N-monosubstituted methoxy-oligo(ethylene glycol) carbamate ester prodrugs of resveratrol

Article abstract of DOI:10.3390/molecules200916085

Resveratrol is a natural polyphenol with many interesting biological activities. Its pharmacological exploitation in vivo is, however, hindered by its rapid elimination via phase II conjugative metabolism at the intestinal and, most importantly, hepatic levels. One approach to bypass this problem relies on prodrugs. We report here the synthesis, characterization, hydrolysis, and in vivo pharmacokinetic behavior of resveratrol prodrugs in which the OH groups are engaged in an N-monosubstituted carbamate ester linkage. As promoiety, methoxy-oligo(ethylene glycol) groups (m-OEG) (CH₃-[OCH₂CH₂]_n-) of defined chain length (n = 3, 4, 6) were used. These are expected to modulate the chemico-physical properties of the resulting derivatives, much like longer poly(ethylene glycol) (PEG) chains, while retaining a relatively low MW and, thus, a favorable drug loading capacity. Intragastric administration to rats resulted in the appearance in the bloodstream of the prodrug and of the products of its partial hydrolysis, confirming protection from first-pass metabolism during absorption.

Full text of DOI:10.3390/molecules200916085

Molecules 2015, 20, 16085-16102; doi:10.3390/molecules200916085
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
N-Monosubstituted Methoxy-oligo(ethylene glycol) Carbamate
Ester Prodrugs of Resveratrol
Andrea Mattarei 1,†,*, Michele Azzolini 2,3,4,†, Mario Zoratti 2,3, Lucia Biasutto 2,3
and Cristina Paradisi 1
1
Department of Chemical Sciences, University of Padova, Via F. Marzolo, Padova 135131, Italy;
E-Mail: cristina.paradisi@unipd.it
2
Department of Biomedical Sciences, University of Padova, Viale G. Colombo, Padova 335121,
Italy; E-Mails: michele.azzolini@gmail.com (M.A.); zoratti@bio.unipd.it (M.Z.);
lucia.biasutto@cnr.it (L.B.)
3
CNR Neuroscience Institute, Viale G. Colombo, Padova 335121, Italy
4
NÓOS srl, Via Campello sul Clitunno, Roma 3400181, Italy
†
These authors contributed equally to this work.
* Author to whom correspondence should be addressed; E-Mail: andrea.mattarei@unipd.it;
Tel.: +39-049-5272; Fax: +39-049-5829.
Academic Editors: Emilie Combet and Maria Rosário Bronze
Received: 28 July 2015 / Accepted: 28 August 2015 / Published: 3 September 2015
Abstract: Resveratrol is a natural polyphenol with many interesting biological activities.
Its pharmacological exploitation in vivo is, however, hindered by its rapid elimination via
phase II conjugative metabolism at the intestinal and, most importantly, hepatic levels. One
approach to bypass this problem relies on prodrugs. We report here the synthesis,
characterization, hydrolysis, and in vivo pharmacokinetic behavior of resveratrol prodrugs
in which the OH groups are engaged in an N-monosubstituted carbamate ester linkage. As
promoiety, methoxy-oligo(ethylene glycol) groups (m-OEG) (CH3–[OCH2CH2]n–) of defined
chain length (n = 3, 4, 6) were used. These are expected to modulate the chemico-physical
properties of the resulting derivatives, much like longer poly(ethylene glycol) (PEG)
chains, while retaining a relatively low MW and, thus, a favorable drug loading capacity.
Intragastric administration to rats resulted in the appearance in the bloodstream of the
prodrug and of the products of its partial hydrolysis, confirming protection from first-pass
metabolism during absorption.

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Keywords: resveratrol; prodrugs; methoxy-oligo(ethylene glycol); poly(ethylene glycol);
polyphenols; bioavailability; carbamate ester
1. Introduction
Evolution has endowed organisms with a powerful apparatus designed to detoxify “xenobiotic”
substances which reach the cytoplasm [1–3]. These compounds tend to be hydrophobic/apolar to pass
the cell membrane. In specific cells, such as enterocytes and hepatocytes, they are modified by the addition
of polar groups (Phase I metabolism). One of the most common processes is the insertion of hydroxyl
groups by P450 oxidases [4,5]. These hydroxyls are then targeted by conjugative enzymes of Phase II
metabolism, in particular sulfotransferases [6,7] and uridine diphosphate-glucuronosyltransferases [8,9], to
produce water-soluble, inactivated metabolites which can be re-exported via Organic Anion/Cation
Transporters (OATPs) [10] and ATP-Binding Cassette (ABC)/Multi Drug Resistance (MDR) efflux
transporters [11–13] and eliminated.
Plant polyphenols already possess, by definition, a number of hydroxyl groups. They are, thus,
ready-made substrates for Phase II metabolism. Indeed, the modifications just mentioned have been
long recognized and studied for representative members of this huge family of natural compounds,
including resveratrol, which is of specific interest here. Rapid glucuronidation and sulfation in
enterocytes, hepatocytes, and other cells (e.g., [14–21]) and re-export to the intestinal lumen by MDR
proteins [15,22,23], or possibly OATPs [24], are thought to limit the potential impact of this celebrated
“natural drug” in vivo. This is a pity, given the pleiotropic effects this dietary compound is reported to
have in important pathophysiological respects (for recent reviews see, e.g., [25–30]). Possibly its most
widely discussed activities are as an activator of the deacetylase SIRT1 and a repressor of
inflammation, to which one might add its estrogen-like features (e.g., [31–38]). Pronounced
metabolism may be an important reason why resveratrol’s effects are often unclear in human clinical
studies [39–44]—a weakness shared with other polyphenols. A tool to increase resveratrol’s
bioavailability and body levels may help dispel (or support) doubts, and, more importantly, allow the
development of a full pharmacology of this compound.
One of the strategies used to prevent or delay drug metabolism and enhance bioavailability and
effectiveness is based on prodrugs: the sites undergoing phase II conjugation, in our case the phenolic
hydroxyls, are temporarily protected (–OH → –O–X–R) by removable groups during absorption, first
pass through the liver and distribution, with final regeneration of the active principle following the
removal of protective groups by chemical and/or enzyme-catalyzed hydrolysis.
Three features of the protecting group are most relevant: (1) the reactivity of the bond engaging the
group to be protected must be suitable for the regeneration of the parent molecule in an appropriate
time frame under the conditions prevailing in vivo; (2) the promoiety “capping” the reactive site ought
to favor absorption and distribution to the desired body districts; and (3) the side-products resulting
from its eventual dismantling ought to be innocuous or beneficial.
This strategy is being actively applied to polyphenols (rev.s: [45,46]). A first family of resveratrol
prodrugs was produced and tested [47], in which the phenolic OH groups are masked as

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N,N-disubstituted carbamate esters (–O–C(=O)–N(CH3)R), where R is a methoxy-poly(ethylene
glycol)-350 (mPEG-350) or a butyl-glucosyl group. These derivatives are highly soluble in aqueous
media but too stable to be used as prodrugs. N-monosubstituted carbamates are expected to undergo
faster hydrolysis. However, the ideal promoiety remains to be chosen. Poly(ethylene glycol) chains are
a strong candidate: their presence can increase aqueous solubility of hydrophobic drugs, prolong
circulation time, slow down hydrolysis, and increase stability in the gastrointestinal tract [48–53]. This
type of decoration is particularly valuable for protein/polypeptide drugs and drug-carrying liposomes
(rev.s, e.g., [54–59]), which can be protected from phagocytosis and are less at risk of provoking an
immune response. In general, polymer chains with MW in the range of a few to many kDa are
employed for these modifications, as they perform better vis-à-vis hydration and immunogenicity.
Their size and size dispersion, however, complicate their chemistry in particular for purification and
analytical aspects. Furthermore in the case of small molecules such as resveratrol the incorporation of
such large structures in the prodrug means that the “drug loading capacity” of the construct, i.e. the
amount of active principle associated with a given weight of precursor, is low. Siddalingappa et al.
have recently reported micelle-forming constructs in which the resveratrol kernel was linked to 2- to
20-KDa PEG and PEG-PLA block copolymer chains via ether or carboxyester functionalities (succinyl
linker) [60]. Micelle formation was concluded to enhance stability vis-à-vis ester hydrolysis.
Constructs with MW up to 6.6 KDa were used injected intravenously into rats to study their
pharmacokinetic behavior. We have explored the use of short-chain, defined-MW (monodispersed)
methoxy-oligo(ethylene glycol) (m-OEG) as a convenient alternative to higher-MW PEGs [61]. These
oligomeric moieties turned out to be useful in modulating physicochemical and absorption properties
of derivatives, maintaining many properties of longer PEG chains with a more favorable drug loading
capacity, and to be fully compatible with absorption from the gastrointestinal tract when linked to the
stilbenoid kernel via acetal or ketal bond systems.
Herein we develop this approach reporting the synthesis and characterization of a small library of
resveratrol prodrugs consisting of methoxy-oligo(ethylene glycol) amines linked to the resveratrol
phenolic functions via the N-monosubstituted carbamate linkage (Scheme 1). Hydrolysis assays and
pharmacokinetic studies in rats were then performed in order to evaluate stability, in vivo, absorption
and metabolism of these new prodrugs.
Scheme 1. Resveratrol (1) and methoxy-oligo(ethylene glycol)-carbamate substituted
prodrugs (2–4).

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2. Results and Discussion
2.1. Synthesis
Synthesis of N-monosubstituted carbamate esters is usually carried out in two steps: reaction of the
desired primary amine with phosgene or its equivalent to give a reactive isocyanate derivative,
followed by its coupling with the phenolic function [49]. These procedures tested on resveratrol led to
low yields for the trisubstituted derivatives, probably due to the high reactivity of the isocyanate group
promoting side reactions of polymerization entraining the stilbene double bond function. In this study
we prepared N-monosubstituted resveratrol carbamate esters through conversion of the corresponding
methoxy-oligo(ethylene glycol) amines (2c–4c) to the activated 4-nitrophenyl carbamates (Scheme 2).
Isolation of the activated 4-nitrophenyl carbamate esters (2d–4d) followed by transesterification with
resveratrol (1) provided the desired trisubstituted N-(methoxy-oligo(ethylene glycol)) carbamate
prodrugs of resveratrol (2–4) in good yields (75%–88%) under mild conditions along with traces of di-
and monosubstituted resveratrol conjugates, which were separated from the desired product by flash
chromatography. Methoxy-oligo(ethylene glycol) amines (2c–4c) (key intermediates for the synthesis
of 2–4) were synthesized under mild conditions and with excellent yields (96%–98%) by Staudinger
reduction of the corresponding methoxy-oligo(ethylene glycol) azides (2b–4b), in turn obtained by
substitution of the tosylated methoxy-oligo(ethylene glycol) (2a–4a) with sodium azide (Scheme 2).
Scheme 2. Synthesis of resveratrol N-monosubstituted carbamate ester prodrugs. Reagents
and conditions: (i) TsCl, DMAP, Pyridine, DCM, 0 °C to r.t., 6 h; (ii) NaN3, H2O/Acetone
3:1, 75 °C, overnight; (iii) PPh3, THF, r.t., 5 h, then H2O, 80 °C, overnight; (iv)
bis-(4-nitrophenyl) carbonate, DMAP, ACN, 50 °C, 3 h; and (v) resveratrol, ACN, DMAP,
50 °C, 24 h.
2.2. Hydrolysis Studies
The hydrolytic reactivity of the synthesized new resveratrol derivatives was tested in solutions
mimicking gastric and intestinal pH and in blood. All turned out to be stable at pH values close to that
of the human stomach (no reaction over 24 h at 37 °C in 0.1 N HCl), but underwent hydrolysis at
near-neutral pH (at pH 6.8, representing intestinal pH) and in blood, at rates that may be suitable for use
in vivo.

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Kinetic analysis of the data was performed by assuming that hydrolysis to resveratrol occurred via
consecutive losses of the three protecting groups in pseudo-first order processes and by considering
each pair of isomeric intermediates (potentially) resulting from the first and second hydrolysis steps as
a single species, i.e. the two monosubstituted and the two disubstituted intermediates were handled as
species I and II, respectively (Scheme 3).
Scheme 3. Kinetic scheme for stepwise hydrolysis of triprotected (III) derivatives 2–4 to
resveratrol (1). II and I represent, each, the two possible isomeric derivatives resulting
from hydrolysis of one (II) and two (I) carbamoyl linkages, respectively.
Figure 1 shows, as an example, the time course of the four species (III, II, I, and 1) involved in the
reaction of derivative 3 in PBS 0.1 M (panel A) and in whole rat blood (panel B). As hydrolysis
proceeds resveratrol will eventually form with the kinetics dictated by the constants, up to 100%
conversion. The fit of the experimental data was obtained using a set of equations analogous to those
utilized by Kozerski et al. [62] and in our previous work [61].
Figure 1. Hydrolysis of 3 in PBS 0.1 M, at pH 6.8, 37 °C (A) and in whole rat blood (B).
Data are expressed as % of the initially-loaded compound. The fit is for pseudo-first order
kinetics as outlined in Scheme 3 and accompanying text.
The full set of kinetic constants resulting from fits of this type for all the derivatives is presented in
Table 1 and in Figure 2.
Table 1. Kinetic data obtained for hydrolysis of resveratrol derivatives 2–4 in aqueous
PBS 0.1 M, at pH 6.8 and in rat blood at 37 °C.
PBS 0.1 M, at pH 6.8, 37 °C
t1/2 (h) kIII (h−1) kII (h−1)
Blood
kII (h−1)
1.47 ± 0.08 0.43 ± 0.04 0.155 ± 0.009
Derivative
kI (h−1)
t1/2 (h)
0.5
kIII (h−1)
kI (h−1)
2
3
4
6.0
3.5
4.5
0.104 ± 0.002 0.030 ± 0.002 0.005 ± 0.002
0.134 ± 0.007 0.042 ± 0.005 0.007 ± 0.003
0.158 ± 0.002 0.051 ± 0.001 0.025 ± 0.001
0.3
2.0 ± 0.2
0.42 ± 0.06
0.17 ± 0.03
0.22 ± 0.03
0.5
1.25 ± 0.08 0.49 ± 0.05

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Figure 2. Hydrolysis rate constants (37 °C) for compounds 2–4 in PBS 0.1 M, at pH 6.8
(A) and in whole rat blood (B).
Hydrolysis of N-monosubstituted carbamate esters has been studied and reported in literature [63,64].
The stability of the N-monosubstituted carbamate group in acidic solution and its reactivity at higher
pH values, are coherent with a mechanism which envisions the deprotonation of the amidic nitrogen
ArO-C(O)-NHR′ followed by aryloxide elimination, thus regenerating the original phenolic
functionality, ArOH. The isocyanate intermediate adds water and decomposes releasing the amine and
carbon dioxide [63,64] (Scheme 4). As reported by L.W Dittert and T. Higuchi [64] the isocyanate
intermediate formed during the hydrolysis of N-monosubstituted carbamate esters involves a lower
energy of activation compared to the tetrahedral intermediate which is the only possible pathway for
the OH− catalyzed hydrolysis of N,N-disubstituted carbamate esters. This results in higher hydrolysis
rates of N-monosubstituted carbamate esters under physiological pH conditions making this linkage
more suitable for use as prodrug than the N,N-disubstituted carbamate ester bond.
Scheme 4. Mechanism of base-induced hydrolysis of carbamates 2–4.
The sensitivity of the carbamoyl group to pH ought to be taken into account when planning for an
eventual oral administration of compounds of this type, since the pH of saliva in humans can normally
vary from slightly acidic to slightly basic (pH 7.4) and hydrolytic enzymes are present. The residence
time in the mouth would, in any case, be expected to be relatively short. The t1/2 of the compounds in
blood (which has a slightly basic pH and hydrolytic enzymes) is in the order of 20–30 min. (Table 1).
The 4-h period over which the hydrolysis was monitored in whole blood was set by the stability of
blood itself under the experimental conditions. It was in any case sufficient to compute the rate
constants according to the model adopted. As hydrolysis proceeds resveratrol must eventually form
with the kinetics dictated by the constants up to 100% conversion.
The data of Table 1 and Figure 2 clearly show that hydrolysis rates are much larger in blood,
suggesting the involvement of enzyme-catalyzed hydrolysis.

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2.3. Pharmacokinetic Studies
All the N-monosubstituted methoxy-oligo(ethylene glycol) carbamate ester prodrugs were tested for
their in vivo absorption and metabolism. Pharmacokinetic studies were performed with rats, and each
compound was administered as a single intragastric bolus, in an equimolar dose/kg body weight
(88 µmol/kg). Blood samples were taken at different time points, treated as described in the Materials
and Methods section and analyzed.
Administration of compound 2 did not result in the appearance of detectable amounts of resveratrol,
derivatives or any metabolites in blood samples, while measurable levels of stilbene derivatives (all
compounds containing the stilbenic structure detected in blood, which can only originate from the
administered prodrugs) were found in blood samples in the case of compounds 3 and 4. The
pharmacokineticbehavior of the prodrugs-absorption is bound to depend on the promoiety attached to
the core compound. The fully protected and partially de-protected species for compounds 3 and 4 were
consistently found in the samples taken, and the time course of their concentration is shown in Figure 3.
Figure 3. Blood pharmacokinetic profiles after intragastric administration of compounds 3
(A) and 4 (B) to rats. Data represent average values + standard deviation (an error bar of
the same size in the negative direction is also implied but not shown for graphic clarity).
N (number of PK experiments performed for each compound) = 3 in all cases.
Both 3 and 4 were rapidly absorbed, peaking 10 min after their administration. In agreement with
the results previously obtained with acetal prodrugs [61], the derivative with four units of ethylene
glycol (3) was better absorbed than its counterpart with six units (0.3 µM vs. 0.1 µM). In both cases the
di-substituted hydrolysis products were also present.
The results of PK studies confirm the observations reported in [61]: variations in the length of the
methoxy-oligo(ethylene glycol) sequence can have an impact on the absorption of the prodrug. Thus,
compounds 3 and 4, which contain four and six units of ethylene glycol per phenolic oxygen,
respectively, worked better than 2, which contains three. The extent of absorption may be expected to
be related to the affinity of the prodrugs for membranes if the step determining the rate of absorption is
diffusion into the latter, i.e., if the constructs are rather more hydrophilic than lipophilic. While we
have not measured the distribution of the present compounds between water and an oily phase and

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their solubility, it is reasonable to presume that they may follow the same pattern as the corresponding
acetal-based compounds of ref. [61].
3. Experimental Section
3.1. Materials and Instrumentation
Resveratrol was purchased from Waseta Int. Trading Co. (Shangai, China). Other starting materials and
reagents were purchased from Sigma-Aldrich (Milan, Italy), Fluka (Milan, Italy), Merck-Novabiochem
(Milan, Italy), Riedel de Haen (Milan, Italy), J.T. Baker (Milan, Italy), Cambridge Isotope
Laboratories Inc. (Rome, Italy), Acros Organics (Milan, Italy), Carlo Erba (Milan, Italy), and Prolabo
(Milan, Italy) and were used as received. TLCs were run on silica gel supported on plastic
(Macherey-Nagel Polygram®SIL G/UV254, silica thickness 0.2 mm, Duren, Germany) and visualized
by UV detection. Flash chromatography was performed on silica gel (Macherey-Nagel 60, 230–400 mesh
granulometry (0.063–0.040 mm)) under air pressure. The solvents were analytical or synthetic grade
1
and were used without further purification. H-NMR spectra were recorded with a Bruker AC250F
spectrometer operating at 250 MHz and a Bruker AVII500 spectrometer (Milan, Italy) operating at
500 MHz. Chemical shifts (δ) are given in ppm relative to the signal of the solvent. HPLC-UV
analyses were performed with an Agilent 1290 Infinity LC System (Agilent Technologies, Milan,
Italy), equipped with binary pump and a diode array detector (190–500 nm). HPLC/ESI-MS analyses
and mass spectra were performed with a 1100 Series Agilent Technologies (Milan, Italy) system,
equipped with binary pump (G1312A) and MSD SL Trap mass spectrometer (G2445D SL) with ESI
source. ESI-MS positive spectra of reaction intermediates and final purified products were obtained
from solutions in acetonitrile, eluting with a water:acetonitrile = 1:1 mixture containing 0.1% formic
acid. High-resolution mass measurements were obtained using a Mariner ESI-TOF spectrometer
(PerSeptive Biosystems, Framingham, MA, USA). HPLC/ESI-MS analysis was used to confirm the
purity (>95%).
3.2. Hydrolysis Assays
3.2.1. Hydrolysis under Physiological-Like Conditions
The chemical stability of all new compounds was tested in aqueous media imitating gastric (0.1 N
HCl, NormaFix) and intestinal (0.1 M PBS buffer, at pH 6.8) pH conditions. A 5 µM solution of the
compound was prepared from a 5 mM stock solution in DMSO, and incubated at 37 °C for 24 h;
samples withdrawn at different times were analyzed by HPLC-UV. Hydrolysis products were
identified by HPLC/ESI-MS analysis of selected samples. Non-linear curve fitting was performed
using Origin 8.0 data analysis software, using the equations described in [61,62].
3.2.2. Stability in Rodent Whole Blood
Rats were anesthetized and blood was withdrawn from the jugular vein, heparinised and transferred
into tubes containing EDTA. Blood samples (1 mL) were spiked with 5 µM compound (dilution from
a 5 mM stock solution in DMSO), and incubated at 37 °C for 4 h (the maximum period allowed by

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blood stability). Aliquots were taken after 10 min, 30 min, 1 h, 2 h and 4 h and treated as described
below (blood sample treatment and analysis). Cleared blood samples were finally subjected to
HPLC-UV analysis.
3.2.3. Blood Sample Treatment and Analysis
Before starting the treatment, 4,4′-dihydroxybiphenyl was added as internal standard to a
carefully-measured blood volume (25 µM final concentration). Blood was then stabilized with a
freshly-prepared 10 mM solution of ascorbic acid (0.1 vol) and acidified with 0.6 M acetic acid (0.1 vol);
after mixing, an excess of acetone (4 vol) was added, followed by sonication (2 min) and
centrifugation (12,000× g, 7 min, 4 °C). The supernatant was finally collected and stored at −20 °C.
Before analysis, acetone was allowed to evaporate at room temperature using a Univapo 150H
(UniEquip, Martinsried, Germany) vacuum concentrator centrifuge, and up to 40 μL of CH3CN were
added to precipitate residual proteins. After centrifugation (12,000× g, 5 min, 4 °C), cleared samples
were directly subjected to HPLC-UV analysis. Metabolites and hydrolysis products were identified by
comparison of chromatographic retention time with true samples.
The recovery yields of resveratrol and its metabolites have been reported previously [65,66]. For the
new prodrugs the corresponding recoveries, expressed as ratio to the recovery of internal standard,
were as follows: 2: 0.914 ± 0.116 3: 0.903 ± 0.104; 4: 0.967 ± 0.100. Recoveries of partially protected
(disubstituted) derivatives were assumed to be the same as those of the corresponding fully substituted
prodrug. Knowledge of these ratios allowed us to determine the unknown amount of analyte in a blood
sample by measuring the internal standard recovered [54], and using the same calibration curve of
resveratrol, since derivatives have the same absorption coefficient of resveratrol itself (y = 5.3085x,
where y is the concentration of analyte and x is the integrated area of the HPLC-UV peak).
Since sample treatment includes an evaporation/concentration step, and there are no interfering
peaks from the tissue matrix, LOD (limit of detection) and LOQ (limit of quantification) were
determined relative to the analytical part of the method (HPLC-UV analysis) and were the same as
resveratrol (i.e., 0.04 µM and 0.12 µM, respectively; [65]).
3.3. Pharmacokinetic Studies
Derivatives 2–4 were administered to overnight-fasted male Wistar rats from the facility of the
Department of Biomedical Sciences, University of Padova, as a single intragastric dose (88 µmol/kg,
dissolved in 250 µL DMSO) delivered with a commercial stainless steel feeding tube (www.2biol.com)
to have a precise starting time. Wistar rats are commonly used for such studies (e.g., [60]). Relatively
young adults (3–5 months) were used in order to have a fully developed metabolism as well as a body
weight (300–400 g) which would limit the amount of synthetic compound to be used.
Blood samples were obtained by the tail bleeding technique: before drug administration, rats were
anesthetized with isoflurane and the tip of the tail was cut off; blood samples (80–100 µL each) were
then taken from the tail tip at different time points after drug administration. Blood was collected in
heparinized tubes, kept in ice and treated as described above within 10 min. All experiments involving
animals were performed with the permission and supervision of the University of Padova Ethical
Committee for Experimentation on Animals (CEASA) and Central Veterinary Service, in compliance

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with Italian Law DL 116/92, embodying EU Directive 86/609. Significance in comparisons was
assessed using the Wilcoxon Rank Test.
3.4. Synthesis
3.4.1. General Procedure for the Preparation of Methoxy-oligo(ethylene glycol)-p-toluenesulfonates
(2a–4a)
Pyridine (1.09 mL, 13.5 mmol, 2.0 eq.) and DMAP (1.65 g, 13.5 mmol, 2.0 eq.) were added to a
solution of methoxy-oligo(ethylene glycol) (6.75 mmol, 1.0 eq.) in DCM (10 mL), and the mixture was
stirred at 0 °C for 15 min. A solution of tosyl chloride (1.93 g, 10.1 mmol, 1.5 eq.) in DCM (10 mL) was
then added dropwise and the reaction mixture was stirred at room temperature for 6 h. The resulting
mixture was diluted in DCM (150 mL) and washed with 0.5 N HCl (100 mL). The aqueous layer was
washed with DCM (5 × 75 mL) and all the organic fractions were collected, dried over MgSO4 and filtered.
The solvent was evaporated under reduced pressure and the residue was purified by flash chromatography.
2-(2-(2-Methoxyethoxy)ethoxy)ethyl 4-methylbenzenesulfonate (2a). Purified by flash chromatography
using DCM/EtOAc 8:2 as eluent. 95% yield as a colourless oil. 1H-NMR (250 MHz, CDCl3) δ (ppm):
2.43 (s, 3H, Ar–CH3), 3.35 (s, 3H, –O–CH3), 3.49–3.66 (m, 10H, 2 × –O–CH2–CH2–O– + –O–CH2–),
3
3
4.14 (t, 2H, Ts–CH2–, JH–H = 5.75 Hz), 7.32 (d, 2H, Ar–H, JH–H = 8.25 Hz), 7.77 (d, 2H, Ar–H,
3JH–H = 8.25 Hz); C-NMR (62.9 MHz, CDCl3) δ (ppm): 144.7, 132.9, 129.7, 127.9, 71.8, 70.6, 70.5,
13
70.4, 69.2, 68.6, 58.9, 21.6; ESI-MS (ion trap): m/z 337 [M + H2O + H]+. HRMS (ESI+): m/z 319.1222
[M + H]+, calcd for C14H23O6S: 319.1215.
2,5,8,11-Tetraoxatridecan-13-yl 4-methylbenzenesulfonate (3a). Purified by flash chromatography
using DCM/Acetone 8:2 as eluent. 94% yield as a colourless oil. 1H-NMR (250 MHz, CDCl3) δ (ppm):
2.41 (s, 3H, Ar–CH3), 3.33 (s, 3 H, –O–CH3), 3.48–3.67 (m, 14 H, 3 × –O–CH2–CH2–O– + –O–CH2–),
3
3
4.12 (t, 2H, Ts–CH2–, JH–H = 4.90 Hz), 7.30 (d, 2H, Ar–H, JH–H = 8.30 Hz), 7.76 (d, 2H, Ar–H,
3JH–H = 8.00 Hz); 13C-NMR (62.9 MHz, CDCl3) δ (ppm): 144.7, 132.8, 129.7, 127.8, 71.8, 70.6, 70.4,
70.4, 70.4, 70.3, 69.1, 68.5, 58.9, 21.5; ESI-MS (ion trap): m/z 381 [M + H2O + H]+. HRMS (ESI+):
m/z 363.1463 [M + H]+, calcd for C16H27O7S: 363.1477.
2,5,8,11,14,17-Hexaoxanonadecan-19-yl 4-methylbenzenesulfonate (4a). Purified by flash chromatography
using DCM/Acetone 6.5:3.5 as eluent. 98% yield as a colourless oil. 1H-NMR (250 MHz, CDCl3) δ
(ppm): 2.35 (s, 3H, Ar–CH3), 3.27 (s, 3H, –O–CH3), 3.42–3.60 (m, 22H, 2 × –O–CH2–CH2–O– +
–O–CH2–), 4.05 (t, 2H, Ts–CH2–, 3JH–H = 5.00 Hz), 7.25 (d, 2H, Ar–H, 3JH–H = 7.93 Hz), 7.70 (d, 2H,
Ar–H, 3JH–H = 8.34 Hz); 13C-NMR (62.9 MHz, CDCl3) δ (ppm): 144.6, 132.7, 129.6, 127.7, 71.7, 70.5,
70.4, 70.4, 70.3, 70.3, 70.3, 69.1, 68.4, 58.8, 21.4; ESI-MS (ion trap): m/z 451 [M + H]+. HRMS
(ESI+): m/z 451.2008 [M + H]+, calcd for C20H35O9S: 451.2002.
3.4.2. General Procedure for the Preparation of Methoxy-oligo(ethylene glycol)-azides (2b–4b)
Sodium azide (10.72 g, 0.165 mol, 5.0 eq.) were added to a solution of methoxy-oligo(ethylene
glycol)-p-toluenesulfonate (2a–4a) (33 mmol, 1.0 eq.) in a water/acetone solution (1:3, 65 mL), and

Molecules 2015, 20
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the mixture was stirred at 75 °C overnight. The mixture was then diluted in DCM (250 mL) and
washed with water (250 mL). The aqueous layer was washed with DCM (5 × 100 mL) and all the
organic fractions were collected, dried over MgSO4 and filtered. The solvent was evaporated under
reduced pressure and the residue was purified by flash chromatography.
1-Azido-2-(2-(2-methoxyethoxy)ethoxy)ethane (2b). Purified by flash chromatography using
1
DCM/Acetone 9:1 as eluent. 99% yield as a colourless oil. H-NMR (250 MHz, CDCl3) δ (ppm):
3.32–3.36 (m, 5H, –O–CH3 + –O–CH2–CH2–N3), 3.48–3.58 (m, 2H, –O–CH2–CH2–N3), 3.48–3.58 (m,
8H, 2 × –O–CH2–CH2–O–); 13C-NMR (62.9 MHz, CDCl3) δ (ppm): 71.7, 70.5, 70.5, 70.4, 69.9, 58.8,
50.5; ESI-MS (ion trap): m/z 190 [M + H]+. HRMS (ESI+): m/z 190.1207 [M + H]+, calcd for
C7H16N3O3: 190.1192.
13-Azido-2,5,8,11-tetraoxatridecane (3b). Purified by flash chromatography using DCM/Acetone
85:15 as eluent. 97% yield as a colourless oil. 1H–NMR (250 MHz, CDCl3) δ (ppm): 3.32–3.36 (m, 5H,
–O–CH3 + –O–CH2–CH2–N3), 3.48–3.52 (m, 2H, –O–CH2–CH2–N3), 3.58–3.65 (m, 12H, 3 ×
13
–O–CH2–CH2–O–); C-NMR (62.9 MHz, CDCl3) δ (ppm): 71.8, 70.5, 70.5, 70.5, 70.4, 70.3, 69.9,
58.9, 50.5; ESI-MS (ion trap): m/z 234 [M + H]+. HRMS (ESI+): m/z 234.1460 [M + H]+, calcd for
C8H20N3O4: 234.1454.
19-Azido-2,5,8,11,14,17-hexaoxanonadecane (4b). Purified by flash chromatography using
1
DCM/Acetone 8:2 as eluent. 96% yield as a colourless oil. H-NMR (250 MHz, CDCl3) δ (ppm):
3.28–3.32 (m, 5H, –O–CH3 + –O–CH2–CH2–N3), 3.43–3.48 (m, 2H, –O–CH2–CH2–N3), 3.53–3.61 (m,
20H, 5 × –O–CH2–CH2–O–); 13C-NMR (62.9 MHz, CDCl3) δ (ppm): 71.6, 70.4, 70.4, 70.3, 70.3, 70.3,
70.3, 70.3, 70.2, 69.7, 58.7, 50.4; ESI-MS (ion trap): m/z 322 [M + H]+. HRMS (ESI+): m/z 322.1989
[M + H]+, calcd for C13H28N3O6: 322.1978.
3.4.3. General Procedure for the Preparation of Methoxy-oligo(ethylene glycol)-amines (2c–4c)
Triphenylphosphine (10.88 g, 41.5 mmol, 1.25 eq.) in anhydrous THF (25 mL) was added dropwise
to a solution of methoxy-oligo(ethylene glycol)-azide (2b–4b) (33.0 mmol, 1.0 eq.) in anhydrous THF
(25 mL), and the solution was stirred at RT for 5 h. Distilled water (20 mL) was then added and the
mixture was heated under reflux (80 °C) and vigorously stirred overnight. The resulting mixture was
evaporated under reduced pressure and the residue was purified by flash chromatography.
2-(2-(2-Methoxyethoxy)ethoxy)ethanamine (2c). Purified by flash chromatography using
DCM/MeOH = 9:1 (+ 1% Et3N) as eluent. 97% yield as a pale yellow oil. 1H-NMR (250 MHz, CDCl3)
δ (ppm): 1.57 (s, 2H, –CH2–NH2), 2.79 (t, 2H, 3JH–H = 5.25 Hz, –CH2–CH2–NH2), 3.31 (s, 3H, –O–CH3),
3.41–3.51 (m, 4H, –O–CH2–CH2–O– + –O–CH2–CH2–NH2), 3.52–3.63 (m, 6H, –O–CH2–CH2–O– +
–O–CH2–CH2–O–); 13C-NMR (62.9 MHz, CDCl3) δ (ppm): 73.2, 71.6, 70.3, 70.3, 70.0, 58.8, 41.5; ESI-MS
(ion trap): m/z 164 [M + H]+. HRMS (ESI+): m/z 164.1275 [M + H]+, calcd for C7H18NO3: 164.1287.
2,5,8,11-Tetraoxatridecan-13-amine (3c). Purified by flash chromatography using DCM/MeOH = 9:1
(+ 1% Et3N) as eluent. 96% yield as a pale yellow oil. 1H-NMR (250 MHz, CDCl3) δ (ppm): 1.54 (s, 2H,
–CH2–NH2), 2.71 (t, 2H, 3JH–H = 5.25 Hz, –CH2–CH2–NH2), 3.22 (s, 3H, –O–CH3), 3.34–3.41 (m, 4H,

Molecules 2015, 20
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–O–CH2–CH2–O– + –O–CH2–CH2–NH2), 3.45–3.51 (m, 10H, 2 × –O–CH2–CH2–O– + –O–CH2–CH2–O–);
13C-NMR (62.9 MHz, CDCl3) δ (ppm): 40.7, 39.3, 38.0, 37.9, 37.9, 37.9, 37.6, 26.4, 9.1; ESI-MS (ion
trap): m/z 208 [M + H]+. HRMS (ESI+): m/z 208.1552 [M + H]+, calcd for C9H22NO4: 208.1549.
2,5,8,11,14,17-Hexaoxanonadecan-19-amine (4c). Purified by flash chromatography using
DCM/MeOH = 9:1 (+ 1% Et3N) as eluent. 98% yield as a pale yellow oil. 1H-NMR (250 MHz, CDCl3)
δ (ppm): 1.78 (s, 2H, –CH2–NH2), 2.76 (t, 2H, 3JH–H = 5.25 Hz, –CH2–CH2–NH2), 3.26 (s, 3H, –O–CH3),
3.39–3.46 (m, 4H, –O–CH2–CH2–O– + –O–CH2–CH2–NH2), 3.51–3.56 (m, 18H, 4 × –O–CH2–CH2–O– +
13
–O–CH2–CH2–O–); C-NMR (62.9 MHz, CDCl3) δ (ppm): 72.8, 71.6, 70.3, 70.3, 70.3, 70.2, 70.2,
70.2, 70.0, 58.7, 41.4; ESI-MS (ion trap): m/z 296 [M + H]+. HRMS (ESI+): m/z 296.2077 [M + H]+,
calcd for C13H30NO6: 296.2073.
3.4.4. General Procedure for the Preparation of Activated 4-Nitrophenyl Methoxy-oligo(ethylene
glycol) Urethanes (2d–4d)
A solution of methoxy-oligo(ethylene glycol) amine (2c–4c) (8.2 mmol, 1.0 eq.) and DMAP (2.00 g,
16.4 mmol, 2.0 eq.) in acetonitrile (15 mL) was added dropwise to a solution of bis(4-nitrophenyl)
carbonate (2.74 g, 9.0 mmol, 1.1 eq.) in acetonitrile (15 mL) and the resulting solution was stirred at
50 °C for 3 h. The reaction mixture was then diluted in DCM (150 mL) and washed with 0.5 N HCl
(100 mL). The aqueous layer was washed with DCM (5 × 100 mL) and all the organic fractions were
collected, dried over MgSO4 and filtered. The solvent was evaporated under reduced pressure and the
residue was purified by flash chromatography.
4-Nitrophenyl (2-(2-(2-methoxyethoxy)ethoxy)ethyl)carbamate (2c). Purified by flash chromatography
1
using DCM/Acetone = 9:1 as eluent. 87% yield as a pale yellow oil. H-NMR (250 MHz, CDCl3) δ
(ppm): 3.31 (s, 3H, –O–CH3), 3.37–3.43 (m, 2H, –O–CH2–CH2–NH–), 3.49–3.63 (m, 10H, 2 ×
3
–O–CH2–CH2–O– + –O–CH2–CH2–NH–), 6.06 (t, 1H, –NH–, JH–H = 5 Hz), 7.25 (d, 2H, Ar–H,
3JH–H = 9.25 Hz), 8.15 (d, 2H, Ar–H, 3JH–H = 9.25 Hz); 13C-NMR (62.9 MHz, CDCl3) δ (ppm): 155.9,
153.1, 144.3, 124.8, 121.8, 71.6, 70.2, 70.2, 69.9, 69.3, 58.7, 40.8; ESI-MS (ion trap): m/z 329 [M + H]+.
HRMS (ESI+): m/z 329.1357 [M + H]+, calcd for C14H21N2O7: 329.1349.
4-Nitrophenyl 2,5,8,11-tetraoxatridecan-13-ylcarbamate (3c). Purified by flash chromatography using
DCM/Acetone = 85:15 as eluent. 85% yield as a pale yellow oil. 1H-NMR (250 MHz, CDCl3) δ (ppm):
3.17 (s, 3H, –O–CH3), 3.24–3.30 (m, 2H, –O–CH2–CH2–NH–), 3.35–3.50 (m, 14H, 3 × –O–CH2–
CH2–O– + –O–CH2–CH2–NH–), 6.21 (t, 1H, –NH–, 3JH–H = 5 Hz), 7.15 (d, 2H, Ar–H, 3JH–H = 9.25 Hz),
3
8.03 (d, 2H, Ar–H, JH–H = 9.25 Hz); 13C-NMR (62.9 MHz, CDCl3) δ (ppm): 13C-NMR (63 MHz,
CDCl3) δ (ppm) 156.1, 153.2, 144.3, 124.9, 121.9, 71.7, 70.4, 70.3, 70.2, 70.1, 69.4, 58.7, 41.0; ESI-MS
(ion trap): m/z 373 [M + H]+. HRMS (ESI+): m/z 373.1619 [M + H]+, calcd for C16H25N2O8: 373.1611.
4-Nitrophenyl 2,5,8,11,14,17-hexaoxanonadecan-19-ylcarbamate (4c). Purified by flash
chromatography using DCM/Acetone gradient from 8:2 to 6:4 as eluent. 84% yield as a pale yellow
1
oil. H-NMR (250 MHz, CDCl3) δ (ppm): 3.30 (s, 3H, –O–CH3), 3.37–3.50 (m, 4H, –O–CH2–CH2–
NH–), 3.55–3.61 (m, 20H, 5 × –O–CH2–CH2–O–), 6.04 (t, 1H, –NH–, 3JH–H = 5 Hz), 7.26 (d, 2H, Ar–H,

Molecules 2015, 20
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3JH–H = 9.25 Hz), 8.17 (d, 2H, Ar–H , 3JH–H = 9.25 Hz); 13C-NMR (62.9 MHz, CDCl3) δ (ppm): 125.8,
124.9, 121.8, 115.5, 71.6, 70.3, 70.3, 70.3, 70.2, 70.0, 69.3, 58.8, 40.9; ESI-MS (ion trap): m/z 461
[M + H]+. HRMS (ESI+): m/z 461.2137 [M + H]+, calcd for C13H30NO6: 461.2135.
3.4.5. General Procedure for the Preparation of 3,4′,5-N-Monosubstituted-methoxy-oligo(ethylene
glycol) Resveratrol Carbamate Esters (2–4)
A solution of resveratrol (0.24 g, 1.1 mmol, 1.0 eq.) and DMAP (0.52 g, 4.2 mmol, 4.0 eq.) in ACN
(15 mL) was added to a solution of the activated 4-nitrophenyl methoxy-oligo(ethylene glycol)
urethane (2c–4c) (4.8 mmol, 4.5 eq) in ACN (5 mL) and the resulting mixture was allowed to react
under vigorous stirring at 50 °C for 24 h. The reaction mixture was diluted with DCM (150 mL) and
washed with 0.5 N HCl (100 mL). The aqueous layer was washed with DCM (5 × 75 mL) and all the
organic fractions were collected, dried over MgSO4 and filtered. The solvent was evaporated under
reduced pressure and the residue was purified by flash chromatography.
(E)-5-(4-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)carbamate)-1,3-phenylene bis((2-(2-(2-methoxyethoxy)
ethoxy)ethyl)carbamate) (2). Purified by flash chromatography using DCM/Acetone = 6.5:3.5 as
eluent. 75% yield as a pale yellow oil. 1H-NMR (250 MHz, CDCl3) δ (ppm): 3.08 (m, 9 H, 3 × –O–CH3),
3.39 (s, 9H, 3 × –O–CH3) 3.42–3.69 (m, 36H, 6 × –O–CH2–CH2–O– + 3 × –O–CH2–CH2–NH–), 5.80
(m, 3H, 3 × –NH–), 6.86 (t, 1H, 4JH–H = 2.0 Hz, H–4), 6.90–7.12 (m, 6H, H–2, H–3′, H–5′, H–6, H–7,
H–8), 7.45 (d, 2H, 3JH–H = 8.75 Hz, H–2′, H–6′); 13C–NMR (62.9 MHz, CDCl3) δ (ppm): 154.6, 154.3,
151.6, 150.7, 139.1, 133.9, 129.2, 127.4, 127.1, 121.8, 116.3, 114.3, 71.8, 70.5, 70.2, 70.2, 69.8, 59.0,
40.9; ESI-MS (ion trap): m/z 818 [M + Na]+. HRMS (ESI+): m/z 796.3873 [M + H]+, calcd for
C38H58N3O15: 796.3868.
(E)-5-(4-(5,8,11-Tetraoxatridecan-13-ylcarbamate)styryl)-1,3-phenylene bis(2,5,8,11-tetraoxatridecan
-13-ylcarbamate) (3). Purified by flash chromatography using DCM/Acetone = 5:5 as eluent. 88%
1
yield as a colourless oil. H-NMR (250 MHz, CDCl3) δ (ppm): 3.35 (s, 9H, 3 × –O–CH3), 3.41–3.56
(m, 12H, 3 × –O–CH2–CH2–NH–), 3.60–3.68 (m, 36H, 9 × –O–CH2–CH2–O–), 5.81–5.88 (m, 3H,
3
3 × –NH–), 6.80–7.14 (m, 7H, H–2, H–4, H–3′, H–5′, H–6, H–7, H–8), 7.44 (d, 2H, JH–H = 8.75 Hz,
H–2′, H–6′); 13C–NMR (62.9 MHz, CDCl3) δ (ppm): 154.5, 154.2, 151.5, 150.6, 138.9, 133.7, 129.1,
127.3, 126.9, 121.7, 116.2, 114.2, 71.7, 70.4, 70.3, 70.3, 70.1, 70.0, 69.6, 58.8, 40.8; ESI-MS (ion
trap): m/z 928 [M + H]+. HRMS (ESI+): m/z 928.4669 [M + H]+, calcd for C44H70N3O18: 928.4654.
(E)-5-(4-(2,5,8,11,14,17-Hexaoxanonadecan-19-ylcarbamate)styryl)-1,3-phenylene bis(2,5,8,11,14,17-
hexaoxanonadecan-19-ylcarbamate) (4). Purified by flash chromatography using DCM/Acetone from
1
4:6 to 2:8 as eluent. 76% yield as a colourless oil. H-NMR (250 MHz, CDCl3) δ (ppm): 3.33 (s, 9H,
3 × –O–CH3), 3.38–3.51 (m, 12H, 3 × –O–CH2–CH2–NH–), 3.57–3.68 (m, 60H, 15 × –O–CH2–CH2–
O–), 5.80–5.86 (m, 3H, 3 × –NH–), 6.80–7.12 (m, 7H, H–2, H–4, H–3′, H–5′, H–6, H–7, H–8), 7.41
3
(d, 2H, JH–H = 8.75 Hz, H–2′, H–6′); 13C-NMR (62.9 MHz, CDCl3) δ (ppm): 154.4, 154.1, 151.5,
150.6, 138.9, 133.8, 129.1, 127.3, 127.0, 121.7, 116.2, 114.2, 71.7, 70.4, 70.4, 70.3, 70.3, 70.3, 70.2,
69.7, 58.8, 40.9; ESI-MS (ion trap): m/z 1193 [M + H]+. HRMS (ESI+): m/z 1192.6231 [M + H]+,
calcd for C56H94N3O24: 1192.6227.

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4. Conclusions
It was previously shown [47] that the N,N-disubstituited carbamate protecting group is too stable to
regenerate resveratrol at convenient rates under physiological conditions. This work also marks
progress in comparison with ref. [60]. The ether and ester bonds used in [60] were too stable or too
unstable for use in prodrugs, respectively. The only carboxylic ester derivative hydrolyzing relatively
slowly in plasma was the PEG–PLA copolymer conjugate which formed supramolecular structures in
aqueous solution and presumably after i.v. injection [60]. How this construct would behave after oral
or intragastric administration was not tested. The N-monosubstituted carbamate ester derivatives
synthesized herein show, instead, optimal stability for use as prodrugs: high stability under acidic
conditions to withstand passage through the stomach, slow hydrolysis at intestinal pH, and faster
hydrolysis in blood. The selected promoieties, short monodispersed methoxy-oligo(ethylene glycol)
chains, may be a convenient tool for modulating the properties and, thus, improving the performance
of the resulting prodrugs.
Acknowledgments
This work was supported by grants from the Fondazione Cassa di Risparmio di Padova e Rovigo
(CARIPARO) (“Developing a Pharmacology of Polyphenols”), from the Italian Ministry of the
University and Research (PRIN n. 20107Z8XBW_004), and by the CNR Project of Special Interest
on Aging.
Author Contributions
A.M., C.P., M.Z., L.B. designed the study and wrote the manuscript; A.M. carried out the
syntheses; M.A., L.B. performed the hydrolysis studies and pharmacokinetics experiments. All authors
reviewed the manuscript.
Conflicts of Interest
The authors have applied for a patent covering the compounds described in this paper.
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