Papulacandins and chaetiacandin: a stereoselective route to their basic skeleton by a palladium-mediated arylation of 4,6-O-benzylidene-3-O-tert- butyldimethylsilyl-l-tributyl-stannyl-D-glucal.

Palladium(0)-catalysed coupling of 1,5-anhydro-4,6-O-benzylidene-3-O-tert- butyldimethylsilyl-2-deoxy-1-tributylstannyl-D-arabino-hex-1-eni tol (7) with 3,5-dibenzyloxy-2-bromobenzyl alcohol gave 1,1(2)-anhydro-4,6-O-benzylidene-3-O-tert-butyldimethylsilyl-2-deoxy-1-( 4,6- dibenzyloxy-2-hydroxymethyl-phenyl)-alpha-D-arabino-hexopyranos e (13). The same reaction buffered by sodium carbonate provided 1,5-anhydro-4,6- O-benzylidene-3-O-tert-butyldimethylsilyl-2-deoxy-1-(4,6-dibenzyloxy+ ++-2- hydroxymethyl-phenyl)-D-arabino-hex-1-enitol (11). Stereoselective oxidative spiroacetalisation of 11 provided 1,1(2)- anhydro-4,6-O-benzylidene-3-O-tert-butyldimethylsilyl-1-(4,6-dibenzyl oxy-2- hydroxymethyl-phenyl)-alpha-D-glucopyranose (15), the basic tricyclic structure of papulacandins. In a model study, 15 was converted in three steps into 1,1(2)-anhydro-1-(4,6-dihydroxy-2-hydroxymethylphenyl)-3-O- octadecanoyl-alpha-D-glucopyranose (24), a structural analogue of papulacandin D. Moreover, stereoselective hydroboration-oxidation of 11 furnished 2-(4,6-O-benzylidene-3-O-tert-butyldimethylsilyl-beta-D- glucopyranosyl)-3,5-dibenzyloxy-1-hydroxymethylbenzene (26), the structural skeleton of the chaetiacandin 3.