Viridin

Herein we describe concise enantioselective chemical syntheses of (-)-viridin and (-)-viridiol. Our convergent approach couples two achiral fragments of similar complexity and employs an enantioselective intramolecular Heck reaction to set the absolute stereochemical configuration of an all-carbon quaternary stereocenter. To complete the syntheses of these base- and nucleophile-sensitive natural products, we conduct carefully orchestrated site- and diastereoselective oxidations and other transformations. Our work is the first to generate these targets as single enantiomers.

The asymmetric total synthesis of (-)-viridin and (-)-viridiol, antifungal metabolites, was achieved in 17 and 18 steps from a commercially available starting material. An intramolecular [3+2] cycloaddition was applied to an easily available l-ribose derivative in order to construct the highly substituted D ring containing the key chiral cis-triol fragment. Co-catalyzed metal-hydride H atom transfer (MHAT) radical cyclization was utilized to form the C-ring and the all-carbon quaternary center at C-10. This convergent strategy provides a scalable approach to prepare viridin and viridiol for biological studies.

Biotransformation of viridin, an antifungal produced by biocontrol agent, with non-viridin producing microorganisms is studied. The results show that some environmental non-targeted microorganisms are able to reduce it in the known phytotoxin viridiol, and its 3-epimer. Consequently, this reduction, which happens in some cases by detoxification mechanism, could be disastrous for the plant in a biocontrol of plant disease. However, a process fermentation/biotransformation could be an efficient approach for the preparation of this phytotoxin.