Generation of N-H Imines from α-Azidocarboxylic Acids through Ru-Catalyzed Decarboxylation

A new method for the synthesis of N-H imines from α-azidocarboxylic acids was developed, which proceeds through decarboxylative C-C bond cleavage catalyzed by a commercial diruthenium complex ([CpRu(CO)₂]₂) under visible light irradiation at room temperature within several minutes. The reactive products undergo condensation, which forms cyclic trimers (2,4,6-trialkylhexahydro-1,3,5-triazines) or linear N,N'-bis(arylmethylidene)arylmethanediamines in quantitative yields. Alternatively, the N-H imines can be trapped with benzylamine and 2-(aminomethyl)aniline, providing stable N-benylimines and tetrahydroquinazolines, respectively. Subsequent oxidation of tetrahydroquinazolines produced quinazolines.

Sulfonium ion-promoted traceless Schmidt reaction of alkyl azides

Schmidt reaction by sulfonium ions is described. General primary, secondary, and tertiary alkyl azides were converted to the corresponding carbonyl or imine compounds without any trace of the activators. This bond scission reaction through 1,2-migration of C-H and C-C bonds was accessible to the one-pot substitution reaction.

Modular Chemical Synthesis of Streptogramin and Lankacidin Antibiotics

Continued, rapid development of antimicrobial resistance has become worldwide health crisis and a burden on the global economy. Decisive and comprehensive action is required to slow down the spread of antibiotic resistance, including increased investment in antibiotic discovery, sustainable policies that provide returns on investment for newly launched antibiotics, and public education to reduce the overusage of antibiotics, especially in livestock and agriculture. Without significant changes in the current antibiotic pipeline, we are in danger of entering a post-antibiotic era.In this Account, we summarize our recent efforts to develop next-generation streptogramin and lankacidin antibiotics that overcome bacterial resistance by means of modular chemical synthesis. First, we describe our highly modular, scalable route to four natural group A streptogramins antibiotics in 6-8 steps from seven simple chemical building blocks. We next describe the application of this route to the synthesis of a novel library of streptogramin antibiotics informed by in vitro and in vivo biological evaluation and high-resolution cryo-electron microscopy. One lead compound showed excellent inhibitory activity in vitro and in vivo against a longstanding streptogramin-resistance mechanism, virginiamycin acetyltransferase. Our results demonstrate that the combination of rational design and modular chemical synthesis can revitalize classes of antibiotics that are limited by naturally arising resistance mechanisms.Second, we recount our modular approaches toward lankacidin antibiotics. Lankacidins are a group of polyketide natural products with activity against several strains of Gram-positive bacteria but have not been deployed as therapeutics due to their chemical instability. We describe a route to several diastereomers of 2,18-seco-lankacidinol B in a linear sequence of ≤8 steps from simple building blocks, resulting in a revision of the C4 stereochemistry. We next detail our modular synthesis of several diastereoisomers of iso-lankacidinol that resulted in the structural reassignment of this natural product. These structural revisions raise interesting questions about the biosynthetic origin of lankacidins, all of which possessed uniform stereochemistry prior to these findings. Finally, we summarize the ability of several iso- and seco-lankacidins to inhibit the growth of bacteria and to inhibit translation in vitro, providing important insights into structure-function relationships for the class.

Modular Approaches to Lankacidin Antibiotics

Lankacidins are a class of polyketide natural products isolated from Streptomyces spp. that show promising antimicrobial activity. Owing to their complex molecular architectures and chemical instability, structural assignment and derivatization of lankacidins are challenging tasks. Herein we describe three fully synthetic approaches to lankacidins that enable access to new structural variability within the class. We use these routes to systematically generate stereochemical derivatives of both cyclic and acyclic lankacidins. Additionally, we access a new series of lankacidins bearing a methyl group at the C4 position, a modification intended to increase chemical stability. In the course of this work, we discovered that the reported structures for two natural products of the lankacidin class were incorrect, and we determine the correct structures of 2,18-seco-lankacidinol B and iso-lankacidinol. We also evaluate the ability of several iso- and seco-lankacidins to inhibit the growth of bacteria and to inhibit translation in vitro. This work grants insight into the rich chemical complexity of this class of antibiotics and provides an avenue for further structural derivatization.