P-Methoxybenzyl glycosides

SCHEME.—Syntheses of allyl glycosides of distal disaccharide units of GPLs from M. avium serovars 4 and 20 involving regioselective substitutions in the preparations of glycosyl acceptors. MPM = 4-methoxyphenylmethyl [p-methoxybenzyl] NPM = 2-nitrophenylmethyl [o-nitrobenzyl]. 

Protic acids will also cleave p-methoxy benzyl ethers under comparatively mild conditions. At 90 cC, acetic acid is sufficient352 but the reaction works at room temperature with as little as 0.5% trifluoroacetic acid in dichloromethane.353 p-Methoxybenzyl groups can be removed selectively with trifluoroacetic acid in the presence of secondary glycosidic linkages, azides, or phenylthio glycosides [Scheme 4.189],354 The survival of a trisaccharide under the deprotection conditions indicates that p-methoxybenzyl groups may be useful in oligosaccharide synthesis. 

More specific anomeric protecting groups are the 2-trimethylsilylethyl (TMSE) [25] and p-methoxyphenyl glycosides [26], both introduced by glycosylation reactions on protected derivatives (Scheme 3.5). The former is cleaved by treatment with BF3-etherate conditions in which TBDMS ethers survive. p-Methoxyphenyl glycosides are cleaved by CAN-oxidation similarly to p-methoxybenzyl ethers. 

Similarly, DDQ oxidation of the p-methoxybenzyl group of 269 in the presence of alcohol 270, a strategy reported by Ito and Ogawa [119] in O-glycoside s)uithesis, provides benzylidene acetal 271 [120] (O Scheme 57). 

In a third variant on this theme, methyl 3,4,6-tri-O-benzyl-l-thio-a-D-mannopyranoside is converted into a p-methoxybenzyl acetal by first etherifying the 2-OH group with p-methoxyben/yl bromide and then treating the resulting p-methoxybenzyl ether with a hydroxyl compound in the presence of 2,3 -dichloro-5,6-dicyano-l,4-benzoquinone (DDQ). Activation of the methylthio anomeric center with methyl triflate then produces the (3-D-mannopyranoside as the only possible glycoside, again because of the acceptor being tethered to the (3 side of the mannopyranoside.56... 

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