Stereoselective syntheses of racemic quercitols and bromoquercitols starting from cyclohexa-1,4-diene: gala-, epi-, muco-, and neo-quercitol

The efficient synthesis of gala-, epi-, neo-, and muco-quercitols and some brominated quercitols starting from cyclohexa-1,4-diene is reported. Treatment of the dibromide, obtained by the addition of bromine to cyclohexa-1,4-diene, with m-chloroperbenzoic acid (m-CPBA) yielded the dibromoepoxide, which was successfully converted to the desired dibromodiol by treatment with sulfuric acid. The resulting diol was reacted with 2,2-dimethoxypropane to give the dibromoketal. Hydrogen bromide elimination with NaOMe gave the key compound methoxyketal, rel-(3aS,5R,7aS)-5-methoxy-2,2-dimethyl-3a,4,5,7a-tetrahydrobenzo[d][1,3]dioxole. The second key compound, an isomeric methoxyketal, was prepared by ketalization of 4,5-dibromocyclohexane-1,2-diol with dimethoxypropane followed by the reaction with NaOMe. Deprotection of ketal functionality with sulfuric acid followed by acetylation with acetic anhydride in pyridine resulted in the formation of diacetate rel-(1S,2R,5R)-5-methoxycyclohex-3-ene-1,2-diyl diacetate. Epoxidation of the double bonds in isomeric methoxy diacetates and cis-hydroxylation followed by epoxide-opening and deprotection resulted in the formation of various quercitol derivatives. The inhibition activity of eleven quercitols, methoxyquercitols and bromoquercitols was tested against alpha-glycosidase.