First class of phosphorus dendritic compounds containing β-cyclodextrin units in the periphery prepared by CuAAC

Article abstract of DOI:10.3390/molecules25184034

A new class of phosphorus dendritic compounds (PDCs) having a cyclotriphosphazene (P₃N₃) core and decorated with six β-cyclodextrin (βCD) units, named P₃N₃-[O-C₆H₄-O-(CH₂)_n-βCD]₆, where n = 3 or 4 was designed, and the synthesis was performed using copper (I) catalyzed alkyne-azide cycloaddition (CuAAC). To obtain the complete substitution of the P₃N₃, two linkers consisting of an aromatic ring and an aliphatic chain of two different lengths were assessed. We found that, with both linkers, the total modification of the periphery was achieved. The two new obtained dendritic compounds presented a considerably high water solubility (>1 g/mL). The compounds comprised in this new class of PDCs are potential drug carrier candidates, since the conjugation of the βCD units to the P₃N₃ core through the primary face will not only serve as surface cover but, also, provide them the faculty to encapsulate various drugs inside the βCDs cavities.

Full text of DOI:10.3390/molecules25184034

molecules
Article
First Class of Phosphorus Dendritic Compounds
Containing β-Cyclodextrin Units in the Periphery
Prepared by CuAAC
Kendra Sorroza-Martínez 1, Israel González-Méndez 1,*, Mireille Vonlanthen 1,
Kathleen I. Moineau-Chane Ching 2,3 , Anne-Marie Caminade 2,3 , Javier Illescas 4
and Ernesto Rivera 1,
*
1
Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior,
Ciudad Universitaria, Mexico City CP 04510, Mexico; kendraivn17@gmail.com (K.S.-M.);
mireille.vonlanthen@gmail.com (M.V.)
CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse,
CEDEX 4, France; kathleen.chane@lcc-toulouse.fr (K.I.M.-C.C.);
2
anne-marie.caminade@lcc-toulouse.fr (A.-M.C.)
3
4
LCC-CNRS, Université de Toulouse, CNRS, 31077 Toulouse, France
Tecnológico Nacional de México/Instituto Tecnológico de Toluca, Avenida Tecnológico S/N Col. Agrícola
Bellavista, Metepec CP 52140, Mexico; fillescasm@toluca.tecnm.mx
Correspondence: israelgonzalezme@gmail.com (I.G.-M.); riverage@unam.mx (E.R.);
Tel.: +52-55-56-22-47-33 (E.R.)
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Jørn Bolstad Christensen
Received: 21 July 2020; Accepted: 28 August 2020; Published: 4 September 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: A new class of phosphorus dendritic compounds (PDCs) having a cyclotriphosphazene (P3N3)
core and decorated with six -cyclodextrin ( CD) units, named P3N3-[O-C6H4-O-(CH2)n- CD]6,
β
β
β
where n = 3 or 4 was designed, and the synthesis was performed using copper (I) catalyzed
alkyne-azide cycloaddition (CuAAC). To obtain the complete substitution of the P3N3, two linkers
consisting of an aromatic ring and an aliphatic chain of two different lengths were assessed. We found
that, with both linkers, the total modification of the periphery was achieved. The two new obtained
dendritic compounds presented a considerably high water solubility (>1 g/mL). The compounds
comprised in this new class of PDCs are potential drug carrier candidates, since the conjugation of
the βCD units to the P3N3 core through the primary face will not only serve as surface cover but, also,
provide them the faculty to encapsulate various drugs inside the βCDs cavities.
Keywords: phosphorus dendritic compounds; β-cyclodextrin; CuAAC reaction
1. Introduction
The use of nanomedicine confers a substantial potentiality for drug delivery and targeted release,
as it increases the safety by reducing toxic effects in nontargeted organs and tissues [1–3]. In recent
years, many researchers have focused on the development of dendrimers, since they have distinctive
properties, such as monodispersity, a large number of easily available functional groups on the surface
and an extraordinary capability to encapsulate host molecules within their hydrophobic environment,
making them ideal nanocarriers for the targeted delivery (with or without ligand) of therapeutic and
diagnostic agents [4–6].
Dendrimers are well-defined hyper-branched three-dimensional macromolecules. Structurally,
dendrimers have a core from which branches (or arms) are derived and end with multiple peripheral
groups that determine their macroscopic properties [7]. Dendrimers can be constructed starting from
the core towards the periphery (divergent synthesis) or through a top-down approach, from the
Molecules 2020, 25, 4034; doi:10.3390/molecules25184034
www.mdpi.com/journal/molecules

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appropriate outer residues (convergent synthesis) [8]. It is clear that, when designing a new dendrimer,
the chemical composition of the core, branches and peripheral groups will have crucial implications
for enhancing possible biomedical applications, either by themselves or as nanocarriers [9].
Among the diverse existing types of dendrimers, phosphorus containing dendrimers,
(i.e., dendrimers having phosphorus derivatives in their structure) play a special role in applications
such as drug delivery, gene therapy and biomaterials, among others [10,11].
In 1994, Caminade et al. described the first method for the synthesis of high-generation phosphorus
dendrimers (PDs), which is still the most widely used, since it allows numerous changes at the
level of the core, branches or terminal groups, conferring biocompatible functions for the use in
biological applications [12]. In this way, PD synthesis can be started from different cores; however,
hexachlorocyclotryphosphazene (P3N3Cl6) is the most used core, since it provides twice the number
of final groups, compared with thiophosphoryl chloride (P(S)Cl3) [10]. Using the P3N3Cl6 core,
modifications to the terminal groups of the PDs can be achieved with different molecules in order to
improve the biocompatible properties. One of the strategies used is the conjugation with different
types of carbohydrates to improve the aqueous solubility and correlate the structures with the impacts
on biological behaviors [13,14]. For instance, the construction of PDs with mannose residues in its
periphery was employed as a strategy for the treatment of lung inflammation [15]. Furthermore,
PDs conjugated with mannopyranoside have been used as multivalent carbohydrate protein-binding
recognition probes [16]. Finally, PDs were used as noncovalent carriers of aminolactitol carbohydrate
for treatment against the HIV virus [17].
With the focus of developing carbohydrate-conjugated dendrimers, diverse modifications of
poly(amidoamines) (PAMAM) dendrimers with cyclodextrin molecules (CDs) have been reported for
multiple applications [18
oligosaccharides, composed of six or more glucopyranose units linked by
Typical native CDs contain six, seven or eight glucose units and are named
Due to the lack of free rotations between glucose units, CDs present a truncated cone shape and a
hydrophobic internal cavity. -cyclodextrin ( CD) is the most studied and frequently used CD, due to
–
21]; however, this strategy has not yet been reported for PDs. CDs are cyclic
(1-4)-glucosidic bonds.
- and -, respectively.
α
α
-,
β
γ
β
β
its low cost, availability and ability to encapsulate a wide range of molecules in its internal cavity.
The variable reactivity of the hydroxyl groups and the remarkable encapsulation properties that can
modify and/or improve the physical, chemical and biological characteristics of host molecules make it
the perfect candidate to be used as building blocks in the construction of drug delivery systems [22–24].
Synthesis mediated by copper-catalyzed azide-alkyne cycloaddition (CuAAC) to produce
1,4-disubstituted 1,2,3-triazoles has been demonstrated to be effective in the design of new dendrimers.
CuAAC coupling confers high specificity, mild reaction conditions and quantitative synthetic yields [25].
The physicochemical properties of the triazole ring are particularly favorable when added to structures
with biological applications, since triazole units act as rigid bonds. Unlike amides, the triazole ring
cannot be hydrolytically cleaved, oxidized or reduced [26].
For all these reasons, in this work, we propose the design, synthesis and characterization of a first
class of phosphorus dendritic compounds (PDCs) decorated with
aliphatic chains as spacers P3N3-[O-C6H4-O-(CH2)n- CD]6 (n = 3 or 4). We selected P3N3 as the core,
and in the first step, alkyne groups were added to the periphery. Finally, the grafting of CD units
βCD units, using aromatic rings and
β
β
was carried out by the CuAAC reaction. The grafting of βCD units on its primary side will allow the
secondary face to be available for the formation of inclusion complexes, so that these new PDCs are
potential candidates as drug nanocarriers and may have applications in nanomedicine.
2. Results and Discussion
2.1. Synthesis
The synthetic route of intermediates HO-C6H4-O-(CH2)n-alkyne (where n = 3 or 4) is shown in
Scheme 1. The intermediates
A and B were prepared via a Williamson etherification reaction between

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the hydroquinone and the respective X-alkynes (X = Cl for n = 3 and I for n = 4), using K2CO3 as a base
and anhydrous N,N-dimethylformamide (DMF) as the solvent at 72 ◦C. Mono and difunctionalized
products were obtained, and they could be separated by column chromatography. Subsequently, the
fraction corresponding to the monofunctionalized product was recrystallized in a cold/hot hexane to
give the compounds A and B [27].
Scheme 1. Synthesis of P3N3-[O-C6H4-O-(CH2)n-βCD]6 (n = 3 or 4) phosphorus dendritic compounds
(PDCs). Conditions: (i) K2CO3, X-alkyne, N,N-dimethylformamide (DMF) anhydrous, 72 ◦C, 36 h.
(ii) (A,B), Cs2CO3, P3N3Cl6, tetrahydrofuran anhydrous (THF) anhydrous, 7 days, room temperature
(RT). (iii) Ts2O/NaOH, H2O, 2 h, RT. (iv) NaN3, DMF anhydrous, 80 ◦C, 48 h. (v) mN3-
H2Asc, dimethylsulfoxide (DMSO):H2O (7:1), 80 ◦C, 7 days.
β
CD, CuSO4 5H2O,
•
The synthetic route of intermediates P3N3-[O-C6H4-O-(CH2)n-alkyne]6 (n = 3 or 4)
C and D is
shown in Scheme 1. The preparation of the intermediates was achieved, as previously reported in the
literature but using phenols with a different substitution pattern [28]. The reaction proceeded through
a substitution reaction of the Cl atoms attached to the P3N3 core; this substitution was carried out in
basic and anhydrous medium using intermediates
The next step in the synthetic route was the formation of mOTs-
), which is shown in Scheme 1. For the synthesis of mOTs- CD, the most efficient technique reported
in the literature was employed to modify a single position of the native CD [29], with minimal
differences in the purification of the final product. It is worth noticing that this synthetic step was
crucial in the design of our PDCs. In particular, the CD tosylation method provided high yields
compared to other previously reported methods [30 31]. Then, the tosyl group of compound was
A
and
B
to obtain compounds
C
and
D, respectively.
β
CD ( ), followed by mN3-
E
βCD
(F
β
β
β
,
E
replaced by an azide group through a nucleophilic substitution reaction to give compound F [32].
Once the necessary intermediates, alkynes (P3N3-[O-C6H4-O-[CH2]n-alkyne]6) and azide
(mN3-βCD) were obtained, the CuAAC reaction was carried out, as shown in Scheme 1. According to
the design of these novel PDCs, we used two linkers with different lengths of the aliphatic chain to
assess whether this factor had an influence on the complete functionalization of all positions of the P3N3
core. To carry out the CuAAC reaction, we used a Cu(I) catalyst synthesized in situ, with CuSO4 as the
copper source and H2Asc as a reducing agent, using dimethylsulfoxide (DMSO) as the ideal solvent,
since it solubilizes the azide and the respective alkynes. To ensure the complete functionalization of the
six core groups of P3N3, an excess of mN3-βCD was added. The final PDCs G and H were purified by
size exclusion chromatography, using water as the eluent, and were obtained in high purity and with
yields of >50%. We found that the length of the linker did not affect the complete functionalization

Molecules 2020, 25, 4034
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of P3N3 with the
proposed linkers.
βCD units, since all six positions of the P3N3 core were functionalized with both
2.2. Characterization
The full characterization of all the synthetic intermediates and final compounds was carried
out. Regarding intermediates and
, in the 1H-NMR spectra (Figures S1 and S4 are available in
A
B
Supplementary Materials (SM)) in DMSO-d6, it is possible to observe the signal corresponding to the
proton of the alkyne group at 2.80 and 2.78 ppm, respectively. Moreover, the signal of the phenol proton
appeared between 8.90 and 8.88 ppm. The structure of intermediates A and B was also confirmed by
the 13C-NMR spectra (Figures S2 and S5 are available in SM), in which the signals corresponding to the
carbons of the alkyne group appeared at 84.56 and 72.34 ppm and at 83.42 and 72.19 ppm, respectively.
The structure of these intermediates was also confirmed by mass spectrometry using the Direct Analysis
in Real Time (DART) technique (Figures S3 and S6 are available in SM). The molecular ions appeared at
177 m/z (A) and 191 m/z (B), which correspond to the molecular weights of the proposed compounds.
The presence of phosphorus in the core allowed simple monitoring by 31P-NMR; furthermore,
this allowed to verify the completion of the reactions in each synthesis step, as well as the integrity
of the entire structure [33]. For the characterization of the intermediates
C and D, a single signal
corresponding to the complete functionalization was observed in the 31P-NMR spectrum (Figures S9
and S13 are available in SM). Furthermore, the complete functionalization of the phosphorous core
was confirmed with the rest of the characterization. In the 1H-NMR spectrum (Figures S7 and S11
are available in SM), the disappearance of the signal corresponding to the phenol proton present
in intermediates
The structure of
A
and
B
, as mentioned above, is clearly observed due to its grafting on P3N3.
C
and
D
was also confirmed by 13C-NMR (Figures S8 and S12 are available in SM)
and Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight (MALDI-TOF) mass spectrometry
(Figures S10 and S14 ESI), where the molecular ions appeared at 1187.623 m/z and 1271.794 m/z,
which correspond to the molecular weights of compounds
C and D, respectively. The structures
of the compounds P3N3-[O-C6H4-O-(CH2)n-alkyne]6 (where n = 3 or 4) were confirmed by all the
characterization data.
The full characterization of two new P3N3-[O-C6H4-O-(CH2)n-βCD]6 (G) and (H) PDCs using
NMR techniques (1H-,13C-NMR and 2D NMR Heteronuclear Multiple-Quantum Correlation (HMQC)
and Homonuclear Correlation Spectroscopy (COSY)) (Figures S15–S17 and S19–S22 are available in SM)
was carried out in DMSO-d6. The classical signals of the linkers and native
with those reported in previous works [17 26]. Moreover, it was possible to observe the differentiation
for some protons of the modified glucopyranose unit of the CD (see Figure 1). The signals of protons
βCD units are in agreement
,
β
H-1’, H-5’ and, particularly, H-6’ appeared at a lower field than those of H-1, H-5 and H-6 of the
nonfunctionalized subunits, due to the change in their chemical environment after the CuAAC reaction.
In the same way, it was possible to identify the H-6” diastereotopic protons, corresponding to one
-CH2-OH fragment contiguous to the substituted one at 3.12 and 2.94 ppm (see HMQC in Figure 2).
Therefore, H-6” and the adjacent OH-6” on the primary face (around 4.33 ppm) appeared significantly
upfield-shifted in comparison to their respective analogs H-6 and OH-6 because of the change in
chemical environment due to the neighboring substitution. In this step, 31P-NMR was used to assess
the completion of the reactions and to assure the complete functionalization of the six positions of
P3N3. This was confirmed by the appearance of a single signal (at 9.31 ppm) in the 31P-NMR spectrum
of P3N3-[O-C6H4-O-(CH2)n-
compared to the signal corresponding to P3N3-[O-C6H4-O-(CH2)n-alkyne]6 (where n = 3, compound
(at 9.93 ppm), because P3N3 is protected by CD molecules (Figure 3). The same behavior was
observed in the 31P-NMR spectrum (Figure S24 is available in SM) of P3N3-[O-C6H4-O-(CH2)n-
CD]6
(where n = 4, compound ). Finally, the structures of these new dendrimers were corroborated by
MALDI-TOF mass spectrometry (Figures S18 and S23 are available in SM). The molecular ions appeared
mainly at 8169.59 m/z and at 8254.30 m/z, corresponding to the molecular weights of the PDCs
βCD]6 (where n = 3, compound G). This signal exhibited an upfield shift
C
)
β
β
H
G

Molecules 2020, 25, 4034
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and
and
H
H
, respectively. Furthermore, the signals at 7034.596 m/z and 7120.029 m/z for compounds
, respectively, correspond to partial ionization, since complete ionization of the molecule is
G
complex when using this technique. This difficulty has been previously reported for other phosphorus
dendrimers [34].
Figure 1. Assignation of the protons in the NMR of P3N3-[O-C6H4-O-(CH2)3-βCD]6 dendritic
compounds (PDCs).
Figure 2.
2D NMR Heteronuclear Multiple-Quantum Correlation (HMQC) spectra of
P3N3-[O-C6H4-O-(CH2)3-βCD]6 PDCs in DMSO-d6.

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1)
2)
Figure 3. Split of 31P-NMR in DMSO-d6 of (
(2) P3N3-[O-C6H4-O-(CH2)3-βCD]6 (PDC G).
1) P3N3-[O-C6H4-O-(CH2)3-alkyne]6 (compound C) and
2.3. Determination of Water Solubility for P3N3-(O-C6H4-O-(CH2)n-βCD)6 PDCs
Among all the special properties of dendrimers, their high solubility makes an important difference
compared to linear polymers. Differences in solubility between hyper-branched and linear polymers
can be of several orders of magnitude, up to 106 times, in some cases [35]. Among all the solvents
used to dissolve dendrimers, water is especially important when considering biological applications.
In recent years, the importance of water-soluble phosphorous-containing dendrimers has increased.
In most cases, the water solubility of these compounds was dependent mainly on the reactivity of
terminal groups in the periphery [36]. Therefore, the determination of the water solubility of our
new PDCs was carried out, according to a method previously reported in the literature [37]. It was
found that, for both PDCs, the solubility in water was >1g/mL. These results represent a considerably
higher solubility compared to the solubility of native
CD derivatives, such as sulfobutylether- CD (>500 mg/mL), O-methyl-
2-hydroxypropyl- CD (>600 mg/mL) [38]. Water-soluble phosphorous-containing dendrimers with
cationic or anionic end groups have been reported; nevertheless, their water solubility was dependent
on the pH [17 35]. On the contrary, the advantage of our new P3N3-[O-C6H4-O-(CH2)n- CD]6 PDCs
βCD (18.5 mg/mL) and of other commercial
β
β
βCD (>500 mg/mL) and
β
,
β
(n = 3 and 4) is that their solubility in water is conferred by the βCD units in standard conditions.
3. Materials and Methods
3.1. General Notes
All starting materials were commercially available reagent grade and were used without any
further purification. Hexachlorocyclotriphosphazene (P3N3Cl6), 5-chloro-1-pentyne, 6-iodo-1-hexyne,
hydroquinone, β-cyclodextrin (βCD), p-toluenesulfonyl chloride (Cl-Ts), p-toluenesulfonic acid (OH-Ts),
N,N-dimethylformamide anhydrous (DMF), potassium carbonate (K2CO3), cesium carbonate (Cs2CO3),
dimethylsulfoxide (DMSO), ascorbic acid (H2Asc), Bio-Gel P-10 medium from Bio-Rad (Hercules,

Molecules 2020, 25, 4034
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CA, USA), potassium iodide (KI), sodium azide (NaN3), sodium hydroxide (NaOH), copper sulfate
pentahydrate (CuSO4 5H2O), diisopropyl ether, dichloromethane (CH2Cl2), ethyl acetate (EtOAc),
•
tetrahydrofuran anhydrous (THF), hexane (HEX), methanol (MeOH), ethanol (EtOH) and acetone
(CH3COCH3) were purchased from Sigma-Aldrich (St. Louis, MO, USA).
3.2. Synthetic Procedures
3.2.1. Synthesis of HO-C6H4-O-(CH2)n-alkyne (n = 3 and 4) A and B
The synthesis of the intermediates
A and B was carried out according to a previously reported
procedure, with some modifications [27 28]. A dry hydroquinone solution (24.03 mmol) in DMF (250 mL)
,
was refluxed for 30 min at 72 ◦C, K2CO3 (30.04 mmol) was added and the mixture was refluxed for 1 h.
To this mixture, alkyne (12.02 mmol) was added dropwise over 2 h. The resulting mixture was refluxed
for 36 h, then cooled to 25 ◦C and filtered. The filtrate was evaporated under reduced pressure. The
resulting brown oil was dissolved in CH2Cl2 (150 mL), and the solution was extracted with water
(3
×
50 mL), the organic phase was dried with anhydrous Na2SO4 and the solvent was evaporated.
The crude product was composed of a mixture of unreacted hydroquinone, the monofunctionalized
and the difunctionalized products, which were separated by column chromatography on silica gel
using hexanes:EtOAc (8:2). Once the fraction corresponding to the monofunctionalized product was
obtained, it was recrystallized from hot/cold hexane, and the final product was recovered by filtration;
the solid was left to dry overnight under vacuum. HO-C6H4-O-(CH2)3-alkyne was obtained as a beige
solid (7.17 mmol, 29%). HO-C6H4-O-(CH2)4-alkyne was obtained as a yellow solid (7.88 mmol, 66%).
1
HO-C6H4-O-(CH2)3-alkyne. H-NMR (400 MHz, DMSO-d6,
δ
ppm): 8.90 (s, 1H, PhOH), 6.76 (d, J = 9 Hz,
C-H),
ppm):
2H, Ha), 6.68 (d, J = 9 Hz, 2H, Hb), 3.93 (t, J = 2.4 Hz, 2H, Hc), 2.80 (t, J = 2.4 Hz, 1H, C
≡
2.31 (t, J = 2.4 Hz, 2H, He), 1.86 (t, J = 2.4 Hz, 2H, Hd); 13C-DEPTQ NMR (101 MHz, DMSO-d6,
δ
152.01, 116.50, 116.20, 84.56, 72.34, 67.15, 28.67, 15.29. DART-MS: 177 m/z (M+H)+; 178 m/z (M + 2H)+.
1
HO-C6H4-O-(CH2)4-alkyne. H-NMR (400 MHz, DMSO-d6,
δ
ppm): 8.88 (s, 1H, PhOH), 6.75 (d, J = 9 Hz,
C-H),
2.22 (t, J = 2.4 Hz, 2H, Hf), 1.75 (t, J = 2.4 Hz, 2H, Hd), 1.58 (t, J = 2.4 Hz, 2H, He); 13C-DEPTQ NMR
2H, Ha), 6.67 (d, J = 9 Hz, 2H, Hb), 3.88 (t, J = 2.4 Hz, 2H, Hc), 2.78 (t, J = 2.4 Hz, 1H, C
≡
(101 MHz, DMSO-d6,
δ ppm): 152.19, 151.88, 116.24, 116.14, 83.42, 72.19, 68.10, 28.75, 25.49, 18.26.
DART-MS: 191 m/z (M)+; 192 m/z (M + H)+.
3.2.2. Synthesis of P3N3-[O-C6H4-O-(CH2)n-alkyne]6 (n = 3 and 4) C and D
The synthesis of intermediates
procedure, with some modifications [39]. A solution of intermediate
C
and
D
was carried out according to a previously reported
or (2.84 mmol) in dry THF
A
B
(25 mL) was stirred for 20 min. Cs2CO3 (5.68 mmol) was added to this solution and stirred for
1 h. Afterwards, P3N3Cl6 (0.32 mmol) was added to the reaction mixture, and it was left stirring at
room temperature for 7 days, until the reaction was complete, monitored by 31P-NMR. The reaction
mixture was then centrifuged at 10,000 rpm for 20 min to remove inorganic salts, the supernatant was
recovered and the solvent was evaporated under reduced pressure. The crude product contained
fully functionalized P3N3-[O-C6H4-O-(CH2)n-alkyne]6 and the unreacted monoalkynes, which were
separated by column chromatography on silica gel using CH2Cl2 as the eluent. Once the fraction
corresponding to the functionalized P3N3 was obtained, it was recrystallized from cold isopropyl ether.
P3N3-[O-C6H4-O-(CH2)n-alkyne]6 (n = 3 or 4) were obtained as white solids (0.19 mmol, 59% and
0.12 mmol, 52% for C and D, respectively).
1
P3N3-[O-C6H4-O-(CH2)3-alkyne]6. H-NMR (400 MHz, DMSO-d6,
δ
ppm): 6.80 (d, J = 9.2 Hz, 12H,
), 2.81 (t, J = 2.41 Hz, 6H, C C- ),
d
); 13C-DEPTQ NMR (101 MHz, DMSO-d6,
H
a), 6.76 (d, J = 9.2 Hz, 12H, H
b
), 4.00 (t, J = 2.5 Hz, 12H, H
2.34 (t, J = 2.41 Hz, 12H, H ), 1.90 (t, J = 2.41 Hz, 12H, H
c
≡
H
e

Molecules 2020, 25, 4034
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δ
ppm): 156.34, 144.23, 122.21, 115.83, 84.36, 72.40, 67.11, 28.56, 15.31; 31P-NMR (162 MHz, DMSO-d6,
δ ppm): s, 9.93. MALDI-TOF-MS: 1187.62 m/z (M)+.
1
P3N3-[O-C6H4-O-(CH2)4-alkyne]6. H-NMR (400 MHz, DMSO-d6,
δ
ppm): 6.78 (d, J = 9.2 Hz, 12H,
Ha), 6.72 (d, J = 9.2 Hz, 12H, Hb), 3.94 (t, J = 2.5 Hz, 12H, H
2.24 (t, J = 2.41 Hz, 12H, H ), 1.80 (m, 12H, H ); 1.61 (m, 12H, H
DMSO-d6,
ppm): 156.46, 144.19, 122.20, 115.77, 85.01, 72.17, 68.08, 28.64, 25.48,18.28; 31P-NMR
c
), 2.78 (t, J = 2.41 Hz, 6H, C
≡
C-H),
f
d
e
); 13C-DEPTQ NMR (101 MHz,
δ
(162 MHz, DMSO-d6, δ ppm): s, 10.01. MALDI-TOF-MS: 1271.79 m/z (M + H)+.
3.2.3. Synthesis of 6-O-monotosyl-β-cyclodextrin (mOTs-βCD) E
Following the procedure previously reported [29], in a round-bottom flask, p-toluenesulfonic
acid (0.0101 mol) and p-toluenesulfonyl chloride (0.039 mol) were dissolved in 50 mL of CH2Cl2 and
stirred at room temperature for 12 h. Then, the reaction mixture was filtered, and the filtrate was
concentrated under reduced pressure. The residue was recrystallized (3
the product (Ts2O) was allowed to dry under vacuum. Afterwards, in a round-bottom flask,
×
) with cold hexane, and
CD
β
(0.0051 mol) and Ts2O (0.0076 mol) were dissolved in 150 mL of H2O. The reaction mixture was stirred
for 2 h at room temperature; after this time, a 2.5-M aqueous NaOH solution was added. The reaction
mixture was stirred for 10 min. Subsequently, the mixture was filtered, and the pH of the filtrate was
adjusted to 8 with a saturated solution of ammonium chloride to give a precipitate. The mixture was
filtered, and the precipitate was recrystallized (3
solid (0.0021 mol, 42%).
×
) in acetone. The product E was obtained as a white
1H-NMR (400 MHz, DMSO-d6,
δ
ppm): 7.75 (b, 2H) a, 7.44 (b, 2H) b, 5.83 (d, J = 6.4 Hz, 1H) OH20,
5.78 (b, 6H) OH2, 5.71 (b, 7H) OH3, 4.84 (d, J = 3.9 Hz, 6H) H1, 4.76 (d, J = 3.9 Hz, 1H) H10, 4.50 (m, 6H)
OH6, 4.35 (m, 2H) H60ab, 4.19 (m, 1H) H50, 3.65 (m, 12 H) H6ab, 3.60 (b, 7H) H3, 3.51 (m, 7H) H5,
3.30 (m, 7H) H2, 3.22 (m, 7H) H4, 2.42 (d, 3H) c. 13C-NMR (100 MHz, DMSO-d6,
δ ppm): 145.25,
133.09, 130.29, 128.03, 102.39, 101.74, 81.95, 81.21, 73.43, 73.16, 72.84, 72.48, 70.17, 69.36, 60.28, 21.62.
MALDI-TOF-MS (m/z): 1311.591 (M + Na)+.
3.2.4. Synthesis of 6-O-monoazido-β-cyclodextrin (mN3-βCD) F
Following a previously reported procedure [32], in a round-bottom flask, mOTs-βCD (0.0016 mol),
NaN3 (0.005 mol) and KI (0.0008 mol) were dissolved in 8 mL of anhydrous DMF. The reaction mixture
◦
was stirred at 80 C for 48 h. After this time, DMF was evaporated under reduced pressure, and
the residue was recrystallized in a mixture, H2O:Acetone (1:1), and allowed to dry under vacuum.
The product F was obtained as a white solid (0.0014 mol, 88%).
1H-NMR (400 MHz, DMSO-d6,
δ ppm): 5.74 (m, 7H) OH2, 5.67 (m, 6H) OH3, 5.62 (d, J = 2.4 Hz, 1H)
OH30, 4.88 (d, J = 3.5 Hz, 1H) H10, 4.83 (m, 6H) H1, 4.48 (m, 6H) OH6, 3.77 (m, 2H) H60, 3.68 (m, 12H)
H6, 3.60 (m, 7H) H3, 3.55 (m, 7H) H5, 3.39 (m, 7H) H4, 3.29 (m, 7H) H2. 13C-NMR (100 MHz, DMSO-d6,
δ
ppm): 102.38, 102.04, 83.41, 81.99, 73.50, 73.30, 72.85, 72.67, 72.46, 70.63, 60.30, 51.53. MALDI-TOF-MS
(m/z): 1182.764 (M + Na)+.
3.2.5. Synthesis of P3N3-[O-C6H4-O-(CH2)n-βCD]6 PDCs (n = 3 or 4) G and H
P3N3-(O-C6H4-O-(CH2)n-alkyne)6 (n = 3 or 4) (0.055 mmol) and mN3-
dissolved in DMSO (5 mL); this mixture was degassed by bubbling argon for 10 min. A solution of
CuSO4 5H2O (0.055 mmol) in a DMSO:H2O mixture (0.5:0.5 mL), followed by a solution of H2Asc
βCD (0.496 mmol) were
•
(0.165 mmol) in DMSO:H2O mixture (0.5:0.5 mL), were added dropwise over 5 min. The reaction
mixture was heated to 80 ◦C with vigorous stirring and under the argon atmosphere for 7 days, until the
reaction was complete, monitored by 31P NMR. At the end of this time, the reaction mixture was cooled
and precipitated dropwise into cold acetone (200 mL). The precipitate was filtered under vacuum.

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The obtained solid was purified by size exclusion chromatography using Bio-Gel® P-10 medium
and water as the eluent. Once the corresponding fractions were obtained, they were lyophilized to
evaporate the water. PDCs
G and H were obtained as white solids (0.041 mmol, 61% and 0.048 mmol,
88% for G and H, respectively).
P3N3-[O-C6H4-O-(CH2)3-β δ ppm): 7.82 (s, 6H, H-triazole),
CD]6. 1H-NMR (400 MHz, DMSO-d6,
6.87 (s, 24H, H-a, b), 5.87 (m, 7H, OH-2’), 5.78 (s, 36H, OH-2), 5.70–5.66 (m, 42H, OH-3,3’), 5.05 (s, 7H,
H-1’), 4.84–4.78 (m, 42H, H-1; H-6’), 4.56–4.50 (m, 36H, H-6’; OH-6), 4.33 (s, 6H, OH-6”), 4.02 (m, 18H,
H-c; H-5’), 3.65–3.59 (m, 138H, H-6, H-3,3’, H-5), 3.41–3.28 (m, 84 H, H4,4’, H-2,2’ overlapped with
H2O), 3.12 (m, 7H, H-6”), 2.94 (m, 7H, H-6”), 2.77 (m, 12H, H-e), 2.05 (m, 12H, H-d); 13C-DEPTQ NMR
(101 MHz, DMSO-d6, δ ppm): 156.55, 144.37, 123.37, 122.15, 115.93, 102.98, 102.02, 84.28, 82.36, 73.05,
72.88, 70.72, 68.13, 60.90, 59.80, 50.37, 29.42, 22.51. 31P-NMR (162 MHz, DMSO-d6,
MALDI-TOF-MS: 8169.59 m/z (M + K)+.
δ ppm): s, 9.31.
P3N3-[O-C6H4-O-(CH2)4-β δ ppm): 7.78 (s, 6H, H-triazole),
CD]6. 1H-NMR (400 MHz, DMSO-d6,
6.83 (s, 24H, H-a, b), 5.88 (m, 7H, OH-2’), 5.78-5.72 (s, 36H, OH-2), 5.70-5.63 (m, 42H, OH-3,3’),
5.05 (s, 7H, H-1’), 4.84-4.78 (m, 42H, H-1; H-6’), 4.58-4.49 (m, 36H, H-6’; OH-6), 4.34 (s, 6H, OH-6”),
3.96 (m, 18H, H-c; H-5’), 3.65-3.59 (m, 138H, H-6, H-3,3’, H-5), 3.41-3.28 (m, 84 H, H4,4’, H-2,2’
overlapped with H2O), 3.12 (m, 7H, H-6”), 2.94 (m, 7H, H-6”), 2.66 (m, 12H, H-f), 1.76 (m, 24H, H-d, e);
13C-DEPTQ NMR (101 MHz, DMSO-d6,
δ ppm): 156.56, 147.25, 123.86, 122.11, 115.96, 102.98, 101.98,
84.18, 82.80, 82.33, 81.62, 73.94, 72.91, 68.38, 60.73, 59.77, 50.79, 29.27, 26.29, 22.50. 31P-NMR (162 MHz,
DMSO-d6, δ ppm): s, 9.42. MALDI-TOF-MS: 8254.30 m/z (M + K)+.
3.3. Characterization
1H-, 13C- and 31P-NMR measurements were carried out on a Bruker spectrometer (Bruker, Beerlika,
MA, USA) (400 MHz) using DMSO-d6 as solvent. Chemical shifts are reported in ppm (δ), and the
signals were described as singlet (s), doublet (d), triplet (t) and multiplet (m); coupling constants
(J) were reported in Hz. DART and MALDI-TOF mass measurements were performed on a JEOL
JMS-AX505-HA instrument (Peabody, MA, USA) and on a Bruker Daltonics Flex Analysis instrument
(Bruker, Beerlika, MA, USA), respectively. Matrices of 2,5-dihydroxybenzoic acid (DHB), dithranol
(DIT) and 2-acetylphloroglucinol (THAP) were used for MALDI-TOF.
3.4. Determination of Water Solubility for P3N3-[O-C6H4-O-(CH2)n-βCD]6 PDCs
The determination of the solubility of the P3N3-[O-C6H4-O-(CH2)n-βCD]6 PDCs was carried out
according to a method previously reported by Jozwiakowski and Connors [36], with modifications.
One gram of the compound was placed in three amber vials of 5 mL with a screw cap, and 1 mL of
water was added. The vials were sealed with parafilm to avoid water evaporation. They were stirred
in an oil bath at a constant temperature of 25
the solid phase by filtration through a Milli-Q membrane (pore size of 0.45
±
0.01 ◦C for 48 h. The supernatant was separated from
m) by injection of the
µ
mixture into disposable plastic syringes (Franklin Lakes, NJ, USA) of 3 mL at 25 ◦C. The supernatant
of each sample was placed in three different vials. The samples were lyophilized for 48 h, and the
obtained solids were weighed on a scale with an uncertainty of ±0.0001 g.
4. Conclusions
In this work, two novel phosphorus dendritic compounds (PDCs) containing a P3N3 as the core
and βCD units as terminal groups were designed. The synthesis was carried out using the CuAAC
reaction, giving high yields and products that were purified by a simple method. The complete
functionalization of the P3N3 core of the new molecules was carried out using two aliphatic chains
of different lengths as the linker. We found that there was no significant impact of the aliphatic
chain length, since, with both linkers, the functionalization was complete. This is the first report for

Molecules 2020, 25, 4034
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PDCs of the type P3N3-[O-C6H4-O-(CH2)n-
βCD]6 arising from a novel design of water-soluble PDCs.
The potential use of these new dendrimers as drug carriers for biomedical applications is currently
under study.
Supplementary Materials: The following are available online: NMR & MS spectrum of the compounds.
Author Contributions: E.R.; I.G.-M.; K.I.M.-C.C. and A.-M.C.—design of the dendrimers. K.S.-M.—synthesis
of the dendrimers. K.S.-M. and I.G.-M.—characterization of the dendrimers. K.S.-M. and I.G.-M.—contributed
to manuscript writing. J.I.—data curation. E.R.; M.V.; J.I.; K.I.M.-C.C. and A.-M.C.—edited and revised the
manuscript. E.R.—supervision. All authors have read and agreed to the published version of the manuscript.
Funding: The financial support of the Quebec-Mexico Bilateral Research Program from FQRNT of Quebec
and CONACYT of Mexico (Project 279380) is gratefully acknowledged. KS-M (No. 604166/771826) and IG-M
(No. 510108/288832) are very grateful with Posgrado en Ciencias Qu
granted fellowships.
ímicas UNAM and CONACyT for the
Acknowledgments: We thank Gerardo Cedillo, Lucero R
í
os, Mar
í
a del Carmen Garc
í
a Gonz lez and Francisco
á
Javier Pérez Flores for their assistance with NMR and mass spectrometry measurements, respectively.
Conflicts of Interest: The authors declare no conflict of interest.
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2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).