Diethyl dithioacetal methylation

The formation of the tranu-l, 2-glycofuranosides is effected in neutral solution, where anomerization cannot occur. Treatment of 3,4-O-isopropyl-idene-2,5-di-O-methyl-L-rhamnose diethyl dithioacetal with mercuric chloride in boiling methanol gave" a 61% yield of the -L-furanoside no trace of the 0-l anomer was found. In this experiment, with an acidic solution (no mercuric oxide present), anomerization could have led to the formation of the more stable anomer. In the normal reactions cited, with maintenance of a neutral solution, no anomerization can occur, and yet the more stable anomer, presumably the kinetic product, is still the major anomer formed. 

The u-deoxy sugars studied (6-deoxy and 5-deoxy) have intense peaks for the dithioacetal fragment, and also break down to Fragments A, B, and C. < -Deoxyhexose dithioacetals cannot be differentiated from 5-deoxy-hexose dithioacetals. The deoxy function is, apparently, too far removed from the bonds most susceptible to cleavage to have a significant effect. Mass spectrometry can also be useful in the characterization of terminal branched-chain deoxy sugars. The spectrum of 6-deoxy-5-C-methyl-D-xylo-hexose diethyl dithioacetal 1 contains prominent peaks due to Fragments A, B, and C. Apparently, substitution beyond C-4 does not affect... 

Closely similar results were obtained with various diphenyl dithioacetal acetates (7), with the unsubstituted diethyl dithioacetals (8), and with the aldehydo-pentose peracetates (9) and the tetra-O-acetylaldo-pentose dimethyl acetals (10). Subsequent work in other laboratories has shown the same general principles for the methyl 5-hexulosonates (11) and the pentononitrile tetraacetates (12), two examples where a full series of stereoisomers has been studied. Other workers have investigated isolated examples or partial series (13, 14, 15, 16, 17, 18), and parallel work by x-ray crystallography (19, 20, 21, 22) on acyclic sugar derivatives in the solid state has shown excellent correlation with the general principles outlined here for the molecules in solution. 

The inner disaccharide unit of the trisaccharide hapten of the M. avium serovar 8 GPL148 was assembled in a manner similar to that of the serovar 20, but with reaction of the rhamnosyl trichloroacetimidate (80b) with the benzylidene acetal (81). O-Deacetylation of the product yielded the disaccharide acceptor (84) for the next glycosylation. Incorporation of the pyruvate acetal moiety into the terminal 3-O-methyl-D-glucose residue of 85 was achieved by transacetalation with methyl pyruvate diethyl dithioacetal, with sulfuryl chloride-triflic acid as catalyst. From the mixture of products the desired diastereomer was separated and converted by successive O-debenzylation, acetylation, selective 1-O-deacetylation, and reaction with trichloroacetonitrile into the trichloroacetimidate 86. Reaction of glycosyl donor 86 with acceptor 84, with trimethylsilyl triflate as promoter, afforded fully... 

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