Application of N -Bromosuccinimide in Carbohydrate Chemistry

This article describes the use of N -bromosuccinimide in different organic group transformations in carbohydrate chemistry. A comprehensive discussion on the synthesis of deoxysugars through selective O -benzylidene fragmentation, photobromination, halogenation, oxidation, and polymerisation of different carbohydrate moieties with the aid of N -bromosuccinimide (NBS) is presented. The use of NBS in the most significant glycosylation methods and in oligosaccharide synthesis is also discussed.


Introduction
The success of a synthesis relies not only on the characteristics of the substrate but also on the choice of the reagent employed in the reaction.Different organic reagents can react through different mechanisms and are vital to determining the profile of a reaction; that is, the composition and constitution of the reaction product.This is an essential element of chemistry and it is to be mentioned that the chemistry and reagents are complementary to each other.One such small well-known reagent is N-bromosuccinimide (NBS).
N-Bromosuccinimide is particularly advantageous as a bromine source for radical reactions and electrophilic addition reactions, [1][2][3][4] and as an oxidising agent in the presence of base without any bromine.Using NBS in aqueous dimethoxyethane, Corey and Ishiguro discovered that secondary alcohols can be oxidised selectively in the presence of primary alcohols. 5Using a strong base, such as 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU), NBS also interacts with primary amides to generate a carbamate via the Hofmann rearrangement. 6NBS electrophilically brominates the amine, followed by decarboxylation and the formation of an imine, which is further hydrolysed to produce an aldehyde and ammonia. 7,8The bromination of allylic hydrogen, benzylic hydrogen and carbonyl -hydrogen, namely the Wohl-Ziegler reaction was found to work with NBS. 9 Both the radical route (discussed above) and acid-catalysis are options for applying NBS to -brominate carbonyl compounds. 10everal bifunctional alkanes can be produced by substituting nucleophiles for water. 11 survey of the literature reveals that NBS has been used for the bromination of the side chain of aromatic compounds without radical initiator under microwave conditions, 12 and for regioselective monobromination of the aromatic substrate with the assistance of NBS in ionic liquid. 13BS also acts as a ligand in organometallic chemistry. 14For silyl ethers 15 and THP ethers, NBS in water with cyclodextrin has been utilised as a deprotecting agent. 16Moreover, many other reactions have been carried out using NBS, such as polymerisation and addition of organic amines to alkenes to generate nitrogen-containing organic compounds. 17,18BS has been widely used with various aromatic and aliphatic compounds, and its application in carbohydrate chemistry is also significant. 19,20This review article focuses on the role of NBS in protection, deprotection, and glycosylation reactions in the field of carbohydrate chemistry.

Removal of Functionalities
The production of complex oligosaccharides can be started with anomeric hydroxy sugars or glycosyl hemiacetal derivatives.These may be utilised directly in dehydrative glycosylation processes or converted into reactive glycosyl donors for the production of oligosaccharides or natural products.During their synthesis, orthogonal protection and deprotection of functionalities are necessary to obtain good yields. 21otawia et al. looked into the synthesis of complex oligosaccharides with orthogonal benzyl, allyl, and transient ester-blocking groups. 22In a successful method, phenyl thioglycosides 1-8 were treated with NBS in aqueous acetone within a short period, and converted into O-glucosides 9-16, respectively (Scheme 1).Under these reaction conditions, acetate, benzylidene acetal, tert-butyldiphenylsilyl, benzyl groups, and the O-glycosidic bond (e.g., di-, tetra-, and pentasaccharide thioglycosides) remain unaffected.Here, the electrophilic activation of sulfur primes the generation of an active sulfonium species, which then participates in the glycoside bond-forming reaction. 22t can be challenging to eliminate allyl functionality.In the work of Donohoe and colleagues, Grubbs second-generation catalyst (G2) was used to isomerise the allylic group of 17 to 18 before being removed by NBS to give 19 (Scheme 2).According to Donohoe, G2 herein functions by breaking down into the ruthenium hydride species, which does the dirty work. 23

Scheme 2 Removal of allyl functionality
In the presence of NBS (1.1 equiv) in aqueous acetone, Panchadhayee and Misra hydrolyzed functionalised allyl glycosides 20 to their corresponding glycosyl hemiacetal derivatives 21, forming hemiacetal derivative at room temperature in excellent yield, without using any acid activator, in three minutes (Scheme 3). 24Under the reaction conditions, the protective hydroxy groups of the carbohydrate backbone, such as benzylidene, isopropylidene acetal, benzoyl, 4-methoxybenzyl, benzyl, tert-butyldiphenylsilyl, and acetyl, were unaffected, though the hemiacetal cleaves under Hanessian-Hullar's reaction conditions.Here, the allyl  S. Bera et al.

Review SynOpen
group and the bromonium ion (Br + ) produced by NBS combine to form an addition adduct, which, after allyl glycoside hydrolysis, produces the glycosyl hemiacetal derivatives.

Scheme 4
Oxidative ring-opening of furan with NBS Silva et al. 26 used a n-Bu 2 SnO/NBS mixture in a combination of chloroform and toluene to regioselectively oxidise the 5-hydroxyl group of allofuranose derivative 24 to produce the 5-keto sugar 25 in a moderate yield (Scheme 5).

Scheme 5 Regioselective oxidation of isopropylidene-protected allofuranose
Sun et al. extended the scope of application of the above reaction to hemicellulose extracted from sugarcane bagasse, which was partially acetylated with acetic anhydride using NBS as a catalyst under mild, solvent-free conditions. 27A series of reactions with temperature adjustments showed that the yields ranged from 66 to 84%, and that the degree of substitution (DS) was between 0.27 and 1.15.The degree of substitution increased with an increase of temperature between 18 and 80 °C, and with an increase in the reaction time from 0.5 to 5 h, demonstrating an important use of NBS as a catalyst (Scheme 6).The treatment of natural hemicellulose 26 with acetic anhydride is thus a convenient way to obtain ester derivatives of biopolymers such as 27.

Scheme 6 Acylation of sugarcane bagasse hemicellulose
Under pseudo-first-order conditions and a temperature of 40 °C, the kinetics and mechanism of micellar catalysed NBS oxidation of dextrose (1:1) in an H 2 SO 4 medium were studied. 28According to the findings of the reactions investigated under a variety of experimental settings, NBS exhibits a first-order, fractional-order reliance on dextrose and a negative fractional-order dependence on sulfuric acid.

Scheme 8 endo-Cyclisation with NBS
Ye and associates reported the synthesis of aryl-C-Δ 3glycosides and 2-deoxy--or --C-glycosyl using a ringopening-ring-closure methodology.Other than employing protic acid, Lewis acid, or PhSeCl, the ring-opened product S. Bera et al.

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36 was transformed into aryl-C-glycosyl compound 37 via an NBS-mediated ring-closure process followed by reductive dehalogenation.Functional groups such as halides, bulky substituents, and various glycals (such as galactals, glucals, and xylals) with diverse electronic characteristics could be included without suffering any efficiency losses (Scheme 9). 31

Halogenation
Penta-O-acetyl--D-glucopyranose 38 is photobrominated with NBS in carbon tetrachloride, resulting in crystalline 5-bromo-derivative 39 in high yield through selective replacement of H-5 along with mono-and dibromoacetyl derivative 40, as by-products.However, with bromine as a reagent, in addition to 5-bromo-derivative 39, other byproducts are also obtained as shown in Scheme 10.As the C-5 epimer of the penta-acetate yields identical products, it is hypothesised that the initial bromination occurs via a unique radical at the tertiary site. 32

Scheme 10 Photobromination of penta-O-acetyl--D-glucopyranose
The direct conversion of thioglycosides 44-50 into glycosyl fluorides 51-57 utilising NBS and dimethylaminosulfur trifluoride (DAST) or HF-pyridine was described by Nicolaou and colleagues in 1984 in the presence of different functional groups including O-glycoside bonds (Scheme 11). 33he bromination of chitin was described by adding NBS and Ph 3 P to a solution of chitin and LiBr in dimethylacetamide (DMA) solvent. 34 rapid, stereoselective, and high-yielding technique of haloazidation is the bromoazidation of glycal 58 with NIS/NBS and TMSN 3 as the source of bromide/iodide and azide.However, trans-bromo-azido derivative 59 formed from glycal with NBS and TMSN 3 within 5 minutes at room temperature (Scheme 12) in lower yields with the forma-tion of chromatographically inseparable degraded products.Similar outcomes were obtained by substituting NBS with NIS, with 82% yield.Because the cyclic bromonium intermediate is less stable than the corresponding iodonium ion, it is more likely to be attacked by ring oxygen before being attacked by azide, as evidenced by the low anti-selectivity of NBS. 35heme 12 Azido-bromination of galactal triacetate Lah et al. 36

Fragmentation of O-Benzylidene Functionality in Carbohydrates
One of the most important applications of NBS in carbohydrates is the selective 4,6-O-benzylidene functionality opening of benzylidene acetals 69 to access deoxy sugars.The groups of Hanessian, Hullar, Roberts and Jeppesen studied the same reaction and mechanism with different substituted benzylidene acetals.In 1966, Hanessian was the first to describe the regioselective opening of 4,6-O-benzylidene acetals of O-benzylidene sugars in tetrachloroethane using NBS.When NBS was exposed to compound 69 in tetrachloroethane at 85 °C, the hydroxyl ion produced by the water of hydration competed with the bromide ion to attack the intermediate cyclic ion, resulting in about equal quantities of the 6-bromo product 70a and the 6-OH product 70b (Scheme 14). 37

Scheme 14 Regioselective opening of 4,6-O-benzylidene acetals with NBS
In order to establish the scope and applications of the methyl 4,6-O-benzylidene hexopyranoside series, Hanessian and Plessas, worked on a series of O-benzylidene cleavage with NBS. 38When methyl 4,6-O-benzylidene hexopyranosides 71 were heated at reflux with NBS in carbon tetrachloride or chlorinated hydrocarbon, the main products were methyl 4-O-benzoyl-6-bromo-6-deoxyhexopyranosides 76.This method showed orthogonality with different protecting such as anhydro rings, which remained intact.The authors also established adaptability of the NBS-assist-ed reaction to benzylidene acetals of some disaccharides.For example, the expected product was produced by the NBS reaction of the 4,6:4′,6′-di-O-benzylidene derivative of ,-trehalose dihydrate in a mixture of carbon tetrachloride and tetrachloroethane. 38The authors also described the formation of the corresponding methyl 4-O-benzoyl-6-bromo-6-deoxyhexopyranosides, which are intermediates in the synthesis of biologically important polyfunctionally benzoylated carbohydrate aminodeoxy and deoxy sugar derivatives, by reacting methyl 4,6-O-benzylidene hexopyranosides with NBS (Scheme 15). 39

Scheme 15 NBS-mediated acetal opening of methyl 4,6-O-benzylidene hexopyranosides
A similar finding was made in 1966 by Failla, Hullar and Siskin, 40 who discovered that NBS interacts specifically with substituted benzylidene acetals to produce -bromo benzoates, wherein the bromine occupies the least substituted carbon.To produce substituted 3,7-oxazabicyclo[4.1.0]heptane(83), NBS reacted with D-glucopyranoside derivatives 81a and 81b to give 4-O-benzoyl-6-bromo-6-deoxy derivatives 82 in high yields.It is unlikely that the reaction of benzylidene acetals with NBS would result in 2,6-imino sugar derivatives as long as a suitable leaving group is not transvicinal to the nitrogen function (Scheme 16).procedure was also used to synthesise substituted 2,5-oxazabicyclo[2.

Review SynOpen
When the benzylidene acetals 84a-b were opened with NBS in CCl 4 and barium carbonate in the presence of water and exposed to low-pressure mercury irradiation for 2.5 hours, only the 2,4-dibenzoate 86b was isolated (62% yield), with no 2,3-dibenzoate (86a) detected. 42A pyranoside with an axial benzoyloxy group along with an equatorial hydroxy group was produced when the benzylidene ring was opened under these conditions.It was predicted that similar reactions would be seen with analogous derivatives of L- fucose based on the regiospecific synthesis of 87 from 85ab, respectively.The L-rhamnose derivatives 88a-b reacted under similar conditions to produce a single carbohydrate compound 89 in 75% yield (Scheme 17).

Mechanisms
To search for the probable mechanism of the NBS-mediated Hanessian-Hullar reaction, several groups investigated different types of reaction.Hanessian favoured the ionic mode of fragmentation whereas Hullar initially promoted it as a radical pathway.Gelas also support an ionic mechanism. 43Following the work by Jeppesen, 44 Roberts later revealed the pure radical fragmentations of 4,6-O-benzylidene acetals with preferential cleavage of the primary C6-O6 bond in both the glucose and mannose derivatives. 45ontrol studies were carried out in 2004 by McNulty et al., and they concluded that an ionic process is most likely responsible for the fragmentation (Scheme 18). 46The initial step was the removal of radical hydrogen from the arylidene acetal carbon by a bromine radical to generate acetal radical 91, as NBS can form bromine (Br 2 ) in situ by interaction with HBr, which is present in a low concentration.Following this, propagation takes place during which the acetal radical undergoes Wohl-Ziegler bromination.The Benzylideneacetal (90) breaks down via an ionic pathway, leaving a stable cyclic carbocation (91), which is then opened by nucleophilic attack of the bromide ion at C6.The inversion of stereochemistry at C4 in compound 86b is a clear illustration of the S N 2-pathway of the previous step.Usually, BaCO 3 functions as an acid scavenger.

Application in Carbohydrates Fragmentation of O-Isopropylidene Functionality in Carbohydrates
The importance of the O-glycoside bond in nature and biomolecules is well known.The O-glycoside bond plays a key role in organic synthesis for the assembly of intricate frameworks containing carbohydrate residues.Thus, the methodology for the formation of the O-glycoside bond is of utmost interest.For glycosylation, glycosyl donors such as thioglycosides are often utilised in the presence of promoters such as methyl triflate, N-iodosuccinimide-triflic acid, and iodonium dicollidine perchlorate.47a The group of Nicolaou developed mild and general procedure of O-glycosidation from phenyl thioglycosides by reaction with NBS in CH 2 Cl 2 under anhydrous conditions at 25 °C in the presence of different hydroxy compounds and 4Å molecular sieves, leading to O-glycosides (-and -anomers) in good to excellent yields within 15 minutes (Scheme 19).47b For a demonstration of the applicability of the glycosidation reaction to complex and polyfunctional compounds, tylosin derivative 110 was constructed as shown in Scheme 20.
Additionally, as shown in Scheme 21, this reaction provides a straightforward route to internal acetals 113 and 114 in intramolecular situations, demonstrating the scope and applicability of this method.

Review SynOpen
Sasaki and Tachibana 48 produced a stereocontrolled synthesis of the core trisaccharide 117 of nephritogenic glycopeptide, nephritogenoside, employing -selective glycosylation without neighbouring-group participation.With the anticipated left and right segments 115 and 116, coupling of these two fragments in a combination of NBS and trifluoromethanesulfonic acid (TfOH) in propionitrile at -78 °C gave trisaccharide 117 and its cc-anomer in 96:4 ratio and 74% combined yield (Scheme 22).

Scheme 22 NBS mediated -selective glycosylation
A pioneering publication from Nicolaou's group on NBS application to activate phenyl thioglycosides serves as an early illustration.47b The results show that the method could be used to couple deoxythioglycoside 118 with a hindered sugar acceptor 119, forming the required disaccharide 120 in 75% yield in a 9:1 (/) mixture (Scheme 23), although this work primarily focused on completely substituted sugars.Later, the Roush group used these conditions to construct the AB disaccharide unit of olivomycin A. 49

Scheme 23 NBS for oligosaccharide synthesis through activation of 2deoxythioglycosides
Tatsuta and colleagues achieved selective glycosylation reactions even for conformationally constrained 2-deoxythioglycoside donors.To do this, they looked at what would happen if they protected the C-3 and C-4 alcohols in 2-deoxyfucose thioglycoside 121 by using an isopropylidene acetal.These sugars exhibited exceptionally selective reactions with straightforward glycosyl acceptors after being activated with NBS (Scheme 24). 50

Review SynOpen
Hsieh-Wilson et al. 51 synthesized disaccharide building blocks of natural heparan sulfate.With that aim, the monosaccharides interconversion of IdoA to GlcA in 123 proceeded in poor overall yield due to -elimination or disaccharide decomposition in a notable amount.The epimerization of GlcA in 125 with NBS under UV light led to the production of the C-5 bromo compound 126 in 75% yield.This was followed by -dehalogenation with Et 3 B and Bu 3 SnH at 20 °C to create the epimerized product IdoA-GlcN in 63% yield following Wong and colleagues method. 52However, NBSmediated bromination of 123 resulted in the formation of an epimeric combination of the C-5 bromo molecule 124; subsequent -dehalogenation with AIBN and Bu 3 SnH at 110 °C produced disaccharide GlcN-GlcA in 33% yield (Scheme 25).

Synthesis of Hemiacetals
Qin et al. synthesised glycoconjugates in good yields by activating phenyl and ethyl thioglycosides with the NBS-Me 3 SiOTf system to introduce a C2 spacer arm.Due to the neighbouring tetrachlorophthalimido group, the coupling of phenyl thioglycoside 127 with 2-bromo-or 2-azido-eth-anol was possible without further protection of the free 3hydroxy group (Scheme 26).In the case of the 4-methoxy group, bromination occurs on the aromatic ring in addition to glycosylation.The glycosides 2-bromoethyl and 2-azidoethyl are important intermediates in the synthesis of neoglyco conjugates.As a result, if the aromatic group is not sensitive to bromination, in either the glycosyl donor or acceptor, then NBS (cat.)/Me 3 SiOTf reagent is a good thioglycoside promoter. 53he synthesis of 2,4-diacetamido-2,4,6-trideoxy-D-galactose (DATDG) containing trisaccharide by Emmadi and Kulkarni 54 is given in Scheme 27.The stereoselective coupling of 2,4-diacetamido-2,4,6-trideoxy--D-hexose (DAT-DH) donor with the primary alcohol of an amino acid is a tough challenge.Following treatment with NBS, THF, and H 2 O, the thioglycoside derivative 134 was converted into its corresponding hemiacetal, and the resulting hemiacetal was treated with trichloroacetonitrile and DBU to yield imidate donor 135.

Review SynOpen
Seeberger, Yin and colleagues recently reported the complete synthesis of a highly functionalized trisaccharide repeating unit from P. shigelloides serotype 51.The D-quino- vosamine building block was converted into its corresponding hemiacetal using NBS in aq.THF (Scheme 29).This was then treated with trichloroacetonitrile and DBU to yield the imidate, which was then coupled with linker acceptor 148 using TMSOTf promoter to yield 149 in 82% yields over three steps. 56heme 29 Glycosilyation of selenyl glycoside with NBS S-Glycosyl thiosulfate 151, sometimes known as 'glycosyl Bunte salts', was employed for protection-free intramolecular glycosylation and alcoholysis by direct anomeric activation.NBS was discovered to be an efficient promoter for the solvolysis reactions with ethanol, yielding ethyl D-glu- copyranoside 152 (Scheme 30). 57heme 30 NBS mediated solvolysis of glycosyl Bunte salts Zong and co-workers in 2022, synthesized -galactosylceramide 154 and its C-6 modified analogues from donor 153 (Scheme 31).The hydrolysis of thiolacetals was carried out with NBS.58 Scheme 31 Hydrolysis of thiolacetals using NBS

Conclusions
The regioselective and orthogonal modification of different functionalities in carbohydrates is crucial because of the wide application of such reactions in chemistry and biology.This review article has summarised the usage of Nbromosuccinimide in carbohydrate chemistry for various organic group transformations.NBS facilitates the production of deoxysugars via selective O-benzylidene fragmentation, photobromination, halogenation, oxidation, and polymerisation of various carbohydrate moieties.
Scheme 11 Conversion of thioglycosides into glycosyl fluorides utilising NBS

Scheme 1
Removal of thiophenyl group with NBS

Biographical Sketches Dr. Smritilekha Bera earned
55heme 27 NBS-mediated conversion of thioglycoside into hemiacetalPatil et al. constructed 146 by following a linear glycosylation protocol converting the reducing end to the nonreducing end as a target to study tetraacylated phosphatidylinositol hexamannoside for antituberculosis activity.To that end, a single elongation unit 137 was used.Accordingly, first, the elongation unit was transformed from 137 to 138 utilizing NBS in aq.acetone followed by imidate formation in an overall 94% yield in two steps (Scheme 28).55