Selective bromination of dihydroquinopimaric acid

The reaction of dihydroquinopimaric acid methyl ester with bromine was found to be chemo- and stereoselective. Regardless of the solvent (acetic acid, methanol, dioxane), bromination of the title compound with an equimolar amount of bromine occurs as electrophilic addition at the double C19=C20 bond with formation of 14α-hydroxy- or 14α-methoxy-19R-bromo derivatives. The reaction with excess bromine (3 equiv) leads to the formation of 16S-bromo derivatives. The bromination process is accompanied by formation of epoxy bridge between the C14 and C20 atoms. X-Ray analysis revealed two polymorphic modifications of (16S,19R)-16,19-dibromo-14β,20-epoxydihydroquinopimaric acid methyl ester.