Medium-sized cyclophanes. Part 57. Synthesis, conformations and stereodynamics of [2.n]metacyclophan-1-enes and their conversion to [2.n]metacyclophan-1-ynes

anti- and syn-[2.8]Metacyclophan-1-enes 3a, which are both conformationally rigid structures, were prepared in good yields by a McMurry cyclization of 1,8-bis(5-tert-butyl-3-formyl-2-methoxyphenyl)octane 2a. Similarly, McMurry cyclization of 1,10-bis(5-tert-butyl-2-methoxy-3-formylphenyl)decane 2b afforded (E)- and (Z)-[2.10]-metacyclophan-1-enes 3b in good yields. The assignment of the E and Z structures was confirmed by (1)H-NMR analyses and single crystal X-ray diffraction studies. Bromination of (E)- and (Z)-3b with benzyltrimethyl- ammonium tribromide affords exclusively the cis-adduct 4b to the bridged double bond. When treated with potassium tert-butoxide in refluxing HOBu(t) at 80 degreesC for 3 h, bromine adduct meso-4b gave the dehydrobrominated product [2.10]metacyclophan-1-yne 6b in 93% yield, along with 1-bromo[2.10]metacyclophan-1-ene 5b in 7% yield; the same reaction with the bromine adduct dl-4b gave 29% 6b along with 5b in 71% yield. Similarly, anti- and syn-[2.8]metacyclophan-1-ynes 6a were also prepared by bromination of syn-[2.n]metacyclophan-1-ene syn-3a, followed by the dehydrobromination of the bromine adduct. The characterization and the reaction pathway of these products are discussed. The dynamics of the ring inversion and UV spectra are also presented.