The synthesis of 1,4:11,14-di-oxa-5,9,l7-tri-cis-7,15-di-~18]annulene 4, whose periphery is forced to be twisted by two oxygefiridges is described.
Ever since [18]annulene tri-oxide l&l was reported as the first example of tri-bridged [18]annulene, consequences of the reduction of the number of bridges in the system have not been studied until 2' and A5 were reported recently. These studies indicated that themoreinteresting aspects of bridged annulene containing furan rings come from di-bridged annulene series rather than from tri-bridged ones: viz., 2 proved to be a "mobile dehydroannulene", in which exchange, coalescence, and freezing of the protons on the trans double bond (Ha and Hb) are quite temperature-dependent. On the contrary 3 proved to be a rigid annulene, and it displayed themostprominent diatropism ever recorded, -far beyond the diatropism of [18]annulene itself-, indicating the significance of equivalent Kekul; structures of 3 and & as for benzene. In order to get further knowledges on di-bridged annulenes in this series we addressed ourselves to prepare 4, a position isomer of 3 made up of same building blocks (i.e.,tensp2 carbons and two furan rings). While 3 provides a snag-fitted inner space for two oxygens and two hydrogens in the pseudohexagonal frame, 3 provides onlyapoor accommodation for these atoms dueto the unsymmetrically arranged cavity. Thus, it is clear that coplanar sp2 carbon arrangement is not met in 3. Model considerations indicate that twisting occurs at both the diene and triene parts. Especially, an extensive twisting arises in the C-9 carbon atom as the consequence of steric interaction between the H-7 and the oxygen bridge linked at the C-11 and 14 positions (dihedral angle of C-8 and C-9 is ca. 45" from model). Another torsional strain which exists in the diene moiety expels the H-16 from the inside to the outside positon of the annulene ring. Therefore,