Privileged structures have been widely used as an effective template for the research and discovery of high value chemicals. Coumarin is a simple scaffold widespread in Nature and it can be found in a considerable number of plants as well as in some fungi and bacteria. In the last years, these natural compounds have been gaining an increasing attention from the scientific community for their wide range of biological activities, mainly due to their ability to interact with diverse enzymes and receptors in living organisms. In addition, coumarin nucleus has proved to be easily synthetized and decorated, giving the possibility of designing new coumarin-based compounds and investigating their potential in the treatment of various diseases. The versatility of coumarin scaffold finds applications not only in medicinal chemistry but also in the agrochemical field as well as in the cosmetic and fragrances industry. This review is intended to be a critical overview on coumarins, comprehensive of natural sources, metabolites, biological evaluations and synthetic approaches.

Amino acids are among the most important molecules in nature since they play central roles both as building blocks of proteins and as intermediates in metabolism. The amino acid sequence dictates protein folding, the native three dimensional structure, and protein stability. Furthermore, the peculiar chemical properties of the amino acids forming the active site and their interplay determine protein function and regulation. All amino acids found in proteins, except glycine, possess a stereogenic center at the a-carbon atom. Millions of years of evolution have resulted in the virtually complete homochirality of such a stereogenic center, i.e. the L-enantiomer, in mammals. 1 This selection of the L-amino acids by nature is generally considered to be a result of chance. 2 Since the cornerstone of the protein-ligand recognition is the multi-point attachment theory, it turns out that the configuration of the a-carbon atom of amino acids strongly affects the protein-ligand interaction. Nevertheless, during the last half of the twentieth century, various studies evidenced the presence of Damino acids in some plants and bacteria. 3,4 These compounds were either found in a free state or in peptides and proteins. Most bacteria produce significant amounts of D-alanine (D-Ala) and D-glutamate (D-Glu), which are incorporated into peptidoglycan. 5 Peptidoglycan is a strong and elastic polymer of the bacterial wall, which is capable to counteract the osmotic pressure of the cell, maintaining cell shape and anchoring components of the cell envelope. 6 The number of D-amino acids present in the structure of peptidoglycan seems to constitute a measure of protection against peptidase and protease attacks. So far, no peptidase capable of hydrolyzing a peptide bond characterized by the sequence D-D or D-L amino acids has been isolated in mammals. In addition, several antibiotics produced by prokaryotes (e.g. bacitracin, actinomycin D) contain D-amino acids (Figure 1). It has been recently demonstrated that bacteria synthesize a pool of different D-amino acids, including D-methionine (D-Met) and D-leucine (D-Leu) in Vibrio cholerae and D-tyrosine (D-Tyr) and D-phenylalanine (D-Phe) in Bacillus subtilis. By selectively incorporating them in the peptidoglycan cell wall, bacteria cope to different environmental stresses. 7

A series of Δ(2)-isoxazoline constrained analogues of procaine/procainamide (7a-k and 8a-k) were prepared and their inhibitory activity against DNA methyltransferase 1 (DNMT1) was tested. Among them, derivative 7b is far more potent in vitro (IC(50) = 150 μM) than other non-nucleoside inhibitors and also exhibits a strong and dose-dependent antiproliferative effect against HCT116 human colon carcinoma cells. The binding mode of 7b with the enzyme was also investigated by means of a simple competition assay as well as of docking simulations conducted using the recently published crystallographic structure of human DNMT1. On the basis of the findings, we assessed that the mode of inhibition of 7b is consistent with a competition with the cofactor and propose it as a novel lead compound for the development of non-nucleoside DNMT inhibitors.

Plant polyphenolic compounds are considered a promising source for new antibacterial agents. In this study, we evaluated the antimicrobial activity of a collection of resveratrol-derived monomers and dimers screened as single molecules against a panel of nine foodborne pathogens. The results demonstrated that two monomers (i.e., pterostilbene 2 and (E)-3-hydroxy-4′,5-dimethoxystilbene 9) and three dimers (i.e., δ-viniferin 10, viniferifuran 14 and dehydro-δ-viniferin 15) were endowed with significant antibacterial activity against gram-positive bacteria. The exposure of gram-positive foodborne pathogens to 100 µg/mL of 2, 9 and 15 induced severe cell membrane damage, resulting in the disruption of the phospholipid bilayer. The most promising dimeric compound, dehydro-δ-viniferin 15, was tested against Listeria monocytogenes, resulting in a loss of cultivability, viability and cell membrane potential. TEM analysis revealed grave morphological modifications on the cell membrane and leakage of intracellular content, confirming that the cell membrane was the principal biological target of the tested derivative.

A straightforward one-step biocatalyzed synthesis of different N-acyl amides in water was accomplished using the versatile and chemoselective acyltransferase from Mycobacterium smegmatis (MsAcT). Acetylation of primary arylalkyl amines was achieved with a range of acetyl donors in biphasic systems within 1 hour and at room temperature. Vinyl acetate was the best donor which could be employed in the N-acetylation of a large range of primary amines in excellent yields (85-99%) after just 20 minutes. Other acyl donors (including formyl-, propionyl-, and butyryl-donors) were also efficiently employed in the biocatalytic N-acylation.Finally, the biocatalyst was tested in transamidation reactions using acetamide as acetyl donor in aqueous medium, reaching yields of 60-70%. This work expands the toolbox of preparative methods for the formation of N-acyl amides, describing a biocatalytic approach easy to accomplish under mild conditions in water.

We developed a new class of covalent inhibitors of Plasmodium falciparum glyceraldehyde-3-phosphate dehydrogenase, a validated target for the treatment of malaria, by screening a small library of 3-bromo-isoxazoline derivatives that inactivate the enzyme through a covalent, selective bond to the catalytic cysteine, as demonstrated by mass spectrometry. Substituents on the isoxazolinic ring modulated the potency up to 20-fold, predominantly due to an electrostatic effect, as assessed by computational analysis.

Rhodesain, a cathepsin L-like cysteine protease of T. brucei rhodesiense, is considered a potential target for the treatment of human African trypanosomiasis. Recent findings have confirmed that rhodesain, a lysosomal protease, is essential for parasite survival. Rhodesain is required by T. brucei to cross the blood-brain barrier, degrade host immunoglobulins, and turn over variant surface coat glycoproteins of T. brucei, which impair effective host immune responses. In this Perspective, we discuss the main classes of rhodesain inhibitors, including peptidic, peptidomimetic, and nonpeptidic structures, emphasizing those that have exhibited an optimal match between enzymatic affinity and trypanocidal profile and those for which preclinical investigations are currently in progress.

A robust two-enzyme system composed of an immobilized ketoreductase (KRED1-Pglu) and a glucose dehydrogenase (BmGDH) was developed via immobilization on aldehyde agarose for the stereoselective reduction of different ketones. The immobilized ketoreductase/glucose dehydrogenase system was continuously used in a flow reactor for weeks, even in the presence of concentrations of DMSO up to 20%.