43
Table 3. Amidation of 3-phenylpropanoic acid (1) with ¡-amino
acids 2a-2f without protection of C-terminalsa
Table 4. Synthesis of dipeptides 6 without protection of C-
terminals in ¡-amino acidsa
O
Ph
O
R
1) ClCO2Et, Et3N, THF, 0 °C, 30 min
1) ClCO2Et, Et3N, THF, 0 °C, 30 min
Ph
O
O
Ph
OH
(1.0 mmol)
N
H
6
R
O
2)
, H2O, 0 °C, 30 min
R
O
, H2O, 0 °C, 30 min
2)
P-HN
OH
P-HN
OH
1
5
H2N OH
H2N OH
2
2
O
R
O
R
O
O
Entry P
5
R
2
Dipeptide 6 Yieldb/%
O
O
+
+
EtO
N
H
4
Ph
N
H
Ph
O
Ph
OH
OH
1
2
3
4
5
6
7
8
Boc 5a Me2CH
Fmoc 5b Me2CH
Cbz 5c Me2CH
Cbz 5c Me3C
Cbz 5c HO2CCH2CH2 2e
Cbz 5c H2NCOCH2CH2 2f
2c
2c
2c
2d
6a
6b
6c
6d
6e
6f
77
83
95
83
51
93
88
88
3
1'
Yield of 3/%
Correctedc Crystal Mother solution
Ratio of 3:4:1¤b
Entry R
2
1
2
3
4
5
6
PhCH2
Ph
2a
2b
2c
2d
99
91
91
90
50
73
93
84
89
86
45
73
24:53:22
32:42:26
8:69:23
14:73:13
13:24:64
0:0:100
Me2CH
Me3C
Cbz 5c PhCH2 (L)
Cbz 5c PhCH2 (D)
2a
2g
6g
6h
HO2CCH2CH2 2e
H2NCOCH2CH2 2f
aAll reactions were carried out with 1.0 mmol of N-protected
L-phenylalanine 5, 1.4 mmol of ethyl chloroformate, and 3.0 mmol
of triethylamine in 20 mL of THF. After stirring for 30 min at 0 °C,
1.5 mmol of ¡-amino acid 2 in 20 mL of H2O was added at 0 °C to
aAll reactions were carried out with 1.0 mmol of 1, 1.4 mmol of
ethyl chloroformate, and 3.0 mmol of triethylamine in 20 mL of
THF. After stirring for 30 min at 0 °C, 1.5 mmol of ¡-amino acid
2 in 20 mL of H2O was added at 0 °C to the reaction mixtures.
bDetermined by 1H NMR analysis of the concentrates of the
b
the reaction mixtures. Isolated yields.
c
95% yields. No racemization was observed in our method. We
still examine further investigations about this type of condensa-
tion for preparation of polypeptides.
mother solutions in all cases. Crystal + 3 of the mother solution
(3 + 4 + 1¤).
References and Notes
concentrates of the mother solutions in all entries of Table 2.
The optimized conditions for preparing 3a were treatment of 1
with 1.4 equivalents of ethyl chloroformate and 3.0 equivalents
of triethylamine in THF at 0 °C, followed by addition of 1.5
equivalents of 2a in H2O at 0 °C (Entry 2). Then, Table 3 shows
the results of the reactions of 3-phenylpropanoic acid (1) with
several kinds of ¡-amino acids 2a-2f. The reactions with ¡-
amino acids 2a-2d afforded the corresponding amides 3a-3d in
90-99% yields (Entries 1-4). Although the reaction of 1 with
L-Glu-OH (2e) afforded 3e in 50% yield along with the by-
product 1¤ of 48% yield based on 1 (Entry 5), the reaction with
L-Gln-OH (2f), the amide form of 2e, gave a better yield (73%
in Entry 6).
Finally, condensations of N-protected L-phenylalanine
5a-5c with several kinds of unprotected ¡-amino acids 2a and
2c-2g via mixed carbonic carboxylic anhydrides are shown in
Table 4. We selected tert-butyl carbamate (Boc), 9-fluorenyl-
methyl carbamate (Fmoc), and benzyl carbamate (Cbz) as a
protecting group. L-Val-OH protected by the Cbz group worked
efficiently to afford the corresponding dipeptide 6c in 95% yield
in comparison with those protected by the Boc and Fmoc groups
as shown in Entries 1-3. A bulky ¡-amino acid 2d and L-Gln-
OH (2f) containing amide moiety gave 83% and 93% yields in
Entries 4 and 6, respectively, although L-Glu-OH (2e) was
converted to the dipeptide 6e in 51% yield as indicated in
Entry 5. L- and D-Phe-OH were transformed into the corre-
sponding single diastereomer in 88% yields in Entries 7 and 8,
respectively.3 These results indicate that racemization does not
proceed under the reaction conditions. Furthermore, the amida-
tion of Cbz-L-Phe-L-Val-OH (6c) with L-Ala-OH was carried
out to afford the corresponding tripeptide 7c in 42% yield.
In conclusion, we have developed a convenient peptide
synthesis without protection of C-terminals in ¡-amino acids 2a
and 2c-2g to afford the corresponding dipeptide 6a-6h in 51-
1
For selected reviews on peptide syntheses: a) M. A. Blaskovich,
Handbook on Syntheses of Amino Acids: General Routes for the
Syntheses of Amino Acids, Oxford University Press, New York,
10827. c) T. Wieland, M. Bodanszky, The World of Peptides: A Brief
History of Peptide Chemistry, Springer-Verlag, New York, 1991,
pp. 77-102. For selected reports for peptide syntheses using
unprotected ¡-amino acids: d) P. Gagnon, X. Huang, E. Therrien,
cited therein. e) M. V. Anuradha, B. Ravindranath, Tetrahedron
2
a) W. Chu, Z. Tu, E. McElveen, J. Xu, M. Taylor, R. R. Luedtke,
Girgis, M. Prashad, B. Hu, D. Har, O. Repič, T. J. Blacklock,
Methods of Peptide Bond Formation in The Peptides: Analysis,
Synthesis, Biology, ed. by E. Gross, J. Meienhofer, Academic Press,
New York, 1979, Vol. 1, pp. 263-314.
3
A typical procedure of the amidation of 5c using ethyl chloroformate
is as follows. To a solution of 299 mg (1.0 mmol) of Cbz-L-Phe-OH
(5c) in 20 mL of THF, 134 ¯L (1.4 mmol, 1.4 equivalents) of ethyl
chloroformate and 415 ¯L (3.0 mmol, 3.0 equivalents) of triethyl-
amine were added at 0 °C. After stirring for 30 min at 0 °C, a solution
of 248 mg (1.50 mmol, 1.5 equivalents) of L-phenylalanine (2a) in
20 mL of H2O was added at 0 °C to the colorless suspension. The
mixture was stirred for 30 min at 0 °C, and the colorless clear
solution was concentrated in vacuo. To the residue was added a
1.0 M solution of hydrogen chloride to adjust to pH 2. The resulting
suspension was extracted with 100 mL of ethyl acetate, washed with
10 mL of brine, and dried over anhydrous magnesium sulfate. The
crude product was recrystallized from 40 mL of an 1:3 mixture of
chloroform and hexane to afford 391 mg (88% yield) of 6g (Cbz-L-
Phe-L-Phe-OH). 6g: colorless powder; 1H NMR (CDCl3): ¤ 2.98
(1H, dd, J = 6.4, 14.0 Hz, CHA), 3.02 (2H, d, J = 6.6 Hz, CH2), 3.14
(1H, dd, J = 6.0, 14.0 Hz, CHB), 4.38-4.46 (1H, m, CH), 4.74-4.79
(1H, m, CH), 5.06 (2H, s, CH2), 5.30 (1H, d, J = 7.1 Hz, NH), 6.34
(1H, d, J = 7.3 Hz, NH), 7.02-7.34 (15H, m, C6H5 © 3).
Chem. Lett. 2012, 41, 42-43
© 2012 The Chemical Society of Japan