Synthesis, crystal structure, and resolution of [10](1,6)pyrenophane: an inherently chiral [N]cyclophane

A synthetic approach to a set of three inherently chiral [n]cyclophanes, [n](1,6)pyrenophanes (29a-c, n = 8-10) was investigated. Progress toward 29a was thwarted by the failure of the key dithiacyclophane-forming reaction. For the next higher homologue, the synthesis was completed, but the desired [9](1,6)pyrenophane (29b) could only be partially separated from an isomeric pyrenophane, [9](1,8)pyrenophane (28b), and an unidentified byproduct. Work aimed at the synthesis of the next higher homologue resulted in the isolation of a 7:4 mixture of [10](1,8)pyrenophane (28c) and [10](1,6)pyrenophane (29c), which could not be separated by column chromatography or crystallization. However, single-crystal X-ray structures of 28c and 29c were obtained after manual separation of two crystals with different morphologies from the same batch of crystals obtained from the 7:4 mixture of 28c and 29c. The pyrene system of 29c was found to have a gentle end-to-end bend as well as a significant longitudinal twist. Short intermolecular C(sp(3))-H···π contacts (2.64 to 2.76 Å) between H-atoms on the bridge and the centroids of three of the four six-membered rings of the pyrene system of a neighboring pyrenophane of like chirality give rise to the formation of single enantiomer columns. From a DNMR study of the mixture of 28c and 29c, the bridge in [10](1,8)pyrenophane (28c) was found to undergo a conformational flip from one side of the pyrene system to the other with ΔG(‡) = 14.9 ± 0.2 kcal/mol. A two-stage preparative HPLC protocol was subsequently developed for the separation of 28c and 29c (Chiralpak AD-H column) and then the enantiomers of 29c (Chiralcel OJ-H column). This enabled the measurement of their optical rotations and CD spectra.