Nonplanar aromatic compounds. 5. A strategy for the synthesis of cis-10b,10c-dimethyl-10b,10c-dihydropyrenes. First crystal structure of a cis-10b,10c-dimethyl-10b,10c-dihydropyrene

Through the use of a potentially removable tether, a heavily substituted 10 b,10c-dimethyl-10b,10c-dihydropyrene (DMDHP), 20, was synthesized exclusive ly as the cis-isomer. It exists as the major component (20:1) in an equilib rium with its valence isomer syn-[2.2]metacyclophanediene 19. An X-ray crys tal structure determination of 20, a cis-(2,7)-10b,10c-dihydropyrenophane, provided the first experimental. measurements of the cis-DMDHP skeleton. Th e observed bond alternation in the [14]annulene was found to be larger than that of the corresponding trans-DMDHP framework. Prior MMPI calculations, on which previous discussion of the structure of the cis-DMDHP system had b een based, are in very good agreement with the experimental results. Our ow n DFT calculations predict a more symmetric and more bond equalized structu re than was observed in 20.