Synthesis of pyruvated saccharide fragments related to the aggregation factor of the marine sponge Microciona prolifera

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Abstract
Treatment of phenyl 2,3,6,2′,3′‐penta‐O‐benzoyl‐1‐thio‐β‐D‐lactoside (7) with BF3 in methyl pyruvate afforded the corresponding phenyl 4′,6′‐O‐[(R)‐1‐(methoxycarbonyl)ethylidene]‐1‐thio‐β‐D‐lactoside (9) in 91% yield. The latter was converted via bromide 10 into the corresponding lactal 11 the azidonitration of which afforded an unseparable mixture of the „gluco”︁‐ and „manno”︁‐configurated azido nitrates 12 (97%). As an alternative, benzyl O‐(β‐D‐galactopyrnosyl)‐(1→4)‐2‐azido‐2‐deoxy‐α‐D‐glucopyranoside (15) was prepared in two steps and 91% yield from the corresponding fully acetylated 2‐azido‐2‐deoxylactosyl bromide 13 and was further modified in four steps to give the benzoylated benzyl 4′,6′‐O‐benzylidene‐2‐deoxy‐2‐[(2,2,2‐trichloroethoxycarbonyl)amino]‐α‐D‐lactoside 18 in 78% overall yield. The benzylidene acetal of the latter was cleaved to give the diol 19 the pyruvation of which as for compound 9 afforded the corresponding pyruvated lactosamine derivative 20 (81%). Hydrogenolysis of 20 followed by treatment with trichloroacetonitrile gave the corresponding pyruvated disaccharide trichloroacetimidate 22 (77%). Coupling of the latter with Z‐protected 5‐amino‐1‐pentanol followed by deblocking of the intermediates furnished 5‐aminopentyl 2‐acetamido‐4′,6′‐O‐[(R)‐1‐carboxyethylidene]‐2‐deoxy‐β‐D‐lactoside (2). Similarly, condensation of 22 with 5‐[(benzyloxycarbonyl)amino]‐pentyl 2,4‐di‐O‐benzyl‐β‐L‐fucopyranoside (25) followed by deblocking of the intermediate trisaccharide afforded 5‐aminopentyl O‐{4,6‐O‐[(R)‐1‐carboxyethylidene]‐β‐D‐galactopyranosyl}‐(1→4)‐2‐acetamido‐2‐deoxy‐β‐D‐glucopyranoside‐(1→3)‐β‐L‐fucopyranoside (3). Saccharides 2 and 3, represent fragments of the Microciona prolifera aggregation factor.

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