Medium-sized cyclophanes.: Part 52:: synthesis and structures of [2.n] metacyclophane-1,2-diones

A series of syn- and anti-[2.n]metacyclophan-1-enes, 3, and [2.n]metacyclophane-1,2-diols, 4, were prepared in good yields by a McMurry cyclization of 1,n-bis(5-tert-butyl-3-formyl-2-methoxyphenyl)alkanes 2. Upon increasing the length of the methylene bridge higher yields of 3 were obtained. The assignment of the syn and anti conformations has been confirmed by H-1-NMR analyses. The 1,2-diol derivatives 4 were converted to the 1,2-diones 9 by Swern oxidation. However, [2.2]metacyclophane-1,2-dione, 9a, was found to be quite labile under treatment by silica gel column chromatography and on refluxing in toluene to give the dicarboxylic acid 10 in quantitative yield. Thus, a trapping reaction of diketone 9a was attempted, in which the crude diketone 9a was treated with o-phenylenediamine in ethanol at room temperature for 24 h to afford in almost quantitative yield the desired [2.2]metacyclophane 11 having a quinoxaline skeleton. Similarly, in the case of [2.3]- and [2.4]metacyclophanes, Swern oxidation of the trans-diols trans-4b,c also afforded the desired diketones 9b,c in quantitative yields as stable yellow prisms.