New axially chiral sulfur compounds: Synthesis and conformational stability of enantiopure 4,4'-biphenanthrene-3,3'-dithiol and related atropisomeric derivatives

Enantiopure (R)- and (S)-4,4'-biphenanthrene-3,3'-dithiol 1a has been prepared for the first time through a synthetic procedure involving in the key step a stereoconservative Newman-Kwart thermorearrangement of the bis-N,N-dimethylthiocarbamoyl ester of (R)- and (S)-biphenanthrol 2b, respectively. The atropisomeric conformations of 1a are not interconverted even at temperatures as high as 285°C, whereas the related biphenanthrothiophene 3 is completely racemized in a few minutes at 250°C. The axially chiral backbone of 1a has been incorporated in a set of novel C 2 symmetry sulfur reagents suitable for a variety of stereoselective reactions.