The known syntheses of the 9,lO-phenanthrenequinones cally active sorbents. The barriers to thermal racemization 10a-c were improved by the chlorotrimethylsilane modifi-amount to 90-130 kJ mol-' (Table 2) and are thus higher than cation of the acyloin condensation of biaryl diesters. The novel the barriers for the corresponding phenanthrene hydrocarbons quinones 4a, 4b, and 12 were prepared. For the first time, by roughly 20 kJ mol-l. The reasons for this increase are disenantiomers (M and P) of this class of compounds were en-cussed in terms of repulsive interactions in the transition state riched semipreparatively by liquid chromatography on opti-of helix inversion.Wittig and Zimmermann[21 were the first to consider that 4,5-dimethyl-9,1O-phenanthrenequinone (A, R = Me) might be twisted, i.e. chiral. However, its synthesis from an optically active biphenyl derivative at room temperature yielded only inactive material [']. Newman and his coworkers[31 interpreted the polarographic half-wave potentials for the reduction of some quinones A (R + H) in terms of non-coplanar carbonyl groups, their twist being generated by the substituents in 4-and 5-positions. In 5,6-[6]helicenequinone, too, a torsion about the 5,6-dione fragment was assumed[41. Apparently, no further results concerning twisted ortho-quinones have been published. Therefore, we studied the separability of the enantiomers (M)-A and (P)-A with suitable substitution, the aim being the determination of the unknown rates of racemizati~n [~].For this purpose, the known s y n t h e s e~[~~~'~'~' * ] of methylsubstituted 9,lO-phenanthrenequinones which contain nontrivial steps had to be repeated and, if possible, improved. In view of the rates of racemization, it was desirable to prepare some novel representatives of this class of compounds which might help to understand the factors affecting the heights of the barriers.
W -A
SynthesesInitially, we preferred routes to 9,lO-phenanthrenequinones which, in addition, give access to the corresponding hydrocarbons because the chirality of the latter is also of interest. Photocyclization and dehydrogenation, e.g. the conversion of 1 into 2 (Scheme l), can be performed in spite of hindering substituent~ [~]. If the phenanthrene molecule is