Synthesis of water-soluble, multiple functionalizable dendrons for the conversion of large dendrimers or other molecular objects into potential drug carriers

FULL PAPER

DOI: 10.1002/chem.200401078

Synthesis of Water-Soluble,Multiple Functionalizable Dendrons for the

Conversion of Large Dendrimers or Other Molecular Objects into Potential

Drug Carriers

Stephan Müller and A. Dieter Schlüter*^[a]

Abstract: The synthesis of dendrons

and dendrimers which carry OEG

chains and bidentate ligands and/or

fluorescence tags is described. The or-

thogonally protected functional groups

of the dendrons allow modification of

the substitution pattern and attachment

to larger entities. Both dendrons and

dendrimers are highly water-soluble.

The dendrons should have considerable

potential to convert, for example, com-

mercially available, high-generation

dendrimers into water-soluble, versatile

support materials for antitumor thera-

py.

Keywords:

dendrimers

·

drug

delivery · macromolecules

Introduction

tate N- and O-based ligands for platinum complexation, and

fluorescence tags to study cellular uptake and intracellular

distribution. A selectively addressable functional group at

the focal point allows the dendronꢀs attachment to larger en-

tities. The goal is to provide building blocks for the conver-

sion of any larger and structurally defined molecular object,

like a commercial high-generation dendrimer, into a water-

soluble and versatile support material for antitumor therapy.

Scheme 1 shows the simplest possible application of this

concept in which three dendrons with branched OEG solu-

bilizers and complexation sites X are attached to a 1,3,5-

tris(aminopropyl)benzene through an hydrolytically stable

amide bond to furnish a small dendrimer. A structure like

this serves as proof of principle only and will not gain, of

course, importance as a drug carrier itself.

Dendrimers are versatile, monodisperse synthetic macromo-

lecules with a near-perfect nanoscale structure and are syn-

thesized step-by-step.^[1]They can be selectively decorated

with different motifs and the maximum (covalent)loading

capacity towards a particular function is defined by the

number of surface groups.^[2]Dendrimers are a perfect exam-

ple for macromolecular engineering.^[3]Their solubilities and

glass transition temperatures, for example, can be tailored in

a wide range by simple chemical “surface” modification. It

is thus to be expected that the coverage of a high-generation

dendrimer with water-soluble small dendrons will render the

whole dendrimer water-soluble. Water solubility is an impor-

tant issue whenever biomedical applications like in drug de-

livery come into play.^[4]It is normally achieved by the intro-

duction of either charged groups^[5]or decoration with polar

oligomers of the ethylene glycol family (OEG).^[6]Whereas

especially positively charged compounds tend to be cytotox-

ic,^[7,8]OEGs revealed low toxicity in in vivo applications.^[9]

Herein, we report on the synthesis of differently sized den-

drons which carry OEG chains for water solubility, biden-

Results and Discussion

The synthetic procedures are compiled in Schemes 2–13.

The main reactions used are etherifications, amido-coupling

reactions (e.g., dendrimer assembly), and standard protec-

tion–deprotection protocols. The OEG chains used in this

project are branched and were synthesized according to a

known procedure. Commercially available OEGs were con-

sidered either too short to mediate sufficient water solublity

or are polydisperse which was believed to be disadvanta-

geous in regard to approval matters. The purification of all

compounds was done by standard column chromatography.

First the schemes will be described in general terms and

then addressed in somewhat more detail. Scheme 2 and

[a] S. Müller, Prof. A. D. Schlüter⁺

Institut für Chemie/Organische Chemie

Freie Universität Berlin

Takustrasse 3, 14195 Berlin (Germany)

E-mail: dieter.schluter@mat.ethz.ch

[⁺] New address: Department of Materials

Institute of Polymers, Swiss Federal Institute of Technology

ETH Hçnggerberg, HCI J 541, 8093 Zürich (Switzerland)

Fax : (+41)44-633-1395

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5589

and dendrimers 55–58 whose syntheses are shown in

Scheme 11 and Scheme 12, respectively. Finally, Scheme 13

depicts the divergent synthesis of dendrimer 63 and its de-

protection to 65.

The synthesis of an OEGylated building block with two

orthogonally protected anchor sites, an amine and a carbox-

ylic acid, makes use of a selective alkylation of 1 in its 4-hy-

droxy position.^[10]For this alkylation tert-butoxycarbonyl(N-

Boc)- and benzyloxycarbonyl(N-Cbz)-protected 3-bromo-

propylamines 2a and 2b, respectively, and O-benzyl(O-Bn)-

protected 3-bromopropanol 2c were used in equimolar

amounts to 1 (Scheme 2). The best results were obtained in

Scheme 2. Synthesis of mono-alkylated gallates. Reagents and conditions:

Scheme 1. Structure of a small target dendrimer with branched oligoethy-

leneglycol (OEG)chains for water solubility and functional groups X

which serve as ligands for metal complexation and as anchor sites for at-

taching fluorescence tags.

a)DMF, NaHCO , KI, four days, room temperature.

3

DMF with excess of NaHCO₃at room temperature, which

gave the mono-alkylated products 3a–c. Occasionally, some

bis-alkylated by-product, for example, 4a, was formed. In

such a case, the resulting mixture could not easily be sepa-

rated and was therefore exhaustively benzylated and then

separated by standard column chromatography (Scheme 3).

Catalytic hydrogenation of 5a yielded pure 3a.^[11]

Scheme 3 show the monoalkylation of gallic acid (1)and the

removal of undesired bis-alkylated by-product 4. Scheme 4

and Scheme 5 depict the synthesis of the OEGylated den-

dron 8a and its selective deprotection at both the amine (to

9)and the focal point acid (to 10a)for subsequent function-

alization of 9 with an ethylene–diamine moiety to 12 and

the fluorescence tag 14 to 15,

respectively. Scheme 6 illus-

trates the synthesis of the pro-

tected malonic acid derivatives

20a and 20b which have an ad-

ditional carboxylic acid for at-

tachment to the dendron.

Scheme 7 and Scheme 8 contain

the synthesis of the allyl-pro-

tected dendrons 21, its modifi-

cation to 22, and the decoration

of the latter with the malonic

Scheme 3. Removal of bis-alkylated by-products. Reagents and conditions: a)K ₂CO₃, DMF, benzyl bromide,

acid derivatives 20a and 20b to

give dendrons 23 and 25, re-

spectively, as well as their de-

protected counterparts 24 and

808C, one day; b)Pd/C, methanol, H .

2

26. Scheme 9 summarizes all the reactions leading to den-

drimers 28, 30, 32, 33, and 35 as well as the subsequent de-

protections, all of which serve as model reactions for the an-

ticipated application to larger objects in the future.

Scheme 10 shows the synthesis and selective deprotection of

the orthogonally protected branching unit 40, which is a key

compound for the synthesis of the larger dendrons 49–54

In the next step 3a was decorated with the symmetric

OEGylated glycerol derivative 7^[12]to afford dendron 8a,

which has already

a number of important features

(Scheme 4). Not only is it fully water-soluble but also it car-

ries a selectively addressable amine group and a focal point

ester function for attachment to a larger object. The reac-

tions of 7 with 3a–c were carried out in DMF at 808C using

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Water-Soluble, Multiple Functionalizable Dendrons

FULL PAPER

Scheme 4. Synthesis of water-soluble dendrons. Reagents and conditions: a)K CO₃, DMF, 808C, three days; b)TFA, CH Cl₂, room temperature, 12 h; c)

2

1m KOH, ethanol, room temperature, 12 h.

K₂CO₃as a base. The conversions of 3b and 3c are not

shown in Scheme 4. Compounds 8a–c were obtained in

yields of 58–65% indicating coupling efficiencies of 75–80%

per step. By keeping the temperature at 808C, base-induced

elimination of toluenesulfonic acid from 7 (not shown)was

kept at a minimum. Mitsunobu protocols were also applied,

but did not prove superior due to tedious removal of triphe-

nylphosphine oxide impurities.

further steps. The racemic N-Boc-protected diaminopropio-

noic acid 11 with its ethylenediamine moiety for Pt²⁺com-

plexation was then connected to 9 under standard amide

coupling conditions to give 12 (Scheme 5). Interestingly, this

coupling

succeeded

with

O-(1H-benzotriazole-1-yl)-

N,N,N’,N’-tetramethyluronium tetrafluoroborate (TBTU)as

active ester reagent, but failed with hydroxybenzotriazole

hydrate/N-(3-dimethylaminopropyl)-N’-ethyl-carbodiimide

hydrochloride (HOBt/EDC)and Hydroxysuccinimidyl/dicy-

clohexylcarbodiimide (HSu/DCC). The free benzoic acid 13

was generated from 12 in excellent yields with KOH at

room temperature in ethanol.

Starting from 8a a variety of valuable compounds were

synthesized. This first required selective deprotections at

both the urea and the ester which was achieved by treating

8a with trifluoroacetic acid (TFA)in CH Cl₂(to give 9)and

2

KOH in ethanol (to give 10a), respectively (Scheme 4).

Both deprotections went smoothly and virtually quantita-

tively so that the products could be used as obtained in the

Because of its high fluorescence intensity the 5-dimethyl-

aminonaphthalene-1-sulfonyl (dansyl)group has often been

used for biochemical applications and was also selected here

Scheme 5. Synthesis of dendrons with an ethylenediamine moiety or a fluorescent label. Reagents and conditions: a)DMF, TEA, TBTU; b)TEA,

CH₂Cl₂, room temperature, 2 h; c)1 m KOH, ethanol, room temperature, 12 h.

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5591

A. D. Schlüter and S. Müller

as fluorescence tag. The attach-

ment went through its chloride

(14)and gave 15 in virtually

quantitative yields (Scheme 5).

Saponification of the focal

point ester of 15 cleanly fur-

nished 16.

Malonic acids are suitable bi-

dentate ligands for structurally

defined platinum(ii) complexes.

They may be superior to ethyle-

nediamines in drug-carrier ap-

Scheme 6. Synthesis of protected malonic acid moieties. Reagents and conditions: a)K ₂CO₃, Bu₄NI, benzene,

reflux, one day; b)for 19a: Pd/C, methanol, H₂; c)for 19b: TFA, CH₂Cl₂, room temperature, 1–12 h.

plications because platinated malonates can release Pt²⁺at

pH 4–5 by hydrolysis. To introduce malonic units to den-

drons of the here described type a few aspects needed to be

considered. First, the two carboxylic acid groups of the ma-

lonic part needed to be protected so that a linker for the at-

tachment to the dendron could be introduced at the acidic

carbon. The protecting groups chosen for this purpose

needed to be removable at a later stage in the sequence

without concomitant decarboxylation, which 1,3-dicarboxylic

acids are prone to undergo. Second, the protecting groups

had to be both stable under coupling conditions and orthog-

onal to the deprotection protocol at the focal point ester.

The best candidates were tert-butyl and benzyl esters and,

thus, compounds 20a and 20b were prepared through the

tris-acid esters 19a and 19b (Scheme 6). They, in turn, were

generated from the protected malonic acids 17a and 17b

and orthogonally protected

Scheme 7. Synthesis of a dendron for assembly with protected malonates.

Reagents and conditions: a)allyl bromide, DMF, K CO₃, Bu₄NI, 12 days,

2

room temperature; b)TFA, CH Cl₂, room temperature, 12 h.

2

acrylic acids 18a and 18b under

Michael conditions. This latter

reaction always yielded a mix-

ture of mono- and bis-alkylated

malonates whose separation

was tedious. Therefore, the raw

mixture was directly deprotect-

ed at the linkerꢀs carboxylic

acid and then separated by

column chromatography. In this

way pure 20a and 20b could be

obtained in acceptable yields of

65% (for 20a)and 67% (for

20b), respectively. For the pro-

tection of the benzoic acid at

the focal point, the propenyl

Scheme 8. Synthesis of dendrons with a malonic acid moiety. Reagents and conditions: a)DMF, TEA, TBTU;

ester with its known Pd-mediat-

ed deprotection protocol was

chosen to avoid a detrimental

b)[Pd(PPh )₄], p-toluenesulfonic acid, CH₂Cl₂, methanol, room temperature, 0.5 h.

3

interference with the other esters. Alkylation of the potassi-

um salt of 10a with an excess of allyl bromide gave the de-

sired propenyl benzoate 21 (Scheme 7).

Removal of the N-Boc group in 21 was performed with

TFA in CH₂Cl₂and gave 22. The protected malonic acid

moieties 20a and 20b were then hooked on to 22 under

standard amide coupling conditions to give 23 and 25, re-

spectively (Scheme 8). The best results for these couplings

were obtained with TBTU, though the same products were

also accessible with other active ester reagents (HOBt/EDC,

HSu/DCC). The best results for the deprotection of the allyl

benzoates were achieved by adding a solution of p-toluene-

sulfonic acid in methanol to a solution of the ester and [Pd-

(PPh₃)₄] in CH₂Cl₂. Column chromatography gave the free

benzoic acids 24 and 26 in high yields (24: 94%; 26: 96%) .

In the next step, the various small dendrons were attached

to the trifunctional core molecule 27 (Scheme 9). This as-

sembly served as a test for the conceptꢀs feasibility which is

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Water-Soluble, Multiple Functionalizable Dendrons

FULL PAPER

Scheme 9. Synthesis of OEGylated dendrimers with different surface motifs. Reagents and conditions: a)HOBt, EDC, TEA, CH ₂Cl₂, methanol; b)

HOBt, EDC, TEA, CH₂Cl₂; c)TFA, CH Cl₂, room temperature, 1–12 h; d)Pd/C, methanol, H .

2

to convert large molecular objects into water-soluble drug-

delivery systems by the attachment of appropriately de-

signed dendrons. In all reactions the dendrons were used in

3.5 molar equivalents to 27. In contrast to the amido-cou-

pling protocol for alkanoic acids with primary alkylamines,

the benzoic acids of 10a, 10c, 13, 16, 24, and 26 reacted

much more efficiently with HOBt/EDC than with TBTU.

The active esters of 10a, 10c, 13, and 16 were generated in

situ by employing HOBt/EDC in CH₂Cl₂at room tempera-

ture and adding them at À408C to solutions of 27 and tri-

ethylamine (TEA)in CH ₂Cl₂and methanol. The reaction

temperature was slowly raised to room temperature and the

mixtures stirred until conversions were complete (TLC).

Column chromatography gave the dendrimers 28, 29, 32,

and 33 in high yields (28: 95%; 29: 98%; 32: 92%; 33:

80%). The excess of respective active esters was recovered

as the corresponding methyl esters which in turn were quan-

titatively reconverted into the free benzoic acids by saponifi-

cation with aqueous 1m KOH (not shown). The analogous

recycling of the active esters of 24 and 26 did not give satis-

factory results because the corresponding two ester func-

tions (methyl benzoate and malonate)of the products could

not be saponified with sufficiently high selectivity. There-

fore, the amido-coupling procedure was carried out without

methanol as solvent and quenched with aqueous 1m

NaHCO₃which converted the excess active esters directly

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5593

A. D. Schlüter and S. Müller

into the starting benzoic acids (24 and 26). All dendrimers

tion of the ester functionality of 40 was achieved cleanly

with aqueous 1m KOH in ethanol at room temperature and

gave 43 virtually quantitatively.

1

were characterized by their fully assigned H and ¹³C NMR

and MALDI-TOF mass spectra. The NMR assignment was

achieved with the help of 2D homo- and heteronuclear cor-

related pulse sequences (COSY, HMBC, and HMQC). In

the reflective mode, the MALDI-TOF mass spectra typically

showed the monoisotopic peak of the potassium and sodium

salts and the expected isotopic pattern.

Compound 41 was used to prepare the more complex

dendrons 49, 51, and 53 as well as their deprotected counter-

parts 50, 52, and 54 which all carry different sets of anchor

groups/fluorescence tags (Scheme 11). The reaction condi-

tions resembled those already described and are therefore

not given here. Yields were good to very high throughout

and all compounds were fully characterized (except, of

course, combustion analysis). The attachment of these den-

drons to the trifunctional core molecule 27 (Scheme 12)

gave dendrimers 55, 57, and 58, respectively. The amide-cou-

pling protocols applied were the same as the ones for the

dendrimers assembled in Scheme 9 except that the reactions

required longer times to reach completion. The resulting

dendrimers were purified by column chromatography with

very polar solvent mixtures and the excess of the dendrons

were recycled after saponification. MALDI-TOF mass spec-

trometric measurements in the linear mode afforded the

molecular peaks of the sodium salt. The NMR spectra

showed broad lines and reliable assignments could therefore

not be done. Additionally, the number of signals in the

¹³C NMR spectra was usually too low because of superimpo-

sitions. Peripheral deprotections were so far only tried for

dendrimer 55. Extended exposure to TFA in CH₂Cl₂led to

a complete disappearence of the signal for the tert-butyl

The acid-labile N-Boc-carbamates of 28 and 33 were

quantitatively deprotected with a large excess of TFA in

CH₂Cl₂at room temperature and gave the corresponding

free amines 29 and 34 (as hydrotrifluoroacetates), respec-

tively. Completion of the procedure was monitored by

¹H NMR spectroscopy. The benzyl ether in 30 was cleaved

by catalytic hydrogenation with Pd/C and gave the free alco-

hol 31. The deprotected dendrimers were again character-

1

ized by their H and ¹³C NMR (assignment by 2D-correlated

spectroscopy)and MALDI-TOF mass spectra. The tert-

butyl malonates of 35 could be deprotected to 37 either with

a large excess of TFA in CH₂Cl₂or by catalytic hydrogena-

tion of 36 with Pd/C in methanol (Scheme 9). Both proce-

dures gave pure 37 directly. Irrespective of the method used

the spectroscopic and spectrometric data of 37 were the

same. Specifically there was no indication of decarboxyla-

tion. The MALDI-TOF mass spectrum of 37, however,

showed decarboxylated by-products. It seems therefore that

this is caused by the MALDI process rather than by chemi-

cally induced undesired decarboxylation.

1

group in the H NMR spectrum.

Another important goal of this work was to synthesize

water-soluble dendrons with two different functional groups,

for example, one for Pt-attachment and one for a fluores-

cence tag. For such an endeavor the (almost)orthogonally

protected branching unit 40 was a key compound. Although

known,^[17]improvements of the procedure were necessary

(Scheme 10). The sequence starts from 38 which was easily

prepared on a 100-g scale.^[18]Its subjection to Suzuki–

Miyaura cross-coupling with Cbz-protected allylamine gave

39 on the 15-g scale in yields of 67%. Subsequent coupling

with Boc-protected allylamine gave compound 40 in 92%

yield. TFA deprotection of 40 gave 41, whereby, however,

also some of the Cbz was cleaved off. Compound 41 was pu-

rified by column chromatography. The Cbz group of 40 was

removed by catalytic hydrogenation. The best solvents were

ethanol/ethyl acetate mixtures and methanol. Both free

amines, 41 and 42, were stored under nitrogen. Saponifica-

In a final sequence the known dendrimer 59^[8]was pre-

pared from 43 and 27 (Scheme 13)to use it as starting mate-

rial for dendrimers with peripheral malonates and fluores-

cence tags. The deprotection of the N-Cbz groups of 59 to

60 by catalytic hydrogenation proceeded cleanly without af-

fecting the Boc groups. The free amines of 60 were then

used to hook the dendron 13, resulting in the formation of

61. Deprotection of 61 and reaction of the corresponding

free amine with both compounds 24 and 26 gave the corre-

sponding dendrimers 63 and 64, respectively, in good to ex-

cellent yields. The separation of these dendrimers from the

excess of 24 and 26 by column chromatography was possible

but too tedious. The excess of the respective active ester

was therefore rather intercepted with methanol to give the

corresponding methyl esters. Finally, dendrimer 65, which

provides free malonic acids with potential for platinum com-

plexation, was obtained via deprotection of 63 with TFA in

Scheme 10. Synthesis and selective deprotection of 40. Reagents and conditions: a)9-BBN, toluene, 0 8C, 12 h, then 1m KOH, [Pd(PPh₃)₄], 608C; b)9-

BBN, toluene, 08C, 12 h, then 1m KOH, [Pd(PPh₃)₄], reflux; c)CH ₂Cl₂, TFA, 12 h, room temperature; d)ethyl acetate/ethanol, Pd/C, 1 h, H ₂; e) 1m

KOH, ethanol, room temperature.

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Water-Soluble, Multiple Functionalizable Dendrons

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Scheme 11. Syntheses of dendrons. a)HOBt, EDC, CH ₂Cl₂, TEA, MeOH; b)TBTU, CH ₂Cl₂, DMF, TEA, MeOH; c)MeOH, Pd/C, 1 h, H ₂; d)EtOH,

1m KOH, room temperature, 12 h; e)ethyl acetate/MeOH, Pd/C, H , 1 h.

2

dichloromethane. The progress of the cleavage of the tert-

butyl esters was conveniently monitored with H NMR spec-

and bidendate anchor group(s)for complexation with phar-

maceutically relevant metal complexes like Pt complexes.

These anchor groups are either of the malonic (weak bind-

ing)or diaminopropionic acid type (strong binding.) Some

of the dendrons carry the dansyl substituent as fluorescence

tag to enable cell distribution studies. All dendrons were at-

tached to a trifunctional core which gave a variety of differ-

ently surface-decorated dendrimers combining several func-

tions at the same time.

1

troscopy. In initial experiments it was not possible to depro-

tect the benzyl esters of 64 to gain 65. The typical deprotec-

tion protocol with Pd/C in a hydrogen atmosphere failed.

In summary, a concept has been presented that should

allow the conversion of molecular objects (e.g. large den-

drimers)into non-ionic and yet water-soluble and surface

addressable entities. Several dendrons of different sizes and

complexity were prepared which not only carry either two

or four branched OEG chains but also one or two mono-

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5595

A. D. Schlüter and S. Müller

Scheme 12. Synthesis of dendrimers with different surface motifs. Reagents and conditions: a)CH ₂Cl₂, HOBt, EDC, TEA, MeOH; b)CH ₂Cl₂, TFA,

room temperature.

were referenced to the solvent signal: CDCl₃at d=77.0 ppm, CD₂Cl₂at

Experimental Section

d=53.5 ppm, CH₃OH at d=49.0 ppm. All spectra were recorded at

258C. Mass spectra were recorded on a Varian MAT 711 and CH6 (EI)

General: All starting materials were purchased from commercial sources

and used without further purification. Solvents were dried under standard

conditions. Compounds 1, 14, 17a,b, and 18a,b were commercially avail-

able. The following compounds were prepared according to literature

procedures: 2a,^[13]2b,^[14]2c,^[15]7,^[12]11,^[16]27,^[8]38,^[18]59.^[8]All dansyl-la-

beled compounds were stored and allowed to react in the dark to avoid

oxidative degradation. Whenever possible, reactions were monitored by

thin-layer chromatography (TLC)using TLC silica gel coated aluminum

plates 60F₂₅₄(Merck). Compounds were detected by UV light (254 nm or

366 nm)and/or by treatment with a solution of ninhydrine in ethanol,

anisaldehyde in H₂SO₄followed by heating, or with iodine. Column chro-

matography was preformed using Merck silica gel 60, 0.040–0.063 mm

(230–400 mesh).

¹H NMR spectra were recorded by using a Bruker AC 500 (500 MHz),

an AM 270 spectrometer (270 MHz), or an AB 250 (250 MHz) instru-

ment, and were referenced to the solvent signal: CDCl₃at d=7.24 ppm,

CD₂Cl₂at d=5.32 ppm, CD₃OH at d=3.35 or 4.78 ppm. ¹³C NMR spec-

tra were recorded by using a Bruker AC 500 (125 MHz), an AM 270

spectrometer (67.5 MHz), or an AB 250 (62.5 MHz) instrument, and

or Type CH5DF (FAB), and a Bruker Reflex with delayed extraction

source (MALDI-TOF). Elemental analyses were performed by using a

Perkin-Elmer EA 240. Because of the polarity of the prepared com-

pounds and their ability to complex metal ions, it was generally difficult

to obtain correct data from elemental analysis. Analytical GPC was re-

corded on Waters Styragel HR 1 or HR 3 columns, Waters 2487 UV/VIS

detector at 254 nm to demonstrate the purity of these compounds.

Ethyl

4-(3-tert-butoxycarbonylaminopropoxy)-3,5-dihydroxybenzoate

(3a): Ethyl 3,4,5-trihydroxybenzoate (1; 7.49 g, 37.8 mmol), tert-butyl (3-

bromopropyl)carbamate (2a; 9.00 g, 37.8 mmol), dry KHCO₃(15.14 g,

151.2 mmol), and KI (0.11 g, 0.6 mmol) were suspended in dry DMF

(40 mL). The mixture was degassed by three freeze–pump–thaw cycles,

flushed with N₂, and stirred for four days at room temperature. After fil-

tration, the organic phase was quenched with water (400 mL), neutral-

ized, and extracted seven times with diethyl ether (60 mL). The com-

bined organic phases were extracted three times with 1m NaHCO₃

(100 mL), three times with water (100 mL), and once with brine

(100 mL). The organic phase was dried over MgSO₄, filtered, and the sol-

vent evaporated. The crude product was purified by column chromatog-

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Chem. Eur. J. 2005, 11, 5589 – 5610

Water-Soluble, Multiple Functionalizable Dendrons

FULL PAPER

TLC. After complete deprotection, the mixture was filtered and the sol-

vent removed under reduced pressure. Further purification was not nec-

essary. Yield: 4.28 g (quant)of a colorless solid.

R_f=0.23 (silica gel, hexane/ethyl acetate 3:1); ¹H NMR (CDCl₃,

[D₄]methanol, 270 MHz): d=1.29 (t, ³J(H,H)=7.0 Hz, 3H; -CH₂CH₃),

1.39 (s, 9H; -C(CH₃)₃), 1.83 (m, 2H; b-CH₂), 3.36 (m, 2H; g-CH₂), 4.02

(t, ³J(H,H)=5.2 Hz, 2H; a-CH₂), 4.25 (q, ³J(H,H)=7.0 Hz, 2H;

-CH₂CH₃), 5.17 (br s, 1H; -NH), 7.09 ppm (2H; Ar-H); ¹³C NMR

(CDCl₃, [D₄]methanol, 68 MHz): d=13.85, 28.05, 30.29, 36.65, 60.79,

69.38, 79.50, 109.17, 125.45, 138.05, 149.77, 157.08, 166.80 ppm; MS (EI,

80 eV, 1608C): m/z (%): 354.9 (5) [M]⁺, 310.0 (3)[ MÀOEt]⁺, 255.9 (4),

254.9 (27)[ MÀC₅H₉O₂]⁺, 101.9 (100)[C ₅H₉O₂]⁺; elemental analysis

calcd (%)for C ₁₇H₂₅NO₇(355.16): C 57.45, H 7.09, N 3.94; found: C

57.21, H 7.07, N 3.83.

Ethyl

4-(3-benzyloxycarbonylaminopropoxy)-3,5-dihydroxybenzoate

(3b): Ethyl 3,4,5-trihydroxybenzoate (1; 7.49 g, 37.8 mmol), benzyl (3-

bromopropyl)carbamate (2b; 10.28 g, 37.8 mmol), dry KHCO₃(15.14 g,