Hindered Rotation in Arylnaphthalene Lignans

Abstract

Many arylnaphthalene lignans show biological activity and although few of them contain stereogenic centers, they may nevertheless be chiral if there is hindered rotation about the aryl−naphthalene bond. A relatively high barrier to rotation may give rise to separable rotational enantiomers (atropisomers) which might have quite different pharmacological properties. In order to investigate this possibility we have synthesized the natural products justicidin A, justicidin B, retro-helioxanthin, retro-justicidin B, and helioxanthin as well as four other arylnaphthalenes lignan analogs. We have studied the aryl−naphthalene rotational barrier in these compounds by dynamic NMR and HPLC and find barriers to rotation ranging from 16.9 to 21.5 kcal/mol. This translates to half-lives for individual atropisomers of less than 10 min at room temperature. The experimentally found barriers are compared to those obtained from molecular orbital calculations.