Stereoselective Isomerizations of 4-(2´-Chloro-3´-methoxyphenyl)-2,5-dimethyl-1,3-dioxolanes: Stereochemistry and Conformation of the Product 2-Benzopyrans

Abstract

Stereoselective isomerization of rel-(2R,4S,5R)-4-(2′-chloro-3′-methoxyphenyl)-2,5-dimethyl-1,3-dioxolane 5 with titanium(iv) chloride afforded solely rel-(1R,3R,4S)-5-chloro-4-hydroxy-6-methoxy-1,3-dimethyl-2-benzopyran 17 in high yield in which the conformation adopted by the dihydropyran ring minimized peri-interactions through stereochemistries that were axial for the C-3 methyl, pseudoaxial for the C-4 hydroxy and pseudoequatorial for the C-1 methyl groups. Similar isomerization of the individual rel-(2S,4R,5R)- and rel-(2R,4R,5R)-diastereoisomeric dioxolanes 6 and 7 gave solely the corresponding rel-(1S,3R,4R)-2-benzopyran 25 in which the orientations of the substituents at C-3, C-4 and C-1 were equatorial, pseudoaxial and pseudoequatorial respectively. These observations differed significantly from those previously made for the related isomerizations of the corresponding 4-(2′-chloro-5′-methoxyphenyl)-2,5-dimethyl-1,3-dioxolanes.