Homotropylidene and its derivatives undergo a fast Cope rearrangement. A homotropylidene system (cyclooctatetraene dimer) is reported that racemizes rapidly by such a valence isomerization in solution at room temperature. In the solid state, however, this rearrangement is frozen, and the single crystals precipitating from the racemic solution contain only one enantiomer. Since very large crystals can be grown, the racemic solution in principle can be converted into a single enantiomer by crystallization.

A simple synthetic route to tetrabenzo [16]-and hexabenzo-[24]annulene systems using one-pot multiple Wittig reactions is presented. We report the characterization of the all-transisomer of tetrabenzo [16]annulene by NMR spectroscopy and X-ray diffraction. Several isomers of an unexpected, tetra-

Polygonal systems P consist of exclusively hexagons and pentagons. They correspond (as chemical graphs) to a certain class of polycyclic conjugated hydrocarbons. This class includes protofullerenes. General formulations for the maximum numbers of internal vertices for P systems with p<6 are derived; here p is the number of pentagons. The CnHs chemical formulas of P systems are studied with special emphasis on the semibuckminsterfullerene (C30H12) , circum-semibuckminsterfullerene (C58H16) and corannulene (C20H10) isomers. Numbers of all CnHs isomers for P systems with r< 6 are computed; here r is the total number of polygons.