STEREOSELECTIVE ISOMERIZATIONS OF 3',5'-DIMETHOXYPHENYL)-2,5-DIMETHYL-1,3-DIOXOLANES - TEMPERATURE-DEPENDENT FORMATION OF EITHER ISOCHROMANES OR DIHYDROISOBENZOFURANS

Stereoselective isomerisation of (3',5'-dimethoxyphenyl)-2,5-dimethyl- 1,3-dioxolane 7 with titanium tetrachloride affords S)-4-hydroxy-6,8-d imethoxy-1,3-dimethylisochromane 19 and its C-1 epimer 20 in high yiel d, The former predominates at a reaction temperature of -78 degrees C and the latter at -30 degrees C, Similar isomerisation of the 1:1 mixt ure of rel-(2S,4R,5R)- and (2R,4R,5R)-4-(3',5'-dimethoxyphenyl)-2,5-di methyl- 1,3-dioxolanes 8 and 9 gives R)-4-hydroxy-6,8-dimethoxy-1,3-di methylisochromane 29 and its C-1 epimer 31, with the latter predominat ing at both -78 and -30 degrees C. At 0 degrees C, dioxolane 7 is isom erised to 1'R,3R)-1-(1'-hydroxyethyl)4,6-dimethoxy-3-methyl- 1,3-dihyd roisobenzofuran 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 l)-4 ,6-dimethoxy-3-methyl-1,3-dihydroisobenzofuran 32 and its C-3 epimer 3 3, These furans probably arise through further isomerisation of the in termediate isochromanes at the higher reaction temperatures.