Conformational analysis—XIII: Syn and anti [3.2]-, [3.3]-, [4.2]-, and [4.3]-metacyclophanes

While in 3.3]metacyclophane (19) the aromatic rings preferentially adopt the syn arrangement, its lower and higher homologues, i.e. 2,2]-, 3.2]-, 4.2], and 4.3]-metacyclophane (1, 6, 26 and 30), adopt the anti conformation. Substituted m,n]metacyclophanes do not necessarily behave similarly to the parent hydrocarbons. Substituted compounds exhibiting a different conformation are 3.2]metacyclophane-1,11-dione (7) (syn), 3.3]metacyclophane-2,11-dione (24) and the corresponding bispropylene thioacetal] (25) (anti), 4.2]metacyclophane-2,12-dione (27) (syn), and 4.3]metacyclophane-2,13-dione (31) (syn). Thus, the solution conformation of an m.n]metacyclophane is sensitive both to chain length m.n.] of the bridges and substitution. The ring inversion barriers determined by variable temperature ¹H NMR decrease with increasing length of the bridges and qualitatively correlate with the transanular strain present in the pertinent system.