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

Treatment of phenyl 2,3,6,2′,3′‐penta‐ O ‐benzoyl‐1‐thio‐β‐ D ‐lactoside ( 7 ) with BF 3 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.