Axial Chirality

Glycosides of benzodioxole-indole alkaloid 6-hydroxy-galanthindole (7-(6'-(hydroxymethyl)benzo[d][1',3']dioxol-5'-yl)-1-methyl-1H-indol-6-ol) having axial chirality were isolated from Narcissus cultivar 'Dutch Master'. The structure, including absolute configuration, was determined by means of extensive spectroscopic data such as UV, IR, CD, MS, 1D and 2D NMR spectra, and computational chiroptical methods. The aglycone has a structure containing two aromatic moieties with substituents hindering rotation about the biaryl axis and is connected to a saccharide moiety linked at C-6 and made up of one, two, or three sugars (glucose, alpha-L-rhamnopyranosyl-(1-->6)-beta-D-glucopyranose, and trisaccharide ([beta-D-xylopyranosyl(1-->2)]-[alpha-L-rhamnopyranosyl-(1-->6)]-beta-D-glucopyranose).

A rotationally hindered and thus stereogenic biaryl axis is the structurally and stereochemically decisive element of a steadily growing number of natural products, chiral auxiliaries, and catalysts. Thus, it is not surprising that significant advances have been made in the asymmetric synthesis of axially chiral biaryl compounds over the past decade. In addition to the classic approach (direct stereoselective aryl–aryl coupling), innovative concepts have been developed in which the asymmetric information is introduced into a preformed, but achiral—that is, symmetric or configurationally labile—biaryl compound, or in which an arylC single bond is stereoselectively transformed into an axis. This Review classifies these strategies according to their underlying concepts and critically evaluates their scope and limitations with reference to selected model reactions and applications. Furthermore, the preconditions required for the existence of axial chirality in biaryl compounds are discussed.