Functional [6]pericyclynes: synthesis through [14+4] and [8+10] cyclization strategies

Critical analysis of possible strategies for the synthesis of novel carbo-benzene derivatives suggests several (18-n)+n] routes for the preparation of hexaoxy6]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 hexaoxy6]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 tetraphenylhexaoxy6]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 diphenylhexaoxy6]pericyclynes with four unsubstituted carbinoxy vertices are described. Only an 8+10] strategy allowed the generation of a fragile diphenylhexaoxy6]pericyclyne with four adjacent unsubstituted carbinoxy vertices, which could be partly characterized. These results show that the synthesis of the nonsubstituted hexahydroxy6]pericyclyne, the ring carbo-mer of 6]cyclitol, is a difficult challenge.