Medium-sized cyclophanes.: Part 53.: Synthesis and conformational studies, and photoinduced cyclization of syn-[n.2]metacyclophanenes

A series of [n.2]metacyclophanenes (3) and (6) were prepared in good yields by a McMurry cyclization of 1,n-bis(3-acetyl-4-methoxyphenyl)alkanes (2) and 1,3-bis(3-formyl-4-methoxyphenyl)propane (5), respectively. Compounds 3b, 3c exist in the syn-conformation due to the steric repulsion between the methyl groups at the ethano bridge and the methoxy groups at the aromatic rings while compound 6 prefers the anti-conformation typical of [3.2]metacyclo- phanes. The assignment of syn-conformations has been confirmed by H-1 NMR analyses and X-ray diffraction studies. Photoinduced transannular cyclization of [n.2]metacyclophanenes (3) and (6) in the presence of iodine as an oxidant afforded phenanthrene-anellated polycyclic aromatic hydrocarbons. Apparently, the rate of the photocyclization of anti-6 was found to be much faster than that of syn-3b and almost completed within 1 h. Thus, the different reactivities for the irradiation of syn- and anti-conformer were observed. The reason for the present preference for the formation of trans-dihydrophenanthrene rather than cis-dihydrophenanthrene as the intermediate might be attributable to the more stable chair form transition state than boat one and the conformational fixation to the chair form in the ground and transition state is possible in the anti-conformer.