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

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 2R,3S, hence clarifying the literature confusion of 2S,3R 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 AMI 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 la 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)/D2O at ambient temperature with retention of configuration, indicating considerable chiral stability of this class of biflavonoids. (C) 2002 Elsevier Science Ltd. All rights reserved.