Chemical synthesis of small-ring cyclic oligosaccharides

Immense interest on studies and applications of naturally-occurring cyclic oligosaccharides has attracted novel methods and ingenuities in their chemical synthesis. The conformation of the monosaccharides constituting the macrocyles is important, in order to permit cyclization and facilitate the macrocyclic supramolecular properties. Synthesis of small-ring cyclic oligosaccharides combining the structural and functional features continues to be a challenge. This review article assesses the current chemical methods for the synthesis of small-ring cyclic oligosaccharides, particularly, macrocycles that possess five or fewer monomer units constituting the macrocycle. Methods to retain the stable conformation of individual sugar units on small-ring macrocyles and the benefits to supramolecular properties are discussed.


Introduction
2][3][4] The cylindrical structures that are comprised of a sugar wall, decorated further with polar hydroxy groups at the top and bottom of the wall.The structural features of the cylindrical macrocycles are characterized further by regions of apolar and polar surfaces.Prominent among the cyclic oligosaccharides is the oligo-cyclo-glucosides, generically called as cyclodextrins, wherein glucopyranoside oligomers cyclize with the aid of interglycosidic bonds and form all-sugar macrocycles.The naturally-occurring cyclodextrins are the enzymatic degradation products of starch, the amylose component of which undergoes degradations and enzyme-mediated cyclizations as one of the end-products.The enzymatic syntheses are streamlined largely. 5he exquisite selectivities of enzyme reactions lead to cyclic oligomers with well-defined number of sugar units constituting the macrocycle, without enforcing structural constraints that would alter the stable chair conformation of the sugar moieties, the interglycosidic bond, torsion angles, the hydroxy functionalities occupying upper, lower rims of the cylindrical structure and the cavity sizes and shapes.Importance of these characteristics in naturally-occurring cyclodextrins is analyzed in great detail by Lichtenthaler and co-workers, through systematic molecular dynamics studies. 6Further studies reveal that oligosaccharide macrocycles composed of fewer glucopyranosyl moieties than that of the smallest naturally-occurring macrocycle, namely, -cyclodextrin possessing 6 glucopyranosyl units, would be constrained.Penalties arise due to shrinking of the macrocycle size, the largest being the inability to retain the favorable chair conformation of the individual glucopyranoside ring.Rings with fewer than 6 glucopyranosides would undergo conformational changes towards distorted chair, skew and envelop conformations of individual sugar units, referring to the number of atoms forming the plane and the associated steric constraints encountered by the substituents of the pyranoside moiety.As a result, formation of cyclic oligosaccharides containing 2 to 5 glucopyranosyl units is thought not feasible during the enzymatic synthesis of macrocycles with 6 and more glucopyranosyl units in the ring. 73][14][15][16][17] Presence of multiple hydroxy groups and the requirements to retain the regio-and stereoselectivities in glycosidic bond formation warrant judicious synthetic designs.Cyclic oligosaccharide syntheses remain a challenge, further complicated by the requirement to retain favorable conformation of the sugar moieties, particularly in small ring cyclic oligosaccharides.The theme of this article is to discuss the major synthetic approaches to prepare the small ring cyclic oligosaccharides, with either native glycosidic bond or heteroatoms linking the individual sugar

Chemical Synthesis of Native Cyclodextrins and Small-Ring Cyclic Oligosaccharides
Chemical synthesis of native cyclodextrins, produced naturally by enzymatic reactions on starch, was an early feat awaiting to be demonstrated.9][20] This pioneering work had given the necessary impetus to achieve chemical synthesis of cyclic oligosaccharides through non-enzymatic routes.The synthesis relied on building blocks approach, wherein a convergent assembly of smaller building blocks and an intramolecular cyclization were adopted to secure the first fully synthetic cyclodextrin.As many as 21 steps were involved in the course of the synthesis, initiated from maltose as the substrate, with a good yield of individual synthetic steps.The stepwise synthesis of the linear hexasaccharide precursor 5 and the final step of the ring closing cyclization to afford cyclodextrin 6 are shown in Scheme 1. Maltose derivatives 1 and 2 were the key intermediates for the preparation of the target cyclic oligosaccharide.The Mukaiyama glycosylation between disaccharide fluoride donor 1 and disaccharide acceptor alcohol 2, in the presence of SnCl2 and AgOTf, afforded the protected tetrasaccharide, which upon O-deacetylation furnished tetrasaccharide 3, having a free hydroxy functionality at the reducing end.Iteration of the glycosylation of 3 with the disaccharide donor 1, followed by Odeacetylation led to the formation of linear hexasaccharide 4, possessing an allyl moiety at the reducing end and a hydroxy moiety at the non-reducing of the hexasaccharide.A 5-step synthetic manipulation of 4 was undertaken, namely, (i) protection of primary hydroxy group at the non-reducing end as a chloroacetate; (ii) deprotection of the allyl moiety at the reducing-end; (iii) conversion of the hemiacetal to the corresponding chloride, upon treatment with SO2Cl2; (iv) transformation of chloride to fluoride using AgF and (v) removal of labile chloroacetyl moiety under the Zemplén condition to secure linear hexasaccharide 5 (Scheme 1).Treatment of the linear hexasaccharide fluoride precursor 5 with SnCl2 and AgOTf afforded the protected derivative of 6, as a result of cyclo-glycosylation reaction.Subsequent O-debenzylation by hydrogenolysis led to the isolation of α-cyclodextrin, in an overall yield of 0.3% from maltose.
The successful demonstration of the chemical synthesis of a cyclodextrin led to queries on the preparation of different types of constitutionally varied cyclodextrin analogues.Among these analogues, synthesis of small-ring cyclic oligosaccharides is of prime interest.In the following sections, preparation of macrocycle with less than six sugar units is discussed.
Synthesis of a cyclic oligosaccharide with five sugar units was reported by Nakagawa and co-workers. 21he stereoselective synthesis of cyclo-maltopentaose was achieved by intramolecular cyclization of the linear pentasaccharide precursor.Preparation of the precursor was initiated with glycosylation of 1,6anhydromaltose derived disaccharide acceptor 7 with monosaccharide thiol donor 8 to afford protected derivative 9, which upon O-deacetylation afforded trisaccharide 9 (Scheme 2).Glycosylation of 9 with maltose derived disaccharide thioglycoside donor 10 led to the formation of linear pentasaccharide 11.The anhydrosugar moiety at the reducing end was converted to a thioglycoside and subjected further to secure the linear pentasaccharide 12.

Chemical Synthesis of the Smallest Cyclodextrin
Cyclodextrins, discovered in 1891 by Antoine Villiers, 31 have occupied most interest, mainly on three cyclic oligomers, namely, α-, β-and γ-CD, consisting of six, seven and eight sugar units connected through α-(1→4) glycosidic linkages, respectively.Chemical synthesis of cyclodextrins with less than six sugar units is challenging, due to conformational instability and steric overlap in individual sugar moieties constituting the macrocycle.Existence of cyclodextrins with three and four sugars was considered nearly impossible until Yamada and co-workers achieved the synthesis of 3 and 4 monosaccharide-containing cyclodextrins, using conformationally supple glucose monomers. 32onstraining the conformation of the glycosyl donor is a valuable route to enforce stereoselectivities in glycosidic bond formation.Conformationally-constrained 3,6-O-o-xylylene-bridged glucosyl fluoride 47 as a donor was developed in order to conduct glycosylation with high α-anomeric selectivity (Scheme 10). 33,34The o-xylylene aids to block the β-face of the pyranose ring and facilitates the approach of the nucleophile from αface to form α-anomeric product selectively, under kinetically-controlled reaction conditions.The presence of 2-O-benzyl group at C-2 in 48 hinders the α-face in the locked pyranose ring system in skew conformation ( 3 S1) and resulted in a low yield of 49, with poor α-selectivity in glycosylation with sterically hindered alcohol.Scheme 10.Glycosylation approach involving conformationally-constrained glycosyl donor, constituted with oxylylene bridge. 33 order to overcome the loss of stereoselectivity, the authors identified the possibility of increasing the length of the bridge and fulfil two important factors, namely, (i) increase the steric hindrance at the β-face and (ii) reduce steric hindrance arising from 2-O-benzyl group on the α-face.With these considerations, (1,1'-(ethane-1,2-diyl) dibenzene-2,2'-bis(methylene) (3,6-O-EDB) bridged glycosyl fluoride donor 50 was chosen over 3,6-O-o-xylene bridged conformer for glycosylation.The attempts resulted in glycoside products with excellent α-selective product formation (51), under Suzuki glycosylation condition at room temperature (Scheme 11). 33,34ollowing the ability to enforce the reaction to afford glycosidic bond with excellent -selectivity, the concept was extended to synthesize strained cyclodextrins rings.A supple pyranose system, attached with EDB bridge between two distant oxygen atoms on the pyranose, forms a bicyclic ring which can modulate the conformations of pyranose intermediates during the glycosylation reactions.The EDB bridge was assumed to be perfect in length to modulate the conformations between an equatorial-rich 4 C1 form and an axial-rich 1 C4 conformation.
Scheme 11.Conceptualization of the EDB bridge glycosyl fluoride donor in a glycosylation. 33he target cyclodextrin analogues CD3 and CD4 were prepared from 1,2,4-orthoacetylglucose monosaccharide in a convergent manner.Linear precursors 52 and 53 were synthesized by α-selective glycosylation, using a common disaccharide acceptor, monosaccharide and disaccharide donors, respectively.Scheme 12. Synthesis of small ring cyclodextrins CD3 and CD4 by Yamada and co-workers. 32e incorporation of sterically less-hindered allyl group, compared to benzyl group, at C-2 carbon afforded high α-selectivity during the glycosylations.Intramolecular cyclization of the trisaccharide 52 and tetrasaccharide 53 glycosyl fluoride donors under Suzuki glycosylation condition and subsequent global deprotections afforded the desired small ring cyclic oligosaccharides CD3 and CD4, in 72% and 14% overall yields, respectively (Scheme 12).The synthesis illustrated that small ring cyclodextrins could indeed be formed and made a record at a time when it was considered nearly impossible that such products could ever exist, for reasons of unfavorable conformations on individual sugar units in these macrocycles.
As anticipated, NMR studies showed that the conformation of CD4 tended to be between 4 C1 and 2 H1, wherein pyranose rings were distorted, and flattened, when compared to that of the larger ring cyclodextrins.Further, the three sugar-containing CD3 was found to adopt 2 S0 conformation in each sugar units in D2O solution, as adjudged in early molecular simulation studies of Lichtenthaler and co-workers. 6Single crystal Xray diffraction studies further reiterated the skew conformation in one of the sugar units ( 5 S1) in CD3, with remaining two sugar units retained in between 4 C1 and 0 H1 conformations in the solid state, much away from the stable 4 C1 conformation of naturally-occurring and large ring cyclodextrins.

Preparation of Glycosidic Bond Expanded Small-Ring Cyclic Oligosaccharides
It became imperative to retain the 4 C1 conformation in the individual sugar units, in order to achieve the sugar wall constituting the cyclic oligosaccharides as that in native CDs and to benefit from attendant molecular, supramolecular properties.Small ring cyclic oligosaccharide analogues were conceived with changes in the glycosidic bond linking the individual sugar units.Important to a modification is to (i) retain the glycosidic oxygen in the -anomeric configuration and (ii) involve C-1 and C-4 carbons of the sugar units in the cyclic oligomers.Reducing the conformational instability warranted changes in the glycosidic bond linkage beyond that present in native cyclodextrins.A one-atom extension between the glycosidic oxygen and C-4 carbon was thought invaluable, in order to meet the above requirements.Thus, the glycosidic bond linkage would be extended with a methylene moiety.With this motivation, early studies focused on synthesis and studies of a disaccharide constituted with the extended glycosidic bond linkage.
Synthesis of this C-4 methylene attached D-glucopyranoside derivative 13 was achieved by using Dglucose as starting material, as shown in Scheme 13. 35 At first D-glucose was converted to orthogonally protected derivative 54, possessing a free hydroxy moiety at C-4 carbon.Dess-Martin periodinane (DMP) oxidation of 54 afforded ketone 55, in a good yield.In order to introduce one carbon at C-4 of 55, Wittig methylidenation reaction was performed to furnish olefin 56.Scheme 13.Synthesis of 4-deoxy-4-C-hydroxymethyl glucopyranoside 13. 35 Hydroboration-oxidation of olefin 56 afforded monosaccharide 57, with galacto-configuration.The reaction also led to the tertiary alcohol, an anti-Markovnikov addition side product, in 10-15% yield.The galacto-alcohol 57 was taken through: (i) DMP oxidation to form aldehyde 58; (ii) epimerization of 58 in the presence of Et3N to afford thermodynamically stable aldehyde 59 and (iii) reduction in the presence of NaBH4 in MeOH to afford 60, installed with the hydroxymethylene functionality at C-4 carbon (Scheme 13).
An effort was undertaken to synthesize the disaccharide constituted with the new monomer 60.The corresponding imidate monomer 61 was prepared through deprotection of p-methyoxybenzyl protecting group in 60 with trifluoracetic acid and reaction of the resulting hemiacetal with trichloroacetonitrile (Cl3CCN), in the presence of K2CO3, which served as the glycosyl donor.Glycosylation of acceptor 60 and acetimidate donor 61, in the presence of TMSOTf, afforded disaccharide 62, with -anomeric configuration, in 66% yield.The -anomer was also obtained in ~30% yield.Further, the disaccharide 62 was converted to active thioglycoside disaccharide monomer 63 by (i) removal of p-methoxybenzyl group in the presence of trifluoroacetic acid and (ii) O-acylation of the resulting lactal; (iii) reaction with EtSH in the presence of BF3.OEt2 and (iv) O-deacetylation at the non-reducing end (Scheme 14). 35cheme 14. Synthesis of disaccharide 62 and the corresponding thioglycoside donor 63. 35 It was of interest to evaluate the hydrolytic stabilities of the disaccharides constituted with the new monosaccharide monomer 60.For this purpose, 3 disaccharides possessing 4-deoxy-4-C-hydroxymethyl pyranose moiety were synthesized.The corresponding naturally-occurring disaccharides were used for the comparison of the hydrolytic stabilities.An acid-catalyzed hydrolysis was conducted on 6 disaccharides, in the presence of DCl (2 N) in D2O, at 60 and 70 o C (Scheme 15).Rate of inter-glycosidic bond hydrolysis was monitored by the appearance of new H-4 in 4-deoxy-4-C-hydroxymethyl D-glucopyranose through 1 H NMR spectroscopy.Scheme 15.Acid-catalyzed hydrolysis of disaccharides 64 -69. 35e hydrolysis data was plotted as a function of time and the observed kinetic data was compared with that of native disaccharide analogues, for example, cellobiose (67), lactose (68) and maltose (69), as shown in Table 1.These experiments showed that 4-deoxy-4-C-hydroxymethyl glucopyranose-containing disaccharides possessed increased hydrolytic stability in comparison to the naturally-occurring disaccharide 67-69.Protonation of the glycosidic bond being the first step in the acid hydrolysis, increased hydrolytic stabilities of glycosidic bond expanded disaccharides appeared to undergo slower protonation in these disaccharides than in the case of native disaccharides.
Encouraged by the increased hydrolytic stabilities of glycosidic bond involving 4-deoxy-4-Chydroxymethyl glucopyranosides, synthesis of new cyclic oligosaccharides constituted with this new glycoside was undertaken and properties of the resulting macrocycles were investigated.For this purpose, disaccharide monomer 63 turned out to be superior for cyclization reactions than the corresponding monosaccharide monomer.A one-pot cyclo-oligomerization was carried out by utilizing thioglycoside monomer 63, in the presence of NIS/TMSOTf or AgOTf (Scheme 16).The cycloglycosylation reaction led to the formation of fully benzyl protected cyclic di-70 and tetrasaccharide 71, in 11% and 64% yields, respectively, through intra-and intermolecular cyclization reaction.Scheme 16.Cyclo-glycosylation of disaccharide monomer 63 and synthesis of cyclic disaccharide (70) and tetrasaccharide (71), possessing the expanded glycosidic bond linkage. 36 varying the promoter from TMSOTf to AgOTf, as well as, changing the monomer concentration of 63 from 3 mM to 20 mM did not improve the yield of benzyl protected cyclic tetrasaccharide 71 considerably.After purification, cyclic tetrasaccharide 71, O-debenzylation was performed by hydrogenolysis, which led to the formation of fully free hydroxy group containing cyclic tetramer 72, in 58% overall yield. 36nergy minimized structure obtained through molecular modeling studies showed that cyclic tetramer 72 possessed an ellipsoid structure, in which all pyranoside moieties adopted 4 C1 conformation with primary hydroxy groups at the wider side of the rim and secondary hydroxy groups at the narrower side of the rim, respectively.The molecular dynamics simulation revealed (i) such a small ring cyclic oligosaccharide fully adopts the stable chair conformation and (ii) the location of primary and secondary hydroxy groups differed with that present in native CDs.
Interestingly, the new cyclic tetramer 72 is amphiphilic in nature, soluble in both organic solvents, as well as, in aqueous solutions.As a result, water insoluble pyrene can be solubilized in aq.solutions containing 72.Pyrene was admixed with aqueous solution of cyclic tetramer 72 in varying concentrations and its extent of solubilization was determined by UV-Vis spectroscopy.It was found that a concentration of up to 14.7 µM pyrene can be solubilized in 0.9 mM of cyclic host 72 in aqueous medium.Further, it was found that organic solvent insoluble L-tyrosine can be solubilized in organic solutions of 72.A 1:1 molar ratio of cyclic tetramer 72 to L-tyrosine was determined from integration values of 1 H NMR spectrum of the inclusion complex.The amphiphilic character of the glycosidic bond expanded cyclic tetrasaccharide 72, with free hydroxy groups in the pyranoside, is un-common in cyclic oligosaccharide macrocycles at large, and opens a new direction for host-guest studies. 36 glycosidic bond expanded linear trisaccharide monomer 77 constituted with hydroxy acceptor site at the non-reducing end and thiocresyl donor moiety at the reducing end was synthesized from 4-deoxy-4-Chydroxymethyl D-glucopyranose 60.Derivative 60 was converted to a thioglycoside monomer 73 by performing: (i) O-acetylation at the non-reducing end; (ii) selective removal of p-methoxybenzyl group; (iii) acetylation of anomeric lactal and (iv) thioglycosylation using p-thiocresol, as shown in Scheme 17. 37 Glycosylation of acceptor 60 with thioglycoside 73, in the presence of NIS and TfOH, afforded protected derivative of 74, which upon O-deacetylation condition facilitated isolation of the α-anomer 74 (Scheme 17).Scheme 17. Synthesis of disaccharide acceptor 74. 37wards synthesis of the trisaccharide monomer 77, further glycosylation of disaccharide 74 and thioglycoside 73, in the presence of NIS/TfOH, followed by O-deacetylation at the non-reducing end afforded trisaccharide 75, in 68% yield (Scheme 18).Finally, trisaccharide 75 was converted to active trisaccharide thioglycoside monomer 77, through formation of lactal 76.First, O-acetylation at the non-reducing end in the presence of Ac2O in pyridine, followed by removal of p-methoxybenzyl group using trifluoroacetic acid afforded lactal 76.Further, O-acetylation of anomeric lactal 76 and subsequent thioglycosylation using pthiocresol, in the presence of BF3.OEt2, led to the formation of thioglycoside 77 (Scheme 18).
Trisaccharide monomer 77 was subjected to one-pot cyclo-glycosylation in dilute condition (20 mM) by the treatment of NIS/TfOH.The crude reaction mixture was taken through O-debenzylation to furnish intramolecular cyclized hydroxy groups containing cyclic trisaccharide 78, in 52% yield after these two steps (Scheme 18).In addition to the cyclic trimer, the crude reaction mixture also revealed the presence of cyclic hexamer and nonamer, as adjudged through mass spectral analysis.However, these higher cyclic oligosaccharide species could not be isolated upon chromatographic purifications.
The new cyclic trimer 78 is highly soluble in aqueous medium, whereas weekly soluble in organic solvents, such as, CHCl3.Further structural studies of 78 became feasible through single crystal X-ray diffraction.Single crystal suitable for the analysis was obtained by slow vapor diffusion of acetone in aqueous solution of the cyclic trimer.Several structural features were identified in the solid state structures.(i) The presence of complete symmetry of the molecule was revealed by the molecule adopting a perfect trigonal symmetry, with P3 space group (Figure 1a).(ii) The pyranoside adopts a perfect 4 C1 conformation, even when the macrocycle is constituted only with 3 sugar units.(iii) The primary hydroxy groups are located at the narrower side of the cone, whereas secondary hydroxy groups are situated at the wider side of the rim, respectively, similar to that of native CDs.(iv) The structure presents a sharper cone shape than native CDs (Figure 1b).(v) A brick wall type arrangement of molecular packing occurs in the solid state, a feature not observed in the native CDs, as shown in Figures 1c and 1d. comparison between this backbone modified cyclic trimer 78 and smallest cyclic CD3 was of interest to address their conformational dissimilarities.There are two distinct features related to their X-ray crystal structures and microenvironments created by these macrocycles: (i) although suppleness of the linear monomeric unit allowed the synthesis of highly strained CD3, the sugar units in CD3 turned into a distorted 5 S1 and between 4 C1 and 0 H1 conformations deviated from 4 C1 conformation, whereas each sugar unit in cyclic trimer 78 possessed a perfect 4 C1 conformation in its single crystal lattice; (ii) The interatomic distances in CD3 revealed almost no cavity inside the macromolecule suggesting its ability to form inclusion complex would be almost nil, whereas trimer 78 contained a micro-cavity which could encapsulate guest molecule (1aminoadamantane) with higher efficacy than native β-CD.
An avenue of immense importance of CDs is their supramolecular properties in aqueous solutions, attendant with their high affinities to hydrophobic guest molecules.The macrocyclic cavity of CDs, possessing the 4 C1 conformation of individual sugar units, confers a hydrophobicity, attenuated further by the polar hydroxy-substituents firmly located at the peripheries and away from the cavities.9][40] Having realized a perfect chair conformation in the case of the new cyclic trisaccharide, a study of the host-guest complexation property became pertinent and important.For this purpose, two guest molecules, namely, 1-aminoadamatane (AMT) and hexamethylene tetramine (HMT), were chosen and the thermodynamics of the complexation were studied by isothermal titration calorimetry (ITC).he encapsulation studies were performed in aq.medium at a cyclic trimer 78 or -CD to AMT of 1:15 molar ratio.ITC studies showed that cyclic trimer 78 binds to AMT in a 1:1 host-guest ratio with binding constant (Ka) 13,200 M -1 , comparatively higher than β-CD, where formation of 1:1 host-guest complexation occurred with Ka 5,400 M -1 .The 1:1 complexation appeared to occur at the wider rim of the macrocycle with both synthetic cyclic trisaccharide 78 and -CD.The higher binding constant with 78 could be attributed to the higher hydrophobicity of this host, which, in turn, is a consequence of the perfect 4 C1 conformation of individual glucopyranoside units constituting the macrocycle. 37he glycosidic expanded cyclic oligosaccharides with 3 and 4 glucopyranoside units showed that these small ring macrocycles compare well with native cyclodextrins in terms of their sizes, shapes and host-guest binding abilities.It became pertinent further to assess formation of 5 sugar containing glycosidic bond expanded cyclic oligomer and to assess the microenvironment offered by such a macrocycle.Towards this aim, an effort was undertaken to synthesize glycosidic bond expanded linear pentasaccharide 80. 41 In order to synthesize linear pentasaccharide 80, thioglycoside derivative 79 was prepared at first from disaccharide 74, by performing four consecutive reactions of: (i) acetyl protection of free hydroxy group; (ii) deprotection of pmethoxybenzyl group; (iii) anomeric O-acetylation and (iv) anomeric glycosylation using p-thiocresol and BF3.OEt2, in an overall 75% yield, as shown in Scheme 19.Scheme 19.Synthesis of linear pentasaccharide monomer 80. 41 Glycosylation of trisaccharide 75 with thioglycoside 79, in the presence of N-iodosuccinimide and triflic acid in toluene at 0 °C, afforded protected pentasaccharide, which upon O-deacetylation led to the formation of linear pentasaccharide 80, in 58% yield (Scheme 19).Along with α-anomeric configuration of the newly formed glycosidic linkage, corresponding -anomer also formed at the newly generated glycosidic bond.Further, O-deacetylation reaction enabled separation of the , -anomeric linear pentasaccharides.
The linear pentasaccharide 80 was taken through multiple steps in order to equip it with an acceptor functionality at the nonreducing end and an activated thioglycoside functionality at the reducing end (Scheme 20).(i) O-Acylation of primary hydroxy group at the non-reducing end; (ii) deprotection of p-methoxybenzyl moiety at the anomeric carbon; (iii) O-acylation at anomeric lactal functionality; (iv) reaction with p-thiocresol and (v) O-deacetylation at the non-reducing end finally afforded the pentasaccharide monomer 81 (Scheme 20).

AUTHOR(S)
Scheme 20.Cycloglycosylation of linear pentasaccharide to the corresponding macrocycle. 41on synthesizing active thioglycoside monomer 81, one-pot cyclo-glycosylation was performed with the concentration of 81 at 20 mM, in the presence of NIS/TfOH, followed by O-debenzylation, using H2/Pd-C, which led to the formation of cyclic pentasaccharide 82, in 67% yield (Scheme 20).The newly synthesized cyclic pentasaccharide was freely soluble in aqueous solution and weakly soluble in EtOH and CHCl3.
In order to determine the structure of cyclic pentamer 82, molecular modeling studies was performed using Gaussian 09 software at the (B3LYP)/6-311g level.Modeling studies showed that cyclic pentamer 82 possessed a distorted ellipsoid structure, with all the pyranoside residues retaining in 4 C1 conformation, in which size of lower rim is ~86% to that of upper rim (Figures 3a,b).imilar to cyclic trimer 78, encapsulation properties of cyclic pentamer 82 was assessed in aq.solution.Thermodynamic study was conducted in aq.medium using AMT as the guest, at a host-to-guest ratio of 1:20, at 30 o C (Figure 3c).Thermodynamic parameters revealed that cyclic pentamer 82 binds AMT in a 1:2 hostguest ratio, with binding affinity of 10,500 M -1 .Cyclic pentamer 82 exhibited binding interactions with two molecules of AMT either from the same face or from the opposite face of the rim, due to marginal difference in size between upper and lower faces.The ellipsoid geometry of 82 compared to regular cone shape structure of cyclic oligomer facilitated 1:2 of host-guest complexation, which is not common in cyclodextrin chemistry.
Synthesis of small ring cyclic oligosaccharide constituted with 2-deoxy glucopyranoside was also demonstrated. 42,43A polycondensation approach was followed in this instance.A 2-deoxyglucopyranosidecontaining disaccharide was synthesized as the cyclo-condensation monomer.The 2-deoxy sugar containing monomer was prepared from maltose, a series of conversions, leading to the formation of the precursor.(i) A protection of 4′-OH and 6′-OH of 83 44 with benzylidene to secure derivative 84; (ii) methylation of hydroxy groups ; (iii) removal of the benzylidene moiety and (iv) selective protection of the primary 6′-OH group as benzoate afforded the required monomer 85 for cyclo-oligomerization (Scheme 21).Scheme 21.Synthesis of 2-deoxy glucopuyranoside containing disaccharide monomer 85. 42 The monomer 85 was thus equipped with an active thioglycoside moiety at the reducing end and the acceptor functionality at C-4 of the non-reducing end.The cyclo-oligomerization was conducted in the subsequent step, at three different monomer concentrations, 2, 10 and 25 mM, in the presence of either NIS/TfOH or AgOTf as the promoter (Scheme 22).MALDI-TOF mass spectrometry and the HPLC analyses of the crude reaction mixture revealed the presence of the cyclic tetrasaccharide 86 and the cyclic hexasaccharide 87, in addition to the linear di-and tetrasaccharides 88 and 89, respectively.With 2 mM and 10 mM concentrations of 85α, the linear saccharides 88 and 89 were isolated in larger amount (88: 40%; 89: 30%).The cyclic products 86 and 87 were obtained in 15% and 7% yields, respectively.Better yields of the cyclic products 86 (40%) and 87 (25%) were isolated when the monomer concentration was 25 mM.The symmetrical structure of 86 was ascertained from their NMR spectra.Only one set of signal was observed in both 1 H NMR and 13 C NMR spectra.In the 1 H NMR spectrum, the signal for H-1 at the 2-deoxy sugar moiety was observed at 4.86 ppm, as a doublet with J1,2a = 3.3 Hz; whereas H-1 corresponding to non-reducing end sugar moiety resonated at 5.89 ppm, as a doublet with J1,2 = 3.6 Hz.In the 13 C NMR spectrum, the C-1 carbon, corresponding to the 2-deoxy sugar moiety, appeared at 98.4 ppm and C-1 carbon, corresponding to the nonreducing end sugar moiety appeared at 95.6 ppm.The C-2 of the 2-deoxy sugar moiety resonated at 33.9 ppm.Benzoyl-deprotected cyclic hexamer 87 formed an inclusion complex with p-nitrophenol, as adjudged by 1 H NMR titration method.

Figure 1 .
Figure 1.(a) ORTEP diagram of cyclic trimer 78; (b) cartoon representation of cyclic trimer with molecular dimensions; (c) CPK model and (d) gauss views of the packing in the solid state.37

Figure 2 .
Figure 2. ITC profile for binding interaction of (a) trimer 78 with AMT at 30 o C in water; (b) β-CD with AMT in water.37

Figure 3 .
Figure 3. Energy minimized molecular modeling structures of cyclic pentamer 82: (a) the CPK model exposing the inside cavity; (b) gauss view mode from the top; (c) ITC profile of AMT and cyclic pentamer 82 interaction.41

N.
Jayaraman joined as a faculty at the Department of Organic Chemistry, Indian Institute of Science, Bangalore, in December 1999, and is a Professor currently.He completed early studies: B.Sc. (University of Madras), M.Sc.(Annamalai University) and doctoral research at Indian Institute of Technology, Kanpur, under the supervision of Professor S. Ranganathan.He was a postdoctoral fellow under Professor Sir J. F. Stoddart, at the University of Birmingham, UK and at University of California Los Angeles, USA.His research themes are focused on synthesis and biophysical studies in the disparate areas of carbohydrates and dendrimers.He was honored with Shanti Swarup Bhatnagar Prize in 2009 and is a Fellow of the Indian Academy of Sciences.This paper is an open access article distributed under the terms of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

Table of Contents 1 .
Introduction 2. Chemical Synthesis of Native Cyclodextrins and Small-Ring Cyclic Oligosaccharides 3. Chemical Synthesis of the Smallest Cyclodextrin 4. Preparation of Glycosidic Bond Expanded Small-Ring Cyclic Oligosaccharides 5. Conclusion

Table 1 .
A comparison of the rate of hydrolysis of disaccharides a Rate of hydrolysis (k) x 10 5 s -1 mol -1