Syntheses of 5,6-benzo- and 5,6-naphtho-(1R*,3R*,4S*,8R*)-4,8-dihydroxy-3-methyl-2-oxabicyclo(2.2.2)oct-5-ene derivatives.

A synthetic method for the title compounds has been developed for the syntheses of sarubicin A (1) and granaticin (2). Tetralones (5) and anthracenones (16) were transformed into 1-hydroxy-1-(1-hydroxyethyl) derivatives of tetralins and tetrahydroanthracenes (8, 18) by the route shown in Charts 2 and 4. The diols were dehydrated to allyl alcohols (11, 19) by acid treatment or by reacting their 1'-monoacetates with thionyl chloride followed by alkaline hydrolysis, the choice of procedure being dependent on the structure of the aromatic ring. cis-Dihydroxylation of the olefins by catalytic osmylation using trimethylamine N-oxide as an oxidant afforded the 1R, 2R, 1'R-triols (12, 20) with 98% stereoselectivity, except in the cases of 11a and 19b, where the stereoselectivities were 95 and 90%, respectively. Oxidative ring closure of the triols to the corresponding oxabicycles (13, 21) was achieved by reaction with N-bromosuccinimide under controlled conditions. The intermediate 4-bromo compounds (14, 15) could be isolated in the reactions of 12c, d and underwent smooth cyclization with silver perchlorate in tetrahydrofuran.