PAPULACANDINS AND CHAETIACANDIN - A STEREOSELECTIVE ROUTE TO THEIR BASIC SKELETON BY A PALLADIUM-MEDIATED ARYLATION OF 4,6-O-BENZYLIDENE-3-O-TERT-BUTYLDIMETHYLSILYL-1-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-enitol (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-hexopyranose (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,(2)-anhydro-4,6-O-benzylidene-3-O-tert-butyldimethylsilyl-1-(4,6-dibenzyloxy-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.