficial method
for the preparation of MBHA resin.
Acknowledgements
This study was supported by a grant from the Korea Health 21
R&D Project, Ministry of Health & Welfare, Republic of Korea
(A050432) and Seoul R&D Program (10538), through Institute of
Bioengineering, Seoul National University, Seoul, Korea.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.06.110.