Add time:08/09/2019         Source:sciencedirect.com

Two syntheses of 6-deoxy-2,4-di-O-methyl-d-allopyranose (11) are described. One route utilised the methylation of methyl 6-deoxy-β-d-allopyranoside 3-carbanilate (4) which occurred with concomitant migration of the ester group to give a mixture of methyl 6-deoxy-2,4-di-O-methyl-β-d-allopyranoside 3-(N-methyl-carbanilate) (5) and methyl 6-deoxy-2,3-di-O-methyl-β-d-allopyranoside 4-(N-methyl-carbanilate) (6). The mixture of 5 and 6 was treated with lithium aluminium hydride to give methyl 6-deoxy-2,4- and 2,3-di-O-methyl-β-d-allopyranoside (7 and 8) which were separated on silica gel and hydrolysed with acid to yield the free sugars 11 and 12, respectively, which were characterised as the crystalline β-diacetates 13 and 14, respectively. An unequivocal synthesis of 11 was accomplished by a series of transformations on methyl 3-O-benzyl-2-O-methyl-α-d-allopyranoside (15). Methyl 6-deoxy-3,4-O-isopropylidene-β-d-allopyranoside (21) was converted into methyl 6-deoxy-3,4-di-O-methyl-β-d-allopyranoside (27) by sequential benzylation, graded hydrolysis with acid, methylation, and hydrogenolysis. Acid hydrolysis of glycoside 27 liberated 6-deoxy-3,4-di-O-methyl-d-allopyranose (28) which was characterised as the crystalline β-diacetate (29).The reaction of methyl 6-deoxy-α-l-mannopyranoside with triphenylboroxole yielded a cyclic benzeneboronic ester having the 2,3-structure 31.

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