Syntheses of acetylenic oligophenylene macrocycles based on a novel Dewar benzene building block approach

A general synthetic approach to strained p-phenylene-based acetylenic macro cycles is described. A key feature in this approach is exploitation of Dewa r benzene as an angular p-phenylene synthon. Thus, 1,4-acetal-bridged 2,5-d ichloro(Dewar benzene) 5, prepared in four steps from dimethyl acetylenedic arboxylate and 1,2-dichloroethylene, is applied as such a building block in the syntheses of strained macrocycles 13 and anti-20. For the synthesis of 13, m-phenylene units are used as spacers and modified Eglington-Glaser co upling is applied for the macrocyclization step. For the synthesis of anti- 20, on the other hand, o-phenylene units are used as spacers and Sonogashir a coupling is applied for the macrocyclization step. Macrocycles 13 and ant i-20 are characterized crystallographically? and their strained nature is r eflected mainly in the deviation of the acetylene units from linearity; the C=C-C angles range from 168.7(3)degrees to 179.9(3)degrees in 13 and from 168.0(5)degrees to 171.4(4)degrees in anti-20, Macrocycle 13 shows unique c onformational property, namely, the p-phenylene units arranged in parallel in the rectangular framework rotate freely about the long axes, as evidence d by the H-1 NMR studies. Macrocycle anti-20 exhibits a Stokes shift of 179 nm, which is exceptionally large for phenylacetylene macrocycles, presumab ly owing to the characteristic stacking structure.