Synthesis and characterization of polyester dendrimers from acetoacetate and acrylate

Article abstract of DOI:10.1021/ol0482184

(Chemical Equation Presented) New aliphatic polyester-type dendrimers were synthesized using a new AB₂-type building block 3, prepared from benzyl acetoacetate and 2 equiv of tert-butyl acrylate by acetoacetic acid ester synthesis. The reiterative [deprotection by HCO₂H, then EDCI/DMAP coupling] sequence using divergent growth method gave [G1]-4tBu-[G5]-64tBu dendrimers. ¹³C NMR relaxation time (T₁) measurements on the carboxy carbons show that the extended chain conformations are predominant in CDCl₃.

Full text of DOI:10.1021/ol0482184

ORGANIC
LETTERS
2005
Vol. 7, No. 4
525-528
Synthesis and Characterization of
Polyester Dendrimers from Acetoacetate
and Acrylate
Yuuki Hirayama, Taisuke Nakamura, Satoshi Uehara, Yohko Sakamoto,†
Kentaro Yamaguchi,‡ Yoshihisa Sei,‡ and Michiko Iwamura*
Department of Biomolecular Science and School of Pharmaceutical Sciences,
Toho UniVersity, Miyama 2-2-1, Funabashi, Chiba 274-8510, Japan, and Chemical
Analysis Center, Chiba UniVersity, 1-33 Yayoicho, Inage-ku, Chiba 263-8522, Japan
michiko@biomol.sci.toho-u.ac.jp
Received September 3, 2004 (Revised Manuscript Received January 11, 2005)
ABSTRACT
New aliphatic polyester-type dendrimers were synthesized using a new AB2-type building block 3, prepared from benzyl acetoacetate and 2
equiv of tert-butyl acrylate by acetoacetic acid ester synthesis. The reiterative [deprotection by HCO2H, then EDCI/DMAP coupling] sequence
using divergent growth method gave [G1]-4tBu−
[G5]-64tBu dendrimers. 13C NMR relaxation time (T1) measurements on the carboxy carbons
show that the extended chain conformations are predominant in CDCl3.
Dendrimers are globular macromolecules consisting of a core
molecule and symmetrically arranged branches with a well-
defined number of reactive end groups matching to each
generation.1,2 Among the dendritic branches such as polyamine,
polyamide, or polyether, etc., polyester dendrimers attract
considerable attention especially for therapeutic applications
where biocompatibility and easy hydrolytic nature inside cells
are important.3 In addition, due to high solubility and
miscibility in ordinary organic solvents and reactivity of end
carboxy or hydroxy groups, they can be used as versatile
tools in various fields.4
In connection with our continuous effort to develop caged
compounds for application to drug delivery systems, we
selected dendrimers suitable for a core to attach sugars and
caged compounds that could recognize particular cells and
release a drug upon irradiation of light.5 In the course of
our study, we were interested to design and synthesize novel
polyester-type dendrimers that could be easy to modify the
(3) (a) Ihre, H.; Padilla De Jesus, O. L.; Fre´chet, J. M. J. J. Am. Chem.
Soc. 2001, 123, 5908-5917. (b) Padilla De Jesus, O. L.; Ihre, H.; Gagne,
L.; Fre´chet, J. M. J.; Szoka, F. C., Jr. Bioconjugate Chem. 2002, 13, 453-
461. (c) Carnahan, M.; Grinstaff, M. W. J. Am. Chem. Soc. 2001, 123,
2905-2906. (d) Carnahan, M. A.; Middleton, C.; Kim, J.; Kim, T.; Grinstaff,
M. W. J. Am. Chem. Soc. 2002, 124, 5291-5293. (e) Morgen, M. T.;
Carnahan, M. A.; Immoos, C. E.; Ribeiro, A. A.; Finkelstein, S.; Lee, S.
J.; Grinstaff, M. W. J. Am. Chem. Soc. 2003, 125, 15485-15489.
(4) For reviews on polyester dendrimers, see: Nummelin, S.; Skrifvars,
M.; Rissanen, K. Dendrimers II; Vo¨gtle, F., Ed. Top. Curr. Chem. 2000,
210, 1-67.
† School of Pharmacetical Sciences.
‡ Chiba University. Present address: Laboratory of Analytical Chemistry
Department of Pharmaceutical Technology, Faculty of Pharmaceutical
Sciences at Kagawa Campus, Tokushima Bunri University, Shido, Sanuki-
city, Kagawa 769-2193.
(1) (a) Fre´chet, J. M. J. Science 1994, 263, 1710-1715. (b) Tomalia, D.
A. Nature 1994, 372, 617-618.
(2) For recent reviews on dendrimers in general, see: (a) Dendrimers
II; Vo¨gtle, F., Ed. Top. Curr. Chem. 2000, 210. (b) Dendrimers and Other
Dendritic Polymers; Fre´chet, J. M. J., Tomalia D. A., Eds.; Wiley: New
York, 2002. (c) Fre´chet, J. M. J. J. Polym. Sci. Part A: Chem. 2003, 41,
3713-3725. (d) Pyun, J.; Zhou, X.-Z.; Drockenmuller, E.; Hawker, C. J.
J. Mater. Chem. 2003, 13, 2653-2660.
(5) (a) Watanabe, S.; Sato, M.; Sakamoto, S.; Yamaguchi, K.; Iwamura,
M. J. Am. Chem. Soc. 2000, 122, 12588-12589. (b) Watanabe, S.; Iwamura,
M. J. Photochem. Photobiol. A: Chem. 2003, 155, 57-62.
10.1021/ol0482184 CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/28/2005

potassium tert-butoxide gave the addition product 1 (96%).
Hydrogenolysis of 1 in the presence of 5% Pd/C in methanol
followed by decarboxylation at 80 °C afforded an almost
quantitative amount of 2. Reduction of 2 by NaBH4 in
ethanol gave the building block 3 in 95% yield. Although a
combination of tert-butyl acetoacetate and benzyl acrylate
gave similar yields of both addition and decarboxylation
products, reduction by NaBH4 resulted in intramolecular
esterification to afford a lactone as a major product. On the
other hand, bulky tert-butyl groups in 3 prevented cyclization.
Scheme 1. Preparation of Building Block 3
Dendrimer synthesis was carried out by a divergent method
using 2 as a core molecule. Synthesis of [G3]-16tBu, 6, is
shown in Scheme 2. Since the use of trifluoroacetic acid
sometimes cleaved the ester bond, formic acid was used for
the deprotection of tert-butyl group. Compound 2 was
deprotected by HCO2H at room temperature followed by
complete evaporation of the acid by azeotropic distillation
with 1,2-dichloroethane under reduced pressure. Deprotected
2 was reacted with 2.05 equiv of 3 in the presence of EDCI
(2.1 equiv) and DMAP (1.0 equiv) in anhydrous dichlo-
romethane to give 4 ([G1]-4tBu) in 90% yield after purifica-
tion by silica gel chromatography. Compound 4 was sub-
sequently deprotected by HCO2H and then reacted with 4.1
functional groups at the periphery and also to be hydrolyzed
in cells by cytosolic esterase. We wish to present here a
synthesis of new polyester-type dendrimers using acetoacetic
ester synthesis to provide a new AB2 type building block
(Scheme 1).
The reaction of benzyl acetoacetate and 2 equiv of tert-
butyl acrylate in THF in the presence of catalytic amount of
Scheme 2. Divergent Synthesis of G1, G2, G3, and G4 Dendrimers
526
Org. Lett., Vol. 7, No. 4, 2005

equiv of 3 in the presence of EDCI (5.2 equiv) and DMAP
(1.8 equiv) to produce 5 ([G2]-8tBu) in 82% yield. Com-
pounds 6 ([G3]-16tBu), 7 ([G4]-32tBu), and 8 ([G5]-64tBu)
were synthesized by repetition of the [deprotection by
HCO2H then EDCI/DMAP coupling] sequence. 8 was
obtained in 49% yield after a nine-step synthesis starting from
2 as shown in Table 1.
Table 1. Total Yields, ESI-MS, and Elemental Analysis Data
for the Dendrimers
yield (%)
from 2
MW
calcd
MS from
ESI-MS
elemental analysise
4
5
6
7
8
88
81
69
60
49
799.2
798a
C, 64.49 (64.64); H, 9.37 (9.33)
1767.1 1768.6a C, 64.38 (64.53); H, 9.30 (9.23)
3704.3 3704.9b C, 64.38 (64.48); H, 9.06 (9.19)
7578.7 7571.1c C, 64.00 (64.46); H, 9.10 (9.17)
15327.6 15342d
C, 64.43 (64.45); H, 9.19 (9.16)
a Observed as [M + Na]+. b Observed as [M + 2Na]2+
.
c Observed as
[M + 3Na]3+
.
d A weak ion-peak corresponding to [M + 3Na]3+ was
observed. e Calculated values in parentheses.
Although a combination of DCC and DPTS can be used
instead of EDCI, the yield of the dendrimer was lower than
that by using EDCI due to the difficulty in separating the
byproduct DCU from the product.
Figure 1. ESI-MS spectra of (a) 6 and (b) 7.
Convergent synthesis or a combination of divergent and
convergent synthesis were also attempted. For example, 7
was synthesized from [G2]-8COOH and OH-[G1] in 52%.
However, because the use of NaBH4 for reduction of a focal
carbonyl group also reduced the ester bonds even under
careful and mild conditions, divergent method was preferred.
All the t-Bu-capped dendrimers are colorless viscous
liquids which are soluble in halogenated solvents, methanol,
or ethyl acetate and insoluble in water. They were mostly
purified by silica gel chromatography using a mixture of
hexane and ethyl acetate (1:1 v/v) as an eluent. The elemental
analysis showed good agreement with the calculated values,
however, ESI-MS offered more reliable molecular weight
data as shown in Table 1.6 Whereas the molecular ion peaks
of 4-7 were clearly observed, only a very weak molecular
ion peak assigned to [M + 3Na]3+ was found among a
number of weak ion peaks for 8. The ESI-MS spectra of 6
and 7 are shown in Figure 1. GPC measurements confirmed
the low polydispersities of 1.08, 1.07, and 1.09 for 6, 7, and
8, respectively.
of the 13C signals of the carboxys, “13C gated decoupling
without NOE technique” was very useful (Figure 2). From
1H and 13C NMR spectra are almost the same regardless
of the generations only differing in the integration ratios of
the signals. For example, while the integration ratio of a
multiplet centered at 4.8 ppm (CH3CH-), a singlet at1.36
ppm ((CH3)3C-) and a doublet at 1.11 ppm (CH3CH-) is
1:12:3 for 5, and the ratio for 8 is 1:9.27:3. Complete
Figure 2. (a)13C NMR of 8 ([G5]-64tBu) in CDCl3; (b) spectra of
carboxy carbons using 13C gated decoupling without NOE tech-
nique.
1
assignments of all the H and 13C signals of 6 and 7 were
carried out using versatile NMR techniques such as TOCSY,
the intensity ratios, the three closely adjacent signals, 172.68,
172.77, and 172.83 ppm, were assigned for the outermost,
the second from the outermost, and the rest of the carboxyl
carbons, respectively.
HOHAHA, HMQC, etc.7 In particular, for the assignment
(6) Due to our instrumental convenience, ESI and CSI (cold spray
ionization) MS were used instead of most commonly used MALDI-MS.
Org. Lett., Vol. 7, No. 4, 2005
527

13C NMR relaxation time (T1) measurements were per-
formed in CDCl3 to give the results shown in Table 2. As
bulky tert-butyls provide mobility around the carbons d
almost to the same extent in regardless of generation from
[G3] to [G5]. These observations suggest that the ester
skeleton of these dendrimers takes predominantly extended
chain conformation at least in CDCl3.
Table 2. 13C NMR Chemical Shifts (ppm) and T1 Relaxation
Time Constants (s) for the Dendrimers [Gn] in CDCl3 at 300 K
In summary, an efficient divergent procedure for synthe-
sizing new aliphatic polyester-type dendrimers from ac-
etoacetic acid and acrylic acidhas been established. The
reiterative [deprotection by HCO2H then EDCI/DMAP
coupling] sequence gave dendrimers ([G1]-[G5]) in fairly
good yield. The tert-butyl groups at the periphery can be
deprotected easily by formic acid to afford the dendrimers
with free COOH, which can be easily modified with various
kinds of functional groups. Studies on biocompatibility of
these new dendrimers for application as a novel drug delivery
system with sugars and caged compounds, or modification
of the periphery with various functionalities are now in
progress.
carbons
δTMS
T1([G3]-16tBu) T1([G4]-32tBu) T1([G5]-64tBu)
a
b
c
d
e
80.27
28.20
172.80
172.89
23.90
10.97
1.34
6.61
1.57
0.37
9.83
1.31
5.64
1.62
0.37
9.67
1.23
5.31
1.62
0.33
the dendrimer generation increases, a slight decrease in T1
is observed for the outermost carboxyl carbons c, methyl
carbons b and quaternary carbons a of tert-butyls, suggesting
the decrease in the mobility at the periphery of the dendrimer
molecules due to the increasing congestion of the tert-butyl
groups. The second from the outermost carboxy carbons d
always show smaller T1 than those of c. These results are
consistent with those previously reported.8,9
Acknowledgment. We thank Drs. Haruhisa Akiyama and
Nobuyuki Tamaoki of Molecular Smart System Group,
Nanotechnology Research Institute, National Institute of
Advanced Industrial Science and Technology (AIST), and
Dr. Ken-ichi Aoki of Faculty of Science, Toho University,
for the GPC measurements. This work was partly supported
by a Grant-in-Aid for Scientific Research (No. 13680677)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
Interestingly, T1 values of the interior carboxy carbons d
are almost constant in 6, 7, and 8, which indicates that the
(7) The detailed analysis of these NMR spectra will be reported elsewhere
in the near future.
(8) (a) Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1998, 120, 10895-
10901. (b) Hecht, S.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1999, 121, 4084-
4085. (c) Meier, H.; Lehmann, M.; Kolb, U. Chem. Eur. J. 2000, 6, 2401-
2406.
Supporting Information Available: Experimental pro-
1
cedures for the synthesis of 3-8 and H and 13C NMR of
6-8 and deprotected 7. This material is available free of
charge via the Internet at http://pubs.acs.org.
(9) Chai, M.; Niu, Y.; Youngs, W. J.; Rinaldi, P. L. J. Am. Chem. Soc.
2001, 123, 4670-4678.
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