Amino triazolo diazepines (ata) as constrained histidine mimics

Article abstract of DOI:10.1021/ol202767k

Two synthetic routes for the synthesis of amino-triazolodiazepine (Ata) scaffolds are presented. The scope of both of these proceeding through key intra- and intermolecular Huisgen cycloaddition reactions is discussed. The replacement of the His-Pro dipep

Full text of DOI:10.1021/ol202767k

ORGANIC
LETTERS
2011
Vol. 13, No. 24
6468–6471
Amino Triazolo Diazepines (Ata) as
Constrained Histidine Mimics
Koen Buysse,† Julien Farard,† Alexandros Nikolaou,‡ Patrick Vanderheyden,‡
Georges Vauquelin, Daniel Sejer Pedersen, Dirk Tourwe, and Steven Ballet*
‡
§
,†
ꢀ †
Laboratory of Organic Chemistry, Vrije Universiteit Brussel, Pleinlaan 2,
B-1050 Brussels, Belgium, Molecular and Biochemical Pharmacology Laboratory,
Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, and Department of
Medicinal Chemistry, University of Copenhagen, Universitetsparken 2,
2100 Copenhagen, Denmark
sballet@vub.ac.be
Received October 14, 2011
ABSTRACT
Two synthetic routes for the synthesis of amino-triazolodiazepine (Ata) scaffolds are presented. The scope of both of these proceeding through
key intra- and intermolecular Huisgen cycloaddition reactions is discussed. The replacement of the His-Pro dipeptide segment in angiotensin IV
by the dipeptide mimetic Ata-Gly and subsequent biological evaluation in two inhibitory enzyme assays validated the use of the Ata moiety as a His
mimic given the equipotency of both peptidic analogs.
The introduction of conformational constraints in bio-
active peptides is an efficient strategy to influence receptor
selectivity, ligand potency, metabolic stability, and mem-
brane permeation.1,2 Peptide cyclization induces a global
constraint, whereas single amino acid modifications can
induce more local or subtle changes. Especially, side chain
constrained analogs of the aromatic amino acids Phe, Tyr,
Trp, and His were shown to be very successful in peptide
medicinal chemistry.3
peptidomimetic chemistry for introducing global and local
conformational constraints.4 1,2,3-Triazoles have been
employed to great effect as replacements of backbone pep-
tide bonds5 or to stabilize turn6,7 or helical8 architectures
(4) (a) Angell, Y. L.; Burgess, K. Chem. Soc. Rev. 2007, 36, 1674–
1689. (b) Yoo, B.; Shin, S. B. Y.; Huang, M. L.; Kirshenbaum, K.
Chem.;Eur. J. 2010, 16 (19), 5528–5537. (c) Pedersen, D. S.; Abell, A.
Eur. J. Org. Chem. 2011, 13, 2399–2411.
(5) (a) Tam, A.; Arnold, U.; Soellner, M. B.; Raines, R. T. J. Am.
Chem. Soc. 2007, 129, 12670–12671. (b) Horne, W. S.; Yadav, M. K.;
Stout, C. D.; Ghadiri, M. R. J. Am. Chem. Soc. 2004, 126 (47), 15366–
15367. (c) van Maarseveen, J. H.; Horne, W. S.; Ghadiri, M. R. Org.
Lett. 2005, 7 (20), 4503–4506.
Due to the appealing structural and electronic proper-
ties of 1,2,3-triazoles, they are finding an increased use in
(6) (a) Appukkuttan, P.; Dehaen, W.; Fokin, V. V.; Van der Eycken,
E. Org. Lett. 2004, 6 (23), 4223–4225. (b) Oh, K.; Guan, Z. B. Chem.
Commun. 2006, 29, 3069–3071. (c) Holland-Nell, K.; Meldal, M. Angew.
Chem., Int. Ed. 2011, 50, 5204–5206.
(7) Pokorski, J. K.; Jenkins, L. M. M.; Feng, H. Q.; Durell, S. R.; Bai,
Y. W.; Appella, D. H. Org. Lett. 2007, 9 (12), 2381–2383.
(8) (a) Cantel, S.; Isaad, A. L. C.; Scrima, M.; Levy, J. J.; DiMarchi,
R. D.; Rovero, P.; Halperin, J. A.; D’Ursi, A. M.; Papini, A. M.;
Chorev, M. J. Org. Chem. 2008, 73 (15), 5663–5674. (b) Scrima, M.;
Le Chevalier-Isaad, A.; Rovero, P.; Papini, A. M.; Chorev, M.; D’Ursi,
A. M. Eur. J. Org. Chem. 2010, 3, 446–457. (c) Ingale, S.; Dawson, P. E.
Org. Lett. 2011, 13 (11), 2822–2825.
† Laboratory of Organic Chemistry, Vrije Universiteit Brussel.
‡ Molecular and Biochemical Pharmacology Laboratory, Vrije Universi-
teit Brussel.
§ University of Copenhagen.
€
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Medicinal Chemistry. In The practice of Medicinal Chemistry, 2nd ed.;
Academic Press: London, 2003; pp 233À250. (c) Boguslavsky, V.; Hruby,
V. J.; O’Brien, D. F.; Misicka, A.; Lipkowski, A. J. J. Peptide Res. 2003,
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2011; pp 101À124.
r
10.1021/ol202767k
Published on Web 11/16/2011
2011 American Chemical Society

by cyclization between side chain or backbone modified
amino acid residues.4a,9
regioselective cycloaddition reaction to produce 1,5-sub-
stituted triazoles.13,14 The intermolecular route (Route A,
Figure 2) uses a ruthenium catalyst to form the 1,5-
substituted triazole 9, followed by a lactamization to
generate the desired6. The intramolecular approach on the
other hand (Route B, Figure 2) consists of a thermal
Huisgen cycloaddition reaction of 10, which is only able
to form the desired 1,5-substituted triazole, during which
both cycles, the triazole as well as the lactam ring, are
formed simultaneously. The latter strategy was used by
Pokorski et al.7 to prepare the six-membered lactam 12
from 16 (Scheme 1).
Prior to the synthesis of Ata compounds 6, the feasibility
of pathway A was studied in models lacking the exo-
cyclic amino substituent (Scheme 1). As an alternative to
the intramolecular cyclization used by Pokorski et al.7
to obtain 1,5-triazolo-pyrazinone 12 from 16 (Scheme 1),
the intermolecular RuAAC reaction, followed by lactami-
zation, was investigated. The cycloaddition of Cbz-
protected propargylamine 8a and ethyl 2-azidoacetate 11
was found to give satisfactory yields with both Cp*RuCl-
(PPh3)2 and Cp*RuCl(COD)15 in refluxing dioxane or
toluene, but only poor yields were obtained in THF
(Table S1 in Supporting Information (SI)).
Wehavepreviously reportedthesynthesis and successful
incorporation of conformationally constrained amino-
azapinoneanalogsofphenylalanine(1, Aba (amino-benzo-
azepinone), Figure 1) and tryptophane (2, Aia (amino-
indolo-azepinone)), in bioactive peptides.10,11 To complement
this we envisaged that an amino-triazolodiazepinone such
as 3 (Ata) could potentially serve as a conformationally
constrained amino acid analog to replace histidine residues
in peptide sequences. Triazole histidine mimics suchasaza-
His 4 and 5 have previously been described,12 but the syn-
thesis of small ring cyclic structures such as 3 has not yet
been reported. Herein we present the synthesis of histidine
analogs Ata 3, based on two different synthetic strategies
employing intermolecular and intramolecular Huisgen
cycloaddition reactions. The scope of both strategies in
terms of yields, stereocontrol, and synthetic drawbacks
is discussed.
Scheme 1. A Model Study of Intramolecular RuAAc vs Intra-
molecular Thermal Huisgen Cyclization
Figure 1. Conformationally constrained amino acids Aba 1,
Aia 2, Ata 3, and structures of reported His mimicks 4 and 5.12.
In order to validate 3 as a histidine mimic, it was intro-
duced in the angiotensin IV (AT IV) peptide sequence and
evaluated as an inhibitor of insulin regulated aminopepti-
dase (IRAP) and aminopeptidase-N (AP-N) activity.
Surprisingly, the reaction of azide 13 with acetylene 8a
required longer refluxing in toluene and gave a lower yield.
After removal of the Cbz protecting group, pyrazinone 12
(10) Ballet, S.; Frycia, A.; Piron, J.; Chung, N. N.; Schiller, P. W.;
Kosson, P.; Lipkowski, A. W.; Tourwe, D. J. Pept. Res. 2005, 66 (5),
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J. C. J. Med. Chem. 2009, 52 (1), 95–104.
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Diederichsen, U. Eur. J. Org. Chem. 2009, 4593–4599.
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Sharpless, K. B.; Fokin, V. V.; Jia, G. C. J. Am. Chem. Soc. 2005, 127
(46), 15998–15999.
Figure 2. Retrosynthetic strategies A and B for the preparation
of Ata scaffold 6 from common precursors.
€
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Osborne, H.; Taylor, D. K.; Abell, A. D. Bioorg. Med. Chem. Lett. 2010,
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Zhao, H.; Lin, Z.; Jia, G.; Fokin, V. V. J. Am. Chem. Soc. 2008, 130
(28), 8923–8930.
The key reaction for the construction of Boc-protected
scaffold 6 suitable for peptide synthesis (Figure 2) is the
Org. Lett., Vol. 13, No. 24, 2011
6469

formed spontaneously. In contrast the seven-membered
lactam in 15 did not form spontaneously and required
refluxing of 14 in ethanol in the presence of a catalytic
amount of piperidine.
N-propargyl amino acid methyl esters 8dÀf in dioxane to
give triazolyl amino acid esters 22dÀf in moderate to
excellent yields. Hydrogenolysis of 22dÀf afforded 9dÀf
ready for cyclization to give the desired triazolodiazepi-
nones 6dÀf.
Based on these results, the synthesis of 6c using the
intermolecular cycloaddition reaction (Scheme 2) was
started from azide 17 which was prepared in two steps
from Boc protected serine according to the method of
Panda et al.16 The RuAAC reaction of 17 with Fmoc-
protectedN-propargyl amine 8byieldedtriazole18in satis-
fying yields. Simultaneous base-promoted deprotection of
the fluorenylmethyloxycarbonyl and hydroxamate groups
was performed prior to the carbodiimid-induced cycliza-
tion and provided the desired Boc-aminotriazoloazepi-
none (Boc-Ata) 6c in 55% yield over the two steps. The
preparation of an Ata scaffold 6 bearing an amino acid
based substituent on N-2, using N-propargyl-Phe methyl
ester 8d, was not successful by this approach.17,18 Whereas
the cycloaddition reaction did yield triazole 19 in 80%
yield, no conditions were found to selectively hydrolyze
the Weinreb amide in the presence of the methyl ester. The
free carboxylic acid in 9d was needed for subsequent
lacamization.
Scheme 3. Intermolecular RuAAC synthesis of Boc-Ata 6dÀf
A variety of cyclization conditions were evaluated on
amino acid 9d (see Table S2 in SI). The use of DCC gave
the desired 6d, however contaminated with the N-acylurea
adduct. The application of EDC or TBTU resulted in par-
tial epimerization (80/20), which could be suppressed using
a combination of EDC and HOAt in DMF.20 Using these
optimized conditions Boc-Ata 6dÀf were obtained in high
yields.
Therefore, Boc-β-azido-Ala 7, obtained from Boc-β-
amino-Ala,7,14,19 was converted to its benzyl ester 21, since
free carboxylic acids have been reported to be poorly com-
patible with RuAAC catalysts (Scheme 3). The RuAAC
reaction was carried out by refluxing azide 21 and
In addition to the successful intermolecular RuAAC
route (Scheme 3), the intramolecular cycloaddition reac-
tion was also investigated. This pathway eliminates two
reaction steps (ester protection and deprotection) that are
required in Route A (Figure 2). Ata scaffold 6c (R = H)
served as a test case. Boc-β-azido-Ala19 7 was coupled to
propargyl amine 8c to produce 10c (Scheme 4). In the
following intramolecular cycloaddition, the only possible
regiochemical outcome is the 1,5-substituted triazole 6c in
principle making the use of a catalyst for intramolecular
RuAAC21 unnecessary.
Scheme 2. Preparation of Boc-Ata-H 6c via Route A
Scheme 4. Intramolecular Thermal Huisgen Cycloaddition
Synthesis of Boc-Ata 6c,d,fÀh
(16) Panda, G.; Rao, N. V. Synlett 2004, 4, 714–716.
(17) Sudhir, V. S.; Baig, R. B. N.; Chandrasekaran, S. Eur. J. Org.
Chem. 2008, 2424–2429.
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Org. Lett., Vol. 13, No. 24, 2011

Triazole precursor 10c was stirred in refluxing dioxane
with and without a Ru-catalyst, and comparable yields
(50%) and reaction times confirmed that the intramole-
cular cycloadditions worked equally well, despite the fact
that the amide bond was exclusively in the trans conforma-
tion. Since secondary amides exhibit a cisÀtrans equili-
brium, the influence of the amide bond conformation was
investigated in the N-benzyl-substituted analog 10g.
The coupling reaction between 8g17 and 7 proved to be
tedious, probably due to steric encumbrance, as 10g was
only obtained in 25% yield using EDC/HOBt in DMF.
The use of phosphonium reagents resulted in even lower
yields. Conversely, the subsequent Huisgen cycloaddition
went smoothly to give a quantitative yield of Ata 6g. A
study of the reaction conditions of the cylization step (see
Table S3 in SI) indicated that the cycloaddition is best
carried out catalyst-free, in dioxane at 100 °C for 1 h, to
give 6g quantitatively.
Toimprovethe overall yield of theprocesswe performed
the coupling and subsequent cycloaddition in one pot
without purification of compound 10g. After overnight
coupling using EDC and HOBt in DMF, refluxing for 1 h
gave Ata 6g in a significantly better 42% yield over two
steps (compared to 25% overall for the stepwise process).
Subsequently, various Ata-Xxx dipeptidomimetics were
prepared starting from the N-propargyl substituted amino
acid esters 8d,f and h.17 To obtain 6f (Boc-Ata-Gly-OMe),
the coupling of7 to 8f was performed overnight usingEDC
and HOAt in DMF at room temperature, followed by
overnight refluxing to give a 40% yield over the two steps
(see Table S4 in SI) showing no epimerization after (S)-
NIFE22 derivatization and subsequent LCMS analysis.
Likewise, Boc-Ata-Ala-OMe 6h was prepared using these
conditions from 8h17 and 7. Unfortunately, a disappoint-
ing yield was obtained for 6h (12% over two steps), and
moreover the 1H NMR spectrum indicated the presence of
a mixture of diastereoisomers (dr 75/25), due to epimeriza-
tion. Similarly, 6d (Boc-Ata-Phe-OMe) gave a poor yield
(10%) showing substantial epimerization (dr 60/40) by
HPLC. An intramolecular cycloaddition reaction on N-(2-
propynyl)amino acid esters under thermal conditions was
reported by Sudhir and co-workers,17 but the enantiopu-
rity of the resulting triazoles was not reported. A compar-
ison of the two pathways indicates that for the preparation
of 6c (Boc-Ata, R = H) and 6f (Boc-Ata-Gly-OMe) the
intermolecular pathway A gives overall yields of 25%
and 12% (calculated from Boc-β-amino-Ala) versus 53%
and 21% for the intramolecular pathway B. However,
for the preparation of the dipeptide mimetics Ata-Ala and
Ata-Phe, theintramolecular pathway isclearlyinferior with
regard to overall yield, but especially because of the sub-
stantial epimerization.
To illustrate the potential of the Ata template as a con-
strained histidine mimic, the His-Pro dipeptide segment in
AT IV (H-Val-Tyr-Ile-His-Pro-Phe-OH) was replaced by
the Ata-Gly dipeptidomimetic. After saponification of the
methyl ester in 6f, this building block was incorporated
in the AT IV sequence using standard solid phase pep-
tide synthesis (see SI). In enzyme assays measuring the
inhibition of insulin regulated aminopeptidase (IRAP) and
aminopeptidase-N (AP-N) activity, the H-Val-Tyr-Ile-
Ata-Gly-Phe-OH analog was equipotent to the native AT
IV (pKi (IRAP) = 7.093 versus 7.142 for AT IV; Figure S1
in SI). In conclusion, we have developed a versatile synthe-
sis of a new constrained dipeptidomimetic using the azideÀ
alkyne cycloaddition reaction. The best pathway to the
dipeptide mimetics is the intermolecular Ru-catalyzed
cycloaddition, followed by lactamization. The potential
of the new Ata scaffold to replace His in a bioactive
peptide was demonstrated in AT IV, where this substitu-
tion yielded an analog which was equipotent with the
native peptide.
Acknowledgment. This work was supported by the In-
stitute for Innovation through Science and Technology
(IWT Vlaanderen), the Research Foundation-Flanders
(FWO), and the Queen Elisabeth Research Foundation.
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Supporting Information Available. Experimental pro-
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pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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