Total synthesis and structural refinement of the cyclic tripyrrole pigmentnonylprodigiosin

The first total synthesis of the cyclic prodigiosin derivative 4 is describ ed, which constitutes a potential lead compound for the development of immu nosuppressive agents. The key steps of this approach comprise a palladium-c atalyzed Suzuki cross coupling reaction of the rather unstable pyrrole boro nic 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 a chieved by using the ruthenium indenylidene complex 21 as precatalyst. X-ra y data of product 18.HCl thus formed suggest that the tautomeric form B pro perly describes the electron distribution within the heteroaromatic segment of this alkaloid, in which the central ring constitutes the azafulvene uni t of the pyrrolylpyrromethene chromophore.