The efficient preparation of di- and tripeptides by coupling N-(Cbz- or Fmoc-α-aminoacyl)benzotriazoles with unprotected amino acids

Article abstract of DOI:10.1055/s-2006-926287

N-(Cbz- or Fmoc-α-aminoacyl)benzotriazoles 2 and N-protected peptidoylbenzotriazoles 6 are coupled in aqueous acetonitrile solution with free amino acids or dipeptides to prepare: (i) 22 chirally pure dipeptides 5a-v (in an average yield of 82%) from N-(Cbz- or Fmoc-α-aminoacyl)benzotriazoles 2 and unprotected amino acids, (ii) five chiral tripeptides 7a-e (in an average yield of 75%) from N-protected peptidoylbenzotriazoles 6 and unprotected amino acids, (iii) one chiral tripeptide 7g (62%) from N-(Cbz- or Fmoc-α- aminoacyl)benzotriazole 2a and the free dipeptide 8. In all, N-(Cbz- or Fmoc-α-aminoacyl)benzotriazole derivatives of 17 of the 20 naturally occurring amino acids were used, including those containing the following unprotected side chain functionalities: alcoholic -OH (Ser), indole -NH (Trp), imidazole -NH, phenolic -OH (Tyr), -CONH₂ (Gln, Asn), -SH (Cys), -CO₂H (Glu, Asp), and -S-S (Cystine). Support for the complete retention of chirality was obtained by parallel experiments involving D-Ala, L-Ala, and DL-Ala for the preparation of di- and tripeptides. This and other evidence for chiral integrity was supported by NMR and HPLC analyses. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-926287

PAPER
411
The Efficient Preparation of Di- and Tripeptides by Coupling N-(Cbz- or
Fmoc-a-aminoacyl)benzotriazoles with Unprotected Amino Acids
Coupling of
N
-(Cb
l
z- or
F
a
oc
-
a
-ami
n
noacyl)benzotriazo
R
les
w
ith Unprote
.
cted Amino A
K
cids atritzky,* Parul Angrish, Kazuyuki Suzuki
Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
Fax +1(352)3929199; E-mail: katritzky@chem.ufl.edu
Received 18 April 2005
typically difficult to separate due to problems with wash-
ing and drying, are often unstable, and are difficult to
store.1d,5 It has been reported6 that undesired side reactions
occur during the preparation of N-hydroxysuccinimide es-
Abstract: N-(Cbz- or Fmoc-a-aminoacyl)benzotriazoles 2 and N-
protected peptidoylbenzotriazoles 6 are coupled in aqueous acetoni-
trile solution with free amino acids or dipeptides to prepare: (i) 22
chirally pure dipeptides 5a–v (in an average yield of 82%) from N-
(Cbz- or Fmoc-a-aminoacyl)benzotriazoles 2 and unprotected ami- ters by coupling N-protected amino acids and N-hydrox-
no acids, (ii) five chiral tripeptides 7a–e (in an average yield of
75%) from N-protected peptidoylbenzotriazoles 6 and unprotected
amino acids, (iii) one chiral tripeptide 7g (62%) from N-(Cbz- or
Fmoc-a-aminoacyl)benzotriazole 2a and the free dipeptide 8. In all,
N-(Cbz- or Fmoc-a-aminoacyl)benzotriazole derivatives of 17 of
the 20 naturally occurring amino acids were used, including those
containing the following unprotected side chain functionalities: al-
coholic –OH (Ser), indole –NH (Trp), imidazole –NH, phenolic
–OH (Tyr), –CONH2 (Gln, Asn), –SH (Cys), –CO2H (Glu, Asp),
and –S-S (Cystine). Support for the complete retention of chirality
was obtained by parallel experiments involving D-Ala, L-Ala, and
DL-Ala for the preparation of di- and tripeptides. This and other ev-
ysuccinimide with carbodiimide. Reactions with phenyl
esters can be complicated by difficulty in the complete re-
moval of the liberated acidic phenol, especially when the
peptides produced do not precipitate out.6
We have extensively applied N-acylbenzotriazoles for N-
acylation7 C-acylation8 and O-acylation.9 During this
work, peptide coupling of N-(Cbz-aminoacyl)benzotri-
azoles with unprotected amino acids produced peptides in
85–95% yield with minimal racemization.10 N-(Cbz-a-
Aminoacyl)benzotriazoles prepared from N-protected a-
idence for chiral integrity was supported by NMR and HPLC anal- amino acids with unfunctionalized side-chains such as
yses.
Ala, Val, and Phe coupled successfully with unprotected
amino acids (Ala, Val, Phe, Ser, Trp) in the presence of
Et3N in partially aqueous solution (CH3CN/H2O). N-Cbz-
Peptidoylbenzotriazoles from N-protected dipeptides, N-
Cbz-Ala-Phe-OH and N-Cbz-Phe-Ala-OH reacted analo-
gously. Recently, we prepared N-(Cbz- or Fmoc-a-ami-
noacyl)benzotriazoles derived from Tyr, Trp, Cys, Met,
and Gln containing unprotected side-chain functional-
ity,10b and successfully coupled them with a-amino acids
(L-Ala, L-Phe) in partially aqueous medium to form the
expected peptides. NMR and HPLC analysis supported
the preservation of the original chirality. These findings
were further supported by NMR and HPLC comparison of
the diastereomeric dipeptides prepared by coupling N-
(Cbz-Tyr, Trp, Cys, Met, Gln and Fmoc-Trp, Met
acyl)benzotriazole with H-DL-Ala-OH, H-DL-Phe-OH.10b
Key words: N-(protected a-aminoacyl)benzotriazole, acylating re-
agent, dipeptides, N-Cbz-peptidoylbenzotriazole, tripeptides
Many biologically active peptides function as enzymes,
inhibitors, immune bodies, peptide hormones, and antibi-
otics. Peptides are also implicated in the onset or continu-
ation of various diseases. Other roles include participation
in mushroom toxins and snake venoms. Large peptides
can be found and isolated from natural sources, but small
peptides are usually prepared synthetically.
In the formation of peptide bonds by chemical synthesis,
protection of the non-participating amino group is manda-
tory. Other functional groups, such as alcohols (–OH in
Ser) or phenols (–OH in Tyr), are also frequently protect-
ed, especially in reactions with highly activated interme-
diates (e.g. acid halides, mixed anhydrides, or O-
acylisoureas).1 However, such functional groups (but not
the thiol group in cysteine) can be left unprotected when
selective acylating agents of moderate activity such as
azides, or active esters are utilized.1a–d
Herein we report (i) the extension of our preparation of N-
(Cbz- or Fmoc-a-aminoacyl)benzotriazoles to Ser, Asn,
Glu, Asp, Met, and Cystine (with unprotected alcoholic,
primary amide, CO2H, SMe, and S–S functional groups),
(ii) peptide coupling of these N-(Cbz- or Fmoc-a-amino-
acyl)benzotriazoles with free a-amino acids including
Ala, Phe, Val, and also Trp, Ser, Gln, Met, Glu, and Cys,
all with unprotected side-chain functionality, (iii) prepara-
tion of N-protected peptidoylbenzotriazoles from N-pro-
tected dipeptides, and (iv) preparation of N-protected
tripeptides carrying unprotected side-chain functional-
ities.
Peptide coupling of unprotected amino acids has been
achieved by acyl azides,1d,2 N-hydroxysuccinimide es-
ters,3 and p-nitrophenyl esters4 of N-protected amino ac-
ids with minimal racemization. However, there are
problems associated with these protocols. Acyl azides are
SYNTHESIS 2006, No. 3, pp 0411–0424
x
x
.
x
x
.2
0
0
5
Advanced online publication: 11.01.2006
DOI: 10.1055/s-2006-926287; Art ID: M02505SS
© Georg Thieme Verlag Stuttgart · New York

412
A. R. Katritzky et al.
PAPER
3a
Preparation of N-(Cbz- and Fmoc-a-aminoacyl)ben-
zotriazoles from N-Cbz- or Fmoc-a-Amino Acids
N
NH
O
H2N
Preparations of N-(Cbz- or Fmoc-a-aminoacyl)benzotri-
azoles 2a–f, 2h–j, 2n were carried out by the previously
reported procedure:10 the N-Cbz- or Fmoc-a-amino acid
(1a–f, 1h–j, 1n) was treated with a mixture of four equiv-
alents of 1H-benzotriazole and one equivalent of thionyl
chloride in THF at room temperature for two hours to
yield the desired 2a–f, 2h–j, 2n in 72–95% (Scheme 1).
Compounds 2k–m were synthesized utilizing one equiva-
lent of the dicarboxylic acid (1k–m) with eight equiva-
lents of 1H-benzotriazole and two equivalents of thionyl
chloride in THF at room temperature for two hours. Cbz-
Asn-Bt (2e) was directly crystallized from methanol to re-
move unreacted 1H-benzotriazole. Attempted isolation of
2e from 1H-benzotriazole by acid or base wash failed.
Fmoc-Ser-Bt (2j) was isolated from a mixture by washing
with 4 N HCl solution; we could not isolate 2j by washing
with saturated Na2CO3 solution or column chromatogra-
phy using ethyl acetate–hexanes (2:1) and the prolonged
reaction caused some decomposition of 2j. Compounds
2e–f, 2j–n are novel and were characterized using 1H and
13C NMR spectroscopy, elemental analysis, and ORP.
CH2Cl2
20 °C
Cbz NH HN
4a (82%)
3b
N
NH
N
NH
O
O
H2N
CH2Cl2
20 °C
Cbz NH HN
Cbz NH Bt
4b (96%)
2g
N
NH
O
H
N
3c
CH2Cl2
20 °C
Cbz NH
N
4c (96%)
Scheme 2 Preparation of Cbz-L-His-amides 4a–c
tained from reactions of crude 2g with amines 3a–c
(Scheme 2).
NMR analysis of compounds 4a–c revealed no detectable
racemization (<5%). 1H NMR analysis of compounds 4a–
c showed an evident doublet for the amide NH proton. No
signal from the corresponding diastereomer was observed
in the NMR analysis of the methyl protons of 4a and 4b.
1H NMR showed a clear doublet for the methyl protons
present in compounds 4a and 4b at 1.33 and 1.28 ppm, re-
spectively, suggesting LL-configuration is more down-
R1
O
R1
O
BtH, SOCl2
20 °C
Pg NH
Bt
Pg NH OH
2a–j
1a–j
O
O
Bt
O
OH
O
n
n
BtH, SOCl2
20 °C
13
field shifted as compared to LD-configuration. C NMR
Cbz NH
Bt
Cbz NH OH
gave singlets for all carbonyl and methyl carbons for 4a
and 4b, further supporting the enantiopurity of the com-
pounds.
1l (n = 1)
1m (n = 2)
2l (n = 1)
2m (n = 2)
Cbz-Pro-Bt (2n) was prepared using the same methodolo-
gy, although a complicated spectrum was observed in the
NMR analysis due to the formation of rotamers. The pres-
ence of an additional protection-free amino group in the
side-chain functionality of L-Lys and L-Arg necessitates
the preparation of doubly N-protected (L-Arg and L-Lys
acyl)benzotriazoles, since we plan experiments in this di-
rection.
Cbz
NH
Cbz
NH
Cbz
HN
Cbz
O
S
BtH, SOCl2
20 °C
O
HN
S
S
S
Bt
Bt
O
OH
HO
O
2k
1k
Bt = Benzotriazol-1-yl
Pg = Protecting group (Z, Fmoc)
Amino acids with R1: Trp, Tyr, Gln, Cys, Met, Asn, Ser, and His.
Scheme 1 Preparation of N-(Cbz- and Fmoc-aminoacyl)benzotri-
azoles 2a–n
Preparation of N-Protected Dipeptides with Addi-
tional Unprotected Side-Chain Functionality
Preparation of Cbz-His-Bt (2g) was carried out in the
same manner as described above. Although the formation
of 2g in the reaction mixture was supported by 1H NMR
spectra, 2g could not yet be isolated in pure form. Com-
pound 2g underwent decomposition when washed with
saturated Na2CO3 solution or when subjected to column
chromatography. Thus, formation of 2g was further sup-
ported by the isolation of N-Cbz-amino amides 4a–c ob-
Peptide-coupling reactions were carried out between N-
(Cbz- or Fmoc-a-aminoacyl)benzotriazoles derived from
Trp, Tyr, Gln, Met, Ser, Cystine, Asp, and Glu 2a–c,f,h–
m with diverse L- and D-amino acids (Ala, Val, Phe, Trp,
Ser, Tyr, Gln, Glu, and Cys) in partially aqueous solution
(MeCN–H2O) in the presence of Et3N for one hour
(Scheme 2 and Table 2). The crude products were washed
with 6 N HCl solution to remove the by-product, BtH. The
dipeptides 5f–h prepared from N-Cbz-Gln-Bt (2c) were
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York

PAPER
Coupling N-(Cbz- or Fmoc-a-aminoacyl)benzotriazoles with Unprotected Amino Acids
413
isolated by precipitation from the reaction mixture upon for each of the corresponding diastereomeric mixtures 5a
1
acidification. Enantiopure dipeptides 5a–u (LL-configura- + 5a¢, 5i + 5v, and 5q + 5q¢. For example, the H NMR
tion) and 5v (LD-configuration) were obtained in 70–95% analysis of 5q indicated an evident doublet for the methyl
yields in high purity without the use of any chromato- protons at 1.29 ppm, whereas two sets of doublets were
graphic techniques (Table 1 and Scheme 3).
obtained at 1.27 and 1.29 ppm for the corresponding dia-
stereomeric mixture 5q + 5q¢.
Diastereomeric mixtures 5a + 5a¢, 5i + 5v, 5q + 5q¢ (LL-
and DL-configuration) were prepared from N-(Cbz-L-Trp, The methyl protons of 5i (LL) were observed at 1.28 ppm,
L-Met, L-CyS-S acyl)benzotriazoles and DL-Ala-OH, us- whereas the signal for the methyl protons for 5v (LD) ap-
ing the same conditions as mentioned for the enantiopure peared at 1.26 ppm. These results were then carefully
LL-dipeptides (Table 2). Compounds 5a–v and the diaste- compared with the diastereomeric mixture 5i + 5v, which
reomeric mixtures 5a + 5a¢, 5i + 5v, and 5q + 5q¢ were ob- gave a multiplet at 1.24–1.29 ppm, further highlighting
13
tained in 70–95% yields and were further recrystallized the chirality of the enantiopure dipeptides 5i and 5v. C
1
from CHCl3–hexane for H and 13C NMR spectroscopy, NMR for each diastereomeric mixture 5a + 5a¢, 5i + 5v,
elemental analysis, and ORP.
and 5q + 5q¢ displayed doublets for most aliphatic and
carbonyl carbons, but no significant difference was ob-
served for the aromatic carbons.
NMR Studies for the Chirality of 5a–v
NMR analysis showed no detectable racemization (<5%) HPLC Analysis for the Chirality of 5a, 5i, and 5q
for the enantiopure LL-dipeptides 5a–u and the LD-dipep-
tide 5v. For compounds 5a–v, 1H NMR analysis for each The enantiopurity of the dipeptides 5a, 5i, and 5q was fur-
compound revealed two sets of doublets for the two –NH ther confirmed with HPLC analyses using Chirobiotic T
protons ranging from 7.5–8.6 ppm. However, for each of column [detection at 254 nm, flow rate 1.0 mL/min, and
the diastereomeric mixtures 5a + 5a¢, 5i + 5v, 5q + 5q¢, the MeOH–H2O (1:1) as solvent]. For each of the LL-dipep-
two NH-proton signals appeared as four sets of doublets. tides 5a, 5i, 5q, the HPLC results showed a single peak.
The methyl protons of the L-Ala fragment showed as a By contrast, two peaks were observed for the correspond-
clear doublet in each of the enantiopure LL-dipeptides 5a, ing diastereomeric mixtures 5a + 5a¢, 5i + 5v, 5q + 5q¢,
5i, and 5q, whereas it was observed as two sets of doublets confirming the enantiopurity of the LL-dipeptides 5a, 5i,
Table 1 Conversion of N-Protected a-Amino Acids into N-Protected (a-Aminoacyl)benzotriazoles
23
Entry
1
Reagent
Product
Yield (%)a
Mp (°C)
100–101
165–166
161–162
144–147
142–143
105–107
[a]D
N-Cbz-L-Trp-OH (1a)b
N-Cbz-L-Tyr-Bt (1b)b
N-Cbz-L-Gln-OH (1c)b
N-Cbz-L-Cys-OH (1d)b
N-Cbz-L-Asn-OH (1e)
N-Cbz-L-Met-OH (1f)
N-Cbz-L-His-OH (1g)
N-Fmoc-L-Trp-OH (1h)b
N-Fmoc-L-Met-OH (1i)b
N-Fmoc-L-Ser-OH (1j)
N-Cbz-L-CyS-S-diOH (1k)
N-Cbz-L-Asp-diOH (1l)
N-Cbz-L-Glu-diOH (1m)
N-Cbz-L-Pro-OH (1n)e
N-Cbz-L-Trp-Bt (2a)b
N-Cbz-L-Tyr-Bt (2b)b
N-Cbz-L-Gln-Bt (2c)b
N-Cbz-L-Cys-Bt (2d)b
N-Cbz-L-Asn-Bt (2e)
N-Cbz-L-Met-Bt (2f)
N-Cbz-L-His-Bt (2g)
N-Fmoc-L-Trp-Bt (2h)b
N-Fmoc-L-Met-Bt (2i)b
N-Fmoc-L-Ser-Bt (2j)
N-Cbz-L-CyS-S-diBt (2k)
N-Cbz-L-Asp-diBt (2l)
N-Cbz-L-Glu-diBt (2m)
N-Cbz-L-Pro-Bt (2n)e
95
86
72
76
72
95
90c
90
87
68
88
86
92
74
+48.8
+64.3
–37.5
–169.0
–30.1
–49.3
2
3
4
5
6
c
d
7
–
–
8
88–90
98–100
100–101
180–182
140–143
150–152
oil
+17.6
–103.9
–44.8
–151.7
+29.3
–6.0
9
10
11
12
13
14
–139.6
a Isolated yield.
b Previously prepared.10b
c Crude yield, no melting point available.
d Characterized by the preparation of amino amides, see experimental section.
e Rotamers.
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York

414
A. R. Katritzky et al.
PAPER
Table 2 Preparation of N-Cbz-Dipeptides 5a–v from N-Protected (a-Aminoacyl)benzotriazoles 2a–c,f,h–m, and Free Amino Acids
23
Entry
1
Reactant
2a
2a
2a
2a
2b
2c
Amino acid
L-Ala
L-Cys
L-Ser
Product
Yield (%)a
90
Mp (°C)
140–144
140–144
133–138
188–190
114–119
211–213
132–134
186–188
137–138
120–122
172–173
119–120
114–115
157–159
148–150
131–132
158–160
98–100
[a]D
Cbz-L-Trp-L-Ala-OH (5a)
Cbz-L-Trp-L-Cys-OH (5b)
Cbz-L-Trp-L-Ser-OH (5c)
Cbz-L-Trp-L-Trp-OH (5d)
Cbz-L-Tyr-L-Trp-OH (5e)
Cbz-L-Gln-L-Ala-OH (5f)
Cbz-L-Gln-L-Gln-OH (5g)
Cbz-L-Gln-L-Val-OH (5h)
Cbz-L-Met-L-Ala-OH (5i)
Cbz-L-Met-L-Met-OH (5j)
Cbz-L-Met-L-Trp-OH (5k)
Cbz-L-Met-L-Glu-OH (5l)
Fmoc-L-Trp-L-Ser-OH (5m)
Fmoc-L-Met-L-Ser-OH (5n)
Fmoc-L-Met-L-Glu-OH (5o)
Fmoc-L-Ser-L-Ala-OH (5p)
(Cbz-L-CyS-S-L-Ala-OH)2 (5q)
(Cbz-L-CyS-S-L-Val-OH)2 (5r)
Cbz-L-Asp-L-Ala-OH (5s)b
Cbz-L-Glu-L-Phe-OH (5t)b
Cbz-L-Glu-L-di-Val-OH (5u)
Cbz-L-Met-D-Ala-OH (5v)
Cbz-L-Trp-DL-Ala-OH (5a + 5a¢)c,d
Cbz-L-Met-DL-Ala-OH (5i + 5v)d
(Cbz-L-CyS-S-DL-Ala-OH)2 (5q + 5q¢)c,d
–23.9
–28.5
–11.4
–15.6
+2.4
2
86
3
86
4
L-Trp
L-Trp
L- Ala
L-Gln
L-Val
L-Ala
L-Met
L-Trp
L-Glu
L-Ser
85
5
90
6
72
–2.1
7
2c
47
+26.7
+5.5
8
2c
66
9
2f
95
–9.0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
2f
95
–8.4
2f
82
+8.2
2f
94
–8.2
2h
2i
87
–13.7
–2.3
L-Ser
88
2i
L-Glu
L-Ala
L-Ala
L-Val
L-Ala
L-Phe
L-Val
D-Ala
DL-Ala
DL-Ala
DL-Ala
93
–18.1
–2.3
2j
72
2k
2k
2l
96
–102.2
–80.4
–57.8
+15.2
–11.1
–33.4
–18.4
–5.4
91
75
112–113
168–170
87–88
2m
2m
2f
88
71
95
157–158
93–95
2a
2f
94
92
141–142
153–154
2k
91
–118.4
a Isolated yields.
b Mono-substituted product.
c a¢ and q¢ are LD-configuration.
d Diastereomeric mixture.
5q. These results also indicated shorter retention times for The reactions were carried out at –10 °C by following the
the LL-configuration as compared to the LD-configuration same simple procedure as used for 2, until the starting ma-
(Table 5).
terials 5 were completely consumed. The products 6 were
obtained with no detectable racemization, as evidenced by
their H and 13C NMR spectra. Initially, a reaction of 5i
was carried out at 20 °C following the procedure used for
2a–n, but the product 6c was then obtained with partial ra-
cemization. Compounds 6a,c,d, and the mixture 6c + 6d
1
Preparation of N-Protected Peptidoylbenzotriazoles
from N-Protected Dipeptides
N-Cbz-Dipeptides 5a,e,i,v, and the diastereomeric mix- were obtained by an aqueous workup, but 6b was purified
ture 5i + 5v were successfully converted into the corre- by column chromatography due to the loss of 6b when
sponding benzotriazole derivatives 6a–d, and the subjected to base wash. Compound 6a–d and the mixture
diastereomeric mixture 6c + 6d respectively (Scheme 4). 6c + 6d were obtained in 76–87% yields without the use
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York

PAPER
Coupling N-(Cbz- or Fmoc-a-aminoacyl)benzotriazoles with Unprotected Amino Acids
415
R1
Pg NH HN
HO
O
R1
O
corresponding diastereomer were observed in the NMR
spectrum of 6c and 6d, suggesting the enantiopurity of the
N-protected peptidoyl benzotriazoles. The NMR analysis
also gave a singlet for the methyl protons (SMe) from N-
Cbz-L-Met fragment. 13C NMR displayed a singlet for
each carbonyl carbon for compounds 6a–d. The methyl
carbons of compounds 6a,c,d also appeared as singlets in
13C NMR.
R2
O
CH3CN–H2O
20 °C
R2
+
Pg NH Bt
H2N
OH
O
2a–c,f,h–m
5a–p (LL)
5v (LD)
O
O
S
O
NH
NH
H
H
OH
O
Cbz
N
Cbz
N
O
O
HO
HO
R1
Cbz NH NH
HO
O
R1
O
S
S
R2
R2
Cbz NH HN
BtH, SOCl2
S
Cbz NH NH
O
O
O
THF, –10 °C
O
O
HO
5s
Bt
6a–d, 6c + 6d
Cbz NH HN
Cbz NH HN
O
O
O
5a,e,i,v,i+v
HO
HO
5q
5r
Scheme 4 Preparation of N-Cbz-dipeptidoylbenzotriazoles 6a–d
and diastereomeric mixture 6c + 6d; for detailed structures, see Table
3
O
OH
OH
NH
O
On the other hand, the mixture 6c + 6d displayed a com-
plicated NMR spectrum. In 1H NMR, two sets of doublets
were obtained for all –NH protons. In addition, an unre-
solved triplet for the methyl protons from L-Ala and two
singlets for the methyl protons (SMe) of the N-Cbz-L-Met
fragment were also observed in the 1H NMR. All aliphatic
carbons with the exception of the benzyl CH2 carbon and
two carbonyl carbons appeared as sets of two singlets in
13C NMR.
O
Ph
O
O
Cbz NH HN
Cbz NH HN
HO
O
5t
HO
5u
Scheme 3 Preparation of LL- and LD-dipeptides 5a–v; for designa-
tion of R1 and R2, refer to Table 2
of any chromatographic techniques with the exception of
6b (Table 3). Compounds 6a–d and the mixture 6c + 6d
were recrystallized using CHCl3–hexane and character-
ized using 1H and 13C NMR spectroscopy, elemental anal-
ysis, and ORP.
Preparation of N-Protected Tripeptides Carrying
Unprotected Side-Chain Functionality
Tripeptides 7a–e, 7g and the diastereomeric mixture 7c +
7f were prepared by two different methods: (i) peptide-
coupling reactions between compounds 6a–c, and the
mixture 6c + 6d with an unprotected amino acid afforded
7a–e and the diastereomeric mixture 7c + 7f (Scheme 5
and Table 4) and (ii) peptide-coupling reaction between
2a and the unprotected dipeptide 8 gave 7g (Scheme 6).
NMR Evidence for the Chirality of 6a–d
NMR analysis demonstrated the absence of racemization
for compounds 6a–d. 1H NMR showed two clear doublets
for the two –NH protons of each of compounds 6a–d. The
observed chemical shift for the methyl protons of L-Ala in
6c was 1.57 ppm, whereas the methyl protons in 6d were
seen at 1.55 ppm. In addition, no signals from the other
The coupling reactions were performed using the method
(i) at –10 °C, and NMR analysis showed minimal racem-
ization. However, extensive racemization was observed
Table 3 Conversion of N-Cbz-Dipeptides 5a,e,i,v, and the Mixture 5i + 5v into N-Cbz-Dipeptidoylbenzotriazoles 6a–d, 6c + 6d
23
Reactant
Product
Yield (%)a
Mp (°C)
176–177
152–154
104–105
135–137
96–97
[a]D
Cbz-L-Trp-L-Ala-OH (5a)
Cbz-L-Trp-L-Trp-OH (5d)
Cbz-L-Met-L-Ala-OH (5i)
Cbz-L-Met-D-Ala-OH (5v)
Cbz-L-Met-DL-Ala-OH (5x)
a Isolated yields.
Cbz-L-Trp-L-Ala-Bt (6a)
Cbz-L-Trp-L-Trp-Bt (6b)
Cbz-L-Met-L-Ala-Bt (6c)
Cbz-L-Met-D-Ala-Bt (6d)
Cbz-L-Met-DL-Ala-Bt (6c + 6d)
78
76
85
87
84
–27.7
+10.1
–19.3
–7.6
–10.6
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York

416
A. R. Katritzky et al.
PAPER
R1
O
Table 5 HPLC Analyses
R2
R1
O
Entry
5a
Compound
tR (min)a
Cbz NH NH
CH3CN–H2O
R3
O
R2
O
Cbz NH HN
+
HN
Cbz-L-Trp-L-Ala-OH
Cbz-L-Trp-DL-Ala-OH
Cbz-L-Met-L-Ala-OH
Cbz-L-Met-DL-Ala-OH
Cbz-L-CyS-S-L-Ala-OH
Cbz-L-CyS-S-DL-Ala-OH
Cbz-L-Met-L-Ala-L-Ala-OH
Cbz-L-Met-DL-Ala-L-Ala-OH
11.0
H2N
OH
O
R3
–10 °C
Bt
HO
5a + 5a¢
5i
11.0, 12.9
11.7
O
7a–e, and 7c + 7f
6a–d
5i + 5v
5q
11.7, 12.3
11.6
Scheme 5 Preparation of tripeptides 7a–e and 7c + 7f from N-Cbz-
dipeptidoylbenzotriazoles 6a–d, 6c + 6d with unprotected amino
acids
5q + 5q¢
7c
11.6, 12.2
12.4
Table 4 Preparation of Tripeptides 7a–e and 7c + 7f from N-Cbz-
Dipeptidoylbenzotriazoles 6a–d and 6c + 6d with Unprotected
Amino Acids
7c + 7f
12.4, 15.6
a For conditions, see the experimental section.
Reac-
tant
Amino Product
Acid
Yield
(%)a
Previously, preparation of tripeptides was also achieved
by the fragment-coupling method using diverse modes of
C-terminal activation of the dipeptide-like acyl azides,11a
carbodiimides,11b p-nitrophenyl esters,11c and alkyl chlo-
roformates.11d These methods employed amino acid esters
bearing alkyl side-chains, but no such reaction utilizing
unprotected amino acids in aqueous solution was report-
ed. Although these known methods produced tripeptides
in two steps, involving the peptide coupling and hydroly-
sis of the esters, the efficiency and the overall yield were
moderate in the range of 51–62%. Thus, these methodol-
ogies are not particularly attractive due to prolonged reac-
tion times, harsh reaction conditions, and low yields,
especially with functionalized amino acids. These associ-
ated drawbacks with previous methods strongly suggest
that our method offers an added advantage for the synthe-
sis of tripeptides in shorter reaction times, utilizing mild
reaction conditions.
6a
6b
6c
6c
6c
L-Cys Cbz-L-Trp-L-Ala-L-Cys-OH (7a)
L-Trp Cbz-L-Trp-L-Trp-L-Trp-OH (7b)
L-Ala Cbz-L-Met-L-Ala-L-Ala-OH (7c)
86
87b
86
L-Ser
Cbz-L-Met-L-Ala-L-Ser-OH (7d)
83c
92d
L-Trp Cbz-L-Met-L-Ala-L-Trp-OH (7e)
6c + 6d L-Ala Cbz-L-Met-DL-Ala-L-Ala-OH (7c + 7f)e 88
a Isolated yields.
b Diastereomeric ratio 3:1.
c Diastereomeric ratio 4:1.
d Diastereomeric ratio 4:1.
e Diastereomeric mixture.
when the coupling was carried out under similar condi-
tions at room temperature.
NH
NMR and HPLC Evidence for the Chirality of 7a–e
O
NMR analysis demonstrated no detectable racemization
Cbz NH HN
for the preparation of 7a and 7c, and 7b, 7d, 7e with some
NH
O
O
1
degree of racemization. H NMR of 7a and 7c showed
CH3CN–H2O
Et3N, –10 °C
HN
HO
+ H2N HN
O
clear doublets for all –NH and the methyl protons from L-
Ala. For compounds 7b,d,e, –NH protons appeared as two
sets of doublets, indicating diastereomeric mixtures; the
ratios of the two sets of doublets for 7b,d,e were summa-
rized in Table 4. The different diastereomeric ratios for
compounds 7b,d,e suggest differences in the extent of ra-
cemization; however, no precise reason could be attribut-
ed to the observation.
O
HO
Cbz NH Bt
O
2a
8
7g (62%)
Scheme 6 Preparation of tripeptide 7g from N-Cbz-(a-amino-
acyl)benzotriazole 2a and unprotected dipeptide 8
The second methodology utilizes a coupling reaction be-
tween N-protected (a-aminoacyl)benzotriazoles 2a and
an unprotected dipeptide 8. The desired coupling reaction
was achieved using dipeptides having less sterically hin-
dered amino acid fragments like Gly, L-Leu (Scheme 5).
However, analogous reactions failed to give the desired
tripeptides, probably due to the hydrolysis of N-(Cbz- or
Fmoc-aminoacyl)benzotriazoles when bulky amino acid
To confirm the extent of racemization, Cbz-L-Met-L-Ala-
L-Ala-OH (7c) and Cbz-L-Met-DL-Ala-L-Ala-OH (7c +
7f) were subjected to HPLC analysis. A single peak was
observed for 7c at 12.4 min, whereas the diastereomeric
mixture 7c + 7f noticeably separated into two peaks at
12.4 and 15.6 min (Table 5).
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York

PAPER
Coupling N-(Cbz- or Fmoc-a-aminoacyl)benzotriazoles with Unprotected Amino Acids
417
N-(Cbz- and Fmoc-diaminoacyl)benzotriazoles 2k–m; General
Procedure
fragments such as L-Trp and L-Met were present at the N-
terminus of dipeptides. This hydrolysis could be due to the
steric hindrance at the N-terminus of dipeptides, leading
to competition between the hydrolysis of N-(Cbz- or
Fmoc-aminoacyl)benzotriazole and the coupling reaction.
SOCl2 (0.73 mL, 10 mmol) was added to a soln of 1H-benzotriazole
(4.760 g, 40 mmol) in anhyd THF (30 mL) at 20 °C, and the reaction
mixture was stirred for 20 min at 40–50 °C. To the reaction mixture
at 0 °C, the N-protected amino acid 1 (5 mmol), dissolved in anhyd
THF (5 mL) was added dropwise, and stirred for 2 h at 20 °C. The
white precipitate formed during the reaction was filtered off, and the
filtrate was concentrated under reduced pressure. The residue was
diluted with EtOAc (100 mL) and the soln was washed with sat.
Na2CO3 soln (3 × 50 mL), sat. NaCl soln (50 mL), and dried over
MgSO4. Removing solvents under reduced pressure gave products
2k–m, which were recrystallized from CHCl3–hexanes for elemen-
tal analysis.
In conclusion, we have prepared N-(Cbz- or Fmoc- Trp,
Tyr, Gln, Asn, Cys, Met, Ser, Glu, Asp, Pro, His, and Cys-
tine acyl)benzotriazoles 2a–n and N-protected peptidoyl-
benzotriazoles 6a–d, and the diastereomeric mixture 6c +
6d. These C-activated amino acid derivatives with unpro-
tected side-chain functionalities undergo peptide cou-
plings with unprotected amino acids or amino acid esters
to give numerous di- and tripeptides in an average yield of
82% and 75%, respectively. The compounds were ob-
tained without using any chromatography for purification.
The original chirality of the resulting products was pre-
served in >97% ee, evidenced by NMR and HPLC. Both
N-(Cbz- or Fmoc-aminoacyl)benzotriazoles and N-pro-
tected peptidoylbenzotriazoles are easy to handle due to
their solidity and stability under ambient atmosphere, and
their coupling reactions can be carried out either in a pure-
ly organic solvent or in a partly aqueous solution.
N-[(1S)-2-Benzotriazolyl-1-(carbamoylmethyl)-2-oxoeth-
yl](phenylmethoxy)carboxamide (N-Cbz-Asn-Bt, 2e)
White microcrystals; yield: 1.323 g (72%); mp 142–143 °C;
[a]D23 –30.1 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 2.77 (dd, J = 15.4, 7.6 Hz, 1 H), 2.91 (dd,
J = 15.4, 5.7 Hz, 1 H), 5.03 (s, 2 H), 5.80–5.82 (m, 1 H), 7.00–7.05
(m, 1 H), 7.33 (s, 5 H), 7.53 (s, 1 H), 7.60–7.66 (m, 1 H), 7.77–7.82
(m, 1 H), 8.20–8.28 (m, 3 H).
13C NMR (DMSO-d6): d = 36.8, 51.3, 66.1, 114.1, 120.4, 126.9,
128.0, 128.2, 128.6, 130.9, 131.3, 136.8, 145.5, 156.1, 170.6, 171.4.
Anal. Calcd for C18H17N5O4: C, 58.85; H, 4.66; N, 19.06. Found: C,
58.60; H, 4.52; N, 18.23.
Melting points were determined on a capillary point apparatus
equipped with a digital thermometer. NMR spectra were recorded
1
in CDCl3 or DMSO-d6 with TMS for H (300 MHz) and 13C (75
Benzyl N-[(1S)-1-(1H-1,2,3-benzotriazol-1-ylcarbonyl)-3-
(methylsulfanyl)propyl]carbamate (N-Cbz-Met-Bt, 2f)
White microcrystals; yield: 1.461 g (76%); mp 105–107 °C;
[a]D23 –35.6 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 2.05 (s, 3 H), 2.01–2.15 (m, 1 H), 2.22–
2.28 (m, 1 H), 2.60–2.72 (m, 2 H), 5.06 (s, 2 H), 5.62–5.69 (m, 1 H),
7.36 (s, 5 H), 7.62–7.67 (m, 1 H), 7.79–7.84 (m, 1 H), 8.22–8.31 (m,
3 H).
MHz) as the internal reference. N-Cbz- and Fmoc-amino acids and
amino acids, purchased from Fluka and Acros, were used without
further purification. HPLC analyses were performed on Beckman
system gold programmable solvent module 126, using Chirobiotic
T column (4.6 × 250 mm), detection at 254 nm, flow rate of 1.0 mL/
min and MeOH–H2O (1:1) as eluting solvent.
N-(Cbz- and Fmoc-aminoacyl)benzotriazoles 2a–f, 2h–j, 2n;
General Procedure
13C NMR (DMSO-d6): d = 14.2, 29.5, 29.7, 53.4, 65.9, 114.0, 120.2,
126.7, 127.8, 127.9, 128.4, 130.7, 131.1, 136.6, 145.3, 156.3, 171.9.
SOCl2 (0.37 mL, 5 mmol) was added to a soln of 1H-benzotriazole
(2.380 g, 20 mmol) in anhyd THF (15 mL) at 20 °C, and the reaction
mixture was stirred for 20 min at 40–50 °C. To the reaction mixture
at 0 °C, the N-protected amino acid 1 (5 mmol), dissolved in dry
THF (5 mL), was added dropwise, and stirred for 2 h at 20 °C. The
white precipitate formed during the reaction was filtered off, and the
filtrate was concentrated under reduced pressure. The residue was
diluted with EtOAc (100 mL) and the solution was washed with 6N
HCl soln (3 × 50 mL) or sat. Na2CO3 soln (3 × 50 mL), sat. NaCl
soln (50 mL), and dried over MgSO4. Removing solvents under re-
duced pressure gave products 2a–f,2h–j,2n, which were recrystal-
lized from CHCl3–hexanes for elemental analysis. In case of 2e, the
product was obtained by washing with Et2O, followed by recrystal-
lization using MeOH. Compounds 2a–d,2h–i are previously report-
ed by us.10b Compounds 2e–f,2j,2n are novel and fully
characterized by NMR and elemental analysis.
Anal. Calcd for C19H20N4O3S: C, 59.36; H, 5.24; N, 14.57. Found:
C, 59.48; H, 5.19; N, 14.62.
Benzyl N-[(1S)-2-(1H-1,2,3-benzotriazol-1-yl)-1-(1H-imidazol-
4-ylmethyl)-2-oxoethyl]carbamate (Cbz-His-Bt, 2g)
Crude yield: 95%; no mp and optical rotation are available. Charac-
terization of 2g was carried out by the preparation of amino amides
4a–c.
Crude 1H NMR (DMSO-d6): d = 3.35–3.43 (m, 1 H), 3.46–3.50 (m,
1 H), 5.06 (s, 2 H), 5.88 (q, J = 8.3 Hz, 1 H), 7.30–7.37 (m, 5 H),
7.54 (s, 1 H), 7.65–7.70 (m, 1 H), 7.82–7.87 (m, 1 H), 8.30 (q,
J = 8.3 Hz, 2 H), 8.54 (d, J = 7.2 Hz, 1 H), 9.13 (s, 1 H).
9H-Fluoren-9-ylmethyl-N-[(1S)-2-(1H-1,2,3-benzotriazol-1-yl)-
1-(hydroxymethyl)-2-oxoethyl]carbamate (N-Fmoc-Ser-Bt, 2j)
White microcrystals; yield: 1.457 g (68%); mp 100–101 °C;
[a]D23 –44.8 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 4.18–4.23 (m, 1 H), 4.30–4.60 (m, 4 H),
5.73–5.75 (m, 1 H), 7.26–7.30 (m, 2 H), 7.38 (t, J = 7.4 Hz, 2 H),
7.60–7.69 (m, 3 H), 7.79 (t, J = 7.6 Hz, 1 H), 7.87 (d, J = 7.7 Hz, 2
H), 8.06–8.22 (m, 1 H), 8.25–8.29 (m, 1 H), 8.52–8.58 (m, 1 H).
Benzyl N-[(1S)-2-(1H-1,2,3-benzotriazol-1-yl)-1-(1H-imidazol-
4-ylmethyl)-2-oxoethyl]carbamate (2g)
SOCl2 (0.37 mL, 5 mmol) was added to a soln of 1H-benzotriazole
(2.380 g, 20 mmol) in anhyd THF (15 mL) at 20 °C, and the reaction
mixture was stirred for 20 min at 20 °C. To the reaction mixture, N-
Cbz-L-His-OH (1g) (1.455 g, 5 mmol) was added, and stirred for 3
h at 20 °C. The white precipitate formed during the reaction was fil-
tered off, and the solvent was completely removed under reduced
pressure. The residue obtained was dissolved in min amount of
CH2Cl2 (10 mL) and was subsequently used for the next reaction.
13C NMR (DMSO-d6): d = 46.5, 54.2, 60.6, 60.8, 66.0, 113.8, 120.0,
120.2, 125.1, 126.8, 127.0, 127.6, 130.5, 131.2, 140.7, 143.5, 145.3,
155.9, 168.1.
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York

418
A. R. Katritzky et al.
PAPER
ESI-FTICR-MS: m/z [M + Na]+ calcd for C24H20N4O4: 451.1377;
Amino Amides 4a–c; General Procedure
found: 451.1417.
Amine 3 was added to an ice-cold soln of crude 2g (1 mmol) in
CH2Cl2 (10 mL) dropwise, until the reaction mixture turned alkaline
(pH 8–9). The reaction mixture was then stirred at 20 °C until the
starting material was completely consumed as observed on 1H
NMR. It was washed with sat. Na2CO3 soln (3 × 20 mL) and sat.
NaCl soln (20 mL), and then dried over MgSO4. Evaporation of the
solvent gave the desired products 4a–c, which were further recrys-
tallized from CHCl3–hexane, unless specified otherwise.
Benzyl N-[(1S)-1-({[(2S)-2-(benzylcarbonyl)amino-3-(1H-1,2,3-
benzotriazol-1-yl)-3-oxopropyl]disulfanyl}methyl)-2-(1H-1,2,3-
benzotriazol-1-yl)-2-oxoethyl]carbamate (N-Cbz-Cystine-diBt,
2k)
White microcrystals; yield: 3.056 g (88%); mp 180–182 °C;
[a]D23 –109.6 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 3.18 (dd, J = 13.8, 9.9 Hz, 2 H), 3.55 (dd,
J = 13.8, 10.4 Hz, 2 H), 5.06 (s, 2 H), 5.90–5.93 (m, 2 H), 7.28–7.38
(m, 10 H), 7.59–7.64 (m, 2 H), 7.80 (t, J = 7.6 Hz, 2 H), 8.12 (d,
J = 8.2 Hz, 2 H), 8.21 (d, J = 8.3 Hz, 2 H), 8.39 (d, J = 7.4 Hz, 2 H).
13C NMR (DMSO-d6): d = 22.0, 30.8, 36.6, 53.5, 58.7, 66.8, 126.4,
127.4, 128.0, 128.2, 128.3, 129.1, 135.6, 137.3, 150.4, 171.0, 172.7,
173.6.
Benzyl N-((1S)-1-(1H-imidazol-4-ylmethyl)-2-oxo-2-{[(1S)-1-
phenylethyl]amino}ethyl)carbamate (4a)
White microcrystals; yield: 0.322 g (82%); mp 183–185 °C;
[a]D23 –26.4 (c 1.75, DMF).
1H NMR (DMSO-d6): d = 1.33 (d, J = 6.9 Hz, 3 H), 2.69–2.86 (m,
2 H), 4.31 (dd, J = 13.4, 7.7 Hz, 1 H), 4.83–4.92 (m, 1 H), 5.00 (s,
2 H), 6.74 (s, 1 H), 7.17–7.21 (m, 4 H), 7.25–7.37 (m, 8 H), 7.54 (s,
1 H), 8.35 (d, J = 7.7 Hz, 1 H).
13C NMR (DMSO-d6): d = 22.4, 30.8, 47.7, 54.7, 65.3, 113.2, 125.7,
126.4, 127.6, 127.7, 128.1, 128.3, 134.6, 136.5, 137.1, 144.6, 155.7,
170.5.
Anal. Calcd for C34H30N8O6S2: C, 57.45; H, 4.25; N, 15.76. Found:
C, 57.67; H, 4.26; N, 15.70.
Benzyl N-[(1S)-3-(1H-1,2,3-benzotriazol-1-yl)-1-(1H-1,2,3-ben-
zotriazol-1-ylcarbonyl)-3-oxopropyl]carbamate (N-Cbz-Asp-
diBt, 2l)
Anal. Calcd for C22H24N4O3: C, 67.33; H, 6.16; N, 14.28. Found: C,
67.64; H, 6.38; N, 14.33.
White microcrystals; yield: 2.019 g (86%); mp 140–143 °C;
[a]D23 +21.3 (c 1.66, DMF).
Benzyl N-((1S)-1-(1H-imidazol-4-ylmethyl)-2-oxo-2-{[(1R)-1-
phenylethyl]amino}ethyl)carbamate (4b)
1H NMR (DMSO-d6): d = 4.19 (dd, J = 17.8, 8.3 Hz, 1 H), 4.30 (dd,
J = 17.8, 4.8 Hz, 1 H), 5.06 (s, 2 H), 6.15 (q, J = 7.1 Hz, 1 H), 7.33–
7.36 (m, 5 H), 7.60–7.78 (m, 2 H), 7.79 (t, J = 7.2 Hz, 1 H), 7.83 (t,
J = 7.7 Hz, 1 H), 8.21–8.32 (m, 4 H), 8.54 (d, J = 6.4 Hz, 1 H).
13C NMR (DMSO-d6): d = 36.8, 50.4, 66.0, 113.9, 114.0, 120.1,
120.2, 126.6, 126.8, 127.7, 127.9, 128.3, 130.5, 130.7, 131.0, 131.2,
136.5, 145.4, 145.5, 155.9, 168.9, 170.0.
White microcrystals; yield: 0.377 g (96%); mp 139–140 °C;
[a]D23 +16.2 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.28 (d, J = 7.1 Hz, 3 H), 2.80–2.83 (m,
2 H), 4.25–4.32 (m, 1 H), 4.84–4.92 (m, 1 H), 4.99 (s, 2 H), 6.83 (s,
1 H), 7.17–7.43 (m, 11 H), 7.56 (s, 1 H), 8.27 (d, J = 8.2 Hz, 1 H).
13C NMR (DMSO-d6): d = 22.3, 30.8, 47.7, 54.6, 65.3, 113.3, 125.9,
126.5, 127.6, 127.7, 128.1, 128.3, 134.7, 136.5, 137.6, 144.4, 155.7,
170.5.
Anal. Calcd for C24H19N7O4: C, 61.40; H, 4.08; N, 20.89. Found: C,
61.22; H, 3.94; N, 20.78.
Benzyl N-[(1S)-1-acetyl-4-(1H-1,2,3-benzotriazol-1-yl)-4-oxo-
butyl]carbamate (N-Cbz-l-Glu-diBt, 2m)
Anal. Calcd for C22H24N4O3: C, 67.33; H, 6.16; N, 14.28. Found: C,
67.69; H, 6.45; N, 14.32.
White microcrystals; yield: 2.224 g (92%); mp 150–152 °C;
[a]D23 –4.4 (c 1.66, DMF).
Benzyl N-[(1S)-1-(1H-imidazol-4-ylmethyl)-2-oxo-2-(1-pyrro-
lidinyl)ethyl]carbamate (4c)
1H NMR (DMSO-d6): d = 2.40–2.47 (m, 1 H), 2.55–2.64 (m, 1 H),
3.72 (t, J = 6.8 Hz, 2 H), 5.06 (s, 2 H), 5.74 (q, J = 7.6 Hz, 1 H),
7.32–7.36 (s, 5 H), 7.57–7.61 (m, 2 H), 7.75 (t, J = 7.8 Hz, 1 H),
7.81 (t, J = 7.8 Hz, 1 H), 8.15 (d, J = 8.2 Hz, 1 H), 8.23–8.26 (m, 3
H), 8.44 (d, J = 6.8 Hz, 1 H).
13C NMR (DMSO-d6): d = 24.9, 31.3, 53.5, 65.9, 113.9, 114.0,
120.0, 126.3, 126.7, 127.8, 127.9, 128.3, 130.5, 130.7, 130.8, 131.1,
136.6, 145.3, 145.4, 156.3, 171.4.
White microcrystals; yield: 0.329 g (96%); mp 133–134 °C;
[a]D23 +24.8 (c 1.83, DMF).
1H NMR (CDCl3): d = 1.80–1.87 (m, 4 H), 2.93–3.08 (m, 2 H),
3.23–3.61 (m, 4 H), 4.73 (q, J = 6.9 Hz, 1 H), 5.03 (d, J = 12.4 Hz,
1 H, A part of AB system), 5.07 (d, J = 12.4 Hz, 1 H, B part of AB
system), 6.09 (d, J = 8.2 Hz, 1 H), 6.80 (s, 1 H), 7.30–7.34 (s, 5 H),
7.49 (s, 1 H).
13C NMR (CDCl3): d = 24.2, 26.1, 30.1, 46.3, 46.8, 52.9, 67.1, 77.4,
128.2, 128.3, 128.7, 135.3 (2C), 136.5, 156.2, 170.3.
Anal. Calcd for C25H21N7O4: C, 62.11; H, 4.38; N, 20.28. Found: C,
62.23; H, 4.29; N, 20.28.
Anal. Calcd for C18H22N4O3: C, 63.14; H, 6.48; N, 16.36. Found: C,
63.10; H, 6.54; N, 16.32.
Benzyl (2S)-2-(1H-1,2,3-benzotriazol-1-ylcarbonyl)tetrahydro-
1H-pyrrole-1-carboxylate (N-Cbz-Pro-Bt, 2n)
Colorless oil; yield: 1.664 g (95%); [a]D23 –139.6 (c 1.83, DMF).
LL- and LD-Dipeptides 5a–v and Diastereomeric Mixtures 5a +
5a¢, 5i + 5v, 5q + 5q¢; General Procedure
1H NMR (DMSO-d6) (two rotameric forms): d = 1.90–2.10 (m, 2
H), 2.20–2.84 (m, 1 H), 2.50–2.54 (m, 1 H), 3.52–3.67 (m, 2 H),
4.89–5.04 (m, 1 H), 5.14 (s, 1 H), 5.70–5.82 (m, 1 H), 6.95–6.98 (m,
2 H), 7.41–7.42 (m, 3 H), 7.67 (t, J = 7.7 Hz, 1 H), 7.84 (t, J = 7.6
Hz, 1 H), 8.21–8.33 (m, 2 H).
13C NMR (DMSO-d6): d = 23.3, 24.1, 29.7, 30.7, 46.6, 47.2, 58.9,
59.6, 66.3, 134.0, 120.2, 126.8, 126.9, 127.5, 127.6, 127.8, 127.9,
128.4, 130.6, 131.1, 136.1, 136.7, 145.3, 153.2, 154.2, 171.0.
N-Protected (aminoacyl)benzotriazoles 2a–c,f,h–m (0.5 mmol)
were added at 20 °C to a soln of a-amino acid (0.5 mmol) in
MeCN–H2O (7 mL : 3 mL) in the presence of Et3N (0.6 mmol). The
reaction mixture was then stirred at 20 °C until the starting material
was completely consumed, as observed on TLC using hexane–
EtOAc (2:1) as the eluent. Aq 6 N HCl soln (1 mL) was then added
and the solvent was removed under reduced pressure. The residue
obtained was dissolved in EtOAc (20 mL), and the organic extract
was washed with 6 N HCl soln (3 × 5 mL) and sat. NaCl soln (10
mL) and dried over MgSO4. Evaporation of the solvent gave the de-
Anal. Calcd for C19H18N4O3: C, 65.13; H, 5.18; N, 15.99. Found: C,
64.94; H, 5.23; N, 15.79.
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Coupling N-(Cbz- or Fmoc-a-aminoacyl)benzotriazoles with Unprotected Amino Acids
419
sired product, which was further recrystallized from CHCl3–hex-
ane, unless specified otherwise.
7.65 (d, J = 7.7 Hz, 1 H), 8.28 (d, J = 7.4 Hz, 1 H), 10.80 (s, 1 H),
10.88 (s, 1 H), 12.7 (s, 1 H).
13C NMR (DMSO-d6): d = 27.1, 27.8, 53.0, 55.4, 65.3, 109.7, 110.2,
111.3, 11.4, 118.2, 118.4, 118.6, 120.9, 121.0, 123.7, 123.8, 127.3,
127.5, 127.7, 128.3, 136.1, 137.0, 155.8, 172.0, 173.3.
(2S)-2-{[(2S)-2-{[(Benzyloxy)carbonyl]amino}-3-(1H-indol-3-
yl)propanoyl]amino}propanoic Acid (N-Cbz-L-Trp-L-Ala-OH,
5a)
Anal. Calcd for C30H28N4O5: C, 68.69; H, 5.38; N, 10.68. Found: C,
67.90; H, 5.35; N, 10.65.
White microcrystals; yield: 0.174 g (85%); mp 140–144 °C;
[a]D23 –23.9 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.32 (d, J = 7.2 Hz, 3 H), 2.90 (dd,
J = 14.6, 10.4 Hz, 1 H), 3.15 (dd, J = 14.6, 3.2 Hz, 1 H), 4.22–4.38
(m, 2 H), 4.88–4.98 (m, 2 H), 6.96–7.04 (m, 1 H), 7.06–7.09 (m, 1
H), 7.18 (s, 1 H), 7.22–7.38 (m, 7 H), 7.70 (d, J = 7.7 Hz, 1 H), 8.38
(d, J = 7.1 Hz, 1 H), 10.82 (s, 1 H), 12.60 (s, 1 H).
13C NMR (DMSO-d6): d = 17.2, 27.8, 47.6, 55.2, 65.2, 110.2, 111.3,
118.2, 119.6, 120.8, 124.0, 127.3, 127.5, 127.7, 128.3, 136.1, 137.0,
155.8, 171.8, 174.1.
(2S)-2-{[(2S)-2-{[(Benzyloxy)carbonyl]amino}-3-(4-hydroxy-
phenyl)propanoyl]amino}-3-(1H-indol-3-yl)propanoic Acid
(N-L-Cbz-Tyr-L-Trp-OH, 5e)
White microcrystals; yield: 0.226 g (90%); mp 114–119 °C;
[a]D23 +2.4 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 2.59 (dd, J = 13.4, 11.0 Hz, 1 H), 2.88
(dd, J = 10.0, 3.0 Hz, 1 H), 3.09 (dd, J = 14.6, 8.1 Hz, 1 H), 3.21
(dd, J = 14.6, 5.2 Hz, 1 H), 4.18–4.25 (m, 1 H), 4.51 (q, J = 7.4 Hz,
1 H), 4.86 (d, J = 12.9 Hz, 1 H, A part of AB system), 4.96 (d,
J = 12.9 Hz, 1 H, B part of AB system), 6.63 (d, J = 8.2 Hz, 2 H),
6.98 (t, J = 7.3 Hz, 2 H), 7.04–7.11 (m, 3 H), 7.17–7.18 (m, 3 H),
7.20–7.40 (m, 5 H), 7.55 (d, J = 7.7 Hz, 1 H), 8.24 (d, J = 7.4 Hz, 1
H), 9.18 (s, 1 H), 10.89 (br s, 1 H), 12.61 (s, 1 H).
Anal. Calcd for C22H23N3O5: C, 64.54; H, 5.66; N, 10.26. Found: C,
64.44; H, 5.66; N, 10.03.
(2S)-2-{[(2S)-2-{[(Benzyloxy)carbonyl]amino}-3-(1H-indol-3-
yl)propanoyl]amino}-3-sulfanylpropanoic Acid (N-Cbz-L-Trp-
L-Cys-OH, 5b)
13C NMR (DMSO-d6): d = 27.0, 36.7, 52.9, 56.3, 65.1, 109.6, 111.3,
114.8, 118.2, 118.4, 121.0, 123.7, 127.2, 127.4, 127.6, 128.1, 128.3,
130.1, 136.1, 137.0, 155.8, 171.7, 173.2.
White microcrystals; yield: 0.190 g (86%); mp 140–144 °C;
[a]D23 –28.5 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 2.41 (t, J = 8.2 Hz, 1 H), 2.76–2.88 (m, 2
H), 2.93 (dd, J = 14.0, 10.2 Hz, 1 H), 3.14 (dd, J = 14.0, 2.7 Hz, 1
H), 4.36–4.40 (m, 1 H), 4.47 (q, J = 6.6 Hz, 1 H), 4.94 (s, 2 H), 6.98
(t, J = 7.2 Hz, 1 H), 7.07 (t, J = 7.4 Hz, 1 H), 7.17–7.35 (m, 7 H),
7.45 (d, J = 8.2 Hz, 1 H), 7.66 (d, J = 7.4 Hz, 1 H), 8.33 (d, J = 7.7
Hz, 1 H), 10.82 (s, 1 H), 12.96 (s, 1 H).
13C NMR (DMSO-d6): d = 25.5, 27.7, 54.4, 55.4, 65.3, 110.1, 111.3,
118.2, 118.5, 120.9, 124.0, 127.3, 127.5, 127.7, 128.3, 136.1, 134.0,
155.9, 171.5, 172.1.
ESI-FTICR-MS: m/z [M + Na]+ calcd for C28H27N3O6: 524.1792;
found: 524.1801.
(2S)-2-{[(2S)-5-Amino-2{[(benzyloxy)carbonyl]amino}-5-oxo-
pentanoyl]amino}propanoic Acid (N-Cbz-L-Gln-L-Ala-OH, 5f)
White microcrystals; yield: 0.126 g (72%); mp 211–213 °C;
[a]D23 –2.1 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.28 (d, J = 7.1 Hz, 3 H), 1.62–1.75 (m,
1 H), 1.89–1.99 (m, 1 H), 2.05–2.20 (m, 2 H), 4.00 (q, J = 8.5 Hz, 1
H), 4.19 (quin, J = 6.8 Hz, 1 H), 5.01 (s, 2 H), 6.77 (s, 1 H), 7.25–
7.42 (m, 7 H), 8.20 (d, J = 7.2 Hz, 1 H), 12.60 (br s, 1 H).
Anal. Calcd for C22H23N3O5S: C, 59.85; H, 5.25; N, 9.52. Found: C,
60.03; H, 5.28; N, 9.37.
13C NMR (DMSO-d6): d = 17.2, 27.9, 31.5, 47.4, 54.1, 65.4, 127.7,
(2S)-2-{[(2S)-2-{[(Benzyloxy)carbonyl]amino}-3-(1H-indol-3-
yl)propanoyl]amino}-3-hydroxypropanoic Acid (N-Cbz-L-Trp-
L-Ser-OH, 5c)
127.8, 128.4, 137.0, 155.9, 171.4, 173.8, 174.1.
Anal. Calcd for C16H21N3O6: C, 54.69; H, 6.02; N, 11.96. Found: C,
54.44; H, 6.21; N, 11.84.
White microcrystals; yield: 0.183 g (86%); mp 133–138 °C;
[a]D23 –11.4 (c 1.66, DMF).
(2S)-5-Amino-2-{[(2S)-5-amino-2-{[(benzyloxy)carbonyl]ami-
no}-5-oxopentanoyl]amino}-5-oxopentanoic Acid (N-Cbz-L-
Gln-L-Gln-OH, 5g)
1H NMR (DMSO-d6): d = 2.93 (dd, J = 14.3, 10.3 Hz, 1 H), 3.16
(dd, J = 14.3, 3.3 Hz, 1 H), 3.68 (dd, J = 10.9, 3.8 Hz, 1 H), 3.77
(dd, J = 10.9, 4.9 Hz, 1 H), 4.32–4.41 (m, 2 H), 4.93 (s, 2 H), 6.96
(t, J = 7.4 Hz, 1 H), 7.06 (t, J = 7.4 Hz, 1 H), 7.17–7.40 (m, 8 H),
7.67 (d, J = 8.0 Hz, 1 H), 8.22 (d, J = 7.7 Hz, 1 H), 10.81 (s, 1 H),
12.60 (s, 1 H).
13C NMR (DMSO-d6): d = 27.8, 54.8, 55.4, 61.4, 65.2, 110.2, 112.3,
118.2, 118.6, 120.8, 123.9, 127.4, 127.7, 128.3, 136.1, 137.0, 135.8,
155.8, 172.0, 172.1.
White microcrystals; yield: 0.094 g (47%); mp 132–134 °C;
[a]D23 +26.7 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.72–2.04 (m, 4 H), 2.30–2.34 (m, 4 H),
3.37 (br s, 2 H), 4.07 (q, J = 8.3 Hz, 1 H), 4.19–4.27 (m, 1 H), 5.05
(s, 2 H), 7.36–7.39 (m, 5 H), 7.48 (d, J = 8.0 Hz, 1 H), 8.20 (d,
J = 7.4 Hz, 1 H), 12.32 (s, 1 H).
13C NMR (DMSO-d6): d = 26.2, 27.3, 30.0, 30.1, 51.1, 53.7, 65.4,
127.7, 127.8, 128.3, 137.0, 155.9, 171.6, 173.1, 173.8, 174.0.
Anal. Calcd for C22H23N3O6: C, 62.11; H, 5.45; N, 9.88. Found: C,
62.01; H, 5.43; N, 9.71.
Anal. Calcd for C18H24N4O7: C, 52.94; H, 5.92. Found: C, 52.70; H,
5.65.
(2S)-2-{[(2S)-2-{[(Benzyloxy)carbonyl]amino}-3-(1H-indol-3-
yl)propanoyl]amino}-3-(1H-indol-3-yl)propanoic Acid (N-Cbz-
L-Trp-L-Trp-OH, 5d)
(2S)-2-{[(2S)-5-Amino-2-{[(benzyloxy)carbonyl]amino}-5-oxo-
pentanoyl]amino}-3-methylbutanoic Acid (N-Cbz-L-Gln-L-Val-
OH, 5h)
White microcrystals; yield: 0.225 g (85%); mp 188–190 °C;
[a]D23 –15.6 (c 1.66, DMF).
White microcrystals (from H2O); yield: 0.125 g (66%); mp 186–
1H NMR (DMSO-d6): d = 2.89 (dd, J = 14.2, 10.6 Hz, 1 H), 3.07–
3.15 (m, 2 H), 3.22 (dd, J = 14.2, 4.8 Hz, 1 H), 4.33–4.35 (m, 1 H),
4.51–4.55 (m, 1 H), 4.93 (s, 2 H), 6.94–7.00 (m, 2 H), 7.01–7.08 (m,
2 H), 7.13 (s, 2 H), 7.19–7.38 (m, 8 H), 7.56 (d, J = 7.8 Hz, 1 H),
188 °C; [a]D23 +5.5 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 0.89 (d, J = 6.9 Hz, 6 H), 1.66–1.76 (m,
1 H), 1.85–2.00 (m, 1 H), 2.01–2.23 (m, 3 H), 4.09 (q, J = 8.7 Hz, 1
H), 4.17 (dd, J = 8.2, 5.8 Hz, 1 H), 5.02 (s, 2 H), 6.79 (s, 1 H), 7.28–
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420
A. R. Katritzky et al.
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7.39 (m, 6 H), 7.46 (d, J = 8.2 Hz, 1 H), 7.94 (d, J = 8.5 Hz, 1 H),
12.66 (br s, 1 H).
13C NMR (DMSO-d6): d = 14.8, 26.3, 29.7, 30.1, 31.9, 51.3, 53.8,
65.6, 127.9, 128.0, 128.5, 137.1, 156.1, 171.8, 173.3, 173.9.
13C NMR (DMSO-d6): d = 17.8, 19.1, 27.9, 30.0, 31.6, 54.2, 57.0,
65.4, 127.7, 127.8, 128.4, 137.0, 155.9, 172.0, 173.0, 173.9.
Anal. Calcd for C18H24N2O7S: C, 52.42; H, 5.86; N, 6.79. Found: C,
52.45; H, 5.84; N, 6.70.
Anal. Calcd. for C18H25N3O6: C, 56.98; H, 6.64; N, 11.07. Found:
C, 57.24; H, 6.80; N, 10.96.
(S)-2-[(S)-2-(9H-Fluoren-9-ylmethoxycarbonylamino)-3-(1H-
indol-3-yl)propionylamino]-3-hydroxypropionic Acid (Fmoc-L-
Trp-L-Ser-OH, 5m)
(S)-2-[(S)-2-Benzyloxycarbonylamino-4-methylsulfanylbutyr-
ylamino]propanoic Acid (Cbz-L-Met-L-Ala-OH, 5i)
White microcrystals; yield: 0.168 g (95%); mp 137–138 °C;
[a]D23 –9.0 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.28 (d, J = 7.1 Hz, 3 H), 1.76–1.90 (m,
2 H), 2.03 (s, 3 H), 2.46–2.50 (m, 2 H), 4.11 (d, J = 5.2 Hz, 1 H),
4.19 (t, J = 7.2 Hz, 1 H), 5.02 (s, 2 H), 7.35 (s, 5 H), 7.48 (d, J = 8.2
Hz, 1 H), 8.24 (d, J = 6.9 Hz, 1 H), 12.55 (br s, 1 H).
White microcrystals; yield: 0.223 g (87%); mp 114–115 °C (de-
composed); [a]D23 –13.7 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 2.95 (dd, J = 14.6, 10.2 Hz, 1 H), 3.15
(dd, J = 14.6, 3.4 Hz, 1 H), 3.66 (dd, J = 10.7, 4.1 Hz, 1 H), 3.76
(dd, J = 10.7, 5.0 Hz, 1 H), 4.13 (s, 3 H), 4.30–4.41 (m, 2 H), 5.01
(br s, 1 H), 6.97 (t, J = 7.5 Hz, 1 H), 7.05 (t, J = 6.9 Hz, 1 H), 7.19–
7.42 (m, 6 H), 7.54 (d, J = 8.8 Hz, 1 H), 7.62 (t, J = 8.2 Hz, 2 H),
7.70 (d, J = 7.7 Hz, 1 H), 7.87 (d, J = 7.4 Hz, 2 H), 8.24 (d, J = 7.7
Hz, 1 H), 10.81 (s, 1 H), 12.66 (br s, 1 H).
13C NMR (DMSO-d6): d = 46.5, 54.7, 55.4, 61.3, 65.6, 110.3, 111.2,
118.1, 118.6, 120.0, 120.8, 123.8, 125.3, 127.1, 127.3, 127.6, 136.1,
140.6, 143.8, 155.7, 171.9, 172.0.
13C NMR (DMSO-d6): d = 14.6, 17.0, 29.5, 31.8, 47.5, 53.6, 65.4,
127.7, 127.8, 128.3, 137.0, 155.9, 171.3, 174.0.
Anal. Calcd for C16H22N2O5S: C, 54.22; H, 6.26; N, 7.90. Found: C,
54.53; H, 6.34; N, 8.30.
Anal. Calcd for C29H27N3O6: C, 67.82; H, 5.30; N, 8.18. Found: C,
67.42; H, 5.51; N, 8.39.
(S)-2-[(S)-2-Benzyloxycarbonylamino-4-methylsulfanylbutyr-
ylamino]-3-methylsulfanylbutanoic Acid (Cbz-L-Met-L-Met-
OH, 5j)
(2S)-2-{[(2S)-2-{[(9H-Fluoren-9-ylmethoxy)carbonyl]amino}-
4-(methylsulfanyl)butanoyl]amino}-3-hydroxypropanoic Acid
(Fmoc-L-Met-L-Ser-OH, 5n)
White microcrystals; yield: 0.197 g (95%); mp 120–122 °C;
[a]D23 –8.4 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.76–1.86 (m, 2 H), 1.89–2.00 (m, 2 H),
2.03 (s, 6 H), 2.47–2.51 (m, 4 H), 4.07–4.14 (m, 1 H), 4.28–4.35 (m,
1 H), 5.01 (s, 2 H) 7.33–7.35 (m, 5 H), 7.80 (d, J = 8.8 Hz, 1 H),
8.21 (d, J = 7.8 Hz, 1 H), 12.68 (br s, 1 H).
13C NMR (DMSO-d6): d = 14.6, 29.5, 29.6, 30.6, 31.7, 50.9, 53.7,
65.4, 127.7, 127.8, 128.3, 137.0, 155.9, 171.6, 173.2.
White microcrystals; yield: 0.202 g (88%); mp 157–159 °C;
[a]D23 –2.3 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.78–1.99 (m, 2 H), 2.04 (s, 3 H), 2.49–
2.51 (m, 2 H), 3.66 (dd, J = 11.0, 4.3 Hz, 1 H), 3.77 (dd, J = 11.0,
5.1 Hz, 1 H), 4.10–4.29 (m, 5 H), 5.01 (s, 1 H, OH), 7.30–7.35 (m,
2 H), 7.39–7.44 (m, 2 H), 7.59 (d, J = 8.3 Hz, 1 H), 7.70–7.75 (m, 2
H), 7.89 (d, J = 7.5 Hz, 2 H), 8.08 (d, J = 7.7 Hz, 1 H), 12.64 (br s,
1 H).
Anal. Calcd for C18H26N2O5S2: C, 52.19; H, 6.32; N, 6.76. Found:
C, 52.09; H, 6.38; N, 6.72.
13C NMR (DMSO-d6): d = 14.6, 29.6, 31.9, 46.6, 53.7, 54.6, 61.2,
65.6, 120.1, 125.3, 127.1, 127.7, 140.7, 143.7, 143.9, 155.9, 171.6,
171.9.
(2S)-2-{[(2S)-2-{[(Benzyloxy)carbonyl]amino}-4-(methylsulfa-
nyl)butanoyl]amino}-3-(1H-indol-3-yl)propanoic Acid (Cbz-L-
Met-L-Trp-OH, 5k)
Anal. Calcd for C23H26N2O6S: C, 60.25; H, 5.72; N, 6.11. Found: C,
60.67; H, 5.67; N, 5.75.
White microcrystals; yield: 0.193 g (82%); mp 172–173 °C;
[a]D23 +8.2 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.78–1.88 (m, 2 H), 2.00 (s, 3 H), 2.41–
2.47 (m, 2 H), 3.07 (dd, J = 14.5, 8.0 Hz, 1 H), 3.18 (dd, J = 14.5,
5.1 Hz, 1 H), 4.13–4.14 (m, 1 H), 4.46–4.48 (m, 1 H), 5.01 (d,
J = 12.5 Hz, 1 H, A part of AB system), 5.03 (d, J = 12.5 Hz, 1 H,
B part of AB system) 6.97 (t, J = 6.9 Hz, 1 H), 7.06 (t, J = 7.7 Hz, 1
H), 7.31–7.34 (m, 6 H), 7.46 (br s, 1 H), 7.51 (t, J = 7.7 Hz, 1 H),
8.13 (d, J = 7.4 Hz, 1 H), 10.87 (s, 1 H), 12.67 (br s, 1 H).
13C NMR (DMSO-d6): d = 14.6, 26.9, 29.5, 31.8, 52.9, 53.7, 65.4,
109.6, 111.3, 118.1, 118.4, 120.9, 123.6, 127.2, 127.7, 127.8, 128.3,
136.0, 137.0, 155.9, 171.4, 173.2.
(2S)-2-[(2S)-2-{[(9H-Fluoren-9-ylmethoxy)carbonyl]amino}-4-
(methylsulfanyl)butanoyl]aminopentanedioic Acid (Fmoc-L-
Met-L-Glu-OH, 5o)
White microcrystals; yield: 0.235 g (94%); mp 148–150 °C;
[a]D23 –18.1 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.75–2.01 (m, 4 H), 2.04 (s, 3 H), 2.27–
2.29 (m, 2 H), 2.45–2.48 (m, 2 H), 4.09–4.29 (m, 5 H), 7.32 (t,
J = 7.5 Hz, 2 H), 7.42 (t, J = 7.3 Hz, 2 H), 7.58 (d, J = 8.2 Hz, 1 H),
7.71–7.75 (m, 2 H), 7.89 (d, J = 7.7 Hz, 2 H), 8.19 (d, J = 7.7 Hz, 1
H), 12.42 (br s, 2 H).
13C NMR (DMSO-d6): d = 14.6, 26.2, 29.5, 30.0, 31.8, 46.7, 51.2,
53.6, 65.6, 120.1, 125.3, 127.1, 127.6, 140.7, 143.7, 144.0, 155.9,
171.6, 173.2, 173.8.
Anal. Calcd for C24H27N3O5S: C, 61.39; H, 5.80; N, 8.95. Found: C,
61.42; H, 5.96; N, 8.78.
(2S)-2-{[(2S)-2-{[(Benzyloxy)carbonyl]amino}-4-(methylsul-
fanyl)butanoyl]amino}pentanedioic Acid (Cbz-L-Met-L-Glu-
OH, 5l)
Anal. Calcd for C25H28N2O7S: C, 60.10; H, 5.64; N, 5.60. Found: C,
60.07; H, 5.64; N, 6.06.
White microcrystals; yield: 0.192 g (93%); mp 119–120 °C;
(2S)-2-[((2S)-2-{[(9H-Fluoren-9-ylmethoxy)carbonyl]amino}-3-
hydroxypropanoyl)amino]propanoic Acid (Fmoc-L-Ser-L-Ala-
OH, 5p)
[a]D23 –8.2 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.74–1.87 (m, 4 H), 2.03 (s, 3 H), 2.27–
2.32 (m, 2 H), 2.45–2.48 (m, 2 H), 4.08–4.16 (m, 1 H), 4.18–4.25
(m, 1 H), 5.02 (s, 2 H), 7.33–7.37 (m, 5 H), 7.49 (d, J = 8.0 Hz, 1
H), 8.19 (d, J = 7.7 Hz, 1 H), 12.44 (br s, 2 H).
White microcrystals; yield: 0.143 g (72%); mp 131–132 °C;
[a]D23 –2.3 (c 1.5, DMF).
1H NMR (DMSO-d6): d = 1.31 (d, J = 7.2 Hz, 3 H), 3.55 (dd,
J = 10.5, 7.1 Hz, 1 H), 3.65 (dd, J = 10.5, 5.4 Hz, 1 H), 4.06–4.33
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Coupling N-(Cbz- or Fmoc-a-aminoacyl)benzotriazoles with Unprotected Amino Acids
421
(m, 5 H), 4.44 (br s, 1 H), 7.32–7.40 (m, 2 H), 7.43–7.48 (m, 2 H),
7.45–7.90 (m, 2 H), 7.93 (d, J = 7.4 Hz, 2 H), 8.20 (d, J = 7.1 Hz, 1
H).
13C NMR (DMSO-d6): d = 22.0, 30.8, 36.6, 53.5, 58.7, 66.8, 126.4,
127.4, 127.9, 128.2, 128.3, 129.1, 135.6, 137.3, 150.4, 170.9, 172.7,
173.6.
13C NMR (DMSO-d6): d = 17.3, 46.6, 47.6, 57.1, 61.8, 65.7, 120.1,
(2S)-2-({(2S)-2-{[(Benzyloxy)carbonyl]amino}-5-[(1-carboxy-2-
methylpropyl)amino]-5-oxopentanoyl}amino)-3-methylbu-
tanoic Acid (Cbz-L-Glu-L-di-Val-OH, 5u)
125.4, 127.1, 127.7, 140.7, 143.8, 143.9, 160.0, 170.0, 174.1.
(S)-2-{(S)-2-Benzyloxycarbonylamino-3-[(S)-2-benzyloxycar-
bonylamino-2-((S)-1-carboxyethylcarbamoyl)ethyldisulfan-
yl]propionylamino}propionic Acid
White microcrystals; yield: 0.170 g (71%); mp 87–88 °C;
[a]D23 –11.1 (c 2.08, DMF).
(Cbz-L-Cystine-L-di-Ala-OH, 5q)
1H NMR (CDCl3): d = 0.86 (d, J = 6.2 Hz, 6 H), 0.88 (d, J = 6.2 Hz,
6 H), 1.27 (s, 1 H), 2.17–2.23 (m, 4 H), 2.44 (dd, J = 17.5, 7.9 Hz,
1 H), 2.74 (dd, J = 17.4, 7.4 Hz, 1 H), 4.53 (dd, J = 8.3, 4.5 Hz, 1
H), 4.65 (d, J = 7.4 Hz, 1 H), 5.22 (d, J = 12.6 Hz, 1 H, A part of AB
system), 5.26 (d, J = 12.6 Hz, 1 H, A part of AB system), 6.80 (d,
J = 8.6 Hz, 2 H), 7.30–7.35 (m, 7 H).
13C NMR (CDCl3): d = 14.3, 17.7, 19.1, 22.5, 22.9, 31.1, 31.7, 31.8,
57.5, 60.1, 68.8, 128.3, 128.7, 128.8, 135.0, 151.7, 171.2, 174.3,
174.8 (2).
White microcrystals; yield: 0.312 g (96%); mp 158–160 °C;
[a]D23 –102.2 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.29 (d, J = 7.4 Hz, 6 H), 2.86 (dd,
J = 13.0, 10.9 Hz, 2 H), 3.13 (dd, J = 13.0, 3.3 Hz, 2 H), 4.21 (quin,
J = 7.2 Hz, 2 H), 4.31–4.37 (m, 2 H), 5.04 (s, 4 H), 7.33–7.35 (m,
10 H), 7.59 (d, J = 8.6 Hz, 2 H), 8.27 (d, J = 7.1 Hz, 2 H), 12.58 (br
s, 2 H).
13C NMR (DMSO-d6): d = 17.0, 40.6, 47.7, 53.7, 65.6, 127.7, 127.8,
128.3, 136.9, 156.0, 169.9, 173.8.
ESI-FTICR-MS: m/z [M + H]+ calcd for C23H33N3O8: 480.2340;
found: 480.2360.
Anal. Calcd for C28H34N4O10S2: C, 51.68; H, 5.27; N, 8.61. Found:
C, 51.87; H, 5.29; N, 8.26.
(R)-2-[(S)-2-Benzyloxycarbonylamino-4-methylsulfanylbutyr-
ylamino]propanoic Acid (Cbz-L-Met-D-Ala-OH, 5v)
White microcrystals; yield: 0.168 g (95%); mp 157–158 °C;
[a]D23 –33.4 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.26 (d, J = 7.4 Hz, 3 H), 1.71–1.96 (m,
2 H), 2.02 (s, 3 H), 2.41–2.51 (m, 2 H), 4.09–4.15 (m, 1 H), 4.20
(quin, J = 7.3 Hz, 1 H), 5.07 (s, 2 H), 7.06–7.36 (m, 5 H), 7.45 (d,
J = 8.5 Hz, 1 H), 8.17 (d, J = 7.4 Hz, 1 H), 12.57 (br s, 1 H).
(S)-2-{(S)-2-Benzyloxycarbonylamino-3-[(S)-2-benzyloxycar-
bonylamino-2-((S)-1-carboxy-2-methylpropylcarbamoyl)eth-
yldisulfanyl]propionylamino}-3-methylbutyric Acid
(Cbz-L-Cystine-L-di-Val-OH, 5r)
White microcrystals; yield: 0.322 g (91%); mp 98–100 °C;
[a]D23 –80.4 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 0.86 (d, J = 6.9 Hz, 12 H), 2.03–2.09 (m,
2 H), 2.85–2.93 (m, 2 H), 3.11–3.16 (m, 2 H), 4.16 (dd, J = 8.0, 5.8
Hz, 2 H), 4.34–4.41 (m, 2 H), 5.01 (d, J = 12.5 Hz, 2 H, A part of
AB system), 5.04 (d, J = 12.5 Hz, 2 H, A part of AB system), 7.28–
7.38 (m, 10 H), 7.66 (d, J = 8.8 Hz, 2 H), 7.97 (d, J = 8.5 Hz, 2 H),
12.71 (br s, 2 H).
13C NMR (DMSO-d6): d = 14.6, 17.4, 29.6, 31.2, 47.5, 53.7, 65.4,
127.7, 127.8, 128.3, 137.0, 155.9, 171.1, 173.9.
Anal. Calcd for C16H22N2O5S: C, 54.22; H, 6.26; N, 7.90. Found: C,
54.31; H, 6.42; N, 7.79.
13C NMR (DMSO-d6): d = 17.9, 19.0, 30.0, 40.6, 53.9, 57.2, 65.8,
2-{[(2S)-2-{[(Benzyloxy)carbonyl]amino}-3-(1H-indol-3-yl)pro-
panoyl]amino}propanoic Acid (Cbz-L-Trp-DL-Ala-OH, 5a +
5a¢)
127.7, 127.8, 128.3, 136.9, 156.0, 170.2, 172.7.
Anal. Calcd for C32H42N4O10S2: C, 54.38; H, 5.99; N, 7.93. Found:
C, 54.45; H, 6.23; N, 7.63.
White microcrystals; yield: 0.192 g (94%); mp 93–95 °C;
[a]D23 –18.4 (c 1.66, DMF).
(3S)-3-{[(Benzyloxy)carbonyl]amino}-4-{[(1S)-1-carboxyeth-
yl]amino}-4-oxobutanoic Acid (Cbz-L-Asp-L-Ala-OH, 5s)
White microcrystals; yield: 0.127 g (75%); mp 112–113 °C;
[a]D23 –57.8 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.37 (d, J = 7.2 Hz, 3 H), 2.60 (dd,
J = 17.0, 5.8 Hz, 1 H), 3.05 (dd, J = 17.0, 9.4 Hz, 1 H), 4.45–4.50
(m, 1 H), 4.68 (q, J = 6.9 Hz, 1 H), 5.04 (s, 2 H), 7.30–7.38 (m, 5
H), 8.00 (d, J = 8.2 Hz, 1 H), 12.83 (br s, 1 H).
1H NMR (DMSO-d6): d = 1.23 (d, J = 8.0 Hz, 3 H), 1.31 (d, J = 7.5
Hz, 3 H), 2.85–2.95 (m, 2 H), 3.05–3.15 (m, 2 H), 4.16–4.37 (m, 4
H), 4.92–4.94 (m, 4 H), 6.94–7.00 (m, 2 H), 7.03–7.08 (m, 2 H),
7.16 (br s, 2 H), 7.23–7.37 (m, 14 H), 7.63–7.70 (m, 2 H), 8.30 (d,
J = 7.4 Hz, 1 H), 8.36 (d, J = 7.4 Hz, 1 H), 10.81 (s, 2 H), 12.59 (br
s, 2 H).
13C NMR (DMSO-d6): d = 14.0, 17.2, 17.3, 22.1, 27.8, 28.2, 31.0,
47.5, 55.2, 55.3, 65.2, 110.0, 110.1, 111.3, 118.1, 118.6, 120.8,
123.9, 124.0, 127.3, 127.4, 127.5, 127.7, 128.3, 136.0, 136.1, 137.0,
137.1, 155.7, 155.8, 171.5, 171.8, 174.0, 174.1.
13C NMR (DMSO-d6): d = 14.1, 34.7, 47.3, 49.3, 65.8, 127.8, 127.9,
128.3, 136.6, 155.7, 170.3, 173.7, 175.4.
Anal. Calcd for C22H23N3O5: C, 64.54; H, 5.66; N, 10.26. Found: C,
64.70; H, 6.06; N, 9.69.
ESI-FTICR-MS: m/z [M + H]+ calcd for C15H18N2O7: 339.1187;
found: 339.1192.
2-((S)-2-Benzyloxycarbonylamino-4-methylsulfanylbutyryl-
amino)propanoic Acid (Cbz-L-Met-DL-Ala-OH, 5i + 5v)
White microcrystals; yield: 0.63 g (92%); mp 141–142 °C;
[a]D23 –5.4 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.24 (d, J = 6.8 Hz, 3 H), 1.27 (d, J = 6.8
Hz, 3 H), 1.79–1.84 (m, 4 H), 2.03 (s, 6 H), 2.49–2.51 (m, 4 H),
4.09–4.22 (m, 4 H), 5.01 (s, 1 H), 5.02 (s, 1 H), 7.33–7.39 (m, 10
H), 7.43–7.49 (m, 2 H), 8.17 (d, J = 7.4 Hz, 1 H), 8.23 (d, J = 7.1
Hz, 1 H), 12.56 (br s, 2 H).
(4S)-4-{[(Benzyloxy)carbonyl]amino}-5-{[(1S)-1-carboxy-2-
phenylethyl]amino}-5-oxopentanoic Acid (Cbz-L-Glu-L-Phe-
OH, 5t)
White microcrystals; yield: 0.186 g (88%); mp 168–170 °C;
[a]D23 +15.2 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.79–1.84 (m, 1 H), 2.17–2.31 (m, 1 H),
2.36–2.45 (m, 2 H), 2.90 (dd, J = 13.7, 9.8 Hz, 1 H), 3.03 (dd,
J = 13.7, 5.2 Hz, 1 H), 4.42–4.50 (m, 1 H), 4.66 (dd, J = 11.0, 2.1
Hz, 1 H), 5.02 (d, J = 12.9 Hz, 1 H, A part of AB system), 5.12 (d,
J = 12.9 Hz, 1 H, B part of AB system), 7.22–7.33 (m, 10 H), 7.32
(s, 1 H), 8.59 (d, J = 7.6 Hz, 1 H), 12.78 (s, 1 H).
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York

422
A. R. Katritzky et al.
PAPER
13C NMR (DMSO-d6): d = 14.6, 17.0, 17.3, 29.5, 29.6, 31.9, 47.5,
53.5, 53.7, 65.4, 127.7, 127.8, 128.3, 137.0, 155.9, 171.1, 171.2,
173.9, 174.0.
Anal. Calcd for C36H31N7O4: C, 69.11; H, 4.99; N, 15.67. Found: C,
68.97; H, 5.05; N, 15.55.
Benzyl N-{(S)-1-[(S)-2-benzotriazol-1-yl-1-methyl-2-oxoethyl-
carbamoyl]-3-methylsulfanylpropyl}carbamate (Cbz-L-Met-L-
Ala-Bt, 6c)
Anal. Calcd for C16H22N2O5S: C, 54.22; H, 6.26; N, 7.90. Found: C,
54.22; H, 6.20; N, 8.09.
White microcrystals; yield: 1.936 g (85%); mp 104–105 °C;
2-{(S)-2-Benzyloxycarbonylamino-3-[(S)-2-benzyloxycarbon-
ylamino-2-(1-carboxyethylcarbamoyl)ethyldisulfanyl]propion-
ylamino}propionic Acid
[a]D23 –19.3 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.57 (d, J = 7.2 Hz, 3 H), 1.75–1.99 (m,
2 H), 2.05 (s, 3 H), 2.47–2.50 (m, 2 H), 4.17–4.24 (m, 1 H), 5.01 (s,
2 H), 5.59 (quin, J = 6.7 Hz, 1 H), 7.33–7.35 (m, 5 H), 7.53 (d,
J = 8.0 Hz, 1 H), 7.64 (t, J = 7.7 Hz, 1 H), 7.80 (t, J = 7.7 Hz, 1 H),
8.22 (d, J = 8.3 Hz, 1 H), 8.29 (d, J = 8.3 Hz, 1 H), 8.90 (d, J = 5.5
Hz, 1 H).
13C NMR (DMSO-d6): d = 14.6, 16.5, 29.5, 31.7, 48.6, 53.3, 65.4,
113.9, 120.2, 126.7, 127.7, 127.8, 128.3, 130.6, 131.1, 136.9, 145.3,
155.9, 171.9, 172.0.
(Cbz-L-Cystine-DL-di-Ala-OH, 5q + 5q¢)
White microcrystals; yield: 0.296 g (91%); mp 153–154 °C;
[a]D23 –118.4 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.27 (d, J = 6.9 Hz, 6 H), 1.29 (d, J = 6.9
Hz, 6 H), 2.81–2.89 (m, 4 H), 3.10–313 (m, 4 H), 4.18–4.21 (m, 4
H), 4.22–4.34 (m, 4 H), 5.04 (s, 8 H), 7.34–7.36 (m, 20 H), 7.57 (t,
J = 9.2 Hz, 4 H), 8.26–8.31 (m, 4 H), 12.62 (s, 4 H).
13C NMR (DMSO-d6): d = 17.0, 17.3, 40.6, 47.6, 53.6, 65.5, 127.6,
127.8, 128.3, 136.9, 156.0, 169.6, 169.9, 173.7, 173.8.
Anal. Calcd for C22H25N5O4S: C, 58.01; H, 5.53; N, 15.37. Found:
C, 58.07; H, 5.58; N, 15.00.
Anal. Calcd for C28H34N4O10S2: C, 51.68; H, 5.27; N, 8.61. Found:
C, 51.80; H, 5.14; N, 8.81.
Benzyl N-[(S)-1-((R)-2-benzotriazol-1-yl-1-methyl-2-oxoethyl-
carbamoyl)-3-methylsulfanylpropyl]carbamate (Cbz-L-Met-D-
Ala-Bt, 6d)
N-Cbz-Dipeptidoylbenzotriazoles 6a–d and Diastereomeric
Mixture 6a + 6d; General Procedure
The preparation of 6a–d and 6a + 6d was performed at –10 °C un-
der similar conditions as those described for 2a–n. In the case of 6b,
the desired product was isolated by column chromatography, using
EtOAc–hexanes (2:1) as eluent.
White microcrystals; yield: 1.982 g (87%); mp 135–137 °C;
[a]D23 –7.6 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.55 (d, J = 7.2 Hz, 3 H), 1.77–1.91 (m,
2 H), 2.03 (s, 3 H), 2.44–2.50 (m, 2 H), 4.12–4.22 (m, 1 H), 4.97–
5.07 (m, 2 H), 5.58 (quin, J = 6.8 Hz, 1 H), 7.34–7.35 (m, 5 H), 7.47
(d, J = 8.2 Hz, 1 H), 7.64 (t, J = 7.6 Hz, 1 H), 7.81 (t, J = 7.7 Hz, 1
H), 8.23 (d, J = 8.6 Hz, 1 H), 8.29 (d, J = 8.3 Hz, 1 H), 8.85 (d,
J = 5.8 Hz, 1 H).
Benzyl N-{(S)-1-[(S)-2-benzotriazol-1-yl-1-methyl-2-oxoethyl-
carbamoyl]-2-(1H-indol-3-yl)-ethyl}carbamate (Cbz-L-Trp-L-
Ala-Bt, 6a)
White microcrystals; yield: 1.991 g (78%); mp 176–177 °C;
13C NMR (DMSO-d6): d = 14.6, 16.7, 29.5, 31.8, 48.6, 53.5, 65.4,
114.0, 120.2, 126.7, 127.7, 127.8, 128.3, 130.6, 131.0, 136.9, 145.3,
155.9, 171.7, 171.8.
[a]D23 –27.7 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.60 (d, J = 7.4 Hz, 3 H), 2.86–2.92 (m,
1 H), 3.11–3.16 (m, 1 H), 4.39–4.43 (m, 1 H), 4.92 (s, 2 H), 5.62–
5.66 (m, 1 H), 6.94–6.99 (m, 1 H), 7.02–7.07 (m, 1 H), 7.22–7.34
(m, 6 H), 7.42 (d, J = 8.5 Hz, 2 H), 7.62–7.72 (m, 2 H), 7.79–7.84
(m, 1 H), 8.25 (d, J = 8.3 Hz, 1 H), 8.31 (d, J = 8.2 Hz, 1 H), 9.02
(d, J = 5.8 Hz, 1 H), 10.83 (s, 1 H).
13C NMR (DMSO-d6): d = 16.1, 27.3, 48.2, 54.4, 64.7, 109.5, 110.8,
113.5, 117.6, 118.1, 119.7, 120.3, 123.5, 126.2, 126.7, 127.0, 127.1,
127.8, 130.2, 130.6, 135.6, 136.4, 144.8, 155.3, 171.3, 171.9.
Anal. Calcd for C22H25N5O4S: C, 58.01; H, 5.53; N, 15.37. Found:
C, 57.74; H, 5.42; N, 15.09.
Benzyl N-[(S)-1-(2-benzotriazol-1-yl-1-methyl-2-oxoethylcar-
bamoyl)-3-methylsulfanylpropyl]carbamate (Cbz-L-Met-DL-
Ala-Bt, 6c + 6d)
White microcrystals; yield: 1.913 g (84%); mp 96–97 °C;
[a]D23 –10.6 (c 1.66, DMF).
Anal. Calcd for C28H26N6O4: C, 65.87; H, 5.13; N, 16.46. Found: C,
65.83; H, 5.01; N, 16.19.
1H NMR (DMSO-d6): d = 1.55 (d, J = 6.5 Hz, 3 H), 1.57 (d, J = 6.5
Hz, 3 H), 1.78–1.94 (m, 4 H), 2.04 (s, 3 H), 2.05 (s, 3 H), 2.43–2.48
(m, 4 H), 4.20–4.25 (m, 2 H), 4.98–5.06 (m, 4 H), 5.59 (quin, J = 6.8
Hz, 2 H), 7.33–7.37 (m, 10 H), 7.47 (d, J = 8.2 Hz, 1 H), 7.54 (d,
J = 8.0 Hz, 1 H), 7.64 (t, J = 7.7 Hz, 2 H), 7.81 (t, J = 7.6 Hz, 2 H),
8.22 (d, J = 8.3 Hz, 2 H), 8.29 (d, J = 8.3 Hz, 2 H), 8.85 (d, J = 5.8
Hz, 1 H), 8.90 (d, J = 5.5 Hz, 1 H).
13C NMR (DMSO-d6): d = 14.6, 14.7 16.5, 16.7, 29.5, 29.6, 31.7,
31.8, 48.6, 48.7, 53.3, 53.5, 65.5, 114.0, 120.2, 126.7, 126.8, 127.7,
127.8, 128.3, 130.6, 131.1, 131.1, 137.0, 145.3, 155.9, 156.0, 171.8,
171.9, 172.0. Anal. Calcd for C22H25N5O4S: C, 58.01; H, 5.53; N,
15.37. Found: C, 58.15; H, 5.61; N, 15.00.
Benzyl N-{(S)-1-[(S)-2-benzotriazol-1-yl-1-(1H-indol-3-ylmeth-
yl)-2-oxoethylcarbamoyl]-2-(1H-indol-3-yl)ethyl}carbamate
(Cbz-L-Trp-L-Trp-Bt, 6b)
Pale yellow microcrystals; yield: 2.378 g (76%); mp 152–154 °C
(decomposed); [a]D23 +10.1 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 2.89 (dd, J = 14.6, 10.3 Hz, 1 H), 3.14
(dd, J = 14.6, 3.8 Hz, 1 H), 3.31–3.39 (m, 1 H), 3.53 (dd, J = 14.5,
5.2 Hz, 1 H), 4.40–4.48 (m, 1 H), 4.93 (s, 2 H), 5.97 (q, J = 5.7 Hz,
1 H), 6.90–6.97 (m, 2 H), 7.00–7.06 (m, 3 H), 7.22–7.34 (m, 8 H),
7.42 (d, J = 8.5 Hz, 1 H), 7.54 (d, J = 7.7 Hz, 1 H), 7.60–7.68 (m, 2
H), 7.79 (t, J = 7.6 Hz, 1 H), 8.21 (d, J = 8.2 Hz, 1 H), 8.26 (d,
J = 8.5 Hz, 1 H), 8.98 (d, J = 6.3 Hz, 1 H), 10.82 (s, 1 H), 10.89 (s,
1 H).
Tripeptides 7a–f; General Procedure
The preparation of tripeptides 7a–f was carried out using the proce-
dure described for the preparation of 5 at –10 °C instead of 20 °C.
13C NMR (DMSO-d6): d = 27.1, 27.7, 53.8, 55.0, 65.3, 108.7, 110.0,
111.3, 111.4, 114.0, 118.0, 118.1, 118.4, 118.5, 120.1, 120.8, 121.0,
123.9, 124.1, 126.6, 126.9, 127.2, 127.5, 127.7, 128.3, 130.5, 131.0,
136.0, 136.1, 136.9, 145.3, 155.8, 171.4, 172.6.
(S)-2-{(S)-2-[(S)-2-Benzyloxycarbonylamino-3-(1H-indol-3-yl)-
propionylamino]propionylamino}-3-mercaptopropionic Acid
(Cbz-L-Trp-L-Ala-L-Cys-OH, 7a)
White microcrystals; yield: 0.220 g (86%); mp 114–115 °C (de-
composed); [a]D23 –23.0 (c 1.66, DMF).
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York

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Coupling N-(Cbz- or Fmoc-a-aminoacyl)benzotriazoles with Unprotected Amino Acids
423
1H NMR (DMSO-d6): d = 1.25 (d, J = 6.9 Hz, 3 H), 2.44–2.46 (m,
1 H), 2.74–2.99 (m, 3 H), 3.10–3.16 (m, 1 H), 4.29–4.47 (m, 3 H),
4.93 (s, 2 H), 6.94–6.99 (m, 1 H), 7.03–7.08 (m, 1 H), 7.16 (app s,
1 H), 7.23–7.34 (m, 6 H), 7.42 (d, J = 8.0 Hz, 1 H), 7.66 (d, J = 7.7
Hz, 1 H), 8.12 (d, J = 7.7 Hz, 1 H), 8.25 (d, J = 7.5 Hz, 1 H), 10.80
(s, 1 H), 12.90 (s, 1 H).
13C NMR (DMSO-d6): d = 18.0, 25.5, 27.7, 48.1, 54.2, 55.4, 65.2,
110.1, 111.2, 118.1, 118.6, 120.8, 123.9, 127.3, 127.4, 127.6, 128.3,
136.0, 137.0, 155.9, 171.4, 171.7, 172.2.
(S)-2-{(S)-2-[(S)-2-Benzyloxycarbonylamino-4-methylsulfanyl-
butyrylamino]propionylamino}-3-(1H-indol-3-yl)propionic
Acid (Cbz-L-Met-L-Ala-L-Trp-OH, 7e)
White microcrystals; yield: 0.249 g (92%); mp 171–172 °C (de-
composed); [a]D23 +5.2 (c 1.66, DMF).
1H NMR (DMSO-d6): dr = 4:1; d = 1.77 (d, J = 7.4 Hz, 3 H), 1.76–
11.96 (m, 2 H), 2.00 (s, 3 H), 2.42–2.50 (m, 2 H), 3.01–3.08 (m, 1
H), 3.13–3.17 (m, 1 H), 4.08–4.09 (m, 1 H), 4.32–4.34 (m, 1 H),
4.44–4.46 (m, 1 H), 5.01 (s, 2 H), 6.94–6.99 (m, 1 H), 7.02–7.07 (m,
1 H), 7.12–7.16 (m, 1 H), 7.31–7.33 (m, 5 H), 7.48–7.56 (m, 2 H),
8.00 (d, J = 7.1 Hz, 0.8 H), 8.11 (d, J = 7.1 Hz, 1 H), 8.17 (d, J = 8.0
Hz, 0.2 H), 10.83 (s, 1 H), 12.65 (br s, 1 H).
Anal. Calcd for C25H28N4O6S: C, 58.58; H, 5.51; N, 10.93. Found:
C, 58.86; H, 5.58; N, 11.13.
(S)-2-{(S)-2-[(S)-2-Benzyloxycarbonylamino-3-(1H-indol-3-yl)-
propionylamino]-3-(1H-indol-3-yl)propionylamino}-3-(1H-in-
dol-3-yl)propanoic Acid (Cbz-L-Trp-L-Trp-L-Trp-OH, 7b)
White microcrystals; yield: 0.309 g (87%); mp 127–129 °C (de-
composed); [a]D23 –13.6 (c 1.66, DMF).
13C NMR (DMSO-d6): d = 14.6, 18.2, 27.0, 29.6, 31.7, 47.9, 52.9,
53.7, 65.4, 109.5, 111.3, 118.1, 118.3, 120.9, 123.7, 127.7, 127.8,
128.3, 136.0, 137.0, 155.9, 171.0, 171.7, 172.1, 173.1, 173.2.
Anal. Calcd for C27H32N4O6S: C, 59.98; H, 5.97; N, 10.36. Found:
C, 59.84; H, 6.24; N, 10.19.
1H NMR (DMSO-d6): dr = 3:1; d = 2.75–2.85 (m, 2 H), 2.95–3.23
(m, 4 H), 4.24–4.30 (m, 1 H), 4.45–4.56 (m, 1 H), 4.62–4.74 (m, 1
H), 4.91 (s, 2 H), 6.82–7.08 (m, 8 H), 7.17–7.36 (m, 10 H), 7.52–
7.65 (m, 3 H), 8.09 (d, J = 7.9 Hz, 0.75 H), 8.30 (d, J = 8.8 Hz, 1
H), 8.39 (d, J = 8.0 Hz, 0.25 H), 10.72–10.84 (m, 3 H), 12.67 (br s,
1 H).
2-{(S)-2-[(S)-2-Benzyloxycarbonylamino-4-methylsulfanylbut-
yrylamino]propionylamino}propanoic Acid (Cbz-L-Met-DL-
Ala-L-Ala-OH, 7f)
White microcrystals; yield: 0.187 g (88%); mp 176–177 °C (de-
composed); [a]D23 –8.6 (c 1.66, DMF).
13C NMR (DMSO-d6): d = 27.1, 27.7, 53.1, 55.5, 65.2, 109.5, 109.8,
109.9, 110.0, 110.2, 110.3, 111.2, 111.3, 118.1, 118.3, 118.5, 120.8,
120.9, 123.6, 123.8, 127.3, 127.4, 127.6, 128.2, 136.0, 136.8, 136.9,
155.7, 171.4, 171.6, 173.1, 173.3.
1H NMR (DMSO-d6): d = 1.24 (d, J = 6.7 Hz, 1.5 H), 1.26 (d,
J = 6.7 Hz, 1.5 H), 1.29 (d, J = 6.7 Hz, 1.5 H), 1.32 (d, J = 6.7 Hz,
1.5 H), 1.82–1.97 (m, 2 H), 2.06 (s, 3 H), 2.44–2.54 (m, 1 H), 4.11–
4.16 (m, 1 H), 4.20–4.28 (m, 1 H), 4.33 (quin, J = 7.1 Hz, 2 H), 5.11
(s, 2 H), 7.37–7.39 (m, 5 H), 7.55 (d, J = 7.9 Hz, 0.5 H), 7.65 (d,
J = 7.5 Hz, 0.5 H), 8.04–8.06 (m, 1 H), 8.16–8.22 (m, 1 H), 12.57
(br s, 1 H).
Anal. Calcd for C41H38N6O6: C, 69.28; H, 5.39; N, 11.82. Found: C,
68.93; H, 5.47; N, 11.83.
(S)-2-{(S)-2-[(S)-2-Benzyloxycarbonylamino-4-methylsulfanyl-
butyrylamino]propionylamino}propanoic Acid (Cbz-L-Met-L-
Ala-L-Ala-OH, 7c)
13C NMR (DMSO-d6): d = 14.5, 17.0, 17.1, 18.1, 29.5, 31.7, 47.3,
47.4, 47.3, 47.9, 53.7, 54.0, 65.3, 65.4, 127.6, 127.7, 128.2, 136.8,
137.0, 171.0, 171.5, 171.7, 173.7, 173.8.
White microcrystals; yield: 0.183 g (86%); mp 213–214 °C;
Anal. Calcd for C19H27N3O6S: C, 53.63; H, 6.40; N, 9.88. Found: C,
53.45; H, 6.49; N, 9.68.
[a]D23 –13.0 (c 1.66, DMF).
1H NMR (DMSO-d6): d = 1.22 (d, J = 7.6 Hz, 3 H), 1.27 (d, J = 7.4
Hz, 3 H), 1.73–1.91 (m, 2 H), 2.02 (s, 3 H), 2.43–2.51 (m, 2 H),
4.05–4.13 (m, 1 H), 4.18 (quin, J = 7.2 Hz, 1 H), 4.29 (quin, J = 7.2
Hz, 1 H), 5.02 (s, 2 H), 7.33–7.39 (m, 5 H), 7.51 (d, J = 7.9 Hz, 1
H), 8.01 (d, J = 7.5 Hz, 1 H), 8.14 (d, J = 7.2 Hz 1 H), 12.54 (br s,
1 H).
(S)-2-{2-[(S)-2-Benzyloxycarbonylamino-3-(1H-indol-3-yl)pro-
pionylamino]acetylamino}-4-methylpentanoic Acid (Cbz-L-
Trp-L-Gly-L-Leu-OH, 7g)
White microcrystals; yield: 0.163 g (62%); mp 130–131 °C;
[a]D23 –36.5 (c 1.66, DMF).
13C NMR (DMSO-d6): d = 14.6, 17.1, 18.2, 29.6, 31.7, 47.4, 47.8,
1H NMR (DMSO-d6): d = 0.84 (d, J = 7.0 Hz, 6 H), 1.45–1.65 (m,
3 H), 2.94 (dd, J = 14.5, 10.5 Hz, 1 H), 3.13 (dd, J = 14.5, 3.9 Hz, 1
H), 3.70–3.74 (m, 2 H), 4.12–4.13 (m, 1 H), 4.23–4.27 (m, 1 H),
4.93 (s, 2 H), 6.96 (t, J = 7.4 Hz, 1 H), 7.02–7.07 (m, 1 H), 7.18 (br
s, 1 H), 7.23–7.34 (m, 6 H), 7.62 (d, J = 7.7 Hz, 1 H), 7.74–7.77 (m,
2 H), 8.55 (s, 1 H), 10.82 (br s, 1 H), 12.60 (s, 1 H).
13C NMR (DMSO-d6): d = 21.9, 22.0, 24.3, 27.6, 41.5, 42.3, 51.5,
55.8, 65.2, 110.3, 111.2, 118.1, 118.4, 120.7, 123.8, 127.2, 127.4,
127.6, 128.2, 136.0, 137.0, 155.9, 167.9, 172.2, 174.2.
53.8, 65.4, 127.7, 127.8, 128.3, 137.0, 156.0, 171.0, 171.9, 174.0.
Anal. Calcd for C19H27N3O6S: C, 53.63; H, 6.40; N, 9.88. Found: C,
53.62; H, 6.44; N, 9.50.
(S)-2-{(S)-2-[(S)-2-Benzyloxycarbonylamino-4-methylsulfanyl-
butyrylamino]propionylamino}-3-hydroxypropanoic Acid
(Cbz-L-Met-L-Ala-L-Ser-OH, 7d)
White microcrystals; yield: 0.183 g (83%); mp 177–178 °C;
[a]D23 –4.4 (c 1.66, DMF).
Anal. Calcd for C27H32N4O6⋅H2O: C, 61.58; H, 6.51; N, 10.64.
Found: C, 61.19; H, 6.18; N, 10.57.
1H NMR (DMSO-d6): dr = 4:1; d = 1.23 (d, J = 6.9 Hz, 3 H), 1.76–
1.96 (m, 2 H), 2.02 (s, 3 H), 2.44–2.50 (m, 2 H), 3.60–3.74 (m, 2 H),
4.06–4.18 (m, 1 H), 4.25–4.32 (m, 1 H), 4.36 (t, J = 7.2 Hz, 1 H),
5.02 (s, 2 H), 7.33–7.36 (m, 5 H), 7.52 (d, J = 7.7 Hz, 0.8 H), 8.02
(d, J = 7.7 Hz, 1 H), 8.09 (d, J = 8.0 Hz, 0.2 H), 12.61 (br s, 1 H).
13C NMR (DMSO-d6): d = 14.6, 18.3, 29.6, 31.7, 47.9, 53.8, 54.6,
61.3, 65.4, 127.7, 127.8, 127.9, 128.3, 137.0, 156.0, 171.0, 171.8,
172.2.
References
(1) (a) Gross, M.; Meienhofer, J. The Peptides; Academic Press:
New York, 1979. (b) Goodman, M.; Felix, A.; Moroder, L.;
Toniolo, C. Synthesis of Peptides and Peptidomimetics, In
Science of Synthesis, Vol. E22a and E22b; Georg Thieme
Verlag: Stuttgart, New York, 2001. (c) Sheppard, R. C.
Amino-acids, Peptides, and Proteins, Vol. 10; The Chemical
Society: London, 1977. (d) Bodanszky, M.; Klausner, Y. S.;
Ondetti, M. A. Peptide Synthesis, 2nd ed.; John Wiley and
Anal. Calcd for C19H27N3O7S: C, 51.69; H, 6.16; N, 9.52. Found: C,
51.57; H, 6.38; N, 9.19.
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York

424
A. R. Katritzky et al.
PAPER
(8) (a) Katritzky, A. R.; Pastor, A. J. Org. Chem. 2000, 65,
Sons: New York, 1976. (e) Wipf, P. Chem. Rev. 1995, 95,
2115. (f) Humphrey, J. M.; Chamberlin, A. R. Chem. Rev.
1997, 97, 2243. (g) Fletcher, M. D.; Campbell, M. M. Chem.
Rev. 1998, 98, 763.
3679. (b) Katritzky, A. R.; Abdel-Fattah, A. A. A.; Wang,
M. J. Org. Chem. 2003, 68, 4932. (c) Katritzky, A. R.;
Abdel-Fattah, A. A. A.; Wang, M. J. Org. Chem. 2003, 68,
1443. (d) Katritzky, A. R.; Suzuki, K.; Singh, S. K.; He,
H.-Y. J. Org. Chem. 2003, 68, 5720. (e) Katritzky, A. R.;
Suzuki, K.; Singh, S. K. Croat. Chem. Acta 2004, 77, 175.
(9) (a) Katritzky, A. R.; Pastor, A.; Voronkov, M. V. J.
Heterocycl. Chem. 1999, 36, 777. (b) Katritzky, A. R.;
Denisko, O. V.; Fang, Y.; Zhang, L.; Wang, Z. Arkivoc
2001, (xi), 41.
(2) Hegarty, A. F.; McCarthy, D. G. J. Am. Chem. Soc. 1980,
102, 4537.
(3) (a) Schmidt, U.; Utz, R.; Lieberknecht, A.; Griesser, H.;
Potzolli, B.; Bahr, J.; Wagner, K.; Fischer, P. Synthesis
1987, 236. (b) Hofmann, K.; Haas, W.; Smithers, M. J.;
Zanetti, G. J. Am. Chem. Soc. 1965, 87, 631.
(4) Gagnon, P.; Huang, X.; Therrien, E.; Keillor, J. W.
Tetrahedron Lett. 2002, 43, 7717.
(10) (a) Katritzky, A. R.; Suzuki, K.; Singh, S. K. Synthesis 2004,
2645. (b) Katritzky, A. R.; Angrish, P.; Hür, D.; Suzuki, K.
Synthesis 2005, 397.
(11) (a) Fredrich, J.; Young, E. A. J. Med. Chem. 1964, 44, 820.
(b) Fredrich, J.; Young, E. A.; Bowen, D. O. J. Med. Chem.
1965, 56, 274. (c) Kunz, H.; Buchholz, M. Chem. Ber. 1979,
112, 2145. (d) Iwamura, M.; Hodota, C.; Ishibashi, M.
Synlett 1991, 35.
(5) Bauer, H.; Staab, K. H.; Schneider, K. M. Azolides in
Organic Synthesis and Biochemistry; John Wiley and Sons
Ltd: New York, 1998.
(6) Gross, H.; Bilk, L. Tetrahedron 1968, 24, 6935.
(7) (a) Katritzky, A. R.; He, H.-Y.; Suzuki, K. J. Org. Chem.
2000, 65, 8210. (b) Katritzky, A. R.; Yang, H.; Zhang, S.;
Wang, M. Arkivoc 2002, (xi), 39. (c) Katritzky, A. R.;
Yang, B.; Semenzin, D. J. Org. Chem. 1997, 62, 726.
(d) Katritzky, A. R.; Wang, M.; Yang, H.; Zhang, S.;
Akhmedov, N. G. Arkivoc 2002, (viii), 134. (e) Katritzky,
A. R.; Wang, M.; Zhang, S. Arkivoc 2001, (ix), 19.
Synthesis 2006, No. 3, 411–424 © Thieme Stuttgart · New York