Halogeno-sugars

A review of reactions of carbohydrate acetals includes a section on 

A review on stereo- and regio—controlled halogenation and their use in glycosidation reactions has appeared. 

0-benzylidene-3-deoxy-o(-D-altropyranoside (5) gave the three fluoro-sugars (6) - (8), the first of which was converted to the 6-deoxy- 

The 3-0-benzyl-et-D-galactosyl chloride (11) has been synthesized in 80% yield from the corresponding methyl /d-D-galactoside derivative on controlled exposure to dichloromethyl methyl ether with catalytic zinc chloride. Prolonged exposure led to cleavage of the benzyl ether and formation of formate (12) and dichloromethyl ether (13)  

The 2-fluoro-arabinose derivative (15) has been prepared from the ribose N- gridinyl sulphonate (16) using potassium hydrogen difluoride. The fluoro-sugar phosphates (17) and (18) have been 

Reougants I, LI N(SiMt3)g it, Dl BAU LU, ACjO-DI lit, ACjO-DMAP-NEt3 

Hydrogen fluoride solvolyais of cellulose and of xylan gives yields of up to 20% -D-gluco yranosyl fluoride and -D-xylopyranosyl fluoride respectively, 

Several syntheses of 2-deoxy-2-fluoro-D-glucose (2) have been reported, A simple preparation uses the addition of acetyl hypofluorite (prepared in situ) to D-glucal, followed by deacetylation. Another uses triflate displacement in methyl 

6-0-benzylidene-3-0-methyl-2 -0-triflyl-y5-mannopyranoside with caesium fluoride, followed by one step removal of all blocking groups with boron tribromide. Use of Cs F led to the F-18 

6-Trideoxy-3,4,6-trifluoro- C-D-galactopyranosyl fluoride has been prepared from 3-deoxy-3-fluoro- c-D-glucopyranosyl fluoride using the diethylamino-sulphur trifluoride (DAST) procedure, and its stereochemistry determined by detailed multinuclear n.m.r. experiments. The DAST method has also been used to prepare 6,6 -dideoxy-6,6 -difluoro-sucrose from 2,3,4 3 4 -P nta-0-benzoyl-sucrose. The free 1 -hydroxy group was not substituted by fluorine. 

A series of 2-0 -benzoyl-4,6-di-O -benzyl-ot-D-galactopyranosyl halides (4), suitable as precursors for -D-galactopyranosyl residues with 2-0- and 3- -connected units in oligosaccharides, have been prepared by conventional means. 

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From this point on two synthetic routes could be followed compound 1J could be treated either to yield the oxazoline 1J2, or the a-halogeno sugar both being... 

Baker 19 has cited the conversion of tetra-O-acetyl-a-D-glucopyranosyl bromide (a cis halogeno sugar) to the less stable beta (trans) isomer with silver chloride [H. H. Schlubach, Ber., 69, 840 (1926)] as an example of this reaction. Lemieux (see Ref. 224) suggested that, in this anomerization, the mechanism is of the SnI rather than the Ss2 type (proceeding by way of an orthoester ion). 

Mitsunobu conditions [triphenylphosphine/diethyl azodicarboxylate (DEAD)]7 and triphenylphosphite methiochde [(PhO)3P+MeI ] or dihalides [(PhO)3P + XX-j8 have been successfully applied to the synthesis of halogeno sugars. 

The dichlorides and the dibromides of hexaacetylcellobial give acetyl derivatives of the 2-desoxy-2-halogeno sugar when treated with moist silver oxide. These compounds, just as the dihalogenides, isomerize to 2-desoxycellobionic acid55 (LXXIV) with deacetylation when treated with lead oxide. 

Reference to nucleosides containing halogeno-sugars is made in... 

Halogeno-sugar and Amino-sugar Nucleosides Thionucleosides... 

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