Total Synthesis and Structural Refinement of the Cyclic Tripyrrole Pigment Nonylprodigiosin

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Abstract
The first total synthesis of the cyclic prodigiosin derivative 4 is described, which constitutes a potential lead compound for the development of immunosuppressive agents. The key steps of this approach comprise a palladium-catalyzed Suzuki cross coupling reaction of the rather unstable pyrrole boronic acid derivative 17 with the electron rich pyrrolyl triflate 15 followed by a ring-closing metathesis reaction (RCM) of the resulting diene to form the macrocyclic ring of the target molecule. This transformation is best achieved by using the ruthenium indenylidene complex 21 as precatalyst. X-ray data of product 18·HCl thus formed suggest that the tautomeric form B properly describes the electron distribution within the heteroaromatic segment of this alkaloid, in which the central ring constitutes the azafulvene unit of the pyrrolylpyrromethene chromophore.