Synthesis of poly(amino)ester dendrimers via active cyanomethyl ester intermediates

Article abstract of DOI:10.1021/jo101739x

A novel strategy for the synthesis of poly(amino)ester dendrimers was developed on the basis of active cyanomethyl ester intermediates and an iteration of four consecutive steps of deprotection, activation, transesterification, and scavenging.

Full text of DOI:10.1021/jo101739x

pubs.acs.org/joc
end groups on the surface.4-6 Among the various dendrimers,
Synthesis of Poly(amino)ester Dendrimers
via Active Cyanomethyl Ester Intermediates
poly(amino)ester dendrimers are particularly attractive as
drug delivery systems because of their multiple advantageous
features. First, they contain labile ester groups and are thus
expected to be biodegradable via ester hydrolysis. Second,
the amine functionalities present in the dendrimers can serve
as buffers to neutralize the acids generated from the ester
hydrolysis, allowing a benign and nonaggressive environ-
ment during dendrimer degradation. Finally, both the amine
and the ester terminals are ready to undergo a large variety of
chemical modifications which can offer various drug con-
jugations and/or confer specific properties such as solubility,
dendrimer coating, and surface shielding.
†
Camille Bouillon, Aura Tintaru, Valerie Monnier,
ꢀ ꢀ
Laurence Charles, Gilles Quelever, and Ling Peng*
‡
§
ꢀ
‡
†
,†
†Centre Interdisciplinaire de Nanoscience de Marseille, CNRS
UPR 3118, Marseille, France, ‡Laboratoire Chimie Provence,
§
CNRS UMR 6264, Marseille, France, and Federation des
ꢀ ꢀ
Sciences Chimiques, Spectropole, Marseille, France
ling.peng@univmed.fr;
Received September 14, 2010
Despite these appealing advantages, only a few examples
of such dendrimers have been reported so far, mainly due to
their complicated synthesis and limited available synthetic
methods.7-10 The synthesis of poly(amino)ester dendrimers
can be envisaged either via Michael addition of amines yield-
ing amine-bearing dendrimers, via ester formation giving
ester-bearing dendrimers, or by a combination of both.4,5,11-14
However, dendrimer growth via Michael addition of amines
has a fatal drawback in that the ester groups present in the
poly(amino)ester dendrimer may themselves be affected by
the nucleophilic attack of amines, requiring special growing
units containing amine and/or ester functionalities. Mean-
while, the conventional ester formation methods are not
favorable for the synthesis of poly(amino)ester dendrimers
due to the presence of amine functionalities, which often lead
to incomplete ester formation and difficulties with purification.
This is a general problem encountered during the synthesis of
various amine-containing esters, which can be mainly ascribed
to the easy protonation of the amines forming zwitterions
or H-bonds in the reaction medium, resulting in reduced
reactivity and impeded reaction progress as well as increased
difficulties in product isolation.15-17
A novel strategy for the synthesis of poly(amino)ester
dendrimers was developed on the basis of active cyano-
methyl ester intermediates and an iteration of four conse-
cutive steps of deprotection, activation, transesterifica-
tion, and scavenging.
We have recently developed a strategy to synthesize amine-
bearing esters via active cyanomethyl ester intermediates,
which leads to a high product yield and easy purification.17
Cyanomethyl esters are reported to be not only stable but
also reactive enough to undergo transesterification with
Dendrimers have attracted particular attention as drug
delivery vehicles because of their high drug payload confined
within a small nanosized volume.1-3 This special feature is
the result of their unique molecular architecture with cascade-
branched units emanating from a focal point and numerous
ꢀ
(6) (a) Tomalia, D. A.; Frechet, J. M. J. J. Polym. Sci., Part A: Polym.
Chem. 2002, 40, 2719–2728. (b) Frechet, J. M. J. J. Polym. Sci., Part A:
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Polym. Chem. 2003, 41, 3713–3725.
(7) Shi, D. X.; Sha, Y. W.; Wang, F.; Tian, Q. Y. Macromolecules 2008,
41, 7478–7484.
(8) Sha, Y. W.; Shen, L.; Hong, X. Y. Tetrahedron Lett. 2002, 43, 9417–9419.
(9) Xu, D. M.; Zhang, K. D.; Zhu, X. L. Tetrahedron Lett. 2005,46, 2503–2505.
(10) Xu, D.-M.; Zhang, K.-D.; Ning, C.-H.; Zhu, X.-L. J. Appl. Polym.
Sci. 2005, 97, 60–64.
(1) (a) Menjoge, A. R.; Kannan, R. M.; Tomalia, D. A. Drug Discovery
Today 2010, 15, 171–185. (b) Medina, S. H.; El-Sayed, M. E. H. Chem. Rev.
2009, 109, 3141–3157. (c) Nanjwade, B. K.; Bechra, H. M.; Derkar, G. K.;
Manvi, F. V.; Nanjwade, V. K. Eur. J. Pharm. Sci. 2009, 38, 185–196.
(d) Tomalia, D. A.; Reyna, L. A.; Svenson, S. Biochem. Soc. Trans. 2007, 35,
61–67.
(2) Boas, U.; Christensen, J. B.; Heegaard, P. M. H. Dendrimers in
Medicine and Biotechnology-New Molecular Tools; RSC Publishing:
London, 2006.
€
(11) Buhleier, E.; Wehner, W.; Vogtle, F. Synthesis 1978, 155–158.
(12) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin,
S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117–132.
(13) Grinstaff, M. W. Chem. Eur. J. 2002, 8, 2838–2846. and references
;
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(3) Lee, C. C.; MacKay, J. A.; Frechet, J. M. J.; Szoka, F. C. Nat.
Biotechnol. 2005, 23, 1517–1526.
cited therein.
ꢁ
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(14) (a) Ihre, H. R.; Padilla De Jesus, O. L.; Szoka, F. C.; Frechet, J. M. J.
ꢁ
Bioconjugate Chem. 2002, 13, 443–452. (b) Padilla De Jesus, O. L.; Ihre,
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H. R.; Gagne, L.; Frechet, J. M. J; Szoka, F. C. Bioconjugate Chem. 2002, 13,
453–461.
(4) (a) Vogtle, F.; Richardt, G.; Werner, N. Dendrimer Chemistry Concepts,
Synthesis, Properties, Applications; Wiley: Weinheim, 2009. (b) Newcome,
G. R.; Moorefield, C. N.; Vogtle, F. Dendrimers and Dendrons. Concepts,
Synthesis, Applications; Wiley: Weinheim, 2001. (c) Frechet, J. M. J., Tomalia,
D. A., Eds. Dendrimers and Other Dendritic Polymers; Wiley: Amsterdam,
2001.
(15) Bouillon, C. Ph.D. Thesis, Aix-Marseille II University, 2009.
(16) Buschhaus, B.; Hampel, F.; Grimme, S.; Hirsch, A. Chem. Eur. J.
;
2005, 11, 3530–3540.
ꢀ ꢀ
(17) Bouillon, C.; Quelever, G.; Peng, L. Tetrahedron Lett. 2009, 50,
(5) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem.,
Int. Ed. 1990, 29, 138–175.
4346–4349.
DOI: 10.1021/jo101739x
r
Published on Web 11/17/2010
J. Org. Chem. 2010, 75, 8685–8688 8685
2010 American Chemical Society

Bouillon et al.
JOCNote
SCHEME 1. Synthetic Strategy to Construct Poly(amino)ester
Dendrimers Based on Cyanomethyl Ester Intermediates and an
Iteration of (i) Deprotection, (ii) Activation, (iii) Transesterifi-
cation, and (iv) Scavenging
FIGURE 1. Construction units for the synthesis of poly(amino)-
ester dendrimers: 1 as dendrimer core and 2 and 3 as the branching
units.
SCHEME 2. Synthesis of the First-Generation Dendrimers 6
and 7a
various alcohols.18-21 We further explored this strategy for
the preparation of poly(amino)ester dendrimers. Here, we
report a novel and efficient method of synthesizing poly-
(amino)ester dendrimers (Scheme 1) based on the active
cyanomethyl ester intermediates, including an original itera-
tion of four consecutive reactions (Scheme 1): (i) deprotec-
tion of the ester terminals to generate acid functionalities,
(ii) activation of the acid terminals as cyanomethyl esters,
(iii) transesterification with an excess of alcohol bearing ester
terminals and a tertiary amine as the branching point, and
(iv) scavenging of the excess of alcohol. Further construction
of higher generation dendrimers can be simply initiated via
aConditions: (i) CF3COOH, CH2Cl2, rt; (ii) chloroacetonitrile, Et3N,
CH2Cl2, rt; (iii) 2 or 3, DBU, CH3CN, rt; (iv) benzoic anhydride,
DMAP, CH2Cl2, rt.
the regeneration of the terminal acids from the resulting
dendrimer followed by the above-mentioned iterative steps
(Scheme 1). The overall synthesis can be performed under
very mild conditions, allowing the corresponding products
to be obtained in a pure state and with good yields after easy
purification.
(18) Greene, T. W.;Wuts, P. G. M. Protective Groups in Organic Synthesis,
3rd ed.; Wiley: New York, 1999; Chapter 7, pp 518-525 and references cited
therein.
(19) Hennen, W. J.; Sweers, H. M.; Wang, Y. F.; Wong, C.-H. J. Org.
Chem. 1988, 53, 4939–4945.
(20) (a) Ellman, J.; Mendel, D.; Anthony-Cahill, S.; Noren, C. J.; Schultz,
P. G. Methods Enzymol. 1991, 202, 301–336. (b) Robertson, S. A.; Ellman,
J. A.; Schultz, P. G. J. Am. Chem. Soc. 1991, 113, 2722–2729.
(21) Findlow, S.; Gaskin, P.; Harrison, P. A.; Lenton, J. R.; Penny, M.;
Willis, C. L. J. Chem. Soc., Perkin Trans. 1 1997, 5, 751–757.
We present here the synthesis of our prototype poly(amino)-
ester dendrimers based on this novel synthetic strategy by
8686 J. Org. Chem. Vol. 75, No. 24, 2010

Bouillon et al.
JOCNote
SCHEME 3. Synthesis of the Second-Generation Dendrimer 10a
aConditions: (i) CF3COOH, CH2Cl2, rt; (ii) chloroacetonitrile, Et3N, CH2Cl2, rt; (iii) 3, DBU, CH3CN, rt; (iv) benzoic anhydride, DMAP, CH2Cl2, rt.
employing the triester 1 as the dendrimer core and the amine-
and ester-bearing alcohols 2 and 3 as branching units for
dendrimer growth (Figure 1).17
We first deprotected 1 with trifluoacetic acid to yield the
triacid 4 (Scheme 2), which was further transformed to the
activated cyanomethyl ester 5 by treatment with chloro-
acetonitrile in the presence of Et3N in CH2Cl2. The resulting
ester 5 was stable enough to be isolated and fully character-
ized. It also turned out to be sufficiently reactive to undergo
a straightforward transesterification reaction with an excess
of either 2 or 3 in the presence of DBU in CH3CN.17 After
completion of the reaction, the excess of alcohol was scavenged
with benzoic anhydride in the presence of DMAP and con-
verted into the corresponding benzoic esters 20 and 30, re-
spectively (Scheme 2).17 Both 20 and 30 possess very different
FIGURE 2. HRMS ESI analysis of poly(amino)ester dendrimer 10:
a triply protonated molecule at m/z 990.6827, corresponding to the
polarity compared to the corresponding products 6 and 7
(Scheme 2), thus allowing easy product purification. This
first cycle of four iterative steps was performed with an
overall yield of 77% for 7, starting from the core 1 and
molecular weight of 10 (2969.95 g mol-1).
3
branching unit 3, and 48% for 6 with the branching unit 2. It
is worth noting that the use of traditional methods for ester
formation did not allow us to identify or isolate either 6 or 7
due to a low product yield and the presence of many side prod-
ucts as well as a tedious and difficult purification process.15
Therefore, the synthetic strategy using active cyanomethyl
ester intermediates is clearly superior with respect to con-
structing our prototype poly(amino)ester dendrimers 6 and
7. It should also be noted that a higher yield was obtained for
7 compared to 6. This could be reasonably ascribed to the
beneficial effect of the longer spacer between the hydroxyl
group and amine functionality in 3 when compared with 2
(Figure 1), making this branching unit less congested in space
and thus more active in reaction. Consequently, we focused
our attention on the use of the branching unit 3 to construct
higher generation dendrimers.
for a further cycle of dendrimer growth after deprotection of
its terminal tert-butyl groups.
All the key intermediates and compounds described above
were fully characterized by NMR analysis (1H, 13C, 13C-
HMQC) and high-resolution mass spectroscopic analysis
for their structural assignment and purity control. Figure 2
shows the HRMS ESI spectral analysis of 10, which dis-
played a characteristic triply charged peak at m/z 990.6827
(maximum of isotopic pattern) for [M þ 3H]3þ, correspond-
ing to the molecular weight of 10 (2969.95 g mol-1) with an
3
error of -0.3 ppm, therefore confirming the results observed
by NMR analysis.
In conclusion, we have developed a novel and efficient
method of synthesizing poly(amino)ester dendrimers via
ester formation of active cyanomethyl ester intermediates.
The synthetic sequence for dendrimer growth involves four
iterative steps, namely (i) deprotection, (ii) activation, (iii) trans-
esterification, and (iv) scavenging. All reactions can be carried
out conveniently under very mild conditions and do not
require any special manipulation and equipment. Moreover,
the resulting dendrimers can be easily identified, purified,
and obtained in a pure state and with good yields. Compared
We then deprotected 7 using trifluoacetic acid (Scheme 3).
The resulting 8 was then treated with chloroacetonitrile, and
the obtained cyanomethyl ester 9 was further transesteri-
fied with 3 in the presence of DBU. After the excess 3 was
scavenged with benzoic anhydride, the expected ester 10
(Scheme 3) was obtained with 46% isolated yield and ready
J. Org. Chem. Vol. 75, No. 24, 2010 8687

Bouillon et al.
JOCNote
to current methods of poly(amino)ester dendrimer synthesis
based on both Michael addition via amine terminals and
ester formation via activation of acid terminals using DCC
or acid chlorides,7-10 our method using cyanomethyl ester
intermediates has proven to be much more simple and effi-
cient with respect to its substrate scope, mild reaction con-
ditions, easy purification procedure, and high product yields.
In addition, the simple and convenient synthetic procedure
developed here can be readily amended for the solid-phase
synthesis of poly(amino)ester dendrimers and further applied
to the synthesis of other ester-bearing dendrimers with diverse
chemical structures. We are actively working in this direction
with a view to developing efficient synthetic methods for the
preparation of biologically relevant poly(amino)ester den-
drimers for a variety of biological applications.
0.29 mmol) and a large excess of 3 (569 mg, 1.74 mmol) in the
presence of DBU (260 μL, 1.74 mmol) in CH3CN (5 mL),
followed by treatment with benzoic anhydride, 7 was obtained
as a colorless oil (304 mg, yield 84%) after chromatography on
silica gel (EtOAc/cyclohexane): Rf 0.50 (EtOAc/cyclohexane
7:3, v/v); 1H NMR (250 MHz, CDCl3) δ 1.19-1.28 (m, 12 H),
1.37 (s, 54 H), 1.51-1.62 (m, 6 H), 2.29-2.40 (m, 24 H),
2.63-2.70 (m, 18 H), 3.95-4.00 (m, 6 H); 13C NMR (62.9 MHz,
CDCl3) δ 23.5, 26.7, 27.9, 28.3, 32.4, 33.4, 48.9, 49.1, 53.3,
64.3, 80.0, 171.8, 172.3; HMRS (ESI) m/z calcd for the doubly
protonated molecule 7 (C66H122N4O182þ) 629.4372, found
629.4392.
Synthesis of 8. The ester 7 (100 mg, 0.08 mmol) was treated by
TFA (370 μL, 4.80 mmol) in CH2Cl2 (3.3 mL) for 3 days at room
temperature. After removal of the solvent and the excess TFA,
the oily crude residue was triturated with Et2O and filtered,
affording 8 as a white solid of TFA salt (91 mg, yield 83%):
1H NMR (250 MHz, CD3OD) δ 1.44-1.55 (m, 6 H), 1.70-1.83
(m, 12 H), 2.82-2.88 (m, 12 H), 2.95-3.00 (m, 6 H), 3.19-3.25
Experimental Section
(m, 6 H), 3.42-3.51 (m, 18 H), 4.19 (t, 6 H, 3J = 6.2 Hz); 13
C
Synthesis of 4. Compound 1 (6.3 g, 15.40 mmol) was treated
with TFA (35.4 mL, 460 mmol) in CH2Cl2 (300 mL). After the
mixture was stirred for 36 h at room temperature, the solvent
and excess of TFA were removed. The oily crude residue was
then triturated with Et2O and filtered, affording 4 as a white
solid of the TFA salt (5.10 g, yield 96%): 1H NMR (250 MHz,
DMSO-d6) δ 2.77 (t, 6 H, 3J = 7.0 Hz), 3.33 (t, 6 H, 3J = 7.0 Hz);
13C NMR (62.9 MHz, DMSO-d6) δ 28.4, 48.7, 171.9; HMRS
(ESI) m/z calcd for the sodium adduct of 4 (C9H15NO6Naþ)
256.0792, found 256.0792.
NMR (62.9 MHz, CD3OD) δ 24.1, 24.4, 29.1, 50.6, 54.9, 66.2,
67.0, 172.7, 173.8; HMRS (ESI) m/z calcd for the protonated
molecule 8 (C42H73N4O18þ) 921.4914, found 921.4916.
Synthesis of 9. The ester 9 was synthesized according to a
procedure similar to that described for the synthesis of 5, start-
ing with 8 (740 mg, 0.54 mmol) and following treatment with
chloroacetonitrile (1 mL, 16.10 mmol) in the presence of Et3N
(1.5 mL, 10.80 mmol) in CH2Cl2 (10 mL). Compound 9 was
obtained as a pale amber oil (599 mg, yield 81%): Rf 0.78
(EtOAc); 1H NMR (250 MHz, CDCl3) δ 1.16-1.45 (m, 12 H),
1.51-1.62 (m, 6 H), 2.32-2.55 (m, 24 H), 2.68-2.73 (m, 18 H),
4.00 (t, 6 H, 3J = 7.1 Hz), 4.60 (s, 12 H); 13C NMR (62.9 MHz,
CDCl3) δ 23.3, 26.6, 28.3, 32.1, 32.3, 48.2, 48.8, 48.9, 53.3, 64.2,
114.5, 170.7, 172.3; HMRS (ESI) m/z calcd for the protonated
molecule 9 (C54H79N10O18þ) 1155.5568, found 1155.5571.
Synthesis of 10. The ester 10 was obtained in 46% yield (238 mg)
according to a procedure similar to the one described for the
synthesis of 7, starting with 9 (200 mg, 0.17 mmol) and an excess of
3 (1.0 g, 3.12 mmol) in the presence of DBU (467 μL, 3.12 mmol) in
CH3CN (10 mL) for 48 h at room temperature. After workup, the
crude reaction mixture was submitted to a solution of benzoic
anhydride (1.8 g, 8.15 mmol) and DMAP (2.1 g, 16.3 mmol) in
CH2Cl2 (15 mL) for 1 h at rt. After chromatography on silica gel
(EtOAc/cyclohexane), 10 was isolated as a colorless oil: Rf 0.26
(EtOAc); 1H NMR (250 MHz, CDCl3) δ 1.16-1.62 (m, 162 H),
2.27-2.36 (m, 60 H), 2.62-2.70 (m, 42 H), 3.95-4.00 (m, 18 H);
13C NMR (62.9 MHz, CDCl3) δ 23.7, 26.8, 27.0, 28.1, 28.5, 32.4,
33.6, 49.3, 53.5, 64.5, 80.2, 172.0, 172.5, 172.6; MALDI-MS m/z
calcd for 10 (C156H285N10O42) 2969.95, found 2970.0; HMRS (ESI)
Synthesis of 5. To a solution of 4 (1.0 g, 2.88 mmol) in CH2Cl2
(5 mL) were added dropwise first Et3N (3.2 mL, 23.10 mmol)
and then chloroacetonitrile (2.2 mL, 34.60 mmol). The reaction
solution was stirred overnight at room temperature. After
removal of solvent, the obtained residue was diluted with EtOAc
(30 mL), washed with water (2 ꢀ 20 mL) and brine (20 mL),
dried over anhydrous MgSO4, and concentrated in vacuo. The
resulting crude material was purified by flash chromatography
(EtOAc/cyclohexane) to provide 5 as a pale amber oil (967 mg,
1
yield 96%): Rf 0.14 (EtOAc/cyclohexane 1:1, v/v); H NMR
(250 MHz, CDCl3) δ 2.49 (t, 6 H, 3J = 6.3 Hz), 2.69 (t, 6 H, 3J =
6.3 Hz), 4.69 (s, 6 H); 13C NMR (62.9 MHz, CDCl3) δ 32.0, 48.2,
48.7, 114.6, 170.5; HMRS (ESI) m/z calcd for the protonated
molecule 5 (C15H19N4O6þ) 351.1299, found 351.1297.
Synthesis of 6. A solution of 5 (95 mg, 0.27 mmol), 2 (519 mg,
1.64 mmol), and DBU (245 μL, 1.64 mmol) in CH3CN (4 mL)
was stirred at room temperature for 48 h. The solvent was then
removed, and the residue was diluted with EtOAc (20 mL),
washed with water (2 ꢀ 15 mL) and brine (15 mL), dried over
anhydrous MgSO4, and concentrated. The resulting crude material
was treated with benzoic anhydride (370 mg, 1.64 mmol) in the
presence of DMAP (210 mg, 1.64 mmol) in CH2Cl2 (10 mL) for
1 h at room temperature. After removal of the solvent, the residue
was diluted with EtOAc (15 mL), washed with saturated NaHCO3
solution (2 ꢀ 15 mL) and brine (15 mL), dried over anhydrous
MgSO4, and concentrated. After purification by chromatography
on silica gel (EtOAc/cyclohexane), 6 was obtained as a colorless
oil (160 mg, yield 52%): Rf 0.77 (EtOAc); 1H NMR (250 MHz,
CDCl3) δ 1.42 (s, 54 H), 2.34 (t, 12 H, 3J = 7.0 Hz), 2.44 (t, 6 H,
m/z calcd for the triply protonated molecule 10 (C156H285
-
N10O423þ) 990.6830, found 990.6827.
Acknowledgment. This work is funded by the CNRS, the
Association Franc-aise contre les Myopathies, the COST
Action TD0802 “Biological Applications of Dendrimers”,
and the international ERA-Net EURONANOMED
European Research project DENANORNA. C.B. was
supported by a Ph.D. fellowship from the French Ministry
of Education.
3J = 7.0 Hz), 2.67-2.81 (m, 24 H), 4.09 (t, 6 H, 3J = 6.3 Hz); 13
C
NMR (62.9 MHz, CDCl3) δ 28.1, 34.0, 48.9, 49.9, 52.0, 62.5,
80.3, 171.6-172.1; HMRS (ESI) m/z calcd for the doubly proto-
nated molecule 6 (C57H104N4O182þ) 566.3667, found 566.3667.
Synthesis of 7. The ester 7 was synthesized according to a pro-
cedure similar to that described for 6. Starting with 5 (100 mg,
Supporting Information Available: 1H NMR, 13C NMR,
and HRMS spectra of 4-10. This material is available free of
charge via the Internet at http://pubs.acs.org.
8688 J. Org. Chem. Vol. 75, No. 24, 2010