TAUTOMERISM AND STEREOCHEMISTRY OF ISOHYPERICIN, BROMOHYPERICINES, AND GYMNOCHROMES - FORCE-FIELD INVESTIGATIONS

Using an MM2 derived force field the stereochemistry and tautomerism of isohypericin and several bromohypericines, including gymnochrome D and isogymnochrome D, were investigated. The geometries of the ''propeller'' and ''double butterfly'' conformers of isohypericin were similar to those of hypericin. However, an interconversion barrier between the enantiomeric ''propeller'' conformers was found, which was 53 kJ/mol lower in isohypericin. From the ten tautomers of isohypericin the 7,14-dioxo tautomer was calculated to be the most stable one by about 50 kJ/mol. In the bromo derivatives and the gymnochromes the bromine substituents caused a ruffling of the biaryl fragments. The buttressing effect of the bromine led to an enhancement of the interconversion barriers between enantiomeric conformers. The ''double butterfly'' conformer was found to be more stable than the ''propeller'' conformer for tetrabromohypericin and the gymnochromes.