1. 1. General 1. 1. 1. Biaryls-Structural and Biosynthetic Variety The
biaryl axis is the joint central structural element of a broad variety
of structurally (in particular stereochemically) and biosynthetically
inter- esting as well as pharmacologically promising natural products.
The increasing importance of this challenging class of secondary
metabolites, its widespread occurrence, its pharmacological
implications, and, in particular, the phenomenon of axial chirality,
have so far been largely underestimated, if not neglected. This is
reflected by the fact that there is as yet no true comprehensive review
on naturally occurring biaryls in the literature. By this article, we do
not intend to provide such a fully comprehensive review, but still want
to draw the attention of the scientific community on the manifold
rewarding facets of this exciting class of natural products, which is
rapidly growing now, with more and more sophisticated analytical and
synthetic tools becoming available. Natural biaryls occur far more
frequently than initially assumed, among them constitutionally
symmetric, simple dimeric monoterpenoid biphenyls like I (I) (see Fig.
1), binaphthalenes like (-)-gossypol (2) (2), bianthraquinones like
skyrin (3) (3-7), and biaryls with annelated heterocyclic rings like
bismurrayafoline-B (4) (8) and desertorin A (Sa) (9, 10), which
is-although built up from a single monomeric precursor -constitutionally
unsymmetric.