Crocacin A

[structure: see text] The total synthesis of (+)-crocacin D is described. The convergent asymmetric synthesis relies on the use of a Stille cross-coupling between an (E)-vinyl stannane with an (E)-vinyl iodide to establish the (E,E)-dienamide moiety followed by a mild and efficient copper-catalyzed coupling between (+)-crocacin C and a (Z)-vinyl iodide to establish the challenging (Z)-enamide function.

Crocacin was isolated from the biomass of the myxobacterium Chondromyces crocatus, strain Cm c3. It inhibited the growth of a few Gram-positive bacteria and a wide spectrum of yeasts and molds. In beef heart submitochondrial particles, crocacin blocked the electron transport within the bc1-segment (complex III) and caused a red shift in the reduced spectrum of cytochrome b with a maximum at 569 nm.

Extensive molecular modeling based on crystallographic data was used to aid the design of synthetic analogues of the fungicidal naturally occurring respiration inhibitors crocacins A and D, and an inhibitor binding model to the mammalian cytochrome bc(1) complex was constructed. Simplified analogues were made which showed high activity in a mitochondrial beef heart respiration assay, and which were also active against certain plant pathogens in glasshouse tests. A crystal structure was obtained of an analogue of crocacin D bound to the chicken heart cytochrome bc(1) complex, which validated the binding model and which confirmed that the crocacins are a new class of inhibitor of the cytochrome bc(1) complex.