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1. Enzymatic Assembly of Diverse Lactone Structures: An Intramolecular C–H Functionalization Strategy

   https://doi.org/10.1021/jacs.3c11722  

   Wackelin, Daniel J.; Mao, Runze; Sicinski, Kathleen M.; Zhao, Yutao; Das, Anuvab; Chen, Kai; Arnold, Frances H. (January 2024, Journal of the American Chemical Society)

   Lactones are cyclic esters with extensive applications in materials science, medicinal chemistry, and the food and perfume industries. Nature’s strategy for the synthesis of many lactones found in natural products always relies on a single type of retrosynthetic strategy, a C−O bond disconnection. Here, we describe a set of laboratory-engineered enzymes that use a new-tonature C−C bond-forming strategy to assemble diverse lactone structures. These engineered “carbene transferases” catalyze intramolecular carbene insertions into benzylic or allylic C−H bonds, which allow for the synthesis of lactones with different ring sizes and ring scaffolds from simple starting materials. Starting from a serine-ligated cytochrome P450 variant previously engineered for other carbene-transfer activities, directed evolution generated a variant P411-LAS-5247, which exhibits a high activity for constructing a five-membered ε-lactone, lactam, and cyclic ketone products (up to 5600 total turnovers (TTN) and >99% enantiomeric excess (ee)). Further engineering led to variants P411-LAS-5249 and P411-LAS-5264, which deliver six-membered δ-lactones and seven-membered ε-lactones, respectively, overcoming the thermodynamically unfavorable ring strain associated with these products compared to the γ-lactones. This new carbene-transfer activity was further extended to the synthesis of complex lactone scaffolds based on fused, bridged, and spiro rings. The enzymatic platform developed here complements natural biosynthetic strategies for lactone assembly and expands the structural diversity of lactones accessible through C−H functionalization. 

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2. Structural determinants driving homoserine lactone ligand selection in the Pseudomonas aeruginosa LasR quorum-sensing receptor

   https://doi.org/10.1073/pnas.1817239116  

   McCready, Amelia R.; Paczkowski, Jon E.; Henke, Brad R.; Bassler, Bonnie L. (December 2018, Proceedings of the National Academy of Sciences)

   Quorum sensing is a cell–cell communication process that bacteria use to orchestrate group behaviors. Quorum sensing is mediated by signal molecules called autoinducers. Autoinducers are often structurally similar, raising questions concerning how bacteria distinguish among them. Here, we use thePseudomonas aeruginosaLasR quorum-sensing receptor to explore signal discrimination. The cognate autoinducer, 3OC₁₂homoserine lactone (3OC₁₂HSL), is a more potent activator of LasR than other homoserine lactones. However, other homoserine lactones can elicit LasR-dependent quorum-sensing responses, showing that LasR displays ligand promiscuity. We identify mutants that alter which homoserine lactones LasR detects. Substitution at residue S129 decreases the LasR response to 3OC₁₂HSL, while enhancing discrimination against noncognate autoinducers. Conversely, the LasR L130F mutation increases the potency of 3OC₁₂HSL and other homoserine lactones. We solve crystal structures of LasR ligand-binding domains complexed with noncognate autoinducers. Comparison with existing structures reveals that ligand selectivity/sensitivity is mediated by a flexible loop near the ligand-binding site. We show that LasR variants with modified ligand preferences exhibit altered quorum-sensing responses to autoinducers in vivo. We suggest that possessing some ligand promiscuity endows LasR with the ability to optimally regulate quorum-sensing traits. 

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3. Access to unsaturated bicyclic lactones by overriding conventional C(sp3)–H site selectivity

   https://doi.org/10.1038/s41557-023-01295-x  

   Das, Jayabrata; Ali, Wajid; Ghosh, Animesh; Pal, Tanay; Mandal, Astam; Teja, Chitrala; Dutta, Suparna; Pothikumar, Rajagopal; Ge, Haibo; Zhang, Xinglong; et al (November 2023, Nature Chemistry)

   Abstract Transition metal catalysis plays a pivotal role in transforming unreactive C–H bonds. However, regioselective activation of distal aliphatic C–H bonds poses a tremendous challenge, particularly in the absence of directing templates. Activation of a methylene C–H bond in the presence of methyl C–H is underexplored. Here we show activation of a methylene C–H bond in the presence of methyl C–H bonds to form unsaturated bicyclic lactones. The protocol allows the reversal of the general selectivity in aliphatic C–H bond activation. Computational studies suggest that reversible C–H activation is followed by β-hydride elimination to generate the Pd-coordinated cycloalkene that undergoes stereoselective C–O cyclization, and subsequent β-hydride elimination to provide bicyclic unsaturated lactones. The broad generality of this reaction has been highlighted via dehydrogenative lactonization of mid to macro ring containing acids along with the C–H olefination reaction with olefin and allyl alcohol. The method substantially simplifies the synthesis of important bicyclic lactones that are important features of natural products as well as pharmacoactive molecules. 

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4. Enantioselective Synthesis of Medium‐Sized Lactams via Chiral α,β‐Unsaturated Acylammonium Salts

   https://doi.org/10.1002/anie.201802483  

   Kang, Guowei; Yamagami, Masaki; Vellalath, Sreekumar; Romo, Daniel (May 2018, Angewandte Chemie International Edition)

   Abstract Medium‐sized lactams are important structural motifs found in a variety of bioactive compounds and natural products but are challenging to prepare, especially in optically active form. A Michael addition/proton transfer/lactamization organocascade process is described that delivers medium‐sized lactams, including azepanones, benzazepinones, azocanones, and benzazocinones, in high enantiopurity through the intermediacy of chiral α,β‐unsaturated acylammonium salts. An unexpected indoline synthesis was also uncovered, and the benzazocinone skeleton was transformed into other complex heterocyclic derivatives, including spiroglutarimides, isoquinolinones, and δ‐lactones. 

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5. Lactone Ring-Opening Equilibria in Methanol by 1H NMR Analysis: An Assessment of the Ring-Opening Polymerizability of Lactone Monomers

   https://doi.org/10.1021/acs.macromol.2c01141  

   Jin, Mengyuan; Hoye, Thomas R. (January 2023, Macromolecules)

   The purpose of this study was to learn if a convenient 1H NMR method could be developed to serve as a tool for estimating the propensity of a given lactone to participate in ring-opening transesterification polymerization (ROTEP). The methanolysis of each of 18 lactones was initially examined in CD3OD solution in the presence of sulfuric acid as a Brønsted catalyst at ambient temperature. Once equilibrium was established, the ratio of remaining lactone to the ring-opened methyl ester/alcohol could be readily measured by NMR spectroscopy. The observed thermodynamic driving force observed for the methanol ring openings is roughly in line with the extent of ROTEP for the various classes of lactones. This is the case even though the reaction conditions for these methanolyses versus ROTEP reactions are substantially different. Qualitative evaluations of the rates of the ring-opening methanolyses were also performed, and several non-obvious relative reactivities were observed. Finally, employing this simple NMR methanolysis using low concentrations of methanol in CDCl3 is recommended as the preferred protocol for the initial evaluation of the polymerizability of any new lactone monomer that researchers may prepare in the future. 

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