The first structure-activity relationship study on γ-alkylidenebutenolides [ (4Z)- and (4E)-6,7-dihydroxyhepta-2,4-dien-4-olide 6-monobenzoate] (4Z, 3 and 4E, 4) and 7-monobenzoate (4Z, 1 and 4E, 2) which are potent melanogenesis inhibitors (IC50 = 0.3-2.9 μM) isolated from the medicinal plant Melodorum fruticosum, was carried out using the related analogous (7-21). The inhibitory activities of the (4Z)- and (4E)-6,7-dideoxylated compounds (7) completely disappeared, while those of the 6-oxo-7-hydroxy-type compounds (4Z, 20 and 4E, 21) were significantly attenuated compared with those of 1-4. By contrast, the 6,7-dihydroxylated compounds (4Z, 8 and 4E, 9; IC50 = 5.8-1.5 μM) showed inhibitory activities similar to those of 1-4, suggesting that the two hydroxyl groups at positions C6 and C7 on the side chain play an essential role in the onset of potent inhibitory activity. The inhibitory activities of the 6,7-diacylated analogues (10-19, acyl group = Ac, Piv, or Bz; IC50 = 0.7-3.3 μM) were also nearly identical to those of the 6- or 7-monobenzoates (1, 3, and 4), regardless of the geometry of the double bond. However, acylation of the strongest inhibitor (4E, 2; IC50 = 0.3 μM) at position C6, reduced the activity by approximately 1/3 to 1/5. This result suggests the importance of the cooperative role of the acyl moiety at position C7 and the hydroxyl group at position C6 in the E-configured double bond. The total synthesis of the natural products melodorinone A (20) and melodorinone B (21) was also achieved during the synthesis of the γ-alkylidenebutenolide analogues.

In contrast to previous SAR studies of aza-compounds (23vs.24 and 25), the present study using analogues (26a–26c, 27c, and 28a–28c) of salacinol (1) revealed an essential role of the thiosugar ring in effectively inhibiting α-glucosidase.

We describe activity-based protein profiling for analyzing the adenylation domains of non-ribosomal peptide synthetases (ABPP-NRPS) in bacterial proteomes. Using a range of non-proteoinogenic amino acid sulfamoyladenosines, the competitive format of ABPP-NRPS provided substrate tolerance toward non-proteinogenic amino acids. When coupled with precursor-directed biosynthesis, a non-proteinogenic amino acid (O-allyl-L-serine) was successfully incorporated into gramicidin S.

The total synthesis of two decahydroquinoline poison frog alkaloids ent-cis-195A and cis-211A were achieved in 16 steps (38% overall yield) and 19 steps (31% overall yield), respectively, starting from known compound 1. Both alkaloids were synthesized from the common key intermediate 11 in a divergent fashion, and the absolute stereochemistry of natural cis-211A was determined to be 2R, 4aR, 5R, 6S, and 8aS. Interestingly, the absolute configuration of the parent decahydroquinoline nuclei of cis-211A was the mirror image of that of cis-195A, although both alkaloids were isolated from the same poison frog species, Oophaga (Dendrobates) pumilio, from Panama.

Much of our current knowledge on nonribosomal peptide synthetases (NRPSs) is based on studies in which the full NRPS system or each protein domain is expressed in heterologous hosts. Consequently, methods to detect the endogenous activity of NRPSs, under natural cellular conditions, are needed for the study of NRPS cell biology. Here, we describe the in vivo activity-based protein profiling (ABPP) for endogenous NRPSs and its applications to the study of their activities in bacteria. Remarkably, in vitro and in vivo ABPP in the context of the surfactin producer Bacillus subtilis enabled the visualization, tracking, and imaging of an endogenous SrfAB-NRPS with remarkable selectivity and sensitivity. Furthermore, in vivo, ABPP allowed the discovery of the degradation processes of the endogenous SrfAB-NRPS in the context of its native producer bacteria. Overall, this study deepens our understanding of the properties of NRPSs that cannot be addressed by conventional methods.

We describe a novel strategy for small-molecule probe-based fluorescence protein labeling and imaging in the Gram-negative bacterium Escherichia coli, which involves inhibition of efflux pumps.

Abstract An important challenge in natural product biosynthesis is the biosynthetic design and production of artificial peptides. One of the most promising strategies is reprogramming adenylation (A) domains to expand the substrate repertoire of nonribosomal peptide synthetases (NRPSs). Therefore, the precise detection of subtle structural changes in the substrate binding pockets of A domains might accelerate their reprogramming. Here we show that an enzyme‐linked immunosorbent assay (ELISA) using a combination of small‐molecule probes can detect the effects of substrate binding pocket residue substitutions in A‐domains. When coupled with a set of aryl acid A‐domain variants (total of nine variants), the ELISA can analyze the subtle differences in their active‐site architectures. Furthermore, the ELISA‐based screening was able to identify the variants with substrate binding pockets that accepted a non‐cognate substrate from an original pool of 45. These studies demonstrate that ELISA is a reliable platform for providing insights into the active‐site properties of A‐domains and can be applied for the reprogramming of NRPS A‐domains.

Marine tricyclic alkaloids lepadiformine and fasicularin, with a unique perhydropyrrolo[2,1- j]quinoline or perhydropyrido[2,1- j]quinoline framework, were synthesized starting from the B ring of the tricyclic system. This approach includes a highly stereocontrolled diallylation of a cyclic enaminoester and subsequent ring-closing metathesis to construct the A/B ring system, which was transformed into key lactams 32 and 33, and amino alcohol 37. Thus, we achieved formal syntheses of (-)-lepadiformines A, C, and (-)-fasicularin in a divergent manner.