Synthesis of Central Chirality-Containing Triarylmethanols and Triarylmethyl Radicals with Extraordinarily Stable Configurations

Triarylmethanol adopts a propeller-shaped
conformation with either right-handed (P) or left-handed
(M) configuration. Herein, new triarylmethanols with
two chiral centers were obtained via introduction of two cis-hydroxyl groups on the side chains, affording four stereoisomers.
These four stereoisomers were easily separated by silica gel column
chromatography into two pairs of propeller-shaped enantiomers, as
shown by NMR and X-ray crystallographic studies. High-performance
liquid chromatography (HPLC) studies showed that the configurations
of the hydroxyl-bearing triarylmethanols are much more stable than
those of the bulky tert-butyldimethylsilyl-protected
precursors, inconsistent with the general strategy in which the steric
repulsion is largely responsible for the configurational stability.
Similarly, two hydroxyl-bearing tetrathiatriarylmethyl (TAM) radicals
also exhibit excellent configurational stability and are thus separable
by CS-HPLC into four stereoisomers. Interestingly, both helical chirality
from triaryl group (M or P) and
central chirality (R and S) on the
side chain have little effect on their electron paramagnetic resonance
properties. Our present study provides a new strategy to construct
configurationally stable triaryl compounds and demonstrates that the
side chain on TAM radicals is a new site for their structural modifications.