Benzylidene derivative disaccharide synthesis

In a further synthesis of (345) by Rana and Matta [234], the disaccharide (353), prepared by the action of sodium cyanoborohydride — hydrogen chloride [120, 121] on the benzylidene derivative (352), was condensed with the fucosyl bromide (322) to give the trisaccharide (354) in 82 % yield. 

Paulsen s group has transformed the protected streptosyl chloride (or bromide) (521 RS = Ph,H) into dihydrostreptosylstreptidine (522) and related pseudo-disaccharides. ° ° The 2,3-0-benzylidene derivatives (521 R R = Ph,H) yielded a-linked glycosides of L-streptose preferentially, whereas the 2,3-carbonates (521 R R = O) gave / -linked glycosides e.g. with cyclohexanol and l,2,3,4-tetra-0-acetyl-)5-D-glucopyranose). ° This work culminated in the synthesis of D-streptobiosamine (523) and its L-enantiomer (a disaccharide unit of streptomycin) by a route that involved condensation of methyl a-o(or L)-strepto-side 3 -(trimethylene dithioacetal) with the nitrosyl chloride adduct of 3,4,6-tri-O-acetyl-D-(or L)-glucal. ... 

For the synthesis of the D-C portion, two different concepts were followed either by modification of laminaribiose (166) [89] or by a stereospecific P, 1 ->3-glycosylation [20]. Laminaran is isolated from seaweeds or from Poria cocos Wolf, degraded by selective acetolysis, and the lower oligomers separated by preparative HPLC [90]. Following acetylation, the heptaacetyl laminaribiosyl bromide is prepared and transformed into the disaccharide glycal 167 by the classical approach in 93 % yield. The 2-deoxy-2-iodo-a-glycoside is formed by application of the NIS procedure after deprotection and subsequent 4,6-0-benzylidenation, the precursor 168 for the radical formation of the 6,6 -dibromo-6,6 -dideoxy derivative is at hand. This compound may be further reduced to methyl-3-0-(P-D-chinovosyl)-a-D-olivoside (169). 

For the synthesis of the block, 36 was glycosylated with 34 using the inverse procedure [10] affording a high yield of disaccharide (37) (Scheme 5.6). Reductive ring-opening of the benzylidene acetal gave the 4-OH derivative (38), which was coupled with the imidate (35) to yield the trisaccharide block (32). 

A similar series of reactions was performed by Paulsen and HOlck for the preparation of the T-antigenic, unprotected, amino acid-disaccharides 200 and 201, starting from the 4,6-0-benzylidene-N-(benzyloxy-carbonyl) benzyl esters 198 and 199, respectively, by condensation with 110 in the presence of mercury dicyanide - mercury dichloride and molecular sieves 4A, and deprotection of the product. Sinay and co-workers also reported the synthesis of hexa-O-acetyl derivatives of 200 and 201 by application of the sequence of azido-nitration-bromination. 

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