Click Chemistry-Inspired Synthesis and Photophysical Studies of Calix[4]arene-Cored Galactosylated and Mannosylated Glycodendrimers

A prompt and modular copper(I)-catalyzed azide – alkyne cy- cloaddition ʼ click ʼ approach has been exploited for the synthesis of galactose- and mannose-coated calixarene-cored G 1 generation glycoden- drimers. The developed calixarene glycodendrimers were characterized by using spectral techniques ( 1 H NMR, 13 C NMR and IR). In photophysical evaluation, UV and fluorescence spectra of the developed compounds were recorded in CH 3 CN/H 2 O.


Introduction
Calixarenes are a class of macrocyclic compounds that can serve as building blocks for a wide range of applications in organic synthesis due to their unique three-dimensional architecture and ease of functionalization at the upper and/or lower rim. 1,2 Calixarenes have received great significance as receptors in the synthesis and application of supramolecular scaffolds for molecular recognition, sensing, self-assembling, catalysis, and drug discovery. 3,4 Similar in design to cyclodextrin, these molecules exhibit more diversity and adaptability than cyclodextrin. There are two key factors, i. e., catalysis and reactivity, which have a significant impact on the functional behaviour of host-guest chemistry. 5 These complexes have a great ability to detect and remove heavy metal ions from the environment as well as remediate nuclear waste. 6 Typically, calixareneʼs host chemistry produces stable compounds with biomolecules that are more relevant for supramolecular chemistry. 7 Furthermore, calixarenebased biomimetic compounds are extremely helpful in biotechnology, 8 biosensing and chemosensing technologies, 9 catalysis, 10 gene transport, 11 platonic micelles, 12 chiral molecular recognition, 13 etc.
The Cu(I)-catalyzed azide-alkyne coupling reaction (CuAAC), usually known as the click reaction, is the most powerful and popular tool for the regioselective synthesis of 1,4-disubstituted triazoles. 14 Dondoni efficiently demonstrated this ligation tool for the construction of multiple triazolyl glycoconjugates anchoring to calix[4]arene scaffolds. 3 Several reports have been documented for the synthesis of various glycoclusters using this orthogonal azide-alkyne coupling reaction. 15 Regioselective generated triazoles from azide-alkyne coupling are the bioisostere of the amide functional group. The resulting triazole component has been extensively investigated as a useful pharmacophore and also provides an appropriate binding site for various metal ions in molecular sensing. 16 Multivalent glycan-protein interactions play a pivotal role in most of the biological recognition and dissemination transduction processes, including surface sensing and adhesion by bacteria and viruses, drug effector mechanisms, cellular interactions, cell cycle regulation and differentiation, and cancer cell aggregation as well as its metastatic spread. 17,18 Among the multivalent glycocluster architectures, glycodendrimers have received great attention due to their mono-dispersity, ability to organize their size, and various sugar units at the periphery. 19 The versatile nature of these macrocycles (calixarenes) towards their functionalization at the lower rim offers selective coordination for a plethora of metals ions. For instance, G 1 generation glycodendrimers including mannosylated-and galactosylated-dendritic architectures are designed, synthesized, and well elucidated by NMR, IR, and HRMS spectroscopy. Several designed glycoconjugates and glycodendrimers exhibited a vital role in biological and photophysical processes. [20][21][22] Several sensors have recently been constructed using supramolecular architectures. Macrocycle design scaffolds including cyclodextrins, calixarenes, and rotaxanes are typical examples that provide supramolecular platforms for guest molecules or ions. 23 Among all, calixarene-based sensors are employed to determine the presence of cations and anions using diverse techniques, such as electrochemical and photophysical methods. 23,24 Photophysical methods play an important role in examining and identifying particular ions and molecules. Aggregation-induced emission, photoinduced electron transfer (PET), photoinduced charge transfer, and Förster resonance energy transfer are some of the preferred mechanisms used in the photophysical studies. According to the PET mechanism, when an ion binds to the ionophore, the fluorophore emits a signal. 25 Therefore, considering the importance of glycodendrimers, we made a significant effort to amalgamate the synthetic advantages of calixarenes and the multivalency effect of sugar through click coupling of these two moieties to assemble a new class of p-tert-butyl-calix[4]arene-tethered glycosyl dendrimers. We, herein wish to report the CuAAC click-inspired synthesis of novel triazole-appended glycosyl dendrimers 14 and 15 with their possible photophysical investigations via UV-Vis and fluorescence spectroscopy.

Results and Discussion
Acetylated D-galactose derivative 1, obtained from D-galactose, was used to develop 2,3,4,6-tetra-O-acetyl-D-galactopyranosyl azide 2 by treating it with HBr (33 %) in acetic acid followed by azidation in anhydrous DMF while the corresponding acetylated D-mannose derivative 3 was treated with SnCl 4 and TMSN 3 in anhydrous dichloromethane at room temperature to give 2,3,4,6-tetra-O-acetyl-D-mannopyranosyl azide 4 (Scheme 1). The resulting azido derivatives 2 and 4 were characterized by their NMR ( 1 H and 13 C) spectral data which closely matched with the literature.
Previously synthesised 1,3-dichloro-2-(chloromethyl)-4,5,6-tris(prop-2-yn-1-yloxy)benzene compound 8 was combined with acetylated galactose sugar azide 2 in the presence of CuSO 4 /NaAsc in THF/water (1 : 1) to produce the corresponding first-generation dendritic wedge 9. The appearance of three triazole peaks in the NMR spectra confirmed the presence of compound 9. Further, the azide-functionalized dendritic molecule 10 was produced by treating the chlorine-containing dendritic architecture 9 with NaN 3 in dry DMF for 12 h. The same technique was used to produce the azide-functionalized compound 12, which likewise confirmed to IR and NMR spectra (Scheme 3).

Photophysical Study of Glycodendrimers 14 and 15
The photophysical properties of glycodendrimers 14 and 15 were studied by UV-Vis and fluorescence spectroscopies. Absorption and emission spectra of glycodendrimers 14 and 15 are taken in CH 3 CN/H 2 O (70/30, v/v, at r. t. 25°C). In the UV-Vis absorption spectrum, glycodendrimers 14 showed a band at 275 nm and glycodendrimers 15 showed a band at 280 nm due to π−π * transition. 26 The fluorescence properties of glycodendrimers 14 and 15 revealed the most prominent peak at 350 nm (Figure 1). 26 It is anticipated that the new glycodendrimers would show sensitivity towards metal ions and their sensing properties can be studied.

Conclusions
Conclusively, herein we wish to report the CuAAC click-inspired synthesis of novel triazole-appended glycosyl dendrimers 14 and 15 with their possible photophysical investigations via UV-Vis and fluorescence spectroscopy, where 14 and 15 are found to be UV and fluorescent active dendrimers.

Experimental Section
General Pure analytical grade solvents and reagents were used throughout. 60 F-254 silica gel that had been pre-coated on aluminium plates was utilised for thin layer chromatography (TLC) using a UV light and a 5 % H 2 SO 4 /methanol solution (charring solution). Heating the sample in an alkaline potassium permanganate (KMnO 4 ) solution allowed the alkynes to be detected. The developed chemicals were purified using flash column chromatography and silica gel. The spectra for 1 H and 13 C were captured at 500 MHz and 125 MHz, respectively. At room temperature, all NMR spectra were captured and given in ppm, about deuterated solvents. The resonance multiplicity in the 1 H NMR spectra is described as: 's' (singlet), 'd' (doublet), 'ddʼ (double doublet), 't' (triplet), and ʼm' (multiplet) and residual protic solvent of CDCl 3 ( 1 H NMR, 7.26 ppm; 13 C NMR, 77.0 ppm). IR spectra of the compound were recorded in Nujol mulls in KBr pellet. Absorption spectra were recorded on a 8400S and Systronics double beam UV-Visible spectrometer and emission spectra were recorded on a Fluoromax 4CP plus spectrofluorometer with a 10 mm quartz cell at 25°C.

Procedures General Experimental Procedure for the Cu(I)-Catalyzed Azide-Alkyne Cycloaddition Reaction
CuSO 4 •5H 2 O (0.3 equiv per alkyne), sodium ascorbate (0.3 equiv per alkyne), and alkyne-possessing analogues were agitated in a THF/water (1 : 1) solution at 45°C for 12 h. The reaction was monitored with TLC and after its completion, the reaction mixture was run through celite and extracted with ethyl acetate. Additionally, the organic layers were washed with water (10 mL), saturated aq. NH 4 Cl (2 × 10 mL), and then with brine solution (2 × 10 mL). The reaction mixture was concentrated under decreased pressure and the organic layers were recovered. The organic residue was purified by using flash column chromatography (SiO 2 ) which resulted in a satisfactory yield of the desired dendrons and glycodendrimers.