Studies on the epipolythiodioxopiperazine alkaloid verticillin D: Scaled production, streamlined purification, and absolute configuration

Verticillins, epipolythiodioxopiperazine alkaloids that were first described over 50 years ago, have undergone extensive cytotoxic and pharmacological evaluations over the last decade. However, of the 27 verticillin analogues in the literature, the chemistry of verticillin D, which has two additional secondary hydroxy moieties, relative to verticillin A, has remained largely unexplored since its discovery in 1999. With the goal of advancing our understanding of verticillin D, there were three main objectives with this study: improving production, streamlining purification, and assigning absolute configuration via X-ray crystallography. To begin, the production of verticillin D was analyzed across seven fungal strains, and the top producer was further assessed under two fermentation conditions. Clonostachys rosea (strain MSX51257) biosynthesized the highest amount of verticillin D, with production peaking between 15 and 25 days on rice media. Interestingly, in contrast to similar studies that yield verticillin A, the biosynthesis of verticillin D was not accompanied by a suite of structurally related verticillin analogues. As such, the purification of verticillin D was more rapid and could be accomplished without the use of HPLC. These materials were used, in part, to determine the absolute configuration of verticillin D via X-ray crystallography, allowing for assignment of the asymmetric centers at both the 13 and 13' positions as R, which has never been accomplished. This is only the third report of an X-ray structure of a verticillin analogue.