Reductive treatment of stereoisomeric mixtures of variously substituted hexaoxy[6]pericyclynes with SnCl(2)/HCl led to the corresponding substituted carbo-benzenes. Tetramethoxyhexaphenyl[6]pericylynediol and dimethoxyhexaphenyl[6]pericyclynetetrol thus proved to be alternative precursors of hexaphenyl-carbo-benzene, previously described. Another hexaaryl-carbo-benzenic chromophore with 4-pyridyl and 4-anisyl substituents was targeted for its second-order nonlinear optical properties and was obtained by aromatization of a dimethoxy[6]pericyclynetetrol. Two alkynyl substituents in para positions were also found to be compatible with the C(18) carbo-benzene ring, provided that the four remaining vertices are substituted by phenyl groups. In the protected series, bis(trimethylsilylethynyl)hexaphenyl-carbo-benzene (C(18)Ph(4)(C triple bond C-TMS)(2)) could be isolated and fully characterized, even by X-ray crystallography. In the bis-terminal series, the diethynylhexaphenyl-carbo-benzene C(18)Ph(4)(C triple bond C-H)(2) could not be isolated in the pure form. It could, however, be generated by two different methods and identified by the corresponding (1)H NMR spectra. Unsubstituted carbo-benzene C(18)H(6) remains unknown, but tetraphenyl-carbo-benzenes C(18)Ph(4)H(2) with two unsubstituted vertices proved to be viable molecules. Whereas the "para" isomer could be characterized by MS and (1)H and (13)C NMR spectroscopy only in a mixture with polymeric materials, the "ortho" isomer (with adjacent CH vertices) could be isolated, and its structure was determined by using X-ray crystallography. The structure calculated at the B3PW91/6-31G** level of theory turned out to be in excellent agreement with the experimental structure. The (1)H and (13)C NMR chemical shifts of hexa- and tetraphenyl-carbo-benzenes were also calculated at the B3LYP/6-31+G** level of theory and were found to correlate with experimental spectra. The remote NMR deshielding of peripheral protons (through up to five bonds) revealed a very strong diatropic circulation around the C(18) ring, regardless of the substitution pattern. In full agreement with theoretical investigations, it has been demonstrated experimentally that the carbo-benzene ring is "independently" aromatic, in accord with structural-energetic and -magnetic criteria.

Critical analysis of possible strategies for the synthesis of novel carbo-benzene derivatives suggests several [(18-n)+n] routes for the preparation of hexaoxy[6]pericyclyne precursors. Beyond the previously attempted [9+9] symmetrical scheme (n=9), the a priori most selective strategies are those for which n=1, 4, 7, 10, 13, and 16. They involve a cyclizing double-propargylation of a C(18-n) omega-bis-terminal-skipped oligoyne containing (19-n)/3 triple bonds with a C(n) omega-dicarbonyl-skipped oligoyne containing (n-1)/3 triple bonds. To complement the previously used [11+7] strategy, the [14+4] and [8+10] strategies were thus explored. They proved to be efficient, affording seven novel hexaoxy[6]pericyclynes corresponding to six different substitution patterns. These compounds were obtained in 7-15 steps as mixtures of stereoisomers. Thus, by using dibenzoylacetylene as the C(4) electrophile, a [14+4] strategy allowed the synthesis of two hexaphenyl representatives with two or four free carbinol vertices. This approach also afforded tetraphenyl representatives in which the two remaining carbinoxy vertices were substituted with either two alkynyl or one 4-anisyl and one 4-pyridyl groups. By using the hexacarbonyldicobalt complex of butynedial as the C(4) electrophile, a [14+4] strategy also allowed the isolation of a tetraphenylhexaoxy[6]pericyclyne with two adjacent unsubstituted carbinol vertices. A regioisomer with two opposite unsubstituted carbinol vertices was obtained through an [8+10] strategy and its oxidation afforded the corresponding pericyclynedione. Several attempts at synthesizing diphenylhexaoxy[6]pericyclynes with four unsubstituted carbinoxy vertices are described. Only an [8+10] strategy allowed the generation of a fragile diphenylhexaoxy[6]pericyclyne with four adjacent unsubstituted carbinoxy vertices, which could be partly characterized. These results show that the synthesis of the nonsubstituted hexahydroxy[6]pericyclyne, the ring carbo-mer of [6]cyclitol, is a difficult challenge.

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