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.
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