Sequential One-Pot Synthesis of Dipeptides through the Transient Formation of CDI-N-Protected α-Aminoesters

Article abstract of DOI:10.1002/adsc.201700034

The synthesis of dipeptides through a sequential one-pot procedure from commercially available protected amino acids is described. The transformation relies on the use of in situ generated transiently CDI-protected α-amino esters (CDI, e.g. N,N′-carbonyldiimidazole). In addition of being a highly atom-economical process, the couplings take place under very mild and neutral conditions without adding a base to the reaction medium. This protocol provides a concise and less costly route to dipeptide derivatives (12 examples, up to 87% yield) and is compatible with commonly used N-urethane protecting groups. Moreover, no epimerization was detected even when sensitive Boc-Cys(Bn)?OH was used. (Figure presented.).

Full text of DOI:10.1002/adsc.201700034

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DOI: 10.1002/adsc.201700034
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Sequential One-Pot Synthesis of Dipeptides through the Transient
Formation of CDI-N-Protected a-Aminoesters
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Renata Marcia de Figueiredo,a,* Jean-Simon Suppo,a Camille Midrier,a and Jean-
Marc Campagnea
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a
´
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM-ENSCM, Ecole Nationale Superieure de Chimie de
Montpellier,
8 Rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France
Tel.: (+) 33 (0) 4 67 14 72 24 or (+) 33 (0) 4 67 14 72 21
Fax: (+) 33 (0) 4 67 14 43 22; e-mail: renata.marcia_de_figueiredo@enscm.fr
Received: January 20, 2017; Revised: March 6, 2017; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201700034
economy, expensive and hazardous coupling reagents,
racemization issues, wasteful processes and imposed
Abstract: The synthesis of dipeptides through a
sequential one-pot procedure from commercially
available protected amino acids is described. The
transformation relies on the use of in situ generated
transiently CDI-protected a-amino esters (CDI,
e.g. N,N’-carbonyldiimidazole). In addition of being
C!N peptide synthesis direction). These restrictions
have encouraged recent efforts toward novel, non-
classical methods for amide bond formation.[7] How-
ever, all these methods are not always compatible
with peptide synthesis and, when applied, require the
a highly atom-economical process, the couplings
prior modification of at least one amino acid partner
take place under very mild and neutral conditions
(e.g. PꢀAAꢀ[CO] such as CON3, COSR;
without adding a base to the reaction medium. This
protocol provides a concise and less costly route to
dipeptide derivatives (12 examples, up to 87%
[N]ꢀAAꢀOP such as IꢀNH, R’OꢀNHR etc), which
may detract from their attractiveness.
It is thus clear that innovative methods allowing
yield) and is compatible with commonly used N-
amide bond formation for application in peptide
urethane protecting groups. Moreover, no epimeri-
syntheses that sidestep such limitations is of para-
zation was detected even when sensitive Boc-
mount importance.
Cys(Bn)ꢀOH was used.
Recently, our group developed a protocol based on
the coupling of CDI-protected a-amino esters,[8] easily
Keywords: amide bond formation; dipeptides;
prepared from inexpensive CDI[9,10] and amino esters,
base-free procedure; non-activated carboxylic acids;
atom economy
with N-urethane protected amino acids (Sche-
me 1b).[11] The method offers several advantages
compared to traditional strategies (e.g. less costly and
more atom-economical) but its main features rely on
43 The amide group is one of the most prolific functional the absence of epimerization, observed during the
44 groups in contemporary organic and bioorganic synthesis of several dipeptides, as well as the possibil-
45 chemistry.[1] In addition to its quintessential role to ity to realize the peptide couplings through the reverse
46 sustain life (e.g. backbone of proteins), amides are and challenging N!C direction.[12] The mechanism is
47 also present in several natural products, materials and believed to proceed via the formation of a mixed
48 polymers, and are also one of the utmost flourishing carbamic anhydride, that further evolves after CO2
49 moieties in modern drug-like compounds.[2–4] Indeed, extrusion to amide (Scheme 1, eq b).[13] The devised
50 it has been estimated that the “acylation of amine” procedure goes through an inverse activation strategy
51 represents the most commonly used reaction in the (e.g. amine-type activation)[14,15] which is not based on
52 synthesis of bioactive molecules.[5]
the classical carboxylate activation. Very recently,
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Classical amide synthesis methods involve the other approaches involving the use of “activated
54 activation of the carboxylic acid through the use of amines” have been reported.[16–18] Despite remarkable
55 stoichiometric amounts of coupling reagents (Sche- advantages, the main inconvenience of this method is
56 me 1a).[6] Though very efficient, these traditional the need for a stepwise process with the prior syn-
57 methods suffer from several drawbacks (e.g. low atom thesis and purification of the CDI-protected a-amino
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Scheme 1. Amide bond formation for peptide synthesis. a) Classical methods through carboxylic acid activation. b) Our
28 previous work via the use of protected a-aminoesters. c) This work: Sequential, one-pot dipeptide synthesis.
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31 esters (used as equivalents of “activated amines”)
32 followed by the coupling reaction with the amino acid
33 bearing the free carboxylic acid function.
34
In this update, we describe an alternative pathway
35 that offers a more practical, economical and straight-
36 forward access to dipeptides through a sequential,
37 one-pot protocol via the intermediacy of CDI-pro-
38 tected a-aminoesters (Scheme 1c).
39
In the initial experiments, we investigated the
40 coupling reaction of HCl·H2NꢀAlaꢀOMe (1a) with
41 BocꢀPheꢀOH (3a). Using the two-step procedure
42 developed in our prior communication,[11] the desired
43 dipeptide BocꢀPheꢀAlaꢀOMe (4a) was isolated in
44 57% yield (Schema 2a). When the same reaction
45 conditions were used, without isolating the imidazole
46 derivative 2a, dipeptide 4a was isolated in a disap-
47 pointing 7% yield (Scheme 2b). This observation
48 corroborates our previous statement that the presence
49 of a base during the coupling step hampers amide
Figure 1. Synthesis of intermediate 2a without the addition
of a base with concomitant formation of imidazolium salts.
By doing so, the amide bond formation was
50 bond formation.[19] Therefore, the reaction of CDI and attempted with the successive addition of the
51 1a was carried out in the absence of NEt3. Indeed, BocꢀPheꢀOH, and in this case, we were delighted to
52 released imidazole could act as an internal base and observe the dipeptide 4a formation in an enhanced
53 promote the formation of 4a via HCl neutralization of yield of 53% (Scheme 2c). This yield could be further
54 1a. Moreover, this step could be visually monitored improved to 65% by adding 10 mol% of CuBr2 and
55 with the concomitant precipitation of imidazolium HOBt into the reaction medium.[11]
56 chloride salts in the reaction medium (Figure 1).
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Indeed, we have previously witnessed that the
addition of these additives improves the yield of the
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Scheme 2. Optimization work: a) Our prior work via a two-step procedure; b) First trial towards a sequential one-pot process;
c) Optimization studies for the one-pot strategy.
33 coupling reactions. By simply switching the equivalent work-up. Another crucial point to be mentioned is the
34 amounts of 1a and 3a (from 1.0 and 1.5 equiv. to 1.5 absence of epimerization during the coupling reaction.
35 and 1.0 equiv. respectively) the reaction conditions No diasteroisomer formation has been detected on
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36 could be optimized with the formation of the dipep- crude H NMR and when the sensitive cysteine was
37 tide in 87% yield (Scheme 2c).
coupled,
the
expected
dipeptide
BocꢀCys
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After establishing the optimized reaction condi- (Bn)ꢀAlaꢀOMe (4m) was isolated with no detectable
39 tions, we explored the coupling of several commer- epimerization as shown on the chiral HPLC chromato-
40 cially available amino acid residues in this sequential grams.[20] Moreover, avoiding the prior isolation of
41 one-pot transformation (Scheme 3). Classical N-ure- intermediates 2 renders this one-pot procedure much
42 thane (e.g. Fmoc, Cbz and Boc) and O-alkyl (e.g. Me, more convenient and less costly. In general, the
43 Et, tBu) protecting groups are compatible with the dipeptides are obtained in very good yields (48–87%
44 one-pot process. Moreover, amino acid residues bear- overall yields after the 2 sequential steps). The only
45 ing protected functional groups are also well tolerated exception is proline when used as the CDI-protected
46 in the coupling conditions affording the dipeptides in amino acid for which no reaction was detected (4i).
47 good yields. In all cases, comparable yields for the The same outcome was also observed in the stepwise
48 prior stepwise method and this sequential one-pot procedure.[11] In contrast, when proline was used as
49 procedure are reached. One of the prime advances in the free carboxylic acid partner the desired dipeptides
50 the latter case is the easier final dipeptide purification. are obtained in high yields (4c). Curiously, the
51 Indeed, crude proton NMRs of the reaction products sequential procedure did not seem favorable when
52 after acidic-basic work-up reveal the anticipated unprotected threonine was tested, with formation of
53 dipeptide as single product (see supporting informa- very small amounts of the expected dipeptide 4j (<
54 tion). In some cases, a very small amount of sym- 25% vs 48% through the two-step method).[21] Besides,
55 metrical ureas (less than 5%) can be observed. Thus, the procedure was further applied to the synthesis of a
56 the final purification step requires a simple column tripeptide CbzꢀTrpꢀMetꢀAlaꢀOMe (4n) obtained in
57 filtration on a short pad of silica gel after classical
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Scheme 3. Scope studies. Reaction conditions: HCl·H2NꢀAA1ꢀOR (1.5 equiv.), CDI (1.5 equiv.), CH2Cl2 (1 M), r.t., 30 min
then PHNꢀAA2ꢀCO2H (1.0 equiv.), HOBt (10 mol%), CuBr2 (10 mol%), r.t., 20 h. Isolated yields after column
chromatography of the coupling step (isolated compounds 4) (yields for the two-step protocol are given in parentheses). For
sensitive dipeptide 4m, chiral HPLC chromatograms are given (see supporting information for details). NR=no reaction.
54 45% yield with no detected formation of diaster-
Furthermore, in order to highlight the convenience
and practical sides of the method we conducted a
coupling reaction in multi gram-scale (Scheme 4).
55 eoisomers by 1H NMR.
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Acknowledgements
This work was supported by the Agence Nationale de la
Recherche (projet NIPS ANR JCJC, n8 ANR-12-JS07-0008-
01, grant to JSS). RMF and JMC gratefully thank the CNRS
and the ENSCM. Lindsay Mas-Normand (undergraduate
student on practical training) is kindly thanked for her help
with the gram-scale synthesis of compound 4a. The authors
give special thanks to Pascale Guiffrey (responsible of the
analytical platform of the team) for her support.
Scheme 4. Sequential, one-pot gram-scale synthesis of dipep-
tide Boc-Phe-Ala-OMe (4a).
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References
In summary, the synthesis of several dipeptides and
13 a tripeptide through in situ generated CDI-protected
14 a-amino esters based on an inverse activation mode
15 has been successfully achieved. The devised one-pot
16 procedure, improved from the former studies from
17 our laboratories, provides a much easier, practical and
18 cost-effective method to access dipeptides (12 exam-
19 ples) in good yields (up to 87%) and with complete
20 absence of epimerization. Due to the growing interest
21 for peptides in pharmaceuticals and the ever growing
22 need for novel strategies seeking to the amide bond
23 formation, this sequential one-pot process affords a
24 good and reliable alternative that could be added to
25 the challenging non-conventional methods towards
26 amides for peptide bond formation.
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Experimental Section
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To a suspension of CDI (0.75 mmol, 1.5 equiv.) in CH2Cl2 (c
1 M, stabilized with amylene) was added HCl·H2N-AA1-OR
(0.75 mmol, 1.5 equiv.) and the resulting suspension was
stirred for 30 min at ambient temperature (21–248C). The
transient formation of CDI-N-protected a-aminoesters (2)
could be monitored by TLC. Then, to the reaction mixture
39 was successively added PHN-AA2-CO2H (0.50 mmol,
1.0 equiv.), HOBt (0.05 mmol, 10 mol%) and CuBr2
(0.05 mmol, 10 mol%). The resulting blue suspension so
obtained was then stirred at ambient temperature (21–248C)
for additional 20 h. After reaction completion (TLC monitor-
ing), the mixture was diluted with CH2Cl2 (10 mL) and
washed once with aqueous HCl 1.0 N (10 mL). The aqueous
phase was further extracted with CH2Cl2 (10 mL). The
organic layers were combined and washed once with a
saturated aqueous solution of NaHCO3 (10 mL). The organic
phase was separated and then washed with brine (10 mL).
Afterwards, the organic layer was dried over MgSO4, filtered
and concentrated under reduced pressure. Crude NMR of
the compounds after this classical work-up, showed the
expected dipeptides with excellent purity; only a small
amount of symmetrical ureas (<5%) could be observed in
some cases. Purification was performed by simple filtration
on a short pad of silica gel by using a mixture of pentane/
EtOAc as eluent. Dipeptides 4 were isolated as white foams
or solids.
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[19] We have observed that no reaction occurs when
performed in the presence of NEt3 (see ref. 11).
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Org. Chem. 2004, 69, 2565.
We believe that in the one-pot process 2a is formed in a
mixture with trimethylamine hydrochloride salt and
imidazole. The later could act as a base and to place the
sequential reaction in the above illustrated conditions.
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[21] In this particular case, a considerable amount of sym-
metrical urea of alanine residue (25% yield) was
detected by LC/MS analysis.
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R. M. de Figueiredo*, J.-S. Suppo, C. Midrier, J.-M.
Campagne
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