Anthracene–diacetylene cyclic dimers having two alkoxy groups (OMe, OEt, or OPr groups) at the intraannular positions were synthesized by dimerization of 1,8-diethynylanthracene precursors. For the OEt and OPr compounds, anti- and syn-stereoisomers that differed in the direction of the alkyl groups were separated by chromatography. Kinetic measurements showed that the barriers to isomerization from syn-isomer to anti-isomer were 124 and >146 kJ mol−1 for the OEt and OPr compounds, respectively. The molecular structures of these compounds were examined by X-ray analysis and DFT calculations. The preference for anti-isomers is attributable to the intramolecular C–H···O hydrogen bonds. The effects of substituents on the molecular structures and the barriers to isomerization are discussed.
2013 The Chemical Society of Japan