Intramolecular porphyrin pi,pi-stacking: absolute configurational assignment of acyclic compounds with single chiral centers by exciton coupled circular dichroism.

We report a new concept based on exciton coupled circular dichroism (CD) for assigning absolute configurations to a single chiral center *CXYSL, where X is -OH or -NH2, Y is an acyclic chain with terminal OH or -NH2, and S (small) and L (large) represent sterically distinct groups. It consists of a one step attachment of porphyrins to X and Y followed by CD measurement. The key event is intramolecular porphyrin pi,pi-stacking, which converts the flexible, acyclic substrate into a rigid stacked conformation characterized by bisignate exciton split CD curves. Since the stacked conformer is sterically controlled by the two groups, S and L, the sign of the exciton split CD is directly governed by the spatial arrangement of these groups. This approach is applicable to various acyclic compounds with different C/C distances (1,3 approximately 1,15) between the functional groups, but not to 1,2-C/C.