Unique Aromatic Ring Systems have potentiality to show physical, chemical and biological characteristics, and therefore they are attractive to obtain various derivatives according to reasonable molecular design. In this study, we focus on fullerene and squaric acid as the spherical π-electron delocalized unique ring system and the planar non-benzenoid strained ring system.
(1) As for fullerene, various cycloaddition reactions were tried and successful results were obtained. In 1,3-dipolar cycloaddition, the reaction with thiocarbonyl ylide which gave a C_<60> derivative fused with S-containing five membered heterocycle was utilized for introduction of functional groups near the surface by S_N1 substitution, and a series of studies revealed that C_<60> core didi not affect the S_N1 reactivity substancially. Formal [3+2]cycloaddition with isocyanides was found to occur without catalyst, yet the use of Cu_2O was more effective to give dihydroproline- fused C_<60> [4+2]cycloaddition
was carried out with cyclooctatetraene, and the cycloadduct was shown to have C_<60>-based characteristics. The same type of cycloaddition with 3,4-fused pyrrolosulfolene unusually afforded a cycloadduct originating from direct trap of 3,4-dimethylenepyrrole intermediate, and this may reflect high radicophilicity of fullerene. Apart from these cycloaddition, radical and nucleophilic additions aided with inorganic reagents were attempted. The reaction of cyclopropanone silyl acetal was effected by the use of FeCl_3 rathan TiCl_4, also indicating high radicophilicity as above. Under irreversible conditions, inorganic nucleophiles were found to react with C_<60>; the required conditions are to realize trapping of a fullerenyl anion intermediate with a certain reagent.
(2) As for squaric acid, ring transformation was investigated and novel rearrangement induced by the nitrene inter-mediate which was generated at α-position of the cylobutenone ring was developed. In this case, the product was polysubstituted 2-azacyclopentdienone,and its stable structure, despite of intrinsic anti-aromatic nature, was attributed to double resonace. Further synthetic application was performed by cyclization with various mono- and bi- nucleophilic reagent, promising for the formation of nitrogen-heterocycles. Less