Discovery, synthesis, and optimization of teixobactin, a novel antibiotic without detectable bacterial resistance

Discovering new antibiotics with novel chemical scaffolds and antibacterial mechanisms presents a challenge for medicinal scientists worldwide as the ever-increasing bacterial resistance poses a serious threat to human health. A new cyclic peptide-based antibiotic termed teixobactin was discovered from a screen of uncultured soil bacteria through iChip technology in 2015. Teixobactin exhibits excellent antibacterial activity against all the tested gram-positive pathogens and Mycobacterium tuberculosis, including drug-resistant strains. Given that teixobactin targets the highly conserved lipid II and lipid III, which induces the simultaneous inhibition of both peptidoglycan and teichoic acid synthesis, the emergence of resistance is considered to be rather difficult. The novel structure, potent antibacterial activity, and highly conservative targets make teixobactin a promising lead compound for further antibiotic development. This review provides a comprehensive treatise on the advances of teixobactin in the areas of discovery processes, antibacterial activity, mechanisms of action, chemical synthesis, and structural optimizations. The synthetic methods for the key building block l-allo-End, natural teixobactin, representative teixobactin analogues, as well as the structure-activity relationship studies will be highlighted and discussed in details. Finally, some insights into new trends for the generation of novel teixobactin analogues and tips for future work and directions will be commented.

Solvent-Free Approaches in Carbohydrate Synthetic Chemistry: Role of Catalysis in Reactivity and Selectivity

Owing to their abundance in biomass and availability at a low cost, carbohydrates are very useful precursors for products of interest in a broad range of scientific applications. For example, they can be either converted into basic chemicals or used as chiral precursors for the synthesis of potentially bioactive molecules, even including nonsaccharide targets; in addition, there is also a broad interest toward the potential of synthetic sugar-containing structures in the field of functional materials. Synthetic elaboration of carbohydrates, in both the selective modification of functional groups and the assembly of oligomeric structures, is not trivial and often entails experimentally demanding approaches practiced by specialized groups. Over the last years, a large number of solvent-free synthetic methods have appeared in the literature, often being endowed with several advantages such as greenness, experimental simplicity, and a larger scope than analogous reactions in solution. Most of these methods are catalytically promoted, and the catalyst often plays a key role in the selectivity associated with the process. This review aims to describe the significant recent contributions in the solvent-free synthetic chemistry of carbohydrates, devoting a special critical focus on both the mechanistic role of the catalysts employed and the differences evidenced so far with corresponding methods in solution.

Synthesis of Chiral Tetrasubstituted Azetidines from Donor-Acceptor Azetines via Asymmetric Copper(I)-Catalyzed Imido-Ylide [3+1]-Cycloaddition with Metallo-Enolcarbenes

The all-cis stereoisomers of tetrasubstituted azetidine-2-carboxylic acids and derivatives that possess three chiral centers have been prepared in high yield and stereocontrol from silyl-protected Z-γ-substituted enoldiazoacetates and imido-sulfur ylides by asymmetric [3+1]-cycloaddition using chiral sabox copper(I) catalysis followed by Pd/C catalytic hydrogenation. Hydrogenation of the chiral p-methoxybenzyl azetine-2-carboxylates occurs with both hydrogen addition to the C=C bond and hydrogenolysis of the ester.

Polyhydroxylated azetidine iminosugars: Synthesis, glycosidase inhibitory activity and molecular docking studies

An efficient and practical strategy for the synthesis of unknown azetidine iminosugars (2S,3R,4S)-2-((R)-1,2-dihydroxyethyl)-3-hydroxy-4-(hydroxymethyl)azetidine 2, (2S,3r,4R)-3-hydroxy-2,4-bis(hydroxymethyl)azetidine 3 and (2S,3R,4S)-3-hydroxy-4-(hydroxymethyl)-N-methylazetidine-2-carboxylic acid 4, starting from the d-glucose has been reported. The methodology involves preparation of the 3-amino-N-benzyloxycarbonyl-3-deoxy-6-O-tert-butyldimethylsillyl-1,2-O-isopropylidene-α-d-glucofuranose 9, which was converted to the C-5-OMs derivative 11. Intramolecular nucleophilic displacement of the C-5-OMs group with in situ generated 3-amino functionality provided the required key azetidine ring skeletons 10 with additional hydroxymethyl group. Removal of 1,2-acetonide protection, followed by reduction and hydrogenolysis afforded azetidine iminosugar 2. Alternatively, removal of 1,2-acetonide group and chopping of C1-anomeric carbon gave C2-aldehyde that on reduction or oxidation followed by hydrogenolysis gave 2,4-bis(hydroxymethyl) azetidine iminosugars 3 and N-methylazetidine-2-carboxylic acid 4 respectively. The glycosidase inhibitory activity of 2-4 iminosugars was screened against various glycosidase enzymes and compared with a standard miglitol. Amongst synthesized targets, the compound 2 was found to be more potent amyloglucosidase inhibitor than miglitol. These results were supported by molecular docking studies.

Square sugars: challenges and synthetic strategies

The synthesis of square sugars requires innovative strategies based on efficient stereoselective methodologies, from organocatalysis to metal carbene insertion.

Construction of quaternary stereocentres on carbohydrate scaffolds

This review describes a glimpse of the various strategies for constructing stereo-defined quaternary centres in densely functionalised carbohydrates moiety of structurally intriguing and biologically potent natural products and building blocks.

Exploring architectures displaying multimeric presentations of a trihydroxypiperidine iminosugar

The synthesis of new multivalent architectures based on a trihydroxypiperidine α-fucosidase inhibitor is reported herein. Tetravalent and nonavalent dendrimers were obtained by means of the click chemistry approach involving the copper azide-alkyne-catalyzed cycloaddition (CuAAC) between suitable scaffolds bearing terminal alkyne moieties and an azido-functionalized piperidine as the bioactive moiety. A preliminary biological investigation is also reported towards commercially available and human glycosidases.