Oxidation of Silyl Enol Ethers by Using IBX and IBX⋅N-Oxide Complexes: A Mild and Selective Reaction for the Synthesis of Enones

Asymmetric Total Synthesis of Norzoanthamine

We report herein the asymmetric total synthesis of norzoanthamine using radical reactions as key steps for rapid access to the congested carbocyclic core, which is the major synthetic challenge for most zoanthamine alkaloids. (1) The Ueno-Stork radical cyclization was applied to construct the adjacent quaternary centers at the C-9 and C-22 positions; (2) a Co-catalyzed HAT radical reaction was successfully applied to construct the quaternary center at C-12 via Csp³ -Csp² bond formation; (3) a Mn-catalyzed HAT radical reaction was used to stereospecifically reduce the tetra-substituted olefin (C13=C18) and install the contiguous stereocenters in proximity to the quaternary center. A one-pot bio-inspired cyclization step was finally applied to forge the unstable bis-amino acetal skeleton. Our approach can precisely control the stereochemistry of seven vicinal stereocenters and effectively construct the highly congested heptacyclic skeleton.

Enantioselective Total Synthesis of (+)-Alstilobanine C, (+)-Undulifoline, and (-)-Alpneumine H

We report herein the enantioselective total synthesis of three monoterpene indole alkaloids, namely, (+)-alstilobanine C, (+)-undulifoline, and (-)-alpneumine H. The key features of our synthesis include: a) introduction of chirality via enantioselective deprotonation of a prochiral 4-substituted cyclohexanone; b) use of methoxymethyl (MOM) ether as both a hydroxyl protective group and a latent oxonium species for the formation of bridged oxepane and c) domino double reductive cyclization to build both the indole and the piperidine ring at the end of the synthesis. The synthesis confirmed the absolute configuration of these natural products assigned based on the biogenetic hypothesis.

Total syntheses of (±)-penicibilaenes A and B via intramolecular aldol condensation

Concise total syntheses of penicibilaenes A and B were achieved via intramolecular aldol condensation and other key reactions.

Chemoselective α,β-Dehydrogenation of Saturated Amides

We report a method for the selective α,β-dehydrogenation of amides in the presence of other carbonyl moieties under mild conditions. Our strategy relies on electrophilic activation coupled to in situ selective selenium-mediated dehydrogenation. The α,β-unsaturated products were obtained in moderate to excellent yields, and their synthetic versatility was demonstrated by a range of transformations. Mechanistic experiments suggest formation of an electrophilic SeIV species.

Oxidative dehydrogenation of C-C and C-N bonds: A convenient approach to access diverse (dihydro)heteroaromatic compounds

Nitrogen heteroarenes form an important class of compounds which can be found in natural products, synthetic drugs, building blocks etc. Among the diverse strategies that were developed for the synthesis of nitrogen heterocycles, oxidative dehydrogenation is extremely effective. This review discusses various oxidative dehydrogenation strategies of C-C and C-N bonds to generate nitrogen heteroarenes from their corresponding heterocyclic substrates. The strategies are categorized under stoichiometric and catalytic usage of reagents that facilitate such transformations. The application of these strategies in the synthesis of nitrogen heteroarene natural products and synthetic drug intermediates are also discussed. We hope this review will arouse sufficient interest among the scientific community to further advance the application of oxidative dehydrogenation in the synthesis of nitrogen heteroarenes.

Transition Metal-Participated Synthesis and Utilization of N-containing Heterocycles: Exploring for Nitrogen Sources

This account aims to describe our recent efforts on the synthesis and utilization of N-containing heterocycles, where transition metals participate in the synthesis. A variety of nitrogen sources, including amines, amides, hydrazones, pyrimidines, isocyanides, and copper nitrate, have been disclosed for the synthesis of diverse bioactive and pharmacologically interesting N-containing heterocycles under the participation of transition metals. The well-known nitrogen sources, such as amines and amides, were used for the construction of indoles, isatins, and quinolones. Dihydrophthalazines, isoquinolines, indazoles, and pyrazoles were obtained from hydrazones, while various pyrimidine-containing heterocycles were afforded through regioselective C-H functionalizations using pyrimidine as the directing group. Recent research has focused on the chemistry of isocyanides to achieve several kinds of heterocyclic compounds with high efficiency under the catalysis of transition metals (Pd, Rh, Mn, Cu), through oxidative cyanation reactions, sequential isocyanide insertions into C-H, N-H, or O-H bonds, and tandem radical annulation. More recently, an efficient route to isoxazolines has been reported using copper nitrate as a novel nitrogen source.

Discovery and syntheses of "superbug challengers"-platensimycin and platencin

Bacteria have developed resistance to almost all existing antibiotics known today and this has been a major issue over the last few decades. The search for a new class of antibiotics with a new mode of action to fight these multiply-drug-resistant strains, or "superbugs", allowed a team of scientists at Merck to discover two novel antibiotics, platensimycin and platencin using advanced screening strategies, as inhibitors of bacterial fatty acid biosynthesis, which is essential for the survival of bacteria. Though both these antibiotics are structurally related, they work by slightly different mechanisms and target different enzymes conserved in the bacterial fatty acid biosynthesis. This Focus Review summarizes the synthetic and biological aspects of these natural products and their analogues and congeners.

Toward palau'amine: Hg(OTf)2-catalyzed synthesis of the cyclopentane core

Catalytic cyclization: The Hg(OTf)2‐catalyzed N‐selective cyclization of amide carbonyl moieties for the construction of a quaternary carbon center was developed. The Hg(OTf)2‐catalyzed cyclization of cyclopentylidene alcohol with acylhydrazide afforded the desired cyclopenta[c]pyridazinone in good yield. The subsequent eight steps gave the functionalized cyclopentane with the correct stereochemistry that corresponds to the E ring of palau′amine (see scheme).magnified image

Recent advances in the chemistry and biology of naturally occurring antibiotics

Ever since the world-shaping discovery of penicillin, nature's molecular diversity has been extensively screened for new medications and lead compounds in drug discovery. The search for agents intended to combat infectious diseases has been of particular interest and has enjoyed a high degree of success. Indeed, the history of antibiotics is marked with impressive discoveries and drug-development stories, the overwhelming majority of which have their origin in natural products. Chemistry, and in particular chemical synthesis, has played a major role in bringing naturally occurring antibiotics and their derivatives to the clinic, and no doubt these disciplines will continue to be key enabling technologies. In this review article, we highlight a number of recent discoveries and advances in the chemistry, biology, and medicine of naturally occurring antibiotics, with particular emphasis on total synthesis, analogue design, and biological evaluation of molecules with novel mechanisms of action.