Concise solid-phase synthesis of inverse poly(amidoamine) dendrons using AB2 building blocks

Article abstract of DOI:10.1039/c3cc40661j

A concise solid-phase synthesis of inverse poly(amidoamine) dendrons was developed. Upon introduction of AB₂-type monomers, each dendron generation was constructed via one reaction. G2 to G5 dendrons were constructed in a peptide synthesizer in 93%, 89%, 82%, and 78% yields, respectively, within 5 days.

Full text of DOI:10.1039/c3cc40661j

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Concise solid-phase synthesis of inverse poly(amidoamine)
dendrons using AB2 building blocks†‡
Adela Ya-Ting Huang,a Ching-Hua Tsai,a Hsing-Yin Chen,a Hui-Ting Chen,b
Chi-Yu Lu,c Yu-Ting Lina and Chai-Lin Kao*a
Cite this: Chem. Commun., 2013,
49, 5784
Received 25th January 2013,
Accepted 3rd April 2013
DOI: 10.1039/c3cc40661j
www.rsc.org/chemcomm
A concise solid-phase synthesis of inverse poly(amidoamine) den- filtration steps, which reduce the amount of effort required during
drons was developed. Upon introduction of AB2-type monomers, purification. Excess reagents are usually used to ensure that a reaction
each dendron generation was constructed via one reaction. G2 to goes to completion and to reduce the formation of incompletely
G5 dendrons were constructed in a peptide synthesizer in 93%, functionalized products. SPOS approaches provide an efficient way
89%, 82%, and 78% yields, respectively, within 5 days.
for the preparation of dendrimers.3
Poly(amidoamine) (PAMAM) dendrimers are one example of
commercially available dendrimers. PAMAM dendrimers present
amine and amido groups and can, therefore, act as protein mimics.
This unique feature confers a diversity of characteristics on PAMAM,
making it suitable for biological applications. The preparation of
PAMAM dendrimers was first reported by Tomalia et al., who used
methyl acrylate and ethylenediamine as building blocks for sub-
sequent Michael addition and amide formation.4 Although these
two reactions are highly efficient, several side products have been
identified.5 Reducing the side products presents a major challenge
to the preparation of PAMAM dendrimers. Several SPOS approaches
have been reported for the preparation of PAMAM dendrons. SPOS
approaches can improve the efficiency of PAMAM dendron prepara-
tion by reducing the prevalence of incomplete reactions. However,
similar by-products will be observed if the original protocol devel-
oped by Tomalia et al. is used in the SPOS preparation.6 In addition
to incomplete reactions, steric hindrance of Michael addition also
led to the formation of by-products.7 Furthermore, intramolecular
cross-linking between proximal dendron molecules on a resin
provides additional drawbacks to using SPOS approaches for the
preparation of PAMAM dendrons.8 Consequently, it was necessary
to reduce the initial loading rate on the solid support to prevent
cross-linking.9 These steps reduced the efficiency of PAMAM den-
dron formation using SPOS techniques. A newly designed approach
that does not involve Michael addition may overcome these dis-
advantages. Meanwhile, the avoidance of alkanediamine can also
potentially minimize the formation of cross-linked by-products. The
dendron construction protocol was designed for use in a peptide
synthesizer to be simple and free from harsh reaction conditions.
Compound 1, an AB2-type molecule, was designed as a building
block using a free carboxylic acid and protected amines (Fig. 1). The
incorporated 1,3-propylenediamine moiety instead of ethylenediamine
in a classical PAMAM dendrimer was found to make the prepared
dendron possess similar molecular volume and surface area to the
Dendrimers and dendrons are polymeric branched macromolecules
with well-defined regular branching structures that emanate radially
from a central core. Dendrons are wedge-shaped sections of a
dendrimer. Dendrimers with a narrow or even monodisperse mass
distribution are potentially useful in clinical applications that take
advantage of their tuneable peripheral functional groups. However,
dendritic structures are generally tedious and laborious to prepare
and purify. The preparation of pure monodisperse dendrimers or
dendrons remains a challenge in the field of dendrimer chemistry.
The preparation of dendrimers and dendrons must meet two
criteria: the precise installation of building blocks at certain positions
and the introduction of a sufficient number of building blocks into
the dendrimer framework. Dendrimers and dendrons may be prepared
via divergent or convergent synthetic approaches. Although both
are efficient approaches, incompletely functionalized by-products
and structurally defective products are usually found during the
preparation through divergent and convergent approaches, respec-
tively. Furthermore, the structurally similar by-products are difficult
to separate from the desired dendrimers and dendrons. A synthetic
approach that addresses these concerns would be tremendously
helpful in the preparation of dendrimers and dendrons. Various
approaches have been suggested for improving the efficiency of the
productive reactions and reducing the formation of defective products.1
One approach involves solid-phase organic syntheses (SPOS).2 SPOS
approaches rely on an insoluble support to isolate a product through
a Department of Medicinal and Applied Chemistry, Kaohsiung Medical University,
Kaohsiung 807, Taiwan. E-mail: clkao@kmu.edu.tw
b Department of Fragrance and Cosmetic Science, Kaohsiung Medical University,
Kaohsiung 807, Taiwan
c Department of Biochemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan
† This paper is dedicated to Professor Hung-wen (Ben) Liu on the occasion of his
60th birthday.
‡ Electronic supplementary information (ESI) available: Experimental procedures, com-
pound characterization data, and simulation procedure. See DOI: 10.1039/c3cc40661j
c
5784 Chem. Commun., 2013, 49, 5784--5786
This journal is The Royal Society of Chemistry 2013

View Article Online
Communication
ChemComm
revealed that fewer reaction steps were needed and a smaller amount
of the building blocks was required to build dendrons of similar size
from 1.11 The coupling reaction was carried out at the free carboxylic
acid groups by adding amino-functionalized monomers to the resin-
bound molecules in a standard solid phase peptide automated
synthesis machine. In comparison to the structure of a classical
PAMAM dendrimer, an inverse amide bond was formed to yield the
final product. Because comparable amounts of similar functional
groups should be present in the final dendrons prepared using SPOS
or the classical route, the properties, such as solubility12 and the site
isolation effect,13 of these inverse and classical PAMAM dendrons
were expected to be similar.
The preparation of 1 starts from the alkylation of b-alanine (4)
with N-Boc-3-bromopropylamine (5). The conditions tested, includ-
ing the solvent, base, and reaction time, are summarized in Table S1
(ESI‡). The reaction temperature was crucial for the reaction.
Raising the reaction temperature increased the yield. The reaction
time was another important factor that affected the formation of
side products. Reaction times shorter than 48 hours afforded low
reaction yields; however, prolonged reaction times did not improve
the reaction yield. In contrast, longer reaction times slightly reduced
the yield. A variety of bases were tested. The reactions in the
presence of DIPEA and K2CO3 gave similar yields, and lower yields
were observed in the presence of others, such as CsCO3 and DBU.
K2CO3 was selected as optimal because of easier removal of
inorganic base. An iodide source was also necessary for this
reaction. Reactions conducted without the addition of iodide gave
very low yields. Two iodide sources, tetrabutylammonium iodide
(TBNI) or KI, were used to improve the reaction and provide similar
reaction yields. Three solvents, THF, MeCN, and DMF, were investi-
gated and THF gave the best results. The presence of additives, such
as crown ether, did not improve the reaction yield. Product 6 was
prepared by the alkylation of 4 with 5 in the presence of K2CO3 and
KI in THF at 65 1C over 48 h. With compound 6 in hand, the
following hydrolysis under basic conditions gave the desired com-
pound 1. It is known that compounds containing tertiary amine and
carboxylic acid will form zwitterions.14 Compound 1 was collected
by extraction after the reaction had completed. Remarkably, com-
pound 1 was collected from the aqueous layer (Scheme 1).
Fig. 1 Designed building block
1 and the structures of classical PAMAM
dendron (2) and inverse PAMAM dendron (3) as well as their optimized structures
viewed along three molecular axes.
classical PAMAM molecules (Table 1). Based on the simulation
result, the molecular volume and surface area of the classical G3
PAMAM dendron (2) are 3976 Å3 and 2084 Å2. The ones of the
inverse dendron with ethylenediamine (7) are only 2752 Å3 and
1613 Å2 (Table S3, ESI‡). In comparison, volume and area of 3 are
3421 Å3 and 1859 Å2, which are closer to the values of 2 than those
of 7. Moreover, the 1,3-propylenediamine moiety was also expected
to reduce the formation of side-products in SPOS dendrons relative
to the results obtained using the ethylenediamine moiety.10
Furthermore, the tertiary amine provided a focal point for 1,
effectively avoiding unwanted cross-linking during amide bond
formation. Michael addition was not needed in this approach,
thereby precluding the formation of side-products that are typical
of the classical approach. A comparison with the classical approach
The dendrons were prepared using an oxime resin as the solid
support. To prepare the desired dendrons, the hydroxyl group of the
oxime and 1 was coupled in the presence of PyBOP. After the
introduction of 1, Ac2O was used to mask the remaining active
amino groups. The following removal of Boc groups required
acidic conditions that could damage the oxime group. Two
conditions, 25% TFA in CH2Cl2 for 2 h or 30% TFA in CH2Cl2 for
Table 1 Volume and surface area of a G3 classical PAMAM dendron (2) and a
G3 inverse PAMAM dendron (3)a
2
3
Volume (Å3)
3976
2084
3421
1859
Surface area (Å2)
a
Data obtained by analysing the lowest energy structures of the
dendrons which were obtained using quenched MD simulations.
Scheme 1
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 5784--5786 5785

View Article Online
ChemComm
Communication
Scheme 2
1 h, were investigated. The Boc group could not be completely approaches, the method reported here can produce dendrons more
removed under prolonged diluted conditions. In contrast, no Boc rapidly and at a lower cost. Moreover, oxime resins are advanta-
groups were identified in the reaction with 30% TFA.
geous because they support a variety of functionalities. Various
All the steps in the procedure (except for the first coupling products containing esters, amides, carboxylic acids, or methylene
reaction with the oxime resin), including coupling, deactivation, alcohols may be obtained via hydrolysis in the presence of methanol,
and deprotection, could be followed on the solid-phase resin using amines, an aqueous base, or reducing agents, respectively. This
a Kaiser test.15 The iterative incorporation of 1 through this cycle advantage leads to more diverse functionality of the final products.
gradually built up the dendron molecules. Upon reaching the desired This approach sets the stage for the construction of inverse PAMAM
size, the product was collected in the presence of NaOH in dioxane. dendrons as well as for further modification of dendrons with
The final products were collected by simple filtration only. Further diverse functional groups. These advantages may further
purification, such as dialysis, or chromatography, is not necessary. expand the applications of dendrons and assist in fundamental
The reaction yields of the products were calculated with reference to studies of dendrimer chemistry.
the maximum theoretical loading of the resin. The reaction yields
This work was support by National Science Council of the
were 91%, 84%, 79%, 71% for 3a–3d, respectively, in manual Republic of China (NSC 99-2113-40 M-037-010-MY2) and
syntheses. In contrast, 93%, 89%, 82%, 78% yields were obtained NSYSU-KMU Joint Research Project, (NSYSUKMU 102-P027).
for 3a–3d, respectively, using the peptide synthesizer (Scheme 2).
We thank the National Center for High performance computing
The resulting products were characterized using NMR, MALDI- for computer time and facilities and CRRD of KMU.
MS, GPC and HPLC (ESI‡). The PDI values fell between 1.07 and 1.18
Notes and references
(Table S2, ESI‡). In classical preparation, dialysis was reported as a
crucial step for purity.16 In comparison, the purity of our products
which were collected after filtration demonstrates similar purity to
that of dialyzed products. This observation indicated that further
purification was not necessary. Furthermore, this approach prepared
dendrons in an efficient manner. As an example, 3d can be prepared
on a 0.1 mmol scale in 5 days using a peptide synthesizer. These
results suggest that this approach is efficient and yields negligible
side products. Remarkably, it was not necessary to reduce the
loading rate, thereby, increasing the amount of product.
1 M. V. Walter and M. Malkoch, Chem. Soc. Rev., 2012, 41, 4593.
2 T. Kehat, K. Goren and M. Portnoy, New J. Chem., 2007, 31, 1218.
3 (a) E. Delort, N.-Q. Nguyen-Trung, T. Darbre and J. L. Reymond, J. Org.
Chem., 2006, 71, 4468; (b) J. J. Diaz-Mochon, M. A. Fara, R. M. Sanchez-
Martin and M. Bradley, Tetrahedron Lett., 2008, 49, 923.
4 I. J. Majoros, C. R. Williams, D. A. Tomalia and J. R. Baker, Jr.,
Macromolecules, 2008, 41, 8372.
5 J. Peterson, V. Allikmaa, J. Subbi, T. Pehk and M. Lopp, Eur. Polym.
J., 2003, 39, 33.
6 (a) V. V. Swali, N. J. Wells, G. J. Langley and M. Bradley, J. Org. Chem.,
1997, 62, 4902; (b) Y. Taniguchi, K. Shirai, H. Saitoh, T. Yamauchi
and N. Tsubokawa, Polymer, 2005, 2541.
In summary, a convenient approach to the production of
inverse poly(amidoamine) dendrons using manual syntheses
and an automated machine was developed. A newly designed
7 J. Bu, R. Li, C. W. Quah and K. J. Carpenter, Macromolecules, 2004, 37, 6687.
8 A. Baso and M. Bradley, Tetrahedron Lett., 2003, 44, 2699.
9 Y.-M. Chung and H.-K. Rhee, Chem. Commun., 2002, 238.
10 P. P. Zweni and H. Alper, Adv. Synth. Catal., 2004, 346, 849.
AB2-type building block provided the focal point of dendrons and 11 M. V. Walter and M. Malkoch, Chem. Soc. Rev., 2012, 4593.
12 F. Zeng and S. C. Zimmerman, Chem. Rev., 1997, 97, 1681.
13 Y. Rio, G. Accorsi, H. Nierengarten, C. Bourgogne, J.-M. Strub, A. V.
provided a means to reduce the formation of side products.
Iterative coupling of this monomer yielded inverse dendrons with
Dorsselaer, N. Armaroli and J.-F. Nierengarten, Tetrahedron, 2003, 59, 3833.
´ ´
a high yield. The significance of this study is that this protocol was 14 C. Bouillon, G. Quelever and L. Peng, Tetrahedron Lett., 2009, 50, 4346.
15 E. Kaiser, R. L. Colescott, C. D. Bossinger and P. I. Cook, Anal.
Biochem., 1970, 34, 595.
16 D. G. Mullen, A. Desai, M. A. van Dongen, M. Barash, J. R. Baker, Jr.
accomplished using a solid-phase synthesizer that can further
reduce the costs and time associated with preparing dendrons,
particularly higher-generation dendrons. In comparison to classical
and M. M. Banaszak Holl, Macromolecules, 2012, 45, 5316.
c
5786 Chem. Commun., 2013, 49, 5784--5786
This journal is The Royal Society of Chemistry 2013