Synthesis of dendrimeric N-glyoxylamide peptide mimics

Article abstract of DOI:10.1016/j.tetlet.2011.05.019

A range of peptidomimetic dendrimers based on 1,3,5-benzenetricarbonyl trichloride and tris(2-aminoethyl)amine central cores has been synthesised through the facile ring-opening of N-acylisatins with amino acids, alcohols and other amines.

Full text of DOI:10.1016/j.tetlet.2011.05.019

Tetrahedron Letters 52 (2011) 3645–3647
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of dendrimeric N-glyoxylamide peptide mimics
Thanh Le a, Wai Ching Cheah a, Kasey Wood a, David St. C. Black a, Mark D. Willcox b, Naresh Kumar a,
⇑
a School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia
b School of Optometry and Vision Science, The University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
A range of peptidomimetic dendrimers based on 1,3,5-benzenetricarbonyl trichloride and tris(2-amino-
ethyl)amine central cores has been synthesised through the facile ring-opening of N-acylisatins with
amino acids, alcohols and other amines.
Received 7 March 2011
Revised 12 April 2011
Accepted 6 May 2011
Available online 13 May 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Peptidomimetic
Dendrimer
N-Acylisatin
Inefficient drug delivery is a key factor that hinders the efficacy
of many synthetic antibiotics for the treatment of infectious dis-
eases.1,2 The development of effective drug delivery systems is,
therefore, required to enhance the performance of current antibiot-
ics. Recently, the use of dendrimers as DNA carriers and drug deliv-
ery systems has gained considerable attention in pharmaceutical
and medicinal chemistry. Dendrimers are well-defined macromol-
ecules with highly branched architectures that can adopt a spher-
ical three-dimensional structure. In addition, they possess a high
density of functional groups which promote host–guest chemistry
at both the interior and periphery of the dendrimer.2–4 As an exam-
ple, Kohle et al. have described a polyamidoamine dendrimer that
can efficiently carry, on average, 78 molecules of ibuprofen into
cells and then release the drug at an appreciably slower rate
in vitro than pure ibuprofen.5
We were interested in the development of novel peptidomimet-
ics derived from N-acylisatins and have previously reported their
facile ring-opening with various amino-ester hydrochloride salts
to give glyoxylamide peptide mimics that possess significant
anti-bacterial activity.12 We report herein the synthesis of the den-
drimer analogues of these compounds for potential use as an effec-
tive drug delivery system for anti-bacterial agents. Two commonly
used central cores, 1,3,5-benzenetricarbonyl trichloride and tris(2-
aminoethyl)amine,13 were selected for this initial study. 1,3,5-Ben-
zenetricarbonyl trichloride is a highly reactive achiral C3-symmet-
ric rigid structure which has been found to possess a hydrogen-
bonded columnar-type packing which can enhance the length of
the dendrimer assembly. Tris(2-aminoethyl)amine is a tridentate
ligand with three aminoethyl groups attached to the central nitro-
gen atom.
The step-wise, controlled construction of dendrimers frequently
utilizes the formation of the stable amide bond, generating macro-
molecules with comparable numbers of amide bonds as those
found in proteins.6 As a logical extension of this concept, several
amino acid or peptide-based dendrimers have been reported.7–9
The pharmacological utility of peptides derived from natural ami-
no acids is limited, however, due to their poor metabolic stability,
low bioavailability, and toxicity.10,11 Recent advances in peptide
chemistry have focused on peptidomimetics. Peptidomimetics
are small peptide-like molecules which utilize functional groups
such as aromatic rings and pyrrole moieties to stabilize the molec-
ular structure and enhance bioavailability, while maintaining the
favorable biological properties of the natural peptides.10
Dendrimers based on a 1,3,5-benzenetricarbonyl trichloride
core unit were developed as shown in Scheme 1. Isatin (1) and
1,3,5-benzenetricarbonyl trichloride (2) were reacted with sodium
hydride in 1,4-dioxane at 0 °C and the reaction mixture was al-
lowed to warm to room temperature overnight. Tris-isatin 3 was
isolated as a yellow solid in 75% yield after aqueous work-up and
column chromatography. The 1H NMR spectrum of tris-isatin 3
indicated the compound was threefold symmetric, displaying
two doublets at 7.79 and 8.03 ppm, equating to H4 and H7, respec-
tively, and two triplets at 7.39 and 7.81 ppm which corresponded
to H5 and H6. The protons of the benzene moiety appeared as a
singlet at 8.75 ppm.
The ring-opening of the tris-isatin 3 system was initially inves-
tigated using a range of alcohols and amines. Tris-isatin 3 was
found to undergo tandem ring-opening in neat solutions of
methanol, ethanol, and n-butanol to afford benzenetricarbonyl
⇑
Corresponding author. Tel.: +61 2 9385 4698; fax: +61 2 9385 6141.
E-mail address: n.kumar@unsw.edu.au (N. Kumar).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.019

3646
T. Le et al. / Tetrahedron Letters 52 (2011) 3645–3647
O
R
O
O
O
NH
O
O
R
O
O
N
Cl
O
O
O
H
a
b
N
+
O
O
O
O
NH
N
O
O
N
H
O
O
R
O
N
Cl
Cl
O
1
2
3
4
O
O
Scheme 1. Reagents and conditions: (a) NaH, anhydrous 1,4-dioxane, 0 °C to rt, 24 h; (b) alcohol or amine, CH2Cl2, reflux, 6 h.
tris-glyoxylesters 4a–c in 62–76% yield. Similar treatment of tris-
isatin 3 with 3.5 equiv of amines in dichloromethane afforded
the tris-gloxylamides 4d–f. In general, the yield of the tris-gloxyla-
mides 4 was found to decrease as the length of the alkyl chain in-
creased (Table 1).
Table 2
Benzenetricarbonyl tris-gloxylamide peptidomimetics
O
R
H3CO
R
O
OCH3
N
H
With the successful formation of tris-gloxylamides from alco-
hols and amines, attention turned to the use of amino acids for
the preparation of dendrimeric peptidomimetics. The ring-opening
of tris-isatin 3 with glycine methyl ester hydrochloride was con-
ducted under the previously reported conditions13 in a dichloro-
methane–water (v/v = 3:1) biphasic solvent system in the
presence of sodium hydrogen carbonate. Dendrimer 5a was iso-
lated as off-white flakes in 20% yield after purification by column
chromatography.14 The 1H NMR spectrum showed the presence
of a doublet at 3.96 ppm and a singlet at 3.62 ppm, which were as-
signed as the methylene and methyl ester protons, respectively.
High resolution mass spectrometric analysis showed a peak at
863.2166 which was consistent with (MꢀH)+ for compound 5a.
The reaction yield was subsequently increased upon use of a
mono-phasic solution of dichloromethane and triethylamine em-
ployed as the base. Tris-isatin 3 was ring-opened at room temper-
O
NH
O
O
O
NH
O
O
H
N
O
NH
O
O
NH
5
O
R
OCH3
Entry
R
Amino acida
Glycine
Product
Yield (%)
ature over 48 h with
a range of amino acid methyl ester
1
2
H
5a
5b
20
25
hydrochlorides to generate peptidomimetics 5b–d in 25–43%
yields (Table 2). Unfortunately, attempts to crystallise compounds
5a–d from a range of solvents in order to perform X-ray diffraction
studies proved unsuccessful.
CH(CH3)2
L
L
L
-Valine
3
4
CH2CH(CH3)2
5c
35
43
-Leucine
CH(CH2)2SCH3
5d
-Methionine
With the successful preparation of the first generation of tris-
glyoxylamide peptidomimetics, efforts were directed toward the
synthesis of second generation mimics using dipeptides. Ring-
a
As the methyl ester hydrochloride salt.
opening of tris-isatin 3 with L-valine-L-phenylalanine methyl ester
produced. It was, therefore, reasoned that the sterically bulky nat-
ure of the two substrates hindered the reaction.
hydrochloride was conducted under the optimized reaction condi-
tions. However, the desired N-1,3,5-tris-glyoxylamide peptide mi-
mic was not produced and only polymeric by-products were
observed by 1H NMR spectroscopy. Further reaction optimization
was performed by varying the number of equivalents of the dipep-
tide hydrochloric salt and examining the use of a biphasic solvent
system, but in all cases none of the desired compound was
The development of dendrimers based on the second core unit,
tris(2-aminoethyl)amine (7), was subsequently investigated as a
potentially less sterically demanding starting substrate (Scheme 2).
In this case, the ring-opening of N-acylisatin would be achieved by
the dendritic central unit 6 itself, instead of by peptides as in the
previous series. Thus, isatin (1) was reacted with methyl chlorofor-
mate in the presence of sodium hydride to give methyl carbamate
6a in 72% yield. Subsequent ring-opening with tris(2-amino-
ethyl)amine (7) using a modification of the procedure reported
by Orvig et al.15 afforded tris-peptide mimic 8a in 30% yield as yel-
low flakes after column chromatography (Table 3). After some
optimization, we found that the formation of by-products was re-
duced through use of a high dilution reaction mixture (10ꢀ3 M).
The successful formation of trimer 8a was confirmed using X-
ray crystallography (Fig. 1). The X-ray crystal structure revealed
that the three arms of the trimer were orientated downward to
form a cage structure. As such, these structures may potentially
possess interesting metal binding capabilities.
Table 1
Benzenetricarbonyl tris-gloxylamides
Entry
R
Product
Yield (%)
1
2
3
4
5
6
OCH3
OCH2CH3
O(CH2)3CH3
NH(CH2)3CH3
NH(CH2)5CH3
NH(CH2)7CH3
4a
4b
4c
4d
4e
4f
76
67
62
74
59
48

T. Le et al. / Tetrahedron Letters 52 (2011) 3645–3647
3647
O
H
N
O
N
HN
R
O
O
R
NH
O
O
O
O
N
a
HN
b
O
NH2
NH2
NH2
+
O
N
N
H
NH
O
O
R
O
HN
1
6
7
R
O
8
Scheme 2. Reagents and conditions: (a) acid chloride, NaH, 1,4-dioxane, 0 °C to rt, 45 min; (b) DIPEA, CH2Cl2, 0 °C to rt, 24 h.
Table 3
trichloride and tris(2-aminoethyl)amine were investigated as the
central core unit and several examples of both types of dendrimers
were produced.
Tris(aminoethyl)amine gloxylamide peptidomimetics
Entry
R
Product
Yielda (%)
1
2
OCH3
CH2CH2COOCH3
8a
8b
30
28
Acknowledgments
a
Isolated yields.
We thank the University of New South Wales and the Australian
Research Council for their financial support.
References and notes
1. Cohen, S.; Chen, L.; Apte, R. N. React. Polym. 1995, 25, 177–187.
2. Kannan, R. M.; Kolhe, P.; Misra, E.; Kannan, S.; Lai, M. L. Int. J. Pharm. 2003, 259,
143–160.
3. Zimmerman, S. C.; Zeng, F. Chem. Rev. 1997, 97, 1681–1712.
4. Aulenta, F.; Hayes, W.; Rannard, S. Eur. Polym. J. 2003, 39, 1741–1771.
5. Kohle, P.; Misra, E.; Kannan, R. M.; Kannan, S.; Lieh-Lai, M. Int. J. Pharm. 2003,
259, 143–160.
6. Mulders, S. J. E.; Brouwer, A. J.; Liskamp, R. M. J. Tetrahedron Lett. 1997, 38,
3085–3088.
7. Mulders, S. J. E.; Brouwer, A. J.; Van Der Meer, P. G. J.; Liskamp, R. M. J.
Tetrahedron Lett. 1997, 38, 631.
8. Chow, H. F.; Mong, T. K. K.; Chan, Y. H.; Cheng, C. H. K. Tetrahedron 2003, 59,
3815–3820.
9. Chapman, T. M.; Hillyer, G. L.; Mahan, E. J.; Shaffer, K. A. J. Am. Chem. Soc. 1994,
116, 11195–11196.
10. Banerji, B.; Bhattacharya, M.; Madhu, R. B.; Das, S. K.; Iqbal, J. Tetrahedron Lett.
2002, 43, 6473–6477.
11. Gjermansen, M.; Ragas, P.; Sternberg, C.; Molin, S.; Tolker-Nielsen, T. Environ.
Microbiol. 2005, 7, 894.
12. Cheah, W. C.; Black, D. S.; Goh, W. K.; Kumar, N. Tetrahedron Lett. 2008, 49,
2965–2968.
13. Examples of dendrimer central cores: (a) James, T. D.; Shinmori, H.; Takeuchi,
M.; Shinkai, S. Chem. Commun. 1996, 705–706; (b) Newkome, G. R.; Yao, Z.-Q.;
Baker, G. R.; Gupta, V. K.; Russo, P. S.; Saunders, M. J. J. Am. Chem. Soc. 1986, 108,
849–850; (c) Gitsov, I.; Wooley, K. L.; Frechet, J. M. J. Angew. Chem. 1992, 104,
1282–1285; (d) Inoue, K. Prog. Polym. Sci. 2000, 25, 453–571; (e) Ashton, P. R.;
Anderson, D. W.; Brown, C. L.; Shipway, A. N.; Stoddart, J. F.; Tolley, M. S. Chem.
Eur. J. 1998, 4, 781–793; (f) Hajela, S. P.; Johnson, A. R.; Xu, J.; Sunderland, C. J.;
Cohen, S. M.; Caulder, D. L.; Raymond, K. N. Inorg. Chem. 2001, 40, 3208–3216;
(g) Ghosh, S.; Reches, M.; Gazit, E.; Verma, S. Angew. Chem., Int. Ed. 2007, 46,
2002–2004.
14. Representative procedure for 5a: Glycine methyl ester hydrochloride (6.0 mmol)
was added to a stirred solution of 1,3,5-triacylisatin 3 (1.0 mmol) in CH2Cl2
(60 ml) and saturated NaHCO3 in H2O (3 ml) at 5 °C. The reaction was warmed
to room temperature and stirred for 24 h. The organic layer was washed with
aqueous HCl (0.5 M, 25 ml) and H2O (20 ml) before being concentrated under
vacuum. Trituration with CH2Cl2 gave 5a as fine off-white flakes (20%). Mp
178–180 °C, 1H NMR (300 MHz, DMSO-d6): 11.6 (3H, br s, NHCO), 9.24 (3H, t, J
5.6 Hz, CONH), 8.75 (3H, s, H13, H15, H17), 8.16 (3H, dd, J 1.5, 7.9 Hz, H7, H70,
H700), 7.85 (3H, dd, J 1.5, 7.9 Hz, H4, H40, H400), 7.75 (3H, J 1.5, 7.5 Hz, H6, H60,
H600), 7.37 (3H, dt, J 1.5, 7.5 Hz, H5, H50, H500), 3.96 (6H, d, J 6.0 Hz, CH2), 3.62
(9H, s, COOCH3). 13C NMR (75 MHz, DMSO-d6): 191.9, 169.9, 164.5, 164.4,
138.9, 135.6, 135.1, 132.6, 130.0, 124.7, 124.3, 122.8, 52.3, 40.9. HRMS (ESI) m/z
calcd for C42H35N6O15 (MꢀH)+ 863.2166; found 863.2166. Anal. Calcd for
Figure 1. ORTEP diagram of 8a.16
Isatin (1) was then reacted with methyl 4-chloro-4-oxobutyrate
to give the longer chain N-oxobutanoate 6b in 60% yield. Reaction
of 6b with tris(2-aminoethyl)amine (7) produced peptidomimetic
8b in 28% yield. Saponification of the methyl ester groups was sub-
sequently investigated on the basis that the peptide chain could
then be extended through amide coupling with the free acids.
Saponification of 8b with aqueous potassium hydroxide in metha-
nol proved to be problematic, however, producing polymeric by-
products in preference to the desired tris-carboxylic acid. It was
postulated that polymerization was favoured due to the presence
of so many labile groups.
C42H36N6O15: C, 58.33; H, 4.20; N, 9.72. Found: C, 58.30; H, 4.44; N, 9.44.
15. Orvig, C.; Hoveyda, H. R.; Karunaratne, V.; Nichols, C. J.; Rettig, S. J.; Stephens, A.
K. W. Can. J. Chem. 1998, 76, 414–425.
16. Crystallographic data for the structure in this Letter have been deposited with
the Cambridge Crystallographic Data Centre as supplementary publication no.
CCDC 806995 (8a). X-ray crystal structures were obtained by Mohan
Bhadbhade, Crystallography Laboratory, UNSW Analytical Centre, Sydney,
Australia.
In summary, the facile ring-opening of N-acylisatins with alco-
hols, amines, and amino acids has been successfully applied to the
synthesis of peptidomimetic dendrimers. 1,3,5-Benzenetricarbonyl