Synthesis of C 2 -symmetric carbohydrate-based macrocycles by introduction of methylene/ p -xylene linkers

Over the last few decades many carbohydrate embedded macrocycles have drawn the attention of synthetic organic chemists because of their interesting chemical and biological properties. Suitably placed chiral 3-and 5-hydroxyl groups of 1,2-O -(1-methylethylidene)- α - D -xylofuranose, derived from inexpensive D -glucose, have been judiciously exploited to generate different non-natural C 2 -symmetric carbohydrate embedded macrocycles. Different symmetric aromatic and aliphatic hydrophobic spacers have been introduced between highly functionalized cluster of carbohydrate chiral centres through different intra-and intermolecular nucleophilic substitution and ring closing metathesis (RCM) for synthesis of 17 to 24 membered C 2 -symmetric macrocycles.


Introduction
Macrocycles, a class of compounds containing at least one large ring of more than 12 atoms, have gradually evolved as an important class of compounds due to their high prevalence in natural products, 1,2 medicinally important molecules, [3][4][5] and their usefulness in the field of catalysis, [6][7][8] materials science, [9][10][11] and sensing applications. 12,13Currently there are more than 100 natural and synthetic chiral macrocyclic drugs in the market and many of these are present in current blockbuster drug list. 3Macrocycles, with relatively large flexible ring and distributed binding sites, were found to be efficient in interacting with the extended shallow and relatively featureless binding surfaces of complex protein-protein interactions (PPIs) [14][15][16][17] and consequently well suited to target conventionally 'undruggable' 18 disease classes. 4,19,20Furthermore, the flexible cyclic structure of macrocycles was found to favourably impact their membrane permeability, 22 metabolic stability and overall pharmacokinetics. 21,22In spite of these favourable essential drug like properties, macrocycles are under represented in current drug repertoire. 20Relatively lesser abundance in natural resources (compared to the traditional small molecules), difficulty and expenses involved in synthesis/purification (natural or synthetic) of these flexible chiral rings in stereoisomerically pure form and spectroscopically determining their final structures with absolute stereochemistry, are among few major reasons behind such underrepresentation. 20Judicious exploitation of inexpensive, naturally abundant carbohydrates as starting materials for their synthesis can conveniently overcome most of these challenges.Growing interest in macrocycles in the last few decades have resulted in many new synthetic methods, [22][23][24][25][26][27][28] though carbohydrate based synthetic endeavours remain rare.This observation inspired us to synthesize some non-natural macrocyclic structures containing embedded carbohydrates via a simple synthetic methodology.In our work, we have synthesised macrocyclic structures containing two carbohydrate units tethered by some groups so that they can easily conjugate with other important biological 4 molecules leading to biological relevant materials.The said macrocyles can easily be converted to nucleosides 25 by following a known protocol of nucleosidation.The C2symmetric molecules thus formed will be very good precursors for synthesising 20 novel chiral ligands in asymmetric catalysis.Herein we have presented synthesis of some symmetric macrocycles starting from inexpensive D-glucose.For synthesis of the C2-symmetric macrocycles, we planned to use 3-and 5-hydroxyl groups of 1,2-O-(1methylethylidene)-α-D-xylofuranose.However, to avoid the problem of competitive reactivity of the hydroxyl groups in such system we wanted to use easily preparable stable D-glucose derivative 1,2:5,6-Di-Oisopropylidene-α-D-glucofuranose with single free hydroxyl group as our starting material.We have targeted to exploit this free C-3 hydroxyl group to symmetrically bind to symmetric electrophilic spacer through easy nucleophilic substitution.Once the symmetrically protected dicarbohydrate system is generated, deprotection of 5, 6-hydroxyl should generate corresponding di-ol in both side of the carbohydrate residue.The diols then can be cleaved and reduced to corresponding hydroxy compound.These hydroxy moiety can now easily be used for synthesis of various C2-symmetric macrocycles applying various symmetric synthetic operation.

Results and Discussion
Keeping all these facts in mind we have started our synthesis with 1,2:5,6-Di-O-isopropylidene-α-Dglucofuranose (1), which can be easily synthesized in one step from D-glucose and acetone in presence of concentrated sulfuric acid as a catalyst.For the synthesis of our target molecule CSM-1, we first reacted 1,2:5,6-Di-O-isopropylidene-α-D-glucofuranose (1) with dichloromethane in presence of NaOH to form the compound 2 with an excellent yield of 95%.Successive 5, 6-deprotection, oxidation and finally reduction gave compound 5. 30 A 24 membered C2-symmetric macrocycle CSM-1 (59%) was obtained from 5 following the same etherification technique (Scheme 1).The symmetric character of the product CSM-1 was indicated by the presence of one set of peaks due to symmetry of the related protons and carbon atoms in the 1 H-NMR and 13 C-NMR spectra.After the successful synthesis of the 24-membered macrocycle CSM-1, we have synthesized macrocycle CSM-2 by simple reaction of 1,4-bis(bromomethyl)benzene (6) 31,32 with precursor 5 in presence of 50% aqueous NaOH and tetrabutylammonium bromide (TBAB) in dichloromethane with a yield of 65%.The insertion of the aromatic group in the compound CSM-2 was confirmed by the appearance of 4H multiplet, δ value 7.45 -7.35 ppm.Also, appearance of another singlet at 3.14 ppm for the CH2 attached with the oxygen and the aromatic ring confirms the formation of the compound CSM-2.The HRMS (TOF-MS US, positive ion) of CSM-2 showed a sodiated molecule peak at m/z 517.2051 [M + Na] + confirming its said structure.

Scheme 2. Synthesis of C2-symmetric 17 membered macrocycle CSM-2.
One p-xylene moiety was introduced between two diacetone-D-glucose moieties by reaction between 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose (1) and 1,4-bis(bromomethyl)benzene (6) in basic medium in presence of tetrabutylammonium bromide (TBAB).Compound 8 was then subjected to a series of reaction (deprotection, oxidation, reduction) yielding the corresponding C2-symmetric hydroxy intermediate (10). 33agent and conditions: a. 50% aq.NaOH, CH2Cl2, TBAB, 1,4-bis(bromomethyl)benzene ( 6 Final ring closing was done by introducing the second p-xylene moiety by treatment of 10 with 6 under basic medium in presence of tetrabutylammonium bromide (TBAB).So, the attempted synthesis of 22 membered C2-symmetric macrocycle CSM-3 became successful.The formation of compound CSM-3 was confirmed by the appearance of two singlet peaks for eight aromatic hydrogens at δ value 7.32 ppm and 7.05 ppm in the aromatic region of the 1 H-NMR spectrum which is further supported by the appearance of corresponding peak of aromatic carbon atoms in the 13 C-NMR in the range of 127-137 ppm.The symmetrical nature of the compound CSM-3 was evident from a single set of protons and carbon atom in the 1 H-NMR and 13 C-NMR spectra.Further the formation of compound CSM-3 confirmed by mass spectra analysis for which we got the m/z [M + Na] + value 607.2515 that exactly matches with the calculated molecular mass.

Scheme 4. Synthesis of C2-symmetric 20 membered macrocycle CSM-4.
In a similar manner the free hydroxyl groups of 10 was first allylated using allyl bromide to obtain the corresponding allylated compound 11 which on olefin-metathesis reaction [34][35][36] using Grubbs 2 nd generation 37 catalyst finally affords novel C2-symmetric 20 membered macrocycle (CSM-4).The formation of compound 11 was confirmed by appearance of characteristics allylic proton signals at δ value 5.94 ppm in 1 H-NMR.The formation of compound CSM-4 is confirmed from m/z value of 557.2374 in mass spectrum that exactly matches with the calculated molecular mass of the compound.

Conclusions
Four different C2-symmetric macrocycles with varied ring size (17 to 24 membered) have been synthesized from inexpensive easily available stable carbohydrate derived precusor 1,2:5,6-di-O-isopropylidene-α-Dglucofuranose.The symmetric nature of final macrocycles were established by nuclear magnetic spectra that show one set of protons and carbons.Our future goal is to convert these sugar appended C2-symmetric macrocycles into nucleosides and other biomolecular conjugates which are very important from biological point of view.These macrocycles contain many protected hydroxyl groups which on deprotection will increase their aqueous solubility and in consequence their bioavailability.This simple but potentially useful methodology for synthesis of carbohydrate based C2-symmetric macrocycles will provide a tool to the synthetic organic chemist for synthesizing many aforesaid important molecules.

Experimental Section
General.Unless otherwise mentioned NMR spectra were recorded on BRUKER AVANCE III 400 (400 MHz for 1 H; 100 MHz for 13 C) spectrometer.The chemical shifts (δ) are reported in parts per million (ppm) multiplicity (s=singlet, d=doublet, t=triplet, q=quartet, p=pentet and m=multiplet), coupling constant (J values) in Hz relative to CDCl3 (7.28 ppm for 1 H and 77.00 ppm for 13 C central peak).The HRMS (HRMS Model Name: Waters -Xevo G2-XS -QToF) spectra were recorded in TOF-MS (US+).Reactions were monitored by thin-layer chromatography using E. Merck Silica Gel 60 F254 percolated plates.Organic extracts were dried over anhydrous sodium sulfate.Unless otherwise mentioned, 60-120 mesh silica gel was used for column chromatography.Solvents were distilled and dried prior to use.Petroleum ether refers to a fraction boiling between 60-80 o C.