Syntheses and Structures of 1,1-Dimethyl-1,2,6,10b-tetrahydro-6,10b-o-benzenoaceanthrylenes: Sterically Congested Triptycenes Carrying Five-Membered Ring

Sterically compressed title compounds which carry a five-membered ring condensed to the triptycene skeleton were synthesized by the Diels-Alder reaction of 1,1-dimethyl-1,2-dihydroaceanthrylene with a benzyne. The structures of three compounds were determined by X-ray crystallography. The molecules have several abnormal bond lengths and angles in addition to significant deformation of the benzene rings as well as the triptycene skeleton to avoid the eclipsed conformation about C(1)–C(10b) and C(1)–C(2) bonds. The effects of the substituents at the 7—10 positions are rather small, but an interesting bond angle deformation is found on comparison of the C(10a)–C(10)–substituent bond angles. Molecular mechanics calculation was performed for the dimethoxy compound to compare the calculated structure with the X-ray structure. Molecular flexibility of the unsubstituted compound was discussed on the basis of MM2 calculation.