Viridicatumtoxin

The details of the total synthesis of viridicatumtoxin B (1) are described. Initial synthetic strategies toward this intriguing tetracycline antibiotic resulted in the development of key alkylation and Lewis acid-mediated spirocyclization reactions to form the hindered EF spirojunction, as well as Michael-Dieckmann reactions to set the A and C rings. The use of an aromatic A-ring substrate, however, was found to be unsuitable for the introduction of the requisite hydroxyl groups at carbons 4a and 12a. Applying these previous tactics, we developed stepwise approaches to oxidize carbons 12a and 4a based on enol- and enolate-based oxidations, respectively, the latter of which was accomplished after systematic investigations that revealed critical reactivity patterns. The herein described synthetic strategy resulted in the total synthesis of viridicatumtoxin B (1), which, in turn, formed the basis for the revision of its originally assigned structure. The developed chemistry facilitated the synthesis of a series of viridicatumtoxin analogues, which were evaluated against Gram-positive and Gram-negative bacterial strains, including drug-resistant pathogens, revealing the first structure-activity relationships within this structural type.

Penicillium aethiopicum produces two structurally interesting and biologically active polyketides: the tetracycline-like viridicatumtoxin 1 and the classic antifungal agent griseofulvin 2. Here, we report the concurrent discovery of the two corresponding biosynthetic gene clusters (vrt and gsf) by 454 shotgun sequencing. Gene deletions confirmed that two nonreducing PKSs (NRPKSs), vrtA and gsfA, are required for the biosynthesis of 1 and 2, respectively. Both PKSs share similar domain architectures and lack a C-terminal thioesterase domain. We identified gsfI as the chlorinase involved in the biosynthesis of 2, because deletion of gsfI resulted in the accumulation of decholorogriseofulvin 3. Comparative analysis with the P. chrysogenum genome revealed that both clusters are embedded within conserved syntenic regions of P. aethiopicum chromosomes. Discovery of the vrt and gsf clusters provided the basis for genetic and biochemical studies of the pathways.

A new tetracycline-type antibiotic named viridicatumtoxin B along with the known compound viridicatumtoxin has been isolated from the mycelium of liquid fermentation cultures of Penicillium sp. FR11. The structure of viridicatumtoxin B was determined on the basis of MS and NMR data. Viridicatumtoxin B inhibited the growth of Staphylococcus aureus including methicillin-resistant S. aureus and quinolone-resistant S. aureus with MIC (microg/ml) of 0.5, which is similar with that of vancomycin, but 8-64 times higher activity than that of tetracycline.