Absolute configuration, conformation, and chiral properties of flavanone-(3→8″)-flavone biflavonoids from Rheedia acuminata

Abstract

The absolute configurations of three flavanone-(3→8″)-flavone type biflavonoids, (+)-morelloflavone ( 1 ), (+)-morelloflavone-7-sulfate ( 2 ) and (+)-volkensiflavone-7-sulfate ( 3 ) isolated from Rheedia acuminata were confirmed by circular dichroism as 2 R,3 S, hence clarifying the literature confusion of 2 S,3 R absolute configuration for (+)-morelloflavone. The conformations of this class of biflavonoids were studied for the first time by variable temperature NMR, NOESY, and computational calculations using a semi-empirical AM1 method. The coexistence of two conformers for morelloflavone ( 1 ) in solution at room temperature was confirmed by variable temperature NMR experiments. NOESY and computational calculations indicated that in the major and minor conformers 1a and 1b , respectively, the flavone DEF moiety is extended above and below the plane of the A/C-ring of the flavanone ABC unit, respectively. The C-3 proton of the C-ring of morelloflavone ( 1 ) is exchangeable by deuterium in acetone- d 6/D 2O at ambient temperature with retention of configuration, indicating considerable chiral stability of this class of biflavonoids.