2-Bromo-1-ethyl pyridinium tetrafluoroborate (BEP): A powerful coupling reagent for N-methylated peptide synthesis

Article abstract of DOI:10.1246/cl.2000.204

2-Bromo-1-ethyl pyridinium tetrafluoroborate (BEP) was firstly utilized to the synthesis of peptides containing N-methyl amino acid residues both in solution and solid phase, and demonstrated high reactivity, low racemization and excellent yields, which w

Full text of DOI:10.1246/cl.2000.204

204
Chemistry Letters 2000
2-Bromo-1-ethyl Pyridinium Tetrafluoroborate (BEP):
A Powerful Coupling Reagent for N-Methylated Peptide Synthesis
Peng Li and Jie Cheng Xu*
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Feng Lin Road, Shanghai 200032, P. R. China
(Received November 4, 1999; CL-990945)
2-Bromo-1-ethyl pyridinium tetrafluoroborate (BEP) was
firstly utilized to the synthesis of peptides containing N-methyl
amino acid residues both in solution and solid phase, and
demonstrated high reactivity, low racemization and excellent
yields, which were approved by the successful synthesis of a
series of oligopeptides and hindered peptide fragments, such as
the 8-11 segment of Cyclosporine A and the pentapeptide moi-
ety of Dolastatin 15.
Since the first phosphonium salt BOP1 was exploited and
used to peptide synthesis, many onium type coupling reagents
have been designed, synthesized and widely used in peptide
chemistry, such as HOBt-derived onium salts: HBTU,2
PyBOP,3 HBPyU,4 HBPipU,5 HBMDU6 and the corresponding
HOAt-based onium salts7: AOP, PyAOP, HATU, HAPyU,
HAMDU. To further enhance the coupling efficiency during
peptide synthesis, we have designed and synthesized immoni-
um and thiazolium type coupling reagents, such as BOMI,8a
BDMP,8b BPMP, AOMP and BEMT8c based upon our previous
studies.8d-8g We now introduce pyridinium salt,9a,9b 2-bromo-1-
ethyl pyridinium tetrafluoroborate (BEP), of which analogues
1-methyl-2-halopyridinium iodides were shown to be efficient
for the syntheses of esters,9c lactones9d and carboxamides,9e
into this field and demonstrate its high efficiency in peptide
synthesis, especially in the incorporation of hindered N-methyl
amino acids into peptides, such as cyclosporins,10 didemins11
and dolastatins.12
Comparing to other halogenerated coupling reagents
PyClU,13a CIP,13b BOP-Cl13c and PyBroP,13d Pyridinium salt
BEP can be more readily synthesized by the N-alkylation of 2-
bromo pyridine using triethyloxonium tetrafluoroborate with
nearly quantitative yield as colorless shelf-stable crystals.14
HPLC monitoring the model reactions (Z-Me-Val-OH +
Me-Val-OMe⋅HCl→Z-Me-Val-Me-Val-OMe and Z-Gly-Phe-
OH + Val-OCH3⋅HCl → Z-Gly-Phe-Val-OCH3) showed that
reagent BEP behaved higher reactivity and lower racemization
during coupling comparing to CBMI, PyBroP, PyClU and
BTFFH et al. (Table 1 and Figure 1).
To further demonstrate the efficiency of BEP during pep-
tide coupling, a series of oligopeptides were synthesized as
shown in Table 2. We also synthesized successfully two exten-
sively N-alkylated peptide segments. During the synthesis of 8-
11 tetrapeptide fragment of cyclosporine A, the C-terminal was
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
205
intentively protected as benzyl ester to challenge the sponta-
neous diketopiperazine formation. A 48% yield was still
obtained during the synthesis of Fmoc-MeLeu-MeLeu-MeVal-
OBzl by sequential deprotection and coupling of Fmoc-MeLeu-
MeVal-OBzl with Fmoc-MeLeu-OH. According to the report
by Wenger during the synthesis of cyclosporine A using modi-
fied mixed pivalic anhydride method, the same desired tripep-
tide was not obtained at all due to the spontaneous formation of
diketopiperazine.15 To further examine the performance of
reagent BEP in SPPS, The hindered 8-11tetrapeptide of CsA
and the linear undecapeptide of CsO were also synthesized in
solid phase using BEP from Fmoc-MeVal-Wang resin. The
purities of the obtained crude peptides were 95% and ~5%16
respectively.
(1992). b) J. Coste, E. Frerot, and P. Jouin, Tetrahedron
Lett., 32, 1967 (1991).
P. Henklein, M. Beyermann, M. Bienert, and R. Knorr,
“Proceeding of the Twenty-First European Peptide
Symposium” ed. by E. Giralt and D. Andreu, ESCOM,
Science, Leiden, (1991), p. 67.
5
6
Y. Kiso, Y. Fujiwara, T. Kimura, A. Nishitani, and K. Akaji,
Int. J. Pept. Protein Res., 40, 308 (1992).
7
a) L. A. Carpino, A. El-Faham, C. A. Minor, and F.
Albericio, J. Chem. Soc., Chem. Commun., 1994, 201. b) F.
Albericio, M. Cases, J. Alsina, S. A. Triolo, L. A. Carpino,
and S. A. Kates, Tetrahedron Lett., 38, 4853 (1997). c) L. A.
Carpino, J. Am. Chem. Soc., 115, 4397 (1993). d) L. A.
Carpino and A. El-Faham, J. Org. Chem., 59, 696 (1994).
a) P. Li and J. C. Xu, Tetrahedron Lett., 40, 3605 (1999). b)
P. Li and J. C. Xu, Chem. Lett. 1999, in press. c) P. Li and J.
C. Xu, Tetrahedron Lett., 40, 8301 (1999). d) S. Q. Chen
and J. C. Xu, Tetrahedron Lett., 32, 6711 (1991). e) S. Q.
Chen and J. C. Xu, Tetrahedron Lett., 33, 647 (1992). f) S.
Q. Chen and J. C. Xu, Chin. Chem. Lett., 4, 847 (1993). g)
S. Q. Chen and J. C. Xu, Chin. J. Chem., 13, 175 (1995).
a) H. Balli and F. Kersting, Liebigs Ann. Chem., 647, 1
(1961). b) K. Saigo, M. Usui, and K. Kikuchi, Bull. Chem.
Soc. Jpn., 50, 1863 (1977). c) T. Mukaiyama, M. Usui, and
E. Shimada, Chem. Lett., 1975, 1045. d) T. Mukaiyama, M.
Usui and K. Saigo, Chem. Lett., 1976, 49. e) E. Bald, K.
Saigo and T. Mukaiyama, Chem. Lett., 1975, 1163.
8
9
10 R. M. Wenger, Angew. Chem., Int. Ed. Engl., 24, 77 (1985).
11 P. Jouin, T. Poncet, M-N. Dufour, A. Pantaloni, and B.
Castro, J. Org. Chem., 54, 617 (1989) and references cited
therein.
12 G. R. Pettit, S. B. Singh, D. L. Herald, P. Lloyd-Williams,
D. Kantoci, D. D. Burkett, J. Barkoczy, F. Hogan, and T. R.
Wardlaw, J. Org. Chem., 59, 6287 (1994) and references
cited therein.
We also synthesized the hindered pentapeptide moiety of
dolastatin 15 which was a pseudopeptide bearing promising
antineoplastic activity. The Boc group was used for the Nα-
protection, thus a 57.7% overall yield of the protected pen-
tapeptide was obtained via seven coupling steps. No or little N-
carboxyanhydride formed during peptide coupling using BEP,
although Boc-protected amino acids were more readily convert-
ed into NCA than Cbz- or Fmoc-protected derivatives and
resulted in relatively low yield using halogenerated reagents.13d
In general, the pyridinium type coupling reagent BEP was
shown to be a very efficient coupling reagent for the synthesis
of the hindered peptide containing N-methyl amino acid
residues with fast reaction speed, low racemization and good
yields.
13 a) J. Coste, E. Frerot, and P. Jouin, Tetrahedron Lett., 32,
1967 (1991). b) K. Akaji, N. Kuriyama, and Y. Kiso,
Tetrahedron Lett., 35, 3315 (1994). c) C. Van der Auwera
and M. J. O. Anteunis, Bull. Soc. Chim. Belg., 95, 203
(1986). d) J. Coste, E. Frerot, and P. Jouin, J. Org. Chem.,
59, 2437 (1994).
14 Coupling reagent BEP was prepared according to literature9a
1
in 95.2% yield. mp 103-104 °C. H NMR (300 MHz, d6-
acetone) δ 1.69 (t, J = 7.3 Hz, 3H), 5.01 (q, J = 7.3 Hz, 2H),
8.24 (m, 1H), 8.53-8.57 (m, 2H), 9.32 (d, J = 6.9 Hz, 1H)
ppm. IR (KBr): ν = 3106, 1617, 1571, 1500, 1467, 1296,
1050, 786, 718, 521 cm-1. FAB-MS m/z: 186, 188.
15 a) J. M. Humphrey and A. R. Chamberlin, Chem. Rev., 97,
2243 (1997). b) R. M. Wenger, Helv. Chim. Acta., 66, 2672
(1983).
References and Notes
1
2
3
4
B. Castro, J. R. Dromoy, G. Evin, and C. Selve, Tetrahedron
Lett., 1975, 1219.
V. Dourtoglou, J. C. Ziegler, and B. Gross, Tetrahedron
Lett., 1978, 1269.
J. Coste, D. Le-Nguyen, and B. Castro, Tetrahedron Lett.,
16 UV analysis of the Fmoc deprotection step indicated that the
coupling yield of each step was above 90%. The low purity
of the crude product was most likely that the amide bonds
between N-methyl amino acids in long peptide fragment
were prone to undergo hydrolysis under strong acidic medi-
um.7b
31, 205 (1990).
a) S. Q. Chen and J. C. Xu, Tetrahedron Lett., 33, 647