Diastereoselective, Diversifiable Synthesis and Biological Evaluation of the Virginiamycin Inducers, the Virginiae Butanolides

Streptomyces species are renowned for their ability to produce bioactive natural products (NPs) via biosynthetic gene clusters (BGCs). However, many BGCs remain transcriptionally silent under standard laboratory conditions. Among the key regulatory mechanisms for NP biosynthesis are the γ-butyrolactone (GBL) signaling molecules, which have been widely studied for their role in repressor-molecule circuits. While the S. coelicolor butanolides (SCBs) and A-factor from S. griseus have been extensively studied, the virginiae butanolides (VBs) from S. virginiae, which alleviate repression of the biosynthesis of the antibiotic virginiamycins via binding to the cluster situated TetR-like repressor BarA, remain understudied. This is in large part due to limited access to enantiopure VBs. Herein, we report a diastereoselective and diversifiable route to access the VB hormones, starting from a protected (R)-paraconyl alcohol intermediate. A library of VB derivatives was synthesized and tested for their ability to alleviate repression of BarA using a newly developed green fluorescent protein (GFP) reporter assay. The synthesis and assay described herein established the most quantitative structure-activity relationship (SAR) analysis of the VBs to date. Overall, this study provides new tools for probing NP regulation in Streptomyces and enables new strategies for BGC activation using synthetic GBL molecules.