CONCERNING THE ENANTIOMERIZATION BARRIER OF HYPERICIN

The Syntheses of omega-(R)-menthyl and omega,omega-bis-(R)-menthyl der ivatives 2 and 3 of hypericin were achieved, and the corresponding dia stereomers could be separated. The equilibria between the respective d iastereomers are slightly displaced in favor of the chromatographicall y faster moving ones. Kinetic measurements on these easily equilibrati ng diastereomers of 2 and 3 provided an Arrhenius activation energy fo r the interconversion barrier between the two propeller conformers of 83 and 89 kJ/mol. It could be shown that the omega-menthyl residues ar e of minor relevance to the height of this barrier, as is also the cas e for the bay hydroxyl ionization and quinone tautomerization equilibr ia. It was thus concluded that the intrinsic barrier for the propeller conformer enantiomerization of hypericin is in the order of 80 kJ/mol . These results are in accord with those obtained from semiempirical c alculations.