Tautomeric equilibria in phenolic A-ring derivatives of prodigiosin natural products.

The prodigiosin natural products contain a common 4-methoxy-pyrromethene chromophore that is attached to a pyrrole A-ring that has its lone-pair nitrogen electrons in conjugation with the pyrromethene entity. This feature is known to play a key role in the biological activities (anticancer, antimicrobial, and immunosuppressive) of the prodigiosins. In an attempt to alter or improve upon the therapeutic potential of the prodigiosins, we have synthesized two new isomeric analogues that contain phenolic A-ring systems (a para (p)-phenol; an ortho (o)-phenol with respect to the pyrromethene) with lone-pair oxygen electrons in conjugation with the pyrromethene chromophore of the natural product.