Facile synthesis of second-generation dendrons with an orthogonal functional group at the focal point

Article abstract of DOI:10.1080/00397910802627076

Facile synthesis of second-generation dendrons with an aldehyde, epoxy, or t-Boc group at the focal point and nine carboxylic acid groups at the periphery is reported. The scheme includes a coupling of the first-generation dendrons and a two-step, one-pot reaction that proceeds through a Boc deprotection and in situ conjugation at the focal point.

Full text of DOI:10.1080/00397910802627076

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Facile Synthesis of Second-
Generation Dendrons with an
Orthogonal Functional Group at
the Focal Point
Ravi Kumar Cheedarala a , Vijaya Sunkara a & Joon
Won Park a
a Center for Integrated Molecular Systems ,
Department of Chemistry, Pohang University of
Science and Technology (POSTECH) , Pohang, Korea
Published online: 27 Apr 2009.
To cite this article: Ravi Kumar Cheedarala , Vijaya Sunkara & Joon Won Park (2009)
Facile Synthesis of Second-Generation Dendrons with an Orthogonal Functional
Group at the Focal Point, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 39:11, 1966-1980, DOI:
10.1080/00397910802627076
To link to this article: http://dx.doi.org/10.1080/00397910802627076
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Synthetic Communications1, 39: 1966–1980, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802627076
Facile Synthesis of Second-Generation Dendrons
with an Orthogonal Functional Group at the
Focal Point
Ravi Kumar Cheedarala, Vijaya Sunkara, and Joon Won Park
Center for Integrated Molecular Systems, Department of Chemistry,
Pohang University of Science and Technology (POSTECH),
Pohang, Korea
Abstract: Facile synthesis of second-generation dendrons with an aldehyde,
epoxy, or t-Boc group at the focal point and nine carboxylic acid groups at the
periphery is reported. The scheme includes a coupling of the first-generation
dendrons and a two-step, one-pot reaction that proceeds through a Boc
deprotection and in situ conjugation at the focal point.
Keywords: Dendrons, deprotection, divergent, epoxy
INTRODUCTION
Dendrimers are monodispersed, repeatedly branched, and highly sym-
metric macromolecules and are objects of increasing interest because
of their unique molecular architecture. They contain multiple functional
groups that can be efficiently tailored to control their chemical and
physical properties. Dendrons consist of a subgroup of dendrimers
and feature multiple functional groups at the periphery and a single
reactive group at the focal point. Their unique structures make it possi-
ble to graft dendrons to a surface, to another dendron,[1] or to another
macromolecule.[2] The first dendrimers were synthesized divergently by
Received September 18, 2008.
Address correspondence to Joon Won Park, Center for Integrated Molecular
Systems, Department of Chemistry, Pohang University of Science and Technology,
San 31, Hyoja-Dong, Pohang, Korea 790-784. E-mail: jwpark@postech.ac.kr
1966

Facile Synthesis of Second-Generation Dendrons
1967
Buhleier et al.,[3] and a convergent synthesis was introduced by Hawker
[4]
and Frechet, Divergent[5] and convergent[4,6] approaches have been
´
explored actively since then. Repetitive protection–deprotection and
purification processes are involved at each generation level.[7] Several
strategies have been devised to expedite the stepwise activation=coupling
sequences employed in both methods. Double-stage convergent,[8]
double exponential growth,[9] and use of ABn (usually n ꢀ 4) building
blocks termed ‘‘hypermonomers’’[10] have had success. Use of the
orthogonal approach reduces the number of synthetic steps and allows
high-generation dendrimers with a few transformations.[11]
During the past two decades, many types of dendrimers and dendrons
have been designed, synthesized, and utilized in various fields.[12] They
have been used for a wide range of biomedical and industrial applica-
tions, such as drug delivery,[13] gene therapy,[14] tissue repair,[15] and
solubility enhancers.[16] Recently, applications of dendrons as surface
functionalizing agents for silica,[17] glass,[18] gold,[19] poly(dimethylsilax-
oane),[20] and carbon nanotubes[21] have been investigated by our
group[17,21] and others.[22] In particular, (polyether) amide dendrimers[23]
have demonstrated great promise for a variety of biomedical applica-
tions. Interestingly, this class of dendrons shows a great number of
favorable properties.
Our goal is to establish a facile divergent route leading to molecular
building components that will provide a nanoscale-controlled structure
on a surface upon self-assembly. Previously, we utilized certain dendrons
for the surface modification and successfully applied the modified surface
for fabricating DNA microarrays.[24] With this background, the research
has been extended to prepare dendrons with a new class of focal point.
Herein, we describe a one-pot synthesis of a series of second-generation
dendrons with an aldehyde, epoxy, or t-Boc functional group at the focal
point and nine carboxylic acid groups at the periphery, which can be
assembled on the hydroxylated surface, while biomolecule attachment
at the focal point is amenable.[25]
RESULTS AND DISCUSSION
We started our work with monomer 3 [tris(2-methoxycarbonylethoxy-
methyl)aminomethane], widely known as Lin’s amine.[26] The trimethyl
ester 3 was prepared in two steps as shown in Scheme 1. The first step
was Michael addition of acrylonitrile to tris(hydroxymethyl)amino
methane (1), affording the desired trinitrile 2 as a pale yellow oil.
Trinitrile 2, when being refluxed in anhydrous methanol saturated with
dry HCl, gave the corresponding trimethyl ester as the hydrochloride salt,

1968
R. K. Cheedarala, V. Sunkara, and J. W. Park
Scheme 1. Reagents and conditions: (a) acrylonitrile, aq. KOH, 1,4-dioxane, rt,
24 h; and (b) dry HCl, methanol, 4 h, reflux.
which was neutralized by treatment with NaHCO3 to produce free amine
3 in 80% yield.
Monomer 3 is a useful building block for the growth of higher-
generation dendrons via an iterative repeat of EDC (or 1-[3-
(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride) and HOBt
(or 1-hydroxybenzotriazole) mediated coupling and subsequent saponi-
fication. The EDC=HOBt coupling reaction was employed because the
reaction condition did not adversely affect the functionalities present
within both the precursors and the product and the side reactions were
minimal.
On the other hand, another precursor, triacid 6, was prepared in
three steps (Scheme 2), starting from 4-aminobutyric acid (GABA).
Protection of the amine group of GABA with di-tert-butyl dicarbonate
[(Boc)2O] in the presence of NaOH in 1,4-dioxane produced Boc-GABA
4.[27] Coupling of the resulting acid 4 with trimethyl ester 3 in the presence
of EDC and HOBt in methylene chloride afforded t-Boc-GABA-
trimethylester 5 in 86% yield. Hydrolysis of the methyl ester in aq.
NaOH=acetone at room temperature afforded the t-Boc-blocked triacid
6 as a stable white solid in 94% yield.
Scheme 2. Reagents and conditions: (a) (Boc)2O, 1,4-dioxane, 1 N NaOH, 12 h;
(b) 3 (1.2 equiv.), EDC, HOBt, methylene chloride, rt, 24 h; and (c) aq. Acetone,
NaOH, rt, 12 h.

Facile Synthesis of Second-Generation Dendrons
1969
In the next step, trimethyl ester 3 was coupled with triacid 6 under
the same coupling condition to produce t-Boc protected methyl ester 7
as a colorless oil in 80% yield. Subsequently, hydrolysis of 7 in aq.
NaOH=acetone at room temperature and under controlled acidification
gave the t-Boc-blocked nona-acid 7a in 77% yield. It was observed that
t-Boc group was deprotected when conc. HCl was used for the acidifica-
tion. The unwanted side reaction can be avoided by adding chilled
aq. HCl (5% v=v) at a temperature lower than 5ꢁC. The progress of the
reaction was monitored with thin-layer chromatography (TLC), and
1
the structure of compound 7 was confirmed by H, 13C NMR, and
infrared (IR) spectroscopy as well as matrix-assisted laser desorption
ionization–mass spectrometry (MALDI-MS).
The second-generation dendrons with an epoxy functional group at the
focal point were prepared through the deprotection and the coupling reac-
tion with the relevant benzoic acids (Scheme 3). For this end, the Boc nona-
methyl ester 7 was deprotected by treatment with trifluoroacetic acid (TFA)
at 0ꢁC in methylene chloride, resulting in the reactive primary amine group.
Addition of 4-(oxiran-2-yl)benzoic acid (8a) and the coupling reagents (i.e.,
EDC and HOBt) dissolved in methylene chloride to the reaction solution
gave the second-generation dendron 9a[28] in 75% yield. The approach
proved to be efficient and applicable to other cases. Therefore, two ana-
logous benzoic acids [i.e., 4-(oxiran-2-ylmethoxy)benzoic acid (8b) and
4-formylbenzoic acid (8c)] were used to obtain the corresponding dendrons
9b and 9c in 80% and 78% yields, respectively. The employed benzoic acids
8a and 8b were prepared from their corresponding methyl esters.[28,29]
When the saponification of the ester groups of 9a and 9b was carried
out in 50% aq. acetone at room temperature for 12 h in the presence of 3
Scheme 3. Reagents and conditions: (a) EDC, HOBt, methylene chloride, rt, 24 h;
(b) i, TFA, methylene chloride, 0ꢁC, 1.5 h; ii, Et3N; (c) 8, EDC, HOBt, methylene
chloride, rt, 24 h, and (d), i, aq. acetone, NaOH, rt, 12 h; ii, degassed methanol,
NaOH, rt, 4 h.

1970
R. K. Cheedarala, V. Sunkara, and J. W. Park
equiv. of NaOH, 9a and 9b afforded the corresponding acids 10a and 10b
in 86% and 84% yields, respectively. Under these conditions, by-products
were not observable with TLC analysis. In the case of 9c, it was thought
that hydrolysis of methyl ester in the presence of the aldehyde could
result in the formation of Cannizaro products. As a control reaction,
hydrolysis of 4-formylmethylbenzoate was carried out under these reac-
tion conditions, and a mixture of 4-formylbenzoic acid (10%), terepthalic
acid (41%), and 4-hydroxymethyl benzoic acid (40%) was obtained. In
degassed methanol, the hydrolysis resulted in the same mixture, but the
formation of 4-formylbenzoic acid was dominant, and the isolation yield
was 80%. The last condition was employed for the hydrolysis of 9c with 3
equiv. of NaOH at room temperature for 4 h, and the aldehyde-9-acid 10c
was isolated in 65% yield.
The structure of the final dendrons was confirmed by IR, NMR, and
MALDI-MS spectroscopy. The IR spectra of 10 showed strong absorp-
tion peaks at 3300 cmꢂ1 for OH, 1728 cmꢂ1 for CO of the carboxylic acid,
and 1650–1630 cmꢂ1 for CO of the amide group.
1
In H NMR spectra, the assignment of the epoxy, aldehyde, amide,
and carboxylic acid functional group was straightforward. In all second
generation dendron (10) cases, the amide proton was observed between
5.0 ppm and 7.5 ppm as a broad resonance, and the amide proton
attached with the aryl ring appeared from 7.7 ppm to 8.5 ppm. For 10a,
the resonances as two doublets of the doublet (dd) at 2.9 ppm and
3.2 ppm for epoxy CH2 and as a quartet at 3.9 ppm for epoxy CH were
observed. In the case of 10b, signals were recorded as two dds at
3.93 ppm and 4.3 ppm for the exocyclic methylene group, and the signals
for the epoxy ring appeared as two dds at 2.6 ppm and 2.7 ppm for CH2
and a multiplet at 3.1 ppm for CH. O- and m-protons on the phenyl ring
were observed at 7.3–7.9 ppm as two doublets. In the case of 10c, a signal
at 9.96 ppm as a singlet for the aldehyde proton, a broad signal at 7.75–
7.72 ppm for the amide proton attached to the aryl ring, and the signals at
8.1–7.89 ppm as doublet for the aromatic ring protons were observed.
MALDI-TOF-MS was used to confirm the molecular weight of the
target dendrons. The spectra showed the expected molecular ion peaks
([M þ Na]þ) at 1548 for 10a, 1578 for 10b, and 1534 for 10c.
EXPERIMENTAL
General
1H NMR spectra were recorded on a 300-MHz Bruker NMR
spectrometer. Chemical shifts are reported in parts per million

Facile Synthesis of Second-Generation Dendrons
1971
(ppm) using tetramethylsilane (TMS) as the internal standard. 13C NMR
spectra were proton decoupled and recorded on a 300-MHz NMR
spectrometer using the carbon signal of the deuterated solvent as the
internal standard. IR spectra were recorded on a Perkin-Elmer Fourier
transform–infrared (FT-IR) system and MALDI MS spectra were
obtained from Applied Biosystems 4700 proteomics analyzer with
TOF=TOFTM optics. TLC was performed on silica plates with F-254
indicator, and the visualization was accomplished by ultraviolet (UV)
lamp or an iodine chamber. All reagents and chemicals were obtained
from commercial sources and used as received, unless otherwise
mentioned.
Synthesis of Boc-GABA-trimethylester (5)
A solution of tris[((methoxycarbonyl)ethoxy)methyl]-aminomethane (3,
2.2 g, 5.8 mmol) in MC (10 ml) was added to a solution of 4-(tert-butylox-
ycarbonylamino)-butyric acid (4, 1.0 g, 4.9 mM), EDC (1.4 g, 7.3 mmol),
and HOBt (0.67 g, 4.9 mmol) dissolved in methylene chloride (MC)
(20 mL), and stirred at rt for 24 h. After removal of the solvent at reduced
pressure, the residue was dissolved in ethyl acetate (50 mL), washed with
water (2 ꢃ 25 mL), chilled 5% HCl (2 ꢃ 25 mL), water (1 ꢃ 25 mL), sat.
NaHCO3 solution (3 ꢃ 25 mL), water (2 ꢃ 25 mL), and brine (25 mL).
The organic layer was then dried over MgSO4, and the solvent was eva-
porated to get 5 (2.4 g, 86%).
1H NMR (CDCl3, 300 MHz), 5: d 6.36 (s, CONHC, 1H), 4.98 (t,
Boc-NH, 1H), 3.71–3.69 (m, NHCCH2OCH2CH2CO, OCH3, 21H),
3.16–3.14 (q, NHCH2, 2H), 2.62–2.53 (t, OCH2CH2CO, 6H), 2.22–2.18
(t, CH2CH2CH2CONH, 2H), 1.83–1.78 (m, CH2CH2CH2, 2H), 1.43 (s,
Boc, 9H). 13C NMR (CDCl3, 300 MHz), 5: d 173 (CH2CONH), 172
(CH2COO), 156 (CONH), 79.0 [(CH3)3COCONH], 69.1 (NHCCH2O),
66.7 (NHCCH2OCH2), 51.7 (CH2COOCH3), 39.8 (CONHCH2), 34.8
(OCH2CH2COOMe), 34.2 (CH2CH2CONH), 28.4 [(CH3)3COCO], 25.9
(NHCH2CH2CH2CO).
Synthesis of Boc-GABA-triacid (6)
Boc-GABA-trimethyl ester 5 (1 g, 1.7 mmol) was dissolved in a mixture of
acetone (40 mL) and 0.4 N NaOH (40 mL) and stirred at rt for 12 h. After
completion of the reaction, the reaction mixture was concentrated, and
the aqueous solution was washed with ethyl acetate (2 ꢃ 50 mL) and
cooled in an ice bath. The pH was adjusted to 3.5, and the resultant

1972
R. K. Cheedarala, V. Sunkara, and J. W. Park
product was extracted with ethyl acetate (4 ꢃ 50 mL). The combined
organic layer was dried (anh. MgSO4) and evaporated. The crude
product thus obtained was recrystallized in an ethyl acetate–hexane
mixture to get pure 6 (0.87 g, 94%).
1H NMR (CDCl3, 300 MHz), 6: d 8.98 (br s, OH, 3H), 6.92
(s, CONHC, 1H), 6.54–6.46 (t, CONH, 1H), 3.77–3.66 (t,
NHCCH2OCH2CH2CO, 12H), 3.11 (q, NHCH2, 2H), 2.6–2.5 (t,
OCH2CH2CO, 6H), 2.24–2.20 (t, CH2CH2CH2CONH, 2H), 1.84–1.77
(m, CH2CH2CH2, 2H), 1.44 (s, Boc, 9H). 13C NMR (CDCl3,
300 MHz), 6: d 178.3 (CH2CH2COOH), 174.0 (CH2CH2CONH), 156.9
(OCONH), 79.9 [(CH3)3COCONH], 69.4 (NHCCH2O), 65.7
(NHCCH2OCH2), 40.2 (CONHCH2), 34.6 (OCH2CH2COOH), 31.2
(NHCH2CH2CH2CO), 28.3 [(CH3)3COCO], 26.3 (NHCH2CH2CH2CO).
Synthesis of Boc-GABA-nonamethylester (7)
A solution of 3 (1.1 g, 2.9 mmol) in MC (10 mL) was added to a solution
of triacid 6 (0.4 g, 0.76 mmol), EDC (0.57 g, 2.9 mmol), and HOBt (0.31 g,
2.3 mmol) dissolved in MC (10 mL) and stirred at rt for 24 h. The pre-
vious general procedure was followed for the isolation of compound 7
(0.98 g, 80%).
IR (neat, cmꢂ1), 7: 3300, 2877, 1739, 1673, 1526, 1199, 1113. 1H NMR
(CDCl3, 300 MHz), 7: d 6.54 (s, CONHC, 1H), 6.1 (br s, OCH2CH2CONH,
3H), 5.2 (t, Boc-NH,1H), 3.69–3.66 (m, CH2OCH2CH2CONH
CCH2OCH2CH2COOCH3, 75H), 3.15 (q, Boc-NHCH2CH2CH2, 2H),
2.56–2.52 (t, OCH2CH2COOCH3, 18H), 2.42–2.40 (t, CONHCCH2-
OCH2CH2, 6H), 2.25 (t, NHCH2CH2CH2CO, 2H), 1.78 (m,
NHCH2CH2CH2CO, 2H), 1.42 (s, Boc, 9H). 13C NMR (CDCl3,
300 MHz), 7: d 172 (CH2CONH), 171 (CH2COO), 170.4 (CH2CONHC-
CH2O), 156 (CONH), 69.1 (NHCCH2O), 67.5 (NHCCH2OCH2), 66.7
(CONHCCH2OCH2), 59.7 (CONHCCH2OCH2), 51.6 (CH2COOCH3),
37.2 (CONHCH2), 34.7 (OCH2CH2COOMe), 28.4 [(CH3)3COCO].
MALDI-TOF-MS, 7: [Mþ Na]þ ¼ 1628.
Synthesis of Boc-GABA-nona-acid (7a)
Boc-GABA-nonamethylester 7 (0.7 g, 0.43 mmol) was dissolved in a
mixture of acetone (15 mL) and 0.4 N NaOH (15 mL) and stirred at rt
for 12 h. After completion of the reaction, the reaction mixture was
concentrated. The aqueous solution was washed with ethyl acetate
(2 ꢃ 20 mL) and cooled in an ice bath, and pH was adjusted to 3.5.

Facile Synthesis of Second-Generation Dendrons
1973
The resulting product was extracted with ethyl acetate (2 ꢃ 50 mL). The
combined organic layer was dried (anh. MgSO4) and evaporated to
dryness. The crude product thus obtained was recrystallized in an ethyl
acetate–hexane mixture to get pure 7a (0.5 g, 77%).
IR (neat, cmꢂ1), 7a: 3300, 2925, 1716, 1670, 1408, 1179, 1117.
1H NMR (CD3OD, 300 MHz), 7a: d 7.74–7.71 (t, CONHCH2CH2,
CONHC, 1H), 7.49–7.46 (br s, CH2CONH, 1H), 7.33–7.31 (br s, OCH2
CH2CONH, 3H), 3.77–3.67 (m, CH2OCH2CH2CONHCCH2OCH2CH2-
COOH, 48H), 3.05 (q, Boc-NHCH2CH2CH2, 2H), 2.78–2.74 (t, CON-
HCCH2OCH2CH2, 6H), 2.54–2.51 (t, OCH2CH2COOCH3, 18H), 2.25
(t, NHCH2CH2CH2CO, 2H), 1.74 (m, NHCH2CH2CH2CO, 2H), 1.43
(s, Boc, 9H). MALDI-TOF-MS, 7a: [M þ Na]þ ¼ 1502.
Synthesis of 8
Methyl 4-(oxiran-2-yl) benzoate[28] or methyl 4-(oxiran-2-ylmethoxy)-
benzoate[29] (1.0 mmol) were dissolved in mixture of acetone (8 mL) and
0.4 N NaOH (8 mL) and stirred at rt for 2 h. After completion of the reac-
tion, the mixture was concentrated. The aqueous solution was washed
with ethyl acetate (2 ꢃ 10 mL) and cooled in an ice bath, and pH was
adjusted to 4. The resultant product was extracted with ethyl acetate
(3 ꢃ 25 mL). The combined organic layer was dried (anh. MgSO4) and
evaporated to dryness. The crude product thus obtained was recrystal-
lized in an ethyl acetate–hexane mixture to get pure 8 (8a, 0.138 g,
85%, and 8b, 0.135 g, 70%).
1H NMR (CDCl3, 300 MHz), 8a: d 10.2 (br s, COOH, 1H), 8.1 (d,
C6H4, 2H), 7.4 (d, C6H4, 2H), 3.94–3.92 (q, CHCH2O, 1H, epoxy),
3.22–3.19 (dd, CHCH2O, 1H, epoxy), 2.81–2.78 (dd, CHCH2O, 1H,
epoxy).
1H NMR (CDCl3, 300 MHz), 8b: d 12.1 (br s, COOH, 1H), 8.0 (d,
C6H4, 2H), 6.9 (d, C6H4, 2H), 4.3 (dd, C6H4OCH2CH, 1H), 4.0 (dd,
C6H4OCH2CH, 1H), 3.4 (m, CHCH2O, 1H, epoxy), 2.9 (dd, CHCH2O,
1H, epoxy), 2.7 (dd, CHCH2O, 1H, epoxy).
Preparation of 9
Compound 7 (0.24 g, 0.15 mmol) was stirred in a solution of trifluoroace-
tic acid (1.5 mL) in dry CH2Cl2 (1.5 mL) for 1.5 h at 0ꢁC. The reaction
mixture was basified with excess Et3N (pH 12) to generate free amine
ester. In another flask, HOBt (20 mg, 0.15 mmol) and EDC (31 mg,
0.16 mmol) were added to the corresponding benzoic acid (8, 0.13 mmol)

1974
R. K. Cheedarala, V. Sunkara, and J. W. Park
in 5 mL of dry methylene chloride at room temperature. After activation
of carboxylic acid in 20 min, the reagent mixture was added to the reac-
tion flask containing free amine, and the reaction mixture was stirred
under N2 for 24 h. After removal of the solvent at reduced pressure,
the residue was dissolved in ethyl acetate (50 mL) washed with water
(2 ꢃ 25 mL), chilled 5% HCl (2 ꢃ 25 mL), water (1 ꢃ 25 mL), sat.
NaHCO3 solution (3 ꢃ 25 mL), water (2 ꢃ 25 mL), and brine (25 mL).
The organic layer was then dried over MgSO4, and the solvent was
evaporated to get compound 9. Yields: 9a, 0.16 g (75%); 9b, 0.18 g
(80%); 9c, 0.17 g (78%).
1H NMR (CDCl3, 300 MHz), 9a: d 7.87–7.84 (d, C6H4, 2H), 7.79–7.73
(t, CONHCH2CH2CH2, 1H), 7.33–7.31 (d, C6H4, 2H), 6.76 (br s, CON-
HCCH2O, 1H), 6.1 (br s, OCH2CH2CONH, 3H), 3.90 (q, CHCH2O, 1H,
epoxy), 3.69–3.67 (m, CH2OCH2CH2CONHCCH2OCH2CH2CO-OCH3,
75H), 3.5–3.48 (t, CONHCH2CH2CH2, 2H), 3.18 (dd, CHCH2O, 1H,
epoxy), 2.8 (dd, CHCH2O, 1H, epoxy), 2.56–2.52 (t, OCH2CH2CO,
18H), 2.42–2.40 (t, CONHCCH2OCH2CH2, 6H), 2.05–2.04 (t,
NHCH2CH2CH2CO, 2H), 1.97–1.95 (m, CONH CH2CH2CH2, 2H).
13C NMR (CDCl3, 300 MHz), 9a: d 173.5 (CH2CONH), 172.1
(CH2COOCH3), 170.4 (CH2CONHCCH2O), 166.9 (CONHCH2CH2),
140.8 (C6H4), 134.5 (C6H4), 127.1 (C6H4), 125.5 (C6H4), 69 (NHCCH2O),
67.4 (CONHCCH2OCH2), 66.7 (CONHCCH2OCH2), 59.9 (CON-
HCCH2OCH2), 51.9 (CHCH2O, epoxy), 51.6 (CH2OCH2CH2COOCH3),
51.2 (CHCH2O, epoxy), 39.8 (CONHCH2CH2CH2), 37.1 (CON-
HCH2CH2CH2CONH), 34.8 (CH2OCH2CH2COOCH3), 24.7 (CON-
HCH2CH2CH2CONH).
1H NMR (CDCl3, 300 MHz), 9b: d 7.84–7.82 (d, C6H4, 2H), 7.54 (t,
CONHCH2CH2CH2, 1H), 6.94–6.92 (d, C6H4, 2H), 6.74 (br s, CON-
HCCH2O, 1H), 6.15 (br s, OCH2CH2CONH, 3H), 4.30–4.27 (dd,
C6H4OCH2CH, 1H), 3.98–3.95 (dd, C6H4OCH2CH, 1H), 3.76–3.67 (m,
CH2OCH2CH2CONHCCH2OCH2CH2COOCH3, 75H), 3.5–3.48 (q,
CONHCH2CH2CH2, 2H), 3.2–3.1 (m, CHCH2O, 1H, epoxy), 2.92–
2.91 (dd, CHCH2O, 1H, epoxy), 2.77 (dd, CHCH2O, 1H, epoxy), 2.54
(t, OCH2CH2CO, 18H), 2.40 (t, CONHCCH2OCH2CH2, 6H), 1.95 (t,
NHCH2CH2CH2CO, 2H), 1.73 (m, CONH CH2CH2CH2, 2H). 13C
NMR (CDCl3, 300 MHz), 9b: d 173.5 (CH2CONH), 172 (CH2COOCH3),
171 (CH2CONHCCH2O), 166.8 (CONHCH2CH2), 160 (C6H4, C1), 129
(C6H4, C3, C5), 127 (C6H4, C4), 114 (C6H4, C2, C6), 69 (NHCCH2O),
68.8 (C6H4OCH2CH), 67 (CONHCCH2OCH2), 66.7 (CONHCCH2OCH2),
59.9 (CONHCCH2OCH2), 51.6 (CH2OCH2CH2COOCH3), 49.9 (CHCH2O,
epoxy), 44.6 (CHCH2O, epoxy), 39.7 (CONHCH2CH2CH2), 37.1 (CON-
HCH2CH2CH2CONH), 34.6 (CH2OCH2CH2COOCH3), 24.8 (CON-
HCH2CH2CH2CONH).

Facile Synthesis of Second-Generation Dendrons
1975
1H NMR (CDCl3, 300 MHz), 9c: d 10.1 (s, C6H4CHO, 1H), 8.13 (br s,
CONHCH2CH2CH2, 1H), 8.07–8.05 (d, C6H4, 2H), 7.95–7.92 (d, C6H4,
2H), 6.8 (br s, CONHCCH2O, 1H), 6.1 (br s, OCH2CH2CONH,
3H), 3.68–3.65 (m, CH2OCH2CH2CONHCCH2OCH2CH2COOCH3,
75H), 3.55–3.51 (q, CONHCH2CH2CH2, 2H), 2.56–2.52 (t, OCH2-
CH2CO, 18H), 2.42–2.32 (t, CONHCCH2OCH2CH2, 6H), 2.0–1.88
(t, CONHCH2CH2CH2, 2H), 1.68 (m, CONH CH2CH2CH2, 2H). 13C
NMR (CDCl3, 300 MHz), 9c: d 191 (C6H4CHO), 173.7 (CH2CONH),
172 (CH2COOCH3), 171.1 (CH2CONHCCH2O), 166.2 (CON-
HCH2CH2), 139.9 (C6H4), 137.9 (C6H4), 129.5 (C6H4), 127.9 (C6H4),
69.09 (NHCCH2O), 67.5 (CONHCCH2OCH2), 66.7 (CONHCC-
H2OCH2), 59.9 (CONHCCH2OCH2), 51.6 (CH2OCH2CH2COOCH3),
40.0 (CONHCH2CH2CH2), 37.0 (CONHCH2CH2CH2CONH), 34.6
(CH2OCH2CH2COOCH3), 24.5 (CONHCH2CH2CH2CONH).
Preparation of 10
The nona-ester 9 (0.09 mmol) was stirred in 20 mL of acetone and
20 mL 0.4 N NaOH mixture (1:1, 3 equiv.) for 12 h at room tempera-
ture. After removal of acetone at reduced pressure, the aqueous layer
was acidified with 1 N HCl until pH reached 3.5. The resultant nona-
acid 10 was extracted with ethyl acetate. The organic layer was dried
over MgSO4, and the solvent was removed by evaporation. The crude
product was purified by recrystallization in methanol and ethyl acetate
to obtain a pale yellow solid, 10. Yields: 10a, 0.12 g (86%); 10b, 0.12 g
(84%).
FT-IR (neat, cmꢂ1), 10a: 2944, 2910, 1727, 1654, 1634, 1631, 1544,
1
1192, 1111. H NMR (DMSO-d6, 300 MHz): d 12.2 (br s, COOH, 9H),
8.45–8.42 (t, CONHCH2CH2CH2, 1H), 7.87–7.84 (d, C6H4, 2H), 7.37–
7.34 (d, C6H4, 2H), 7.1 (br s, CONHCCH2OCH2CH2CONH, 4H), 3.98
(q, CHCH2O, 1H, epoxy), 3.57–3.54 (m, CH2OCH2CH2CONHC-
CH2OCH2-CH2, 48H), 3.48 (q, CONHCH2CH2CH2, 2H), 3.14 (dd,
CHCH2O, 1H, epoxy), 2.8 (dd, CHCH2O, 1H, epoxy), 2.43–2.39 (t,
CONHCCH2OCH2CH2COOH, 18H), 2.43–2.39 (t, CONHCCH2O-
CH2CH2, 6H), 2.19–2.17 (t, CONH CH2CH2CH2, 2H), 2.1 (m, CONH
CH2CH2CH2, 2H). 13C NMR (CD3OD, 300 MHz): d 175.8 (CH2COOH),
174 (CONH), 158.6 (CONHCH2CH2), 132 (C6H4), 130 (C6H4), 129
(C6H4), 70, 68.8 (NHCCH2O), 68.3, 68 (CONHCCH2OCH2), 64
(CHCH2O, epoxy), 61.5 (CONHCCH2CH2O), 46.8 (CHCH2O, epoxy),
41.3 (CONHCH2CH2CH2), 38 (OCH2CH2CONH), 36 (CH2OCH2CH2-
COOH), 35 (CONHCH2CH2CH2CO), 27 (CONHCH2CH2CH2CO).
MALDI-TOF-MS: [Mþ Na]þ ¼ 1548.

1976
R. K. Cheedarala, V. Sunkara, and J. W. Park
IR (neat, cmꢂ1), 10b: 2944, 2912, 1728, 1654, 1632, 1631, 1544, 1185.
1H NMR (DMSO-d6, 300 MHz): d 12.1 (br s, COOH, 9H), 8.32 (t,
CONHCH2CH2CH2, 1H), 7.81–7.78 (d, C6H4, 2H), 7.54 (t, CON-
HCH2CH2CH2, 1H), 6.99–6.96 (d, C6H4, 2H), 5.1 (br s, OCH2CH2-
CONH, 3H), 4.3 (dd, C6H4OCH2CH, 1H), 3.93 (dd, C6H4OCH2CH,
1H), 3.54–3.50 (m, CH2OCH2CH2CONHCCH2OCH2CH2COOH,
48H), 3.2 (q, CONHCH2CH2CH2, 2H), 3.1 (m, CHCH2O, 1H, epoxy),
2.7 (dd, CHCH2O, 1H, epoxy), 2.6 (dd, CHCH2O, 1H, epoxy), 2.50 (t,
OCH2CH2CO, 18H), 2.40 (t, CONHCCH2OCH2CH2, 6H), 2.26 (t,
NHCH2CH2CH2CO, 2H), 1.97 (m, CONH CH2CH2CH2, 2H).
MALDI-TOF-MS: [M þ Na]þ ¼ 1578.
Preparation of 10c
NaOH (0.067 g, 1.67 mmol, 3.1 equiv.) was added to a solution of 9c
(0.1 g, 0.06 mmol) in 4 mL of degassed methanol and stirred for 4 h at
room temperature. After removal of methanol at reduced pressure, water
was added to the residue, and the aqueous layer was acidified. The pro-
duct 10c was extracted with ethyl acetate. The organic layer was dried
over MgSO4 and then evaporated to get the crude product. The crude
product was isolated as a white solid after recrystallization in methanol
and ethyl acetate. Yield: 0.059 g (65%).
FT-IR (neat, cmꢂ1), 10c: 2925, 2912, 1728, 1654, 1632, 1631, 1545,
1
1185, 1112. H NMR (CD3OD, 300 MHz): d 9.96 (s, C6H4CHO, 1H),
8.1–7.89 (d, C6H4, 4H), 7.75–7.71 (t, NH, 1H), 7.48–7.44 (s, NH, 3H),
5.4 (s, NH, 1H), 3.6–3.5 (m, CH2OCH2CH2CONHCCH2OCH2-
CH2COOH, 48H), 3.4 (q, CONHCH2CH2CH2, 2H), 2.45–2.40 (t,
OCH2CH2CO, 18H), 2.32 (t, CONHCCH2OCH2CH2, 6H), 1.8 (t,
CONHCH2CH2CH2, 2H), 1.7 (m, CONHCH2CH2CH2, 2H). 13C
NMR (CD3OD, 300 MHz): d 193 (C6H4CHO), 175 (CH2COOH), 174
(CONH), 139 (C6H4), 130.7 (C6H4), 129.6 (C6H4), 128 (C6H4), 127
(C6H4), 70.09 (NHCCH2O), 69.7 (CONHCCH2OCH2), 68.8 (CON-
HCCH2OCH2), 58.8 (CONHCCH2OCH2), 40.5 (CONHCH2CH2CH2),
38.09 (OCH2CH2CONH), 35.9 (CH2OCH2CH2COOH), 30.7 (CON-
HCH2CH2CH2CO), 25.8 (CONHCH2CH2CH2CO). MALDI-TOF-MS:
[M þ Na]þ ¼ 1534.
CONCLUSION
In conclusion, we report a facile method for the preparation of (poly-
ether) amide dendrons by the divergent method. A relatively pure form

Facile Synthesis of Second-Generation Dendrons
1977
of the dendrons was prepared without going through the column chroma-
tography step. The approach is synthetically convenient and saves time.
Also, the scheme can be applied for the preparation of dendrons with
various functional groups at the focal point in a one-pot, two-step
reaction. Exploration of this approach can be utilized to allow facile
synthesis of various higher-generation dendrons substituted with various
functional groups at the focal point. All the new dendrons will be of
potential use in surface modifications and drug delivery.
ACKNOWLEDGMENTS
We are grateful for financial support from the Korea Research Founda-
tion (KRF-2004-005-C00007). We thank Prof. Young Ho Rhee for his
valuable suggestions.
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