Amine capture strategy for peptide bond formation by means of quinolinium thioester salts

Article abstract of DOI:10.1021/ja054775p

A new and unprecedented exploitation of quinolinium thioester salts 2 in peptide bond formation is reported. These synthetic tools were assessed during the preparation of a number of dipeptides 3a-f obtained in good yields with complete stereochemical integrity. A sequential mechanism related to a prior amine capture strategy is well-established. Additionally, a tripeptide 3g was prepared according to a "safety-catch" approach, thus demonstrating the important potential of these new synthetic tools in the design of new safety-catch linkers exploitable in Solid-Phase Peptide Synthesis (SPPS). Copyright

Full text of DOI:10.1021/ja054775p

Published on Web 10/19/2005
Amine Capture Strategy for Peptide Bond Formation by Means of
Quinolinium Thioester Salts
Ste´phane Leleu, Mae¨l Penhoat, Alexis Bouet, Georges Dupas, Cyril Papamicae¨l,
Francis Marsais, and Vincent Levacher*
Laboratoire de Chimie Organique Fine et He´te´rocyclique UMR 6014 IRCOF, CNRS, UniVersite´ et INSA de Rouen,
B.P 08, F-76131 Mont-Saint-Aignan Ce´dex, France
Received July 16, 2005; E-mail: vincent.levacher@insa-rouen.fr
To go beyond the limits of the stepwise solid-phase peptide
synthesis (SPPS),1 various convergent methods have been success-
fully investigated allowing the preparation of large peptides.2
Among them, “natiVe chemical ligation” represents today one of
the most promising approaches that has made the total chemical
synthesis of proteins a practical reality.3 The general principle of
this approach relies on an efficient ligation reaction of two unpro-
tected peptide fragments to bring the two coupling sites into close
proximity. As a result of this entropic activation, the resultant “lig-
ated product” undergoes acyl transfer to furnish the final coupled
peptide containing a native peptide bond at the ligation junction.
The “Prior Amine Capture Strategy”, initially reported by Kemp
Figure 1. Amine capture strategy by means of a quinolinium thioester salt.
in 1975, is the first general approach to ligation-based coupling
method.4 In this context, innovative templates for developing new
Table 1. Preparation of Dipeptides 3a-f by Means of Quinolinium
Thioester Salts 2a,ba
amine capture strategies are highly desirable. Our approach reported
herein capitalizes on conjugated additions to quinolinium salts-type
25 to design a new “electrophilic platform” capable of acting as an
amine capture device. The reversible nature of these conjugated addi-
tions would offer the advantage to realize in a single step the acyl
transfer and release of the template, which represents a real benefit
over existing ligation processes previously reported. A thioester
link was selected to join the amino acid residue and the quinoline
template. This functional group is expected to display a good bal-
ance between reactivity and stability, so that intramolecular acyl
transfer occurs through the ligated product intermediate, while re-
maining nonreactive in the absence of this entropic activation.6 As
a consequence of the incapacity of quinolines 1 to form 1,4-adducts,
this precursor may be considered as a “latent capture amine site”
activated by quaternization. In this perspective, quinolinium thioester
salts 2 are appealing candidates, fulfilling all the criteria of a promis-
ing template for developing a new amine capture strategy (Figure 1).
To validate this approach, Boc-Gly-S-quinolinium salt 2a and
Boc-D-Phe-S-quinolinium 2b were selected as models and were
involved in peptide bond formation with various amino esters. Table
1 summarizes the main results obtained after optimization of the
reaction conditions. After screening several solvents, acetonitrile
provided a good compromise between solubility and reactivity.
Coupling reactions proceeded with excellent conversion rates
(>90%) and, importantly, without detectable epimerization. Dipep-
tides 3a-f could be isolated by flash chromatography with
satisfactory yields ranging from 55 to 75%. It has been established
that a reaction time of 5-6 h was generally required to drive the
coupling reaction to completion (Table 1).
conv. %c
(yield %)
entry template
aminoesterb
dipeptide 3a−f
1
2
3
4
5
6
2a
2a
2a
2b
2b
2b
H-L-PheOMe Boc-Gly-L-PheOMe 3a
H-L-Pro-OMe Boc-Gly-L-Pro-OMe 3b
H-L-Phg-OMe Boc-Gly-L-Phg-OMe 3c
>90 (50)
>90(65)
>90(70)
H-L-Phe-OMe Boc-D-Phe-L-Phe-OMe 3d >90(55)
H-L-Ala-OMe Boc-D-Phe-L-Ala-OMe 3e >90(66)
H-L-Pro-OMe Boc-D-Phe-L-Pro-OMe 3f >90(75)
a Reagents and conditions: NEt3 (2equiv), CD3CN, room temperature,
6 h. b Hydrochloride salts are used. c Measured by 1H NMR spectroscopy.
Scheme 1. Experimental Data Supporting an Amine Capture
Strategy Prior to Peptide Bond Formationa
a Reagents and conditions: (a) NEt3 (2eq)/CH3CN/r.t./6h.
A control experiment revealed that the peptide coupling is totally
ineffective in the absence of triethylamine, pointing out the crucial
role of this base. Triethylamine probably takes part at some stage
of the amine capture step for assisting the deprotonation of the
amino ester. This observation strongly argues in favor of an amine
capture prior to peptide bond formation. Finally, indirect confirma-
tion was obtained for the proposed intermediate in a separated UV
spectroscopy experiment performed with quinolinium salt 7.
Whereas UV spectra of quinolinium salt 7 remain unchanged by
addition of benzylamine, sequential additions of NEt3 led to the
formation of the adduct product 8 which was further isolated and
1
fully characterized by H and 13C NMR (Figure 2).
The incapability of quinolines 1a,b to react with amino esters
under the same conditions than those employed with quinolinium
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J. AM. CHEM. SOC. 2005, 127, 15668-15669
10.1021/ja054775p CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
AcOEt) and subsequently coupled with Boc-glycine under standard
activation conditions (EDCI/HOBt/DIEA/CH2Cl2), providing the
dipeptide Boc-Gly-D-Phe-S-quinoline 1d in 73% overall yield
(Scheme 2). At this stage, no residual reactivity of the thioester
was detected, demonstrating that both deprotection and activation
conditions are compatible with a safety-catch approach. After
activation of the amine capture site by quaternization of 1d, the
resulting quinolinium salt 2c was reacted with L-alanine methyl
ester, under the optimized conditions established above, to supply
the desired tripeptide Boc-Gly-D-Phe-L-Ala-OMe 3g in a satisfac-
tory yield (Scheme 2).
In summary, we have demonstrated that quinolinium thioester
salts-type 2 display an attractive potential in peptide synthesis.
Interestingly, a number of experimental observations lend to the
belief that a sequential mechanism related to a prior amine capture
strategy is involved. The “latent reactiVity” of the nonquaternized
quinoline 1 renders this precursor an appealing synthetic tool in
view of developing a new safety-catch linker. These preliminary
results laid down the basis of future developments in SPPS.
Figure 2. Participation of NEt3 in conjugated additions of amines to the
quinolinium salt 7 evidenced by means of UV spectroscopy.
Acknowledgment. We thank the CNRS, the re´gion Haute-
Normandie, and the CRIHAN for financial and technical support.
Scheme 2. Preparation of a Tripeptide 3g Using a Safety-Catch
Supporting Information Available: Experimental procedures and
characterization data for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
Approacha
References
(1) Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149.
(2) For illustrative examples of convergent approaches by coupling of
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Angew. Chem., Int. Ed. Engl. 1991, 30, 590. (b) Hendrix, J. C.; Lansbury,
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(3) For leading references on fragment peptide coupling by prior chemical
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Chemistry, Structures and Biology; Walter, R., Ed.; Ann Arbor Science:
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(5) The reversible nature of these additions has already been sporadically
investigated by us and others, in view of developing new nucleophile-
transferring agents. (a) Leleu, S.; Papamicae¨l, C.; Marsais, F.; Dupas, G.;
Levacher, V. Tetrahedron: Asymmetry 2004, 15, 3919. (b) Kellog, R. M.
Angew. Chem., Int. Ed. Engl. 1984, 23, 782. (c) Mashraqui, S. H.; Kellog,
R. M. J. Am. Chem. Soc. 1983, 105, 7792.
(6) The thioester group has already been widely used in native chemical
ligation by means of a prior thiol capture strategy; see ref 3.
(7) For leading references on safety-catch linker used in solid-phase peptide
synthesis, see: (a) Backes, B. J.; Virgilio, A. A.; Ellman, J. A. J. Am.
Chem. Soc. 1994, 116, 11171. (b) Backes, B. J.; Virgilio, A. A.; Ellman,
J. A. J. Am. Chem. Soc. 1996, 118, 3955. (c) Bourne, G. T.; Golding, S.
W.; McGeary, R. P.; Meutermans, W. D. F.; Jones, A.; Marshall, G. R.;
Alewood, P. F.; Smythe, M. L. J. Org. Chem. 2001, 66, 7706.
a Reagents and conditions: (a) HCl (3 M) in AcOEt, 1 h, 0 °C; (b) Boc-
glycine/EDCI/HOBt/DIEA/CH2Cl2, 20 °C, 12 h; (c) TfOMe/Na2CO3, 20
°C, 1 h; (d) L-alanine methyl ester hydrochloride (1 equiv)/NEt3 (2 equiv)/
CH3CN, room temperature, 6 h.
salts 2a,b provided additional evidence for the proposed sequential
mechanism. With these experimental observations in mind, we then
speculated that quinolines 1a,b may be conceptually considered as
a “masked amine capture site”, which may possibly be used in the
design of a new “safety-catch” linker7 and exploitable in Boc
peptide synthesis (Scheme 1).
To validate the whole sequence of this safety-catch approach,
quinoline 1b was subjected to Boc deprotection conditions (HCl/
JA054775P
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