Organophosphorus esters of 1-hydroxy-2-phenylbenzimidazole: Synthesis and utilization as novel peptide coupling reagents

Article abstract of DOI:10.1055/s-2007-965918

Highly efficient coupling reagents - phosphoric acid diethyl ester 2-phenylbenzimidazol-1-yl ester, diphenylphosphinic acid 2-phenylbenzimidazol-1- yl ester and phosphoric acid diphenyl ester 2-phenylbenzimidazol-1-yl ester - were designed, synthesized and successfully applied in peptide coupling reactions. Their efficiency was evaluated through the synthesis of a range of amides and peptides, and the extent of racemization was studied by HPLC and found to be negligible. The mechanism of action is probably similar to those found for organophoshorus esters of hydroxybenzotriazole: a presumption supported by the isolation and characterization by mass spectrometry and ¹H NMR of some of the proposed intermediates. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-965918

766
PAPER
Organophosphorus Esters of 1-Hydroxy-2-phenylbenzimidazole:
Synthesis and Utilization as Novel Peptide Coupling Reagents
O
rganophosphorus
a
Esters of
1
g
-Hydroxy-2-
n
phenyl-benz
n
imidazole ath D. Kokare,a,b Rahul R. Nagawade,a,b Vipul P. Rane,a,b Devanand B. Shinde*a
a
Department of Chemical Technology, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431004 (MS), India
b
Wockhardt Research Centre, New Drug Discovery, Aurangabad 431210 (MS), India
E-mail: dbschemtech@hotmail.com
Received 19 October 2006; revised 1 December 2006
phosphinyloxy-2-oxopyridine (6),14 BOP 7a15 and AOP
7b16 (Figure 1). During peptide coupling via uronium and
phosphonium salts, the N-protected amino carboxylic
acid first reacts with the coupling reagent to give an active
Abstract: Highly efficient coupling reagents – phosphoric acid di-
ethyl ester 2-phenylbenzimidazol-1-yl ester, diphenylphosphinic
acid 2-phenylbenzimidazol-1-yl ester and phosphoric acid diphenyl
ester 2-phenylbenzimidazol-1-yl ester – were designed, synthesized
and successfully applied in peptide coupling reactions. Their effi- ester, which then reacts with an amino component to give
ciency was evaluated through the synthesis of a range of amides and
the corresponding amide. The latter step is rate-limiting
peptides, and the extent of racemization was studied by HPLC and
and responsible for loss of configuration. The inherent re-
found to be negligible. The mechanism of action is probably similar
activity of the active ester intermediate is therefore a crit-
to those found for organophoshorus esters of hydroxybenzotriazole:
ical aspect of particular uronium or phosphonium salts. In
a presumption supported by the isolation and characterization by
mass spectrometry and 1H NMR of some of the proposed interme-
diates.
connection with our ongoing research, we have designed
and synthesized the organophosphorus esters of substitut-
ed benzimidazoles and demonstrated their ability to act as
amide and peptide coupling reagents with minimal racem-
ization in short reaction times.
Key words: N-hydroxy-2-phenylbenzimidazole, peptide coupling
reagents, HOBt, amide synthesis, minimal racemization
1-Hydroxy-2-phenylbenzimidazole (8) was synthesized
as a white crystalline solid by coupling ortho-nitroaniline
with benzyl bromide using sodium hydride as base,
followed by benzyl deprotection using 10% Pd/C
(Scheme 1).17 The coupling reagents phosphoric acid di-
ethyl ester 2-phenylbenzimidazol-1-yl ester (9a), diphe-
nylphosphinic acid 2-phenylbenzimidazol-1-yl ester (9b)
and phosphoric acid diphenyl ester 2-phenylbenzoimid-
azol-1-yl ester (9c) were synthesized by reaction of 8 with
diethyl chlorophosphate, diphenylphosphorochloridate or
diphenylphosphinic chloride, respectively, using triethyl-
amine base in dichloromethane. The yields obtained for
reagents 9a, 9b and 9c were 67%, 71% and 63% respec-
tively. To avoid the problem of unsatisfactory yield, a
method was developed in which the reagents were synthe-
sized and used in situ for the coupling reactions.
A commonly encountered problem in peptide synthesis
involves an extensive racemization of the amino acid
component, usually through oxazolone formation, unde-
sired side reactions and low coupling rates.1–3 Thus a com-
bination of high efficiency during the formation of the
amide bond, together with a low level of racemization of
the coupled components, represent the main objectives in
the development of new coupling reagents. Over the last
few years, a number of new coupling reagents have been
reported in the literature that have shown considerably im-
proved coupling ability over more traditional methods in-
volving carbodiimide. Generally, in order to prevent the
loss of configuration, additives such as 1-hydroxybenzo-
triazole (1a; HOBt)4 or 7-aza-1-hydroxybenzotriazole (1b;
HOAt)5 have been used. Such reagents have also been
used in many protocols for stepwise peptide assembly, ei-
ther in conjunction with carbodiimide6 or active esters,7 or
through incorporation into stand-alone reagents such as
phosphonium8 or uronium/guanidinium salts.9 Currently
preferred coupling reagents include organophosphorus
derivatives of HOBt and 3,4-dihydro-4-oxo-1,2,3-benzo-
triazine (HODht), which includes diethoxyphosphinyloxy-
benzotriazole (2; DepOBt),10 3-(diethoxyphosphinyloxy)-
3,4-dihydro-4-oxo-1,2,3-benzotriazine (3; DepODht),11
3-O-(2-oxo-1,3,2-dioxaphosphorinanyl)-3,4-dihydro-4-
oxo-1,2,3-benzotriazine (4; DopODht),12 1,2-benzisox-
azolyl diphenyl phosphate,13 3-phenoxyphosphinyloxy-
3,4-dihydro-4-oxo-1,3-quinazoline (5),13 1-diphenoxy-
To standardize the reaction conditions for the coupling re-
actions, N-benzylbenzamide was synthesized by the reac-
tion of benzoic acid with benzylamine using the coupling
reagent 9a in the presence of different solvents and bases.
The results obtained (Table 1) showed that diisopropyl-
ethylamine (DIPEA) was the optimal base, in combina-
tion with dimethylformamide (DMF) as solvent.
Using the standardized reaction conditions, the scope of
these reagents was examined through the synthesis of 27
different amides (Table 2), all of which were character-
1
ized by mass spectrometry and H NMR spectroscopy.
The use of these reagents proved to be superior to existing
methods in respect to yield, reaction time and ease of pu-
rification. Coupling of carboxylic acids with sterically
hindered amines such as isopropylamine and tert-butyl-
amine also gave improved yields (Table 2, entries 22–
SYNTHESIS 2007, No. 5, pp 0766–0772
x
x
.
x
x
.2
0
0
7
Advanced online publication: 08.02.2007
DOI: 10.1055/s-2007-965918; Art ID: Z21206SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Organophosphorus Esters of 1-Hydroxy-2-phenyl-benzimidazole
767
N
N
N
N
O
N
O
N
O
O
O
N
N
O
P
OEt
O P
N
N
N
N
X
O
OEt
O
OH
O
P
OEt
EtO
1a X = CH, HOBt
1b X = N, HOAt
2
3
4
O
N
O
N
N
O
P
N
OPh
–
+
O
PF6
N
NMe2
X
O
O
P
O
P
NMe2
Me2N
OPh
PhO
OPh
5
6
7a X = CH, BOP
7b X = N, AOP
Figure 1 Some organophosphorus esters reported in the literature
NO2
Table 1 Synthesis of N-Benzylbenzamide: Effect of Solvent and
Base on the Coupling Reagent 9a
N
b
a
N
NH2
Solvent
MeCN
MeCN
DCM
DCM
DMF
DMF
DMF
THF
Base
Time (h)
2.5
2.5
2
Yield (%)
O
Bn
Et3N
64
53
71
77
81
96
68
57
61
8a
DIPEA
Et3N
N
N
N
c
N
O
R
O
DIEA
Et3N
1.5
1
OH
P
R
8b
9a R = OEt
9b R = OPh
9c R = Ph
DIPEA
Lutidine
Et3N
0.75
1.25
3.5
3
Scheme 1 Reagents and conditions: (a) NaH, BnBr, 70 °C, 3 h; (b)
H2, Pd/C, MeOH, 15 min; (c) R2POCl, Et3N, CH2Cl2, 0 °C.
THF
DIPEA
27).18 The workup for these reactions was also simplified
since acid-base treatment followed by trituration with an
appropriate solvent (hexane or diethyl ether) directly gave
the pure compound. Furthermore, the N-hydroxy-2-phe-
nylbenzimidazole starting material could be recovered
from the acidic aqueous extract and re-used for the syn-
thesis of reagents.
improvement on existing methods.7a,18,19 In particular, the
coupling of sterically hindered amino acids such as tert-
butoxycarbonyl-protected a-aminoisobutyric acid (Boc-
Aib-OH) with Val-OMe·HCl gave improved yields of
72%, 67% and 71% for reagents 9a, 9b and 9c, respective-
ly (Table 3, entries 10, 11 and 12).19a The extent of race-
mization (DL%) was examined by HPLC and found to be
in the range of 0.4–2.7%, with the lowest being observed
in the synthesis of the Cbz-Val-Val-OMe peptide using
reagent 9b (0.4%; Table 3, entry 2). The optical rotations
of all the synthesized peptides were measured and found
to be in agreement with those reported in the literature.
Young’s test20 was performed for the coupling of Bz-Leu-
OH and Gly-OEt·HCl (Table 3, entries 4, 5 and 6). In the
case of reagent 9b (Table 3, entry 5), this test revealed that
only 1.6% racemization had taken place, which is signifi-
cantly lower than reported methods using HOBt-based
uronium salts [BOP (20%), PyBOP (15%) or HBTU
(12.7%)].21,22 Furthermore, Anteunis tests23 showed that
the coupling of Cbz-Gly-Phe-OH and Val-OMe·HCl
(Table 3, entries 13, 14 and 15) gave diastereomeric ratios
of 10:1, 10:1 and 9.05:0.5 for reagents 9a, 9b and 9c, re-
As can be seen in Table 2, acids with electron-withdraw-
ing substituents required less reaction time for coupling
with amines (Table 2, entries 4, 5 and 6) as compared to
acids with electron-donating substituents (Table 2, entries
10, 11 and 12). A comparative study of yields shows that
reagents 9a and 9b gave slightly higher yields than 9c
(Table 2, cf entries 1, 2, 3 and 10, 11, 12). In some cases,
however, reagent 9c gave higher yields than either 9a or
9b (Table 2, entries 7, 8 and 9).
The applicability of 9a, 9b and 9c as peptide coupling re-
agents was also examined. The reaction conditions were
standardized with the coupling reaction of Cbz-Val-OH
and Val-OMe·HCl using reagents 9a, 9b and 9c. These re-
sults, together with those obtained by applying the opti-
mized protocol to a range of other peptides, are
summarized in Table 3. The yields obtained constitute an
Synthesis 2007, No. 5, 766–772 © Thieme Stuttgart · New York

768
N. D. Kokare et al.
PAPER
Table 2 Synthesis of Amides Using Coupling Reagents 9a, 9b and 9c
Entry Acid
Amine
Amide
Reagent Time (min) Yield (%)
1
2
Benzoic acid (10)
Benzylamine (11)
N-Benzylbenzamide (12)
9a
9b
9c
9a
9b
9c
9a
9b
9c
9a
9b
9c
9a
9b
9c
9a
9b
9c
45
40
45
20
20
20
35
35
30
55
50
55
35
35
40
45
30
40
30
30
30
45
45
50
70
60
60
96
95
91
93
91
95
92
92
94
97
95
88
94
91
93
96
96
91
95
94
94
91
93
88
90
91
84
10
11
12
3
10
11
12
4
4-Nitrobenzoic acid (13)
11
4-Nitro-N-benzylbenzamide (14)
5
13
11
14
6
13
11
14
7
4-Bromobenzoic acid (15)
11
4-Bromo-N-benzylbenzamide (16)
8
15
11
16
9
15
11
16
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Anisic acid (17)
11
4-Methoxy-N-benzylbenzamide (18)
17
11
18
17
11
18
Cinnamic acid (19)
11
N-Benzyl cinnamate (20)
19
19
10
10
10
10
10
10
10
10
10
10
10
10
11
20
11
20
Cyclopropylamine (21)
N-Cyclopropylbenzamide (22)
21
22
22
21
Piperidine (23)
Phenyl (piperidin-1-yl) methanone (24) 9a
23
24
9b
9c
9a
9b
9c
9a
9b
9c
23
24
Isopropylamine (25)
N-Isopropylbenzamide (26)
25
26
25
26
tert-Butylamine (27)
N-tert-Butylbenzamide (28)
27
27
28
28
spectively. These results were confirmed by HPLC, which additives such as HOBt, HOAt and HODht were found to
showed 1.2, 0.9 and 1.7 DL percentages using the reagents be in the range of 3.3–24.6% (Table 4).24 The reagents 9a,
9a, 9b and 9c, respectively. Considering the reaction 9b and 9c gave the desired compound in comparatively
times, yields and ease of workup, the application of these better yields and lower levels of racemization (1.1–2.4%).
reagents thus constitutes an improved method for amide
During peptide coupling via uronium/guanidinium or
and peptide synthesis.
phosphonium salts, the N-protected carboxylic acid first
A comparative study of the three coupling reagents devel- reacts with the coupling reagent to form an active ester,
oped here, together with other commonly used systems, which subsequently reacts with the amino component to
was performed on the coupling reaction of Z-Phg-OH give the corresponding amide. Regarding the coupling re-
with H-Pro-NH2 in DMF, giving the peptide Z-Phg-Pro- agents developed here, when the reagents 9a, 9b and 9c
NH2 (34). The extent of racemization using coupling re- were reacted with the appropriate acid (benzoic acid/Cbz-
agents N,N′-dicyclohexylcarbodiimide (DCC) and N-eth- Val-OH) in cooled DMF, using diisopropylethylamine as
yl-N′-(3-dimethylaminopropyl)carbodiimide (EDC) with base for 10 min (Scheme 2), the active esters (35 for ben-
Synthesis 2007, No. 5, 766–772 © Thieme Stuttgart · New York

PAPER
Organophosphorus Esters of 1-Hydroxy-2-phenyl-benzimidazole
769
Table 3 Synthesis of Peptides Using Reagents 9a–c
Entry
1
Peptide product
Reagent
9a
Time (h)
Yielda (%)
89
Racemizationb (DL%) [a]D25 (c, solvent)
–28.7 (1, EtOH)c
Cbz-Val-Val-OMe (29)
3
1.1
0.4
0.7
1.8
1.6
2.7
2.1
0.8
1.9
–
2
29
9b
9c
3
92
–26.2 (1, EtOH)c
–26.8 (1, EtOH)c
–32.3 (3.1, EtOH)d
–33.8 (3.1, EtOH)d
–31.2 (3.1, EtOH)d
–19.8 (1, EtOH)
–20.4 (1,EtOH)
–21.1 (1,EtOH)
+21.7 (1, EtOH)e
+20.4 (1, EtOH)e
+20.9 (1, EtOH)e
+15.8 (1, EtOH)
+15.2 (1, EtOH)
+16.4 (1, EtOH)
3
29
2.5
3
91
4
Bz-Leu-Gly-OEt (30)
9a
85
5
30
9b
9c
3.5
3.5
2.5
3
78
6
30
84
7
Cbz-Leu-Gly-OEt (31)
9a
81
8
31
9b
9c
67
9
31
3
74
10
11
12
13
14
15
Boc-Aib-Val-OMe (32)
9a
4
72
32
9b
9c
4
67
–
32
4
71
–
Cbz-Gly-Phe-Val-OMe (33)f
9a
3.5
3.5
3.5
76
1.2
0.9
1.7
33f
33f
9b
9c
72
69
a Isolated yields, based upon the starting amino acid, after column chromatography.
b
DL% equal to % of D-isomer multiplied by two.
c Lit.8b [a]D20 –28 (c 1, EtOH).
d Lit.19 [a]D20 –34 (c 3.1, EtOH). Young’s test; epimerization: 2.7% (entry 4), 1.6% (entry 5), 4.6% (entry 6).
e Lit.23 [a]D20 –19.5 (c 1, EtOH).
f Anteunis test23 (1H NMR, 400 MHz); diastereomeric ratios: 10:1 (entries 13 and 14), 9.5:0.5 (entry 15).
zoic acid and 36 for Cbz-Val-OH) could be isolated and diphenyl ester 2-phenylbenzimidazol-1-yl ester (9c) have
1
characterized by mass and H NMR. These results indi- been easily prepared from N-hydroxy-2-phenylbenzimid-
cate that the reaction pathway for these reagents should be azole and used in situ for coupling reactions to synthesize
similar to those of the organophoshorus derivatives of amides and peptides. The advantages of these peptide
HOBt and HOAt.
coupling reagents are their scalability, minimal levels of
racemization, higher yields and use of readily available,
inexpensive starting materials for the synthesis of re-
agents. Another promising feature of these reagents is
In conclusion, phosphoric acid diethyl ester 2-phenyl-
benzimidazol-1-yl ester (9a), diphenylphosphinic acid 2-
phenylbenzimidazol-1-yl ester (9b) and phosphoric acid
O
O
O
RNH2
R
9a/9b/9c
N
N
OH
N
H
O
+
8b
35
O
O
H
N
O
H
N
R
H
N
RNH2
9a/9b/9c
Cbz
N
H
8b
+
Cbz
OH
N
N
Cbz
O
36
Scheme 2 Isolation of active ester intermediates 35 and 36
Synthesis 2007, No. 5, 766–772 © Thieme Stuttgart · New York

770
N. D. Kokare et al.
PAPER
Anal. Calcd for C20H16N2O: C, 79.98; H, 5.37; N, 9.33. Found: C,
80.03; H, 5.41; N, 9.31.
that, after completion of coupling reaction, the by-product
N-hydroxy-2-phenyl benzimidazole could be easily sepa-
rated by acid-base treatment and re-used for the synthesis
of reagents 9a, 9b and 9c. Furthermore, these reagents
give improved yields for coupling of sterically hindered
amino acids such as aminoisobutyric acid with acid-
protected amines. In summary, the reagents 9a, 9b and 9c
have been found to be versatile reagents for the synthesis
of amides and peptides, which complement and extend
previously reported methods.
N-Hydroxy-2-phenylbenzimidazole (8b)
N-Hydroxybenzyl-2-phenylbenzimidazole (8a; 5.2 g 0.17 mol) was
dissolved in MeOH (50 mL), Pd/C (10%, 500 mg) was added and
the mixture was stirred under a H2 atmosphere at r.t. for 15 min. The
reaction mixture was filtered through high-flow celite and the fil-
trate was concentrated and purified by column chromatography
(10% MeOH–CHCl3) to yield the title compound as a white solid.
Yield: 3.55 g (98%).
1H NMR (400 MHz, DMSO-d6): d = 7.2–7.4 (m, 2 H), 7.6–7.8 (m,
5 H), 8.2 (d, J = 8.3 Hz, 2 H), 12.2 (s, 1 H).
Table 4 Effect of Coupling Reagents on the Yield and Extent of
Racemization in the Synthesis of Z-Phg-Pro-NH2 (34) in DMF24
MS (EI, 70 eV): m/z = 211 [M + H]+.
Anal. Calcd for C13H10N2O: C, 74.27; H, 4.79; N, 13.32. Found: C,
73.82; H, 4.71; N, 13.29.
Coupling reagents
DCC/HOBt
EDC/HOBt
DCC/HOAt
EDC/HOAt
DCC/HODht
EDC/HODht
9a
Yield (%)
89.2
81.9
83
DL%
9.3
1-[(Diethoxyphosphoryl)oxy]-2-phenyl-1H-benzimidazole (9a);
Typical Procedure
10.1
3.3
A solution of N-hydroxy-2-phenylbenzimidazole (8b; 0.420 g, 0.02
mol) and Et3N (0.695 mL, 0.05 mol) was stirred in CH2Cl2 (2.5 mL)
and diethyl chlorophosphate (0.212 mL, 0.024 mol) was added at
0 °C. The mixture was stirred until the reaction was complete (mon-
itored by TLC) then the solvent was evaporated and the residue was
stirred with pentane (20 mL) and decanted. The remaining residue
was stirred with Et2O (2 × 15 mL) and decanted three times. The
combined Et2O extracts were concentrated to yield the title com-
pound.
81.4
89.2
80.4
91
7.0
16.7
24.6
1.7
9b
88
1.1
Yield: 462 mg (67%); colorless solid; mp 112–116 °C (dec).
IR (Nujol): 2985, 1298, 1036, 981 cm–1.
9c
93
2.4
1H NMR (400 MHz, CDCl3): d = 1.2 (t, J = 6.2 Hz, 6 H) 4.1 (q, J =
7.4 Hz, 4 H), 7.3 (m, J = 8.1 Hz, 3 H), 7.5 (d, J = 8.1 Hz, 2 H), 7.8
(d, J = 8.2 Hz, 2 H), 8.2 (d, J = 8.2 Hz, 2 H).
The amino acid derivatives were obtained from commercial sources
or synthesized according to the literature. Melting points were de-
MS (EI, 70 eV): m/z = 347 [M + H]+.
1
termined in capillary tubes and are uncorrected. H NMR spectra
Anal. Calcd for C17H19N2O4P: C, 58.96; H, 5.53; N, 8.06. Found: C,
59.13; H, 5.59; N, 8.11.
were recorded on a 400 MHz Varian Gemini spectrometer and are
reported as parts per million (ppm) downfield from TMS internal
standard. Mass spectra were taken with Micromass - QUATTRO-II
of WATER mass spectrometer. The optical rotations were deter-
mined using a Rudolph degipole automatic polarimeter. HPLC was
performed using a Zorbax SB-C18 reverse phase column (0.46 × 25
cm) on Shimadzu instrument equipped with an automatic injector
and a UV-PDA detector (215 nm). The mobile phase consisted of
0.05% TFA and MeCN–H2O (1:1). The products were eluted iso-
cratically at a flow rate of 1 mL/min. Flash column chromatography
was performed with 300–400 mesh silica gel and analytical TLC
was performed on precoated silica gel plates (60F254) with the elu-
tion systems indicated. Solvents and reagents were purified by stan-
dard methods as necessary. Amino acids are of the L-configuration
if not otherwise mentioned.
1-[(Diphenoxyphosphoryl)oxy]-2-phenyl-1H-benzimidazole
(9b)
Yield: 624 mg (71%); white solid; mp 93–95 °C (dec).
IR (Nujol): 2930, 1244, 1105, 1018 cm–1.
1H NMR (400 MHz, CDCl3): d = 7.2–7.4 (m, 13 H), 7.6 (d, J = 8.0
Hz, 2 H), 7.8 (d, J = 8.2 Hz, 2 H), 8.2 (d, J = 8.2 Hz, 2 H).
MS (EI, 70 eV): m/z = 443 [M + H]+.
Anal. Calcd for C25H19N2O4P: C, 67.87; H, 4.33; N, 6.33. Found: C,
67.81; H, 4.41; N, 6.30.
1-[(Diphenylphosphoryl)oxy]-2-phenyl-1H-benzimidazole (9c)
Yield: 516 mg (63%); white solid; mp 131–133 °C (dec).
IR (Nujol): 3016, 1165, 1045, 993 cm–1.
N-Hydroxybenzyl-2-phenylbenzimidazole (8a)
NaH (1.94 g, 0.048 mol) was suspended in THF (30 mL) and o-ni-
troaniline (2.68 g, 0.02 mol) was added portion-wise with cooling.
After 15 min, benzyl bromide (5.8 g, 0.05 mol) was added slowly
and the mixture was heated at 80 °C for 4 h. The reaction mixture
was cooled to r.t., quenched with H2O (25 mL) and extracted into
EtOAc (2 × 15 mL). The organic layer was dried (Na2SO4) and con-
centrated under vacuum. The product was isolated by trituration
with hexane and filtration. Off-white solid; yield: 5.67 g (97%).
1H NMR (400 MHz, CDCl3): d = 7.1–7.4 (m, 13 H) 7.6 (d, J = 8.1
Hz, 2 H), 7.7 (d, J = 8.2 Hz, 2 H), 8.2 (d, J = 8.2 Hz, 2 H).
MS (EI, 70 eV): m/z = 411 [M + H]+.
Anal. Calcd for C25H19N2O2P: C, 73.16; H, 4.67; N, 6.83. Found: C,
72.78; H, 4.71; N, 6.86.
Synthesis of Amides and Peptides; General Procedure
1H NMR (400 MHz, CDCl3): d = 5.1 (s, 2 H), 7.2–7.4 (m, 5 H), 7.6–
7.8 (m, 5 H), 7.8 (d, J = 8.2 Hz, 2 H), 8.2 (d, J = 8.2 Hz, 2 H).
A solution of N-hydroxy-2-phenylbenzimidazole (8b; 420 g, 0.02
mol) and DIPEA (0.83 mL, 0.05 mol) was stirred in DMF (2.5 mL)
and diethyl chlorophosphate (0.212 mL, 0.024 mol) was added with
cooling. The reaction mixture was stirred for 10 min then the acid
MS (EI, 70 eV): m/z = 301 [M + H]+.
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Organophosphorus Esters of 1-Hydroxy-2-phenyl-benzimidazole
771
component (0.016 mol) was added and the reaction was stirred for
a further 10 min at 0 °C. The amine component (0.03 mol) was add-
ed and the mixture was stirred at r.t. until the reaction was complete
(monitored by TLC). Sat. aq NaCl (20 mL) was added and the mix-
ture was extracted with EtOAc (2 × 15 mL). The organic layer was
washed with HCl (2 N, 15 mL), sat. aq NaHCO3 (15 mL) and H2O
(2 × 20 mL), dried (Na2SO4) and the solvent removed to give the
corresponding amide or peptide.
Phenyl (Piperidin-1-yl) Methanone (24)
White solid; mp 167–168 °C.
1H NMR (400 MHz, CDCl3): d = 1.15–1.30 (m, 3 H), 1.40 (m, 2 H),
1.62 (m, 1 H), 2.0 (dd, J = 7.1, 5.4 Hz, 2 H), 4.0 (m, 1 H), 7.4 (m,
3 H), 7.8 (d, J = 8.2 Hz, 2 H).
MS (EI, 70 eV): m/z = 190 [M + H]+.
Anal. Calcd for C12H15NO: C, 76.16; H, 7.99; N, 7.40. Found: C,
76.21; H, 8.04; N, 7.39.
N-Benzylbenzamide (12)
White solid; mp 104–106 °C.
N-Isopropylbenzamide (26)
White solid; mp 137–138 °C.
1H NMR (400 MHz, CDCl3): d = 6.8 (d, J = 5.6 Hz, 2 H), 6.4 (br s,
1 H), 7.35–7.6 (m, 8 H), 7.8 (d, J = 8.1 Hz, 2 H).
1H NMR (400 MHz, CDCl3): d = 1.02 (d, J = 3.6 Hz, 6 H), 4.3 (m,
1 H), 6.0 (s, 1 H), 7.4 (m, 3 H), 7.7 (d, J = 6.8 Hz, 2 H).
MS (EI, 70 eV): m/z = 212 [M + H]+.
MS (EI, 70 eV): m/z = 164 [M + H]+.
Anal. Calcd for C14H13NO: C, 79.59; H, 6.20; N, 6.63. Found: C,
78.93; H, 6.23; N, 6.58.
Anal. Calcd for C10H13NO: C, 73.59; H, 8.03; N, 8.58. Found: C,
73.61; H, 8.11; N, 8.57.
4-Nitro-N-benzylbenzamide (14)
Off-white solid; mp 138–139 °C.
N-tert-Butylbenzamide (28)
White solid; mp 151–153 °C.
1H NMR (400 MHz, CDCl3): d = 4.6 (d,J = 5.9 Hz, 2 H), 6.55 (s,
1 H), 7.35 (m, 5 H), 7.7 (d, J = 6.8 Hz, 2 H), 8.3 (d, J = 6.8 Hz, 2 H).
1H NMR (400 MHz, CDCl3): d = 1.4 (s, 9 H), 6.0 (s, 1 H), 7.4 (m,
3 H), 7.7 (d, J = 5.9 Hz, 2 H).
MS (EI, 70 eV): m/z = 257 [M + H]+.
MS (EI, 70 eV): m/z = 178 [M + H]+.
Anal. Calcd for C14H12N2O3: C, 65.62; H, 4.72; N, 10.93. Found: C,
65.48; H, 4.68; N, 10.91.
Anal. Calcd for C11H15NO: C, 74.54; H, 7.90; N, 9.03. Found: C,
75.26; H, 7.94; N, 9.02.
4-Bromo-N-benzylbenzamide (16)
White solid; mp 117–119 °C.
Cbz-Val-Val-OMe (29)
White solid; mp 110–111 °C.
1H NMR (400 MHz, CDCl3): d = 4.6 (d, J = 5.5 Hz, 2 H), 6.45 (s,
1 H), 7.28 (m, 5 H), 7.55 (d, J = 8.1 Hz, 2 H), 7.64 (d, J = 8.1 Hz,
2 H).
1H NMR (400 MHz, CDCl3): d = 0.87–0.98 (m, 12 H), 2.04–2.19
(m, 2 H), 3.73 (s, 3 H), 4.0–4.10 (m, 1 H), 4.55 (dd, J = 8.5, 4.9 Hz,
1 H), 5.12 (s, 2 H), 5.41 (d, J = 8.5 Hz, 1 H), 6.44 (d, J = 8.0 Hz,
1 H), 7.34 (m, 5 H).
MS (EI, 70 eV): m/z = 291 [M + H]+.
Anal. Calcd for C14H12BrNO: C, 57.95; H, 4.17; N, 4.83. Found: C,
58.37; H, 4.21; N, 4.89.
MS (EI, 70 eV): m/z = 365 [M + H]+.
Anal. Calcd for C19H28N2O5: C, 62.62; H, 7.74; N, 7.69. Found: C,
62.65; H, 7.79; N, 7.67.
4-Methoxy-N-benzylbenzamide (18)
White solid; mp 128–129 °C.
1H NMR (400 MHz, CDCl3): d = 3.8 (s, 3 H), 4.62 (d, J = 5.6 Hz,
2 H), 6.38 (s, 1 H), 6.9 (d, J = 8.2 Hz, 2 H), 7.32 (m, 5 H), 7.78 (d,
J = 8.2 Hz, 2 H).
Bz-Leu-Gly-OEt (30)
White solid; mp 150–151 °C.
1H NMR (400 MHz, CDCl3): d = 0.9 (d, J = 6.2 Hz, 6 H), 1.25 (t,
J = 7.3 Hz, 3 H), 1.58–1.6 (m, 1 H), 1.7 (m, 2 H), 4.0 (d, J = 5.8 Hz,
2 H), 4.25 (q, J = 7.3 Hz, 2 H), 5.1 (s, 1 H), 5.5 (br s, 1 H), 7.25 (m,
5 H).
MS (EI, 70 eV): m/z = 242 [M + H]+.
Anal. Calcd for C15H15NO2: C, 76.67; H, 6.27; N, 5.80. Found: C,
76.61; H, 6.23; N, 5.81.
MS (EI, 70 eV): m/z = 321 [M + H]+.
N-Benzyl Cinnamate (20)
White solid; mp 69–70 °C.
Anal. Calcd for C17H24N2O4: C, 63.73; H, 7.55; N, 8.74. Found: C,
62.01; H, 7.61; N, 8.67.
1H NMR (400 MHz, CDCl3): d = 4.55 (d, J = 5.2 Hz, 2 H), 5.9 (s,
1 H), 6.4 (d, J = 8.2 Hz, 1 H), 7.4 (m, 5 H), 7.8 (d, J = 8.2 Hz, 2 H).
Cbz-Leu-Gly-OEt (31)
White solid; mp 141–143 °C.
MS (EI, 70 eV): m/z = 238 [M + H]+.
1H NMR (400 MHz, CDCl3): d = 0.9 (d, J = 6.1 Hz, 6 H), 1.25 (t,
J = 7.3 Hz, 3 H), 1.58–1.6 (m, 1 H), 1.7 (m, 2 H), 4.0 (d, J = 7.6 Hz,
2 H), 4.25 (q, J = 7.6 Hz, 2 H), 5.12 (m, 2 H), 5.5 (br s, 1 H), 7.25
(m, 5 H).
Anal. Calcd for C16H15NO: C, 80.98; H, 6.37; N, 5.90. Found: C,
80.12; H, 6.39; N, 6.02.
N-Cyclopropylbenzamide (22)
White solid; mp 153–154 °C.
MS (EI, 70 eV): m/z = 351 [M + H]+.
1H NMR (400 MHz, CDCl3): d = 0.66 (dd, J = 3.7 Hz, 2 H), 0.85
(dd, J = 3.7, 5.2 Hz, 2 H), 2.8 (m, 1 H), 7.3 (m, 3 H), 7.8 (d, J = 5.2
Hz, 2 H).
Anal. Calcd for C18H26N2O5: C, 61.70; H, 7.48; N, 7.99. Found: C,
60.89; H, 7.51; N, 8.02.
Boc-Aib-Val-OMe (32)
White solid; mp 111–112 °C.
MS (EI, 70 eV): m/z = 146 [M + H]+.
Anal. Calcd for C10H11NO: C, 74.51; H, 6.88; N, 8.69. Found: C,
74.53; H, 6.93; N, 8.67.
1H NMR (400 MHz, CDCl3): d = 0.87–0.92 (2 × d, J = 7.0 Hz, 6 H),
1.44–1.52 (3 × s, 15 H), 2.16 (m, 1 H), 3.73 (s, 3 H), 4.52 (dd,
J = 8.5, 4.9 Hz, 1 H), 4.93, 7.20 (2 × br s, 2 H).
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N. D. Kokare et al.
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(2) (a) Albericio, F.; Carpino, L. A. Methods Enzymol. 1997,
MS (EI, 70 eV): m/z = 351 [M + H]+.
289, 104. (b) Albericio, F.; Kates, S. A. In Solid-phase
Synthesis, A Practical Guide; Kates, S. A.; Albericio, F.,
Eds.; Marcel Dekker: New York, 2000, 275. (c) Albericio,
F.; Chinchilla, R.; Dodswork, D. J.; Najera, C. Org. Prep.
Proced. Int. 2001, 33, 203.
Anal. Calcd for C15H28N2O5: C, 56.94; H, 8.92; N, 8.85. Found: C,
57.31; H, 8.96; N, 8.67.
Cbz-Gly-Phe-Val-OMe (33)
White solid; mp 123–124 °C.
(3) Albericio, F.; Boffil, J. M.; El-Faham, A.; Kates, S. A. J.
Org. Chem. 1998, 63, 9678.
1H NMR (400 MHz, CDCl3): d = 0.75 (2 × d, J = 7.0 Hz, 6 H), 1.95
(m, 1 H), 2.94 (m, 2 H), 3.57 (s, 3 H), 4.34 (dd, J = 8.0, 5.5 Hz,
1 H), 5.01 (s, 2 H), 6.86 (d, J = 8.0 Hz, 1 H), 7.04–7.26 (m, 10 H).
(4) Konig, W.; Gieger, R. Chem. Ber. 1970, 103, 788.
(5) (a) Carpino, L. A. J. Am. Chem. Soc. 1993, 115, 4397.
(b) Carpino, L. A.; El-Faham, A.; Minor, C. A.; Albericio, F.
J. Chem. Soc., Chem. Commun. 1994, 201. (c) Klose, J.; El-
Faham, A.; Henklein, P.; Carpino, L. A.; Bienert, M.
Tetrahedron Lett. 1999, 40, 2045.
MS (EI, 70 eV): m/z = 399 [M + H]+.
Anal. Calcd for C22H26N2O5: C, 66.32; H, 6.58; N, 7.06. Found: C,
65.71; H, 6.61; N, 7.08.
(6) Rich, D. H.; Singh, J. In The Peptides: Analysis, Synthesis,
Biology, Vol. 1; Gross, E.; Meienhofer, J., Eds.; Academic
Press: New York, 1979, 250.
(7) (a) Konig, W.; Gieger, R. Chem. Ber. 1973, 106, 3626.
(b) Bodanszky, M. In The Peptides: Analysis, Synthesis,
Biology, Vol. 1; Gross, E.; Meienhofer, J., Eds.; Academic
Press: New York, 1979, 143.
Cbz-Phg-Pro-NH2 (34)
White solid; mp 89–91 °C.
1H NMR (400 MHz, CDCl3): d = 2.1–2.2 (m, 4 H), 3.2 (m, 1 H), 3.7
(m, 1 H), 4.65 (m, 1 H), 5.13 (s, 2 H), 5.6 (m, 2 H), 6.25 (m, 1 H),
6.65 (m, 1 H), 7.25–7.6 (m, 10 H).
MS (EI, 70 eV): m/z = 382 [M + H]+.
(8) (a) Castro, B.; Dormoy, J. R.; Evin, G.; Selve, C.
Tetrahedron Lett. 1975, 14, 1219. (b) Coste, J.; Le-Nguyen,
D.; Castro, B. Tetrahedron Lett. 1990, 31, 205.
(9) (a) Dourtoglou, V.; Zieger, J. C.; Gross, B. Tetrahedron Lett.
1978, 15, 1269. (b) Dourtoglou, V.; Gross, B.;
Lambropoulou, V.; Zioudrou, C. Synthesis 1984, 572.
(c) Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gilleessen, D.
Tetrahedron Lett. 1989, 30, 1927.
Anal. Calcd for C21H23N3O4: C, 66.14; H, 6.04; N, 11.02. Found: C,
66.21; H, 6.08; N, 10.86.
2-Phenyl-1-[(phenylcarbonyl)oxy]-1H-benzimidazole (35)
Off-white solid; mp 234–235 °C.
1H NMR (400 MHz, CDCl3): d = 7.2 (m, 2 H), 7.4 (m, 3 H), 7.5–7.7
(m, 7 H), 8.2 (d, J = 7.8 Hz, 2 H).
(10) Kim, S.; Chang, A.; Ko, Y. K. Tetrahedron Lett. 1985, 26,
1341.
MS (EI, 70 eV): m/z = 315 [M + H]+.
Anal. Calcd for C20H14N2O2: C, 76.42; H, 4.49; N, 8.91. Found: C,
77.02; H, 4.56; N, 8.90.
(11) Fan, C.-X.; Hao, X.-L.; Ye, Y.-H. Synth. Commun. 1996, 26,
1455.
(12) Ueda, M.; Oikawa, H. J. Org. Chem. 1985, 50, 760.
(13) Horiki, K. In Peptides: Chemistry, Structures and Biology;
Rivier, J. E.; Marshall, G. R., Eds.; ESCOM: Lieden / The
Netherlands, 1990, 907.
2-Phenyl-1H-benzimidazol-1-yl N-[(Benzyloxy)carbonyl]-l-
valinate (36)
White solid; mp 163–165 °C.
1H NMR (400 MHz, CDCl3): d = 1.02 (d, J = 7.5 Hz, 6 H), 2.2 (m,
1 H), 4.1 (m, 1 H), 4.8 (s, 1 H), 5.41 (s, 2 H), 7.15–7.30 (m, 7 H),
7.5–7.7 (m, 5 H), 8.2 (d, J = 8.4 Hz, 2 H).
(14) Kim, S.; Kim, S. S. J. Chem. Soc., Chem. Commun. 1986,
719.
(15) Castro, B.; Domoy, J.-R. Tetrahedron Lett. 1973, 3243.
(16) (a) Carpino, L. A.; El-Faham, A.; Minor, C. A.; Albericio, F.
J. Chem. Soc., Chem. Commun. 1994, 201. (b) Ehrich, A.;
Rothemond, S.; Brudel, M.; Beyermann, M.; Carpino, L. A.;
Bienert, M. Tetrahedron Lett. 1993, 34, 4781.
(17) Gardiner, J. M.; Procter, J. Tetrahedron Lett. 2001, 42, 5109.
(18) Bailen, M. A.; Chinchilla, R.; Dodsworth, D. J.; Najera, C.
J. Org. Chem. 1999, 64, 8936.
MS (EI, 70 eV): m/z = 444 [M + H]+.
Anal. Calcd for C26H25N3O4: C, 70.41; H, 5.68; N, 9.47. Found: C,
69.86; H, 5.62; N, 9.52.
Acknowledgment
(19) (a) Fernando, A.; Miguel, A. B.; Rafael, C.; David, J. D.;
Carmen, N. Tetrahedron 2001, 57, 9607. (b)Carpino,L. A.;
El-Faham, A.; Albericio, F. Tetrahedron Lett. 1994, 35,
2279.
The authors are thankful to the Head of the Department of Chemical
Technology, Dr. Babasaheb Ambedkar Marathwada University,
Aurangabad 431004 (MS), India for providing laboratory facilities.
(20) Williams, M. W.; Young, G. T. J. Chem. Soc. 1963, 881.
(21) Chen, S.; Xu, J. Tetrahedron Lett. 1992, 33, 647.
(22) Coste, J.; Le-Nguyen, D.; Castro, B. Tetrahedron Lett. 1990,
31, 205.
(23) Van der Auwers, C.; Vandamme, S.; Anteunis, M. J. O.
J. Pept. Protein Res. 1987, 29, 464.
(24) Carpino, L. A.; El-Faham, A. J. Org. Chem. 1995, 60, 3561.
References
(1) Humphrey, J. M.; Chamberlin, A. R. Chem. Rev. 1997, 97,
2243.
Synthesis 2007, No. 5, 766–772 © Thieme Stuttgart · New York