Carbohydrates selective methylation

Of the numerous alkyl protecting groups known [8, 9], only those widely used in synthetic chemistry of carbohydrates will be discussed. Methyl ethers — with hundreds of selective methylations far exceeding the scope of the review — were excluded for their unimportant role as synthetic intermediates (great majority of partial methylations are carried out to complete the list of reference compounds for the methylation analysis of polysaccharides, or simply to prepare ethers widely occurring in biologically important molecules). 

Candida cylindracea, phosphate buffer pH 7, Bu20. The 6-0-acetyl of Q -methyl peracetylglucose was selectively removed. Porcine pancreatic lipase will also hydrolyze acetyl groups from carbohydrates. These lipases are not specific for acetate since they hydrolyze other esters as well. In general selectivity is dependent on the ester and the substrate. ... 

The carbohydrate has sites for ionic interaction (clusters of sialic acid or sulphate residues) and also hydrophobic interaction (clusters of hydrophobic methyl groups offered by fucose residues). Sedimentation velocity has been a valuable tool in the selection of appropriate mucoadhesives and in the characterisation of the complexes [ 138-143]. 

Interesting selectivity has been observed with some substituted carbohydrates. On treatment with sulfur trioxide-pyridine complex in A/,A/-dimethylformamide, methyl 4,6-O-benzylidene-a-D-... 

Selective replacement of primary hydroxyl groups in carbohydrates by iodine atoms has been achieved by using the Rydon reagent, namely, methyltriphenoxyphosphonium iodide.368 Treatment of methyl 3,4-O-isopropylidene-jS-D-galactopyranoside with the phosphonium salt in benzene for 48 hours at room temperature yielded 60% of the 6-deoxy-6-iodo derivative,369 and reaction of thymidine, uridine, and 2,2 -anhydrouridine in N,N-dimethylformamide afforded 5 -deoxy-5 -iodo derivatives in yields of 63, 65, and 31%, respectively.370... 

Benzyl and triphenylmethyl ethers of carbohydrates are preferred over methyl ethers when selective removal of protecting groups is important. The relatively high nucleophilic activity of the 5-hydroxyl group in glycosides and 1,2-O-alkylidene derivatives of 4 permits its benzylation and triphenylmethylation under mild conditions. Thus, treatment of 33 (Ref. 34) and 36 (Ref. 57) with benzyl bromide and... 

A. C. Richardson, Selective acylation of methyl 3-acetamido-3-deoxy-a-L-glucopyranoside Its conversion into the L-galacto- and L-manno-isomers, Carbohydr. Res., 4 (1967) 415421. 

R. S. Bhatt, L. Hough, and A. C. Richardson, Selective chlorination of methyl p-lactoside with mesyl chloride in DMF, Carbohydr. Res., 49 (1976) 103-118. 

Another example of a selective dehalogenation in the carbohydrate field has been reported by Hanessian and Plessas.83(b) Catalytic hydrogenation of methyl 4-0-benzoyl-3-bromo-2,6-dichloro-2,3,6- tri-deoxy-a-D-mannopyranoside over palladium-on-carbon in the presence of barium carbonate gave methyl 4-0-benzoyl-6-chloro-2,3,6-... 

Although hydrazine hydrate in ethanol is effective in achieving denitration of carbohydrate nitrates,243,244 the reagent showed little selectivity in its reactions244 with the 2,3-dinitrates of methyl 4,6-O-ben-zylidene-a- and methyl 4,6-0-ethylidene-a- and -/3-D-glucopyranoside. 

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