Precise Synthesis of Ester-Functionalized Cyclo[6]- and Cyclo[7]furans

Shape-persistent conjugated macrocycles have attracted interest for their unique optoelectronic and self-assembly properties, but the syntheses to obtain these structures can be laborious. In this work, we describe the straightforward synthesis of a recently discovered class of macrocycle, the cyclo[n]furan, using Suzuki-Miyaura cross-coupling of a simple aromatic monomer. We demonstrate that the combination of hexyl 2-bromo-5-(boronic acid pinacol ester)furan-3-carboxylate with tris(dibenzylideneacetone)dipalladium(0), tri-tert-butylphosphonium tetrafluoroborate and cesium fluoride leads to cyclo[6]- and cyclo[7]furan esters in 45% yield (28% and 17%, respectively). Crude ¹H NMR spectroscopy revealed that total conversion to macrocycles was 52 ± 6% over 3 runs, highlighting the robustness of this protocol for cyclofuran synthesis. The oligomerizations are rapid, and model compound studies suggest that a chain-growth mechanism may be operative. The hexyl-substituted cyclo[n]furan esters (n = 6 and 7) are separable via column chromatography. The unique optical and electronic features for each cycle can be partially explained by the size difference for the two systems, as well as the increased conformational flexibility for the larger, ester-functionalized cyclo[7]furan.