Magnesium/hydrazinium monoformate: A new hydrogenation method for removal of some commonly used protecting groups in peptide synthesis

Article abstract of DOI:10.1016/S0040-4039(01)02113-X

Removal of some commonly used protecting groups in peptide synthesis by catalytic transfer hydrogenation employing hydrazinium monoformate and magnesium is described. This method is equally competitive with other methods in deblocking most of the commonly used protecting groups in peptide synthesis. tert-Butyl derived and base labile protecting groups were completely stable under these conditions. The use of Mg/NH₂-NH₂·HCOOH makes this a rapid, low-cost alternative to palladium and reduces the work-up to a simple and extraction operation.

Full text of DOI:10.1016/S0040-4039(01)02113-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 311–313
Magnesium/hydrazinium monoformate: a new hydrogenation
method for removal of some commonly used protecting
groups in peptide synthesis
D. Channe Gowda*
Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore, Karnataka 570 006, India
Received 29 August 2001; revised 23 October 2001; accepted 5 November 2001
Abstract—Removal of some commonly used protecting groups in peptide synthesis by catalytic transfer hydrogenation employing
hydrazinium monoformate and magnesium is described. This method is equally competitive with other methods in deblocking
most of the commonly used protecting groups in peptide synthesis. tert-Butyl derived and base labile protecting groups were
completely stable under these conditions. The use of Mg/NH –NH ·HCOOH makes this a rapid, low-cost alternative to palladium
2
2
and reduces the work-up to a simple and extraction operation. © 2002 Elsevier Science Ltd. All rights reserved.
For the removal of some commonly used protecting
groups in peptide synthesis, catalytic transfer hydro-
genation offers certain advantages over catalytic hydro-
genation procedures, which are generally employed for
this purpose. Among the desirable characteristics of any
deprotection scheme are the following: (1) low cost, (2)
rapidity, (3) selectivity, (4) mild conditions, usually
avoiding strong acid or base and (5) broad applicabil-
ity. A number of improvements in the area of catalytic
peptides. This drawback has been overcome by carrying
out hydrogenolysis at room temperature, with formic
acid as the hydrogen donor, which may also deblock
acid labile protecting groups like such as the tert-butyl-
oxycarbonyl group. In certain cases HCOONH can
4
advantageously replace formic acid, but it is insoluble
in most organic solvents including methanol. We have
recently shown that Zn could be employed conveniently
as a catalyst either with formic acid, or ammonium
formate for the reduction of nitrocompounds to
1
–10
transfer hydrogenation have been reported
which
11
greatly simplify these procedures and some even reduce
the operating conditions to standard room temperature
amines. In this context, a detailed survey was under-
taken to search for other low-cost metals, which are
active with catalytic transfer agents, and we developed
the present system, which utilizes hydrazinium mono-
formate as the hydrogen donor and magnesium as the
catalyst for the removal of some commonly used pro-
tecting groups in peptide synthesis.
5
–10
and pressure.
These employ such catalytic transfer
1,2 3
hydrogenation agents as cyclohexene,
hydrazine,
and ammonium for-
The most useful reactions discovered to date
4
5–8
cyclohexadiene, formic acid
9
,10
mate.
have centered on the use of Pd, with the lesser use of Pt
and Rh, and the other transition metals scarcely at all.
Furthermore, palladium was the only metal used with
all the above said catalytic transfer hydrogenation
agents in peptide synthesis. Thus, when cyclohexene
and hydrazine are employed as hydrogen donors, tem-
peratures of 70 and 50°C, respectively, have to be
maintained during hydrogenolysis, and this requirement
is a disadvantage if the products are heat-sensitive
Using methanol as the solvent, a variety of amino acids
and peptide derivatives were smoothly deprotected as
shown in Table 1. The protecting groups that have been
a
successfully removed include the N -benzyloxycar-
o
o
bonyl, the N -benzyloxycarbonyl and N -2-chloroben-
zyloxycarbonyl of lysine, a C-terminal benzyl ester, the
O-benzyl ether of O-benzyl-tyrosine, serine or
threonine, the nitro group of arginine, the cyclohexyl
ester of aspartic and glutamic acids, the 2,6-
dichlorobenzyl and bromobenzyloxycarbonyl ether of
tyrosine and the benzyloxymethyl and benzyloxycar-
bonyl of histidine. The progress of the hydrogenolysis
could be followed by TLC on silica gel plates.
Hydrazinium monoformate itself gives a wide yellow
Keywords: catalytic transfer hydrogenation; hydrazinium monofor-
mate; magnesium; new hydrogenation system; peptide synthesis;
deprotection.
*
Corresponding author. Tel.: 091-0821-344348 (res.), 091-0821-
15525 ext. 48 (office); fax: 091-0821-421263/518835; e-mail:
dcgowda@yahoo.com
5
0
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02113-X

3
12
D. Channe Gowda / Tetrahedron Letters 43 (2002) 311–313
Table 1. Catalytic transfer hydrogenolysis of some protected amino acid and peptides using hydrazinium monoformate and
Mg
Specific rotation [h]2
5
D
Ref.
Substrate
Product
Time (h) Yield
Mp (°C)
a
(
%)
Found
92 86–88
Reported
86.5–87.5
Found
Reported
Z-Gly
Z-Phe
Z-Trp
Gly
Phe
Trp
0.5
0.5
0.5
–
–
12
13
13
92 269–275 (dec.) 270–275 (dec.) −34.7 (c 1, H O)
92 280–285 (dec.) 280–285 (dec.) −31.8 (c 1, H O)
−34.4 (c 2, H O)
−32.290.5 (c 1,
2
2
2
H O)
2
Boc-Thr(OBzl)
Boc-Ser(OBzl)
Boc-Thr
Boc-Ser
1.0
2.0
90 153–154
88 141–142
154–155
140–142
+11.3 (c 1, CH OH) +11.4 (c 0.99,
14
14
3
CH OH)
3
+13.2 (c 3, CH OH) +13.3 (c 3.06,
3
CH OH)
3
Boc-Asp(OBzl)
Boc-Asp(OcHx)
Boc-Glu(OBzl)
Boc-Glu(OcHx)
Boc-Tyr(OBzl)
Boc-Asp
Boc-Asp
Boc-Glu
Boc-Glu
Boc-Tyr
1.0
2.0
1.0
2.0
0.5
2.0
0.5
95 175–176
93 175–176
95 172–173
94 172–173
89 136–137
89 136–137
95 202–204
176–177
176–177
171–172
171–172
136–138
136–138
203–204
+10.9 (c 1, CH OH) +10.9 (c 1, CH OH) 14
3
3
+10.9 (c 1, CH OH) +10.9 (c 1, CH OH) 14
3
3
+9.1 (c 1, CH OH) +9.1 (c 1, CH OH) 14
3
3
+9.1 (c 1, CH OH) +9.1 (c 1, CH OH) 14
3
3
+3.6 (c 2, CH OH) +3.9 (c 2.04, AcOH) 14
3
Boc-Tyr (2-BrZ) Boc-Tyr
Boc-Lys(Z)
+3.6 (c 2, CH OH) +3.9 (c 2.04, AcOH) 14
3
Boc-Lys
Boc-Lys
−10.9 (c 1, AcOH) −10.7 (c 0.87,
14
14
AcOH)
−10.9 (c 1, AcOH) −10.7 (c 0.87,
AcOH)
Boc-Lys(2-ClZ)
1.5
94 202–204
203–204
Boc-His(Z)
Boc-His(Bom)
Fmoc-Gly
Fmoc-Phe
Z-Phe-Leu-
OC H -t
Boc-His
Boc-His
Fmoc-Gly
Fmoc-Phe
Phe-Leu-OC H -t
1.0
2.0
24
24
0.75
88 192–196
89 192–196
191–195
191–195
–
–
–
+23.5 (c 1, CH OH) +24 (c 1, CH OH) 15
+23.5 (c 1, CH OH) +24 (c 1, CH OH) 15
3 3
3
3
b
b
100
100
–
–
–
–
Oil
–
–
–
–
–
16
90 Oil
4
9
4
9
Boc-Gly-Val-
Gly-Val-Pro-
OBzl
Boc-Gly-Val-Gly-Val-Pro 0.5
93 126–127
127
–
−63.9 (c 1, CH OH) −64.0 (c 1, CH OH) 17
3
3
Boc-Arg(NO )-
Boc-Arg-Pro-Arg-Proc
2.0
90 70–71
89 82–83
−85.0 (c 1, CH OH) –
–
–
2
3
Pro-Arg(NO )-
2
Pro-OBzl
Boc-D
-Lys(2-ClZ)- Boc-
D
-Lys-Pro-Arg-Proc 2.5
–
−46.1 (c 1, CH OH) –
3
Pro-Arg(NO )-
2
Pro-OBzl
a
b
c
Refers to the actual isolated yields; no attempts were made to optimize.
Starting material was recovered.
Satisfactory elementary and amino acid analyses were obtained.
spot with ninhydrin, which sometimes masks the spots
due to the deprotected peptide. In such cases a better
way to monitor the completion of the reaction is to
look for the absence of the spot due to the protected
deprotection. These included the excess of donor
required, the catalyst, solvent, concentration and reac-
tion temperature. An excess of 2–4 equiv. of
hydrazinium monoformate (per protecting group) was
found to be ideal. We observed the optimal ratio of
catalyst to substrate to be 1:1 by weight for each
protecting group to be removed and methanol was the
most effective solvent of choice. The deprotection pro-
ceeds smoothly when the concentrations of amino acid
or peptide substrate are in the range of 0.05–0.25
mmol/mL and at an optimum temperature of ꢀ25°C.
peptide as this is more easily done. The Arg(NO ) is
2
smoothly deblocked into Arg by this treatment. Under
these experimental conditions, the tert-butyloxycar-
bonyl and 9-fluorenylmethyloxycarbonyl groups appear
to be quite stable, as indicated by 90–98% yields of Boc
and Fmoc protected amino acid and peptide derivatives
and the absence of free amino acids, as revealed by the
TLC of the crude products.
It was observed that this method is equally competitive
with other methods of deblocking most of the com-
monly used protecting groups in peptide synthesis. The
advantages of transfer hydrogenation over conventional
hydrogenation have been described. Further,
hydrazinium monoformate is a more effective hydrogen
donor than hydrazine or formic acid and hydrogenoly-
sis can be carried out at room temperature without
The results given in Table 1 demonstrate the feasibility
of using Mg/NH NH ·HCOOH for the removal of
2
2
most of the commonly used protecting groups in pep-
tide synthesis. All the products were characterized by
comparison of their TLC, melting points and specific
rotation with authentic samples. Studies were also car-
ried out to determine the optimum conditions for

D. Channe Gowda / Tetrahedron Letters 43 (2002) 311–313
313
removing any acid sensitive protecting groups like
tert-butyloxycarbonyl, whereas hydrazine requires a
temperature of 50°C and formic acid may deblock the
acid-sensitive tert-butyloxycarbonyl group. An addi-
tional advantage of using hydrazinium monoformate
as the hydrogen donor is its miscibility with most
organic solvents (including methanol) used in peptide
synthesis. The protecting groups viz. 2-ClZ, BrZ, Bom
or OcHx, which are removed by HF treatment at
present, can be conveniently removed by this method.
It was also observed that hydrazinium diformate,
which is a colorless solid, does not possess any hydro-
gen donor properties with Mg. The use of Mg/NH2–
Hydrogenolysis of protected amino acids and peptides
with Mg/NH NH ·HCOOH
2
2
General procedure: To a stirred solution of the appro-
priately protected amino acid derivative or peptide (200
mg) and Mg (200 mg) in methanol (2.5 mL),
hydrazinium monoformate (4 equiv.) was added. The
resulting reaction mixture was stirred at room tempera-
ture. After completion of the hydrogenolysis (moni-
tored by TLC), the mixture was filtered through Celite
and washed with methanol. The combined washings
and filtrate were evaporated; the residue was taken up
in chloroform and washed with 50% saturated NaCl
solution to remove hydrazinium monoformate. The
solvent was removed under reduced pressure and tritu-
rated with di ethyl ether to obtain Boc-protected amino
acid or peptide derivatives. The yields, duration and
physical constants are given in Table 1.
NH ·HCOOH makes this a rapid, low-cost alternative
2
to expensive palladium catalysts and reduces the work-
up to a simple filtration and extraction operation. To
the best of my knowledge this is the first report of
using hydrazinium monoformate as a hydrogen donor
and Mg as the catalyst for the removal of
hydrogenolysable protecting groups in peptide synthe-
sis. Further investigations of other useful applications
particularly in solid phase peptide synthesis are in
progress.
References
1
2
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(
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3
3
HOAc (90:10:3) and CHCl –MeOH–HOAc (85:15:3).
3
12. Bodanszky, M.; Bodanszky, A. In The Practice of Peptide
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tions were determined on a Perkin–Elmer 241MC
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4
, 347–371.
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tralizing slowly, equal moles of hydrazine and formic
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as such in all reactions.
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