Rapid, efficient synthesis of heterobifunctional biodegradable dendrimers

Article abstract of DOI:10.1021/ja071530z

A method for the preparation of heterobifunctional biodegradable dendrimers is described involving the preparation of a symmetric aliphatic ester dendrimer derived from 2,2-bis(hydroxymethyl)propanoic acid bearing a cyclic carbonate periphery, followed by

Full text of DOI:10.1021/ja071530z

Published on Web 05/10/2007
Rapid, Efficient Synthesis of Heterobifunctional Biodegradable Dendrimers
Andrew P. Goodwin, Stephanie S. Lam, and Jean M. J. Fre´chet*
Department of Chemistry, UniVersity of California, MS 1460, Berkeley, California 94720-1460
Received March 4, 2007; E-mail: frechet@berkeley.edu
A number of polymeric systems have been explored for biomedi-
cal applications such as drug delivery,1 gene delivery,2 and in vivo
imaging,3 but the properties of dendrimers translate especially well
into pharmacology.4-6 Their precise synthesis produces a polymer
of low polydispersity, which increases the likelihood of reproducible
behavior within a patient. Moreover, their high degree of branching
provides a multivalent periphery that can be used for the conjugation
of many copies of functional molecules, increasing the efficacy or
spectroscopic sensitivity of each individual dendritic carrier mol-
ecule. In particular, the aliphatic polyester dendrimer derived from
2,2-bis(hydroxymethyl)propanoic acid (bis-HMPA) boasts low
toxicity and immunogenicity7 with its non-ionic and biodegradable
structure.8
Figure 1. General two-step differentiation scheme for cyclic carbonates
derived from 2,2-bis(hydroxymethyl)propanoic acid.
Scheme 1. Synthesis of [G-2] Bis-HMPA Dendrimer with a Cyclic
Carbonate Peripherya
From a single dendrimer scaffold, functional molecules may be
added to provide solubility and compatibility, increased plasma
residence time, targeting, imaging, therapy, or potentially any
combination of these. However, controlled implementation of varied
functions onto a single platform can be difficult because all
peripheral groups of a symmetric dendrimer have the same
reactivity. Dendrimers bearing more than one type of peripheral
group can be prepared, but their synthesis, requiring numerous steps,
can be challenging.9-11 An efficient method would be to grow a
symmetric dendrimer in bulk and then tune its periphery to the
desired application. Since symmetric dendrimers are easier to
construct than their asymmetric counterparts, the initial part of the
synthesis would be more amenable to larger-scale production.
However, the latter process requires that the subsequent differentia-
tion and coupling steps be minimal in number and efficient in
reactivity. Moreover, all synthetic intermediates should be purified
by simple means, preferably without chromatography.
We employed the cyclic carbonate as a symmetric substrate that
can yield a bifunctional product. The reaction of a cyclic carbonate
and an amine has been used previously for polyurethane synthesis12
and has even proven efficient and selective enough to be run in
water with quantitative conversions.13 In the reaction, the amine
opens the ring, forming a carbamate linkage with concomitant
liberation of an alcohol that may then be used for a subsequent
ligation. Thus two different moieties may be added in immediate
succession without any deprotection steps or functional group
conversions. To append these groups to bis-HMPA dendrimers,
synthesis of a cyclic carbonate bis-HMPA monomer was under-
taken. Unfortunately, the direct addition of reactive carbonyl
equivalents such as phosgene, triphosgene, and ethyl chloroformate
to the free bis-HMPA led to a mixture of products. Therefore, the
free acid was first protected as the benzyl ester and then treated
with ethyl chloroformate and triethylamine to yield a carbonate
product that could be recrystallized easily and in good yield
(Scheme 1).14 The acid was then deprotected by hydrogenolysis in
near quantitative yield and required no further purification. To
provide a model platform for testing the reaction, dendrimer 4 with
eight hydroxyl groups was synthesized in two steps from pen-
taerythritol.15,16 DCC coupling of 3 and 4 led to a pure carbonate-
a Reagents and conditions: (a) BnBr, Et3N, CH2Cl2; (b) ethyl chloro-
formate, Et3N, THF; (c) H2, 10% Pd/C, EtOAc, 98%; (d) 3, DCC, DMAP,
DPTS, CH2Cl2, DMF, 81%.
bearing dendrimer after filtration, extraction, and precipitation into
ether. No urea impurities could be found by NMR or IR, and
MALDI-TOF MS showed only the desired product.
The ring-opening reaction with amines proved to be quite facile.
As 5 was soluble in highly polar solvents such as DMF or pyridine,
solvent screening reactions with benzylamine as a model nucleo-
phile revealed that a 1:1 mixture of THF and pyridine worked best,
although pyridine alone could be used as well if desired. Next, the
generality of this reaction was explored. To examine the effect of
steric hindrance of the amine, test reactions were performed with
benzylamine, sec-butylamine, and piperidine. Piperidine and ben-
zylamine proved to be most reactive, with 1H NMR analysis
suggesting full conversion after 1 and 3 h, respectively. MALDI-
TOF analysis of the reaction product showed the presence of a
smaller peak corresponding to a product resulting from seven rather
than eight reactions, suggesting an overall conversion of 95-99%
(see Supporting Information). sec-Butylamine reacted more slowly,
with some starting material still apparent after overnight stirring.
Other nucleophiles were not as reactive. After 16 h stirring, phenol,
aniline, and benzyl alcohol showed only 13, 7, and 12% conversion,
respectively; thus the reaction appeared to favor amines. It should
be noted that no side products could be observed by 1H NMR during
these experiments.
We explored the elaboration of dendrimer 5 to produce den-
drimers with reactive sites amenable to future conjugation. Encour-
aged by the difference in reactivity between amines and phenols,
tyramine was employed to build a dendrimer containing phenolic
substituents for either PET radiolabeling17 or subsequent liga-
tions18,19 (6i). The NMR spectrum of 6i showed no peaks corre-
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J. AM. CHEM. SOC. 2007, 129, 6994-6995
10.1021/ja071530z CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Synthesis of Bifunctional Dendrimer 7a
Table 1. Reactions of Cyclic Carbonate Dendrimer 5a
time
(h)
conversionb
(%)
yieldc
(%)
entry
reagent
6a
6a
6b
6b
6c
6d
6e
6f
benzylamine
benzylamine
sec-butylamine
sec-butylamine
piperidine
aniline
phenol
benzyl alcohol
propargylamine
(MeO)2CHCH2NH2
tyramine
1
3
3
16
1
16
16
16
16
16
16
63
95-99
26
90
95-99
13
7
12
95-99
95-99
95-99
-
92
-
74
96
-
-
-
a Reagents and conditions: (a) 1,1′-carbonyldiimidazole in CH2Cl2, 1
h, then propargylamine, 16 h, 80%.
6g
6h
6i
89
91
94
Acknowledgment. The authors thank NIH (R01-EB002047) for
funding. S.S.L. thanks NSF for a summer research stipend.
Supporting Information Available: Experimental procedures and
characterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
a Performed in 1:1 pyridine/THF. b Conversion per carbonate measured
1
by H NMR. c Dendrimer yield after workup.
sponding to acyclic carbonates, as would be expected if the phenol
had reacted with the cyclic carbonates, and peak integration data
for the carbamate peaks were consistent with those of the inner
dendrimer framework. Reaction of 5 with propargylamine, intended
to provide a dendrimer with alkynes for click chemistry,19 also met
with similar success, affording 6g. Finally, reaction with aminoac-
etaldehyde dimethyl acetal gave a 90% yield of dendrimer 6h with
eight acetals. Deprotection of these acetals would provide free
aldehydes, which could be used for subsequent oxime, hydrazone,
or thiazolidene formation.18c In each reaction, the dendrimer
products were purified simply by washing the reaction mixture with
aqueous acid to remove both the pyridine and any unreacted amine.
The free alcohols were then employed to introduce other
functional substituents. Since oxime formation and click chemistry
are orthogonal to one another, a dendrimer containing both protected
aldehydes and alkynes was synthesized. The free alcohols of 6h
were activated with 1,1′-carbonyldiimidazole, then reacted in situ
with propargylamine (Scheme 2). Excess propargylamine and
propargylurea byproduct were washed away with water, affording
bifunctional dendrimer 7 as a white powder in 80% yield.
This work has demonstrated the rapid and scalable synthesis of
heterobifunctional bis-HMPA dendrimers derived from a cyclic
carbonate periphery. The symmetric carbonate periphery reacted
cleanly, efficiently, and selectively with primary amines to produce
bifunctional dendrimers. Use of this methodology yielded a
dendrimer bearing both alkynes and protected aldehydes for
orthogonal ligation reactions. This was accomplished in nine steps
from commercially available materials, requiring only extraction,
precipitation, or recrystallization to purify the synthetic intermedi-
ates. We are exploring the use of this methodology to construct
future platforms for the delivery of therapeutic agents.
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