HELICAL PHENANTHRENES .3. SYNTHESES, ENRICHMENT OF ENANTIOMERS, AND BARRIERS TO RACEMIZATION OF TWISTED 9,10-PHENANTHRENEQUINONES

The known syntheses of the 9,10-phenanthrenequinones 10a-c were improved by the chlorotrimethylsilane modification of the acyloin condensation of biaryl diesters. The novel quinones 4a, 4b, and 12 were prepared. For the first time, enantiomers (M and P) of this class of compounds were enriched semipreparatively by liquid chromatography on optically active sorbents. The barriers to thermal racemization amount to 90-130 kJ mol-1 (Table 2) and are thus higher than the barriers for the corresponding phenanthrene hydrocarbons by roughly 20 kJ mol-1. The reasons for this increase are discussed in terms of repulsive interactions in the transition state of helix inversion.