Variantes de mycosides caractérisées par des résidus glycosidiques substitués par des chaînes acyles

The hydrogenolysis of peracetylated mycoside G′ cleaves this mycoside between the aromatic nucleus and the acylated sugars. The mass spectrum of one of these sugars is typical of an ethyl 2‐O‐mycolyl‐3‐O‐methyl‐4‐O‐acetyl‐6‐deoxypyanoside (I): The highes peaks (m/e 1243, 1215 and 1187) are due to splitting on both sides of the methyl branching of the mycolic chain. Other important peaks prove that the sugar cycle is pyranic, that the mycolic chain is bound to the hydroxyl carried by C‐2 and that the methoxyl group is on C‐3. The acyl chain (IV) fragments (m/e 995, 967 and homologues) are typical of a mycolic acid with a methoxyl group (m/e 253, 281, 325) and a C22 side chain, as indicated by the +CH=6;Ċ–C22H45 peak. The structure of mycosides A, B and G comprising polar sugars bound through an aromatic nucleus to an apolar aglycone suggests that these substances might function as a carrier or cofactor in the vicinity of the bacterial membrane or close to the bacterial cell wall. That mycolic acids which esterify hydroxyl groups of mycoside sugars are found to exist favors this conclusion, the more so as mycolic substances are constituants of the cell walls (arabinogalactane mycolate) and of the bacterial outer layer (cord factor).