The Long and the Short of It: Regiospecific Syntheses of Isomers of Dicarbomethoxydibenzo-27-crown‑9 and Binding Abilities of Their Pyridyl Cryptands

The two isomers 6 and 9 of cis(4,4′-)-dicarbomethoxydibenzo-27-crown-9 with tri- and tetra-(ethyleneoxy) linkages transposed were synthesized regiospecifically in high yields (94 and 92%, respectively) by the Wang–Pederson–Wessels (WPW) protocol and were converted via the corresponding diols 7 and 10 to the corresponding pyridyl cryptands 3 and 4 by reaction with pyridine-2,6-dicarbonyl chloride. As expected from Corey–Pauling–Koltun (CPK) models, the cryptand with the tri­(ethyleneoxy) arm para to the ester linkages, “short-armed” cryptand 3, did display a higher binding constant (K_a = 2.4 × 10⁵ M^–1) with paraquats than the analogous dibenzo-30-crown-10-based cryptand previously studied; however, the effect was only twofold. Its binding to diquat was reduced by an order of magnitude (1.5 × 10⁵ M^–1), as expected on the basis of its narrower cavity. Also as expected, the cryptand with the tetra­(ethyleneoxy) arm para to the ester linkages, “long-armed” cryptand 4, possessed diminished binding with both paraquats and diquat relative to the 30-crown-based analogue; in these systems, 2:1 H:G complexes were also detected by mass spectrometry. A crystal structure is reported for 3·DQ­(PF₆)₂.