Stereoselective isomerisations of 4-(3′,5′-dimethoxyphenyl)-2,5-dimethyl-1,3-dioxolanes. Temperature-dependent formation of either isochromanes or dihydroisobenzofurans

Abstract

Stereoselective isomerisation of rel-(2R,4S,5R)-4-(3′,5′-dimethoxyphenyl)-2,5-dimethyl-1,3-dioxolane 7 with titanium tetrachloride affords rel-(1R,3R,4S)-4-hydroxy-6,8-dimethoxy-1,3-dimethylisochromane 19 and its C-1 epimer 20 in high yield. The former predominates at a reaction temperature of –78 °C and the latter at –30 °C. Similar isomerisation of the 1 : 1 mixture of rel-(2S,4R,5R)- and rel-(2S,4R,5R)-4-(3′,5′-dimethoxyphenyl)-2,5-dimethyl-1,3-dioxolanes 8 and 9 gives rel-(1R,3R,4R)-4-hydroxy-6,8-dimethoxy-1,3-dimethylisochromane 29 and its C-1 epimer 31, with the latter predominating at both –78 and –30 °C. At 0 °C, dioxolane 7 is isomerised to rel-(1S,1′R,3R)-1-(1′-hydroxyethyl)-4,6-dimethoxy-3-methyl-1,3-dihydroisobenzofuran 25 and its C-3 epimer 26 as the sole reaction products in a 10 : 1 ratio. Dioxolanes 8 and 9 are similarly converted into rel-(1R,1′R,3S)-1-(1′-hydroxyethyl)-4,6-dimethoxy-3-methyl-1,3-dihydroisobenzofuran 32 and its C-3 epimer 33. These furans probably arise through further isomerisation of the intermediate isochromanes at the higher reaction temperatures.