"New Axially Chiral Sulfur Compounds: Synthesis and Conformational Stability of Enantiopure 4,4'-Biphenantrene-3,3'-dithiol and Related Atropisomeric Derivatives

Enantiopure (R)- and (S)-4,4'-biphenanthrene-3,3'-dithiol la 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 la 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 la has been incorporated in a set of novel C2 symmetry sulfur reagents suitable for a variety of stereoselective reactions