One-Carbon Insertion and Polarity Inversion Enabled a Pyrrole Strategy to the Total Syntheses of Pyridine-Containing Lycopodium Alkaloids: Complanadine A and Lycodine

Controllable Skeletal and Peripheral Editing of Pyrroles with Vinylcarbenes

The skeletal editing of azaarenes through insertion, deletion, or swapping of single atoms has recently gained considerable momentum in chemical synthesis. Here, we describe a practical skeletal editing strategy using vinylcarbenes in situ generated from trifluoromethyl vinyl N-triftosylhydrazones, leading to the first dearomative skeletal editing of pyrroles through carbon-atom insertion. Furthermore, depending on the used catalyst and substrate, three types of peripheral editing reactions of pyrroles are also disclosed: α- or γ-selective C-H insertion, and [3+2] cycloaddition. These controllable molecular editing reactions provide a powerful platform for accessing medicinally relevant CF3-containing N-heterocyclic frameworks, such as 2,5-dihydropyridines, piperidines, azabicyclo[3.3.0]octadienes, and allylated pyrroles from readily available pyrroles. Mechanistic insights from experiments and density functional theory (DFT) calculations shed light on the origin of substrate- or catalyst-controlled chemo- and regioselectivity as well as the reaction mechanism.

Late-stage benzenoid-to-troponoid skeletal modification of the cephalotanes exemplified by the total synthesis of harringtonolide

Skeletal modifications enable elegant and rapid access to various derivatives of a compound that would otherwise be difficult to prepare. They are therefore a powerful tool, especially in the synthesis of natural products or drug discovery, to explore different natural products or to improve the properties of a drug candidate starting from a common intermediate. Inspired by the biosynthesis of the cephalotane natural products, we report here a single-atom insertion into the framework of the benzenoid subfamily, providing access to the troponoid congeners - representing the reverse of the proposed biosynthesis (i.e., a contra-biosynthesis approach). Computational evaluation of our designed transformation prompted us to investigate a Büchner-Curtius-Schlotterbeck reaction of a p-quinol methylether, which ultimately results in the synthesis of harringtonolide in two steps from cephanolide A, which we had previously prepared. Additional computational studies reveal that unconventional selectivity outcomes are driven by the choice of a Lewis acid and the nucleophile, which should inform further developments of these types of reactions.

N-Heterocycle-Editing to Access Fused-BN-Heterocycles via Ring-Opening/C-H Borylation/Reductive C-B Bond Formation

Skeletal editing of N-heterocycles has recently received considerable attention, and the introduction of boron atom into heterocycles often results in positive property changes. However, direct enlargement of N-heterocycles through boron atom insertion is rarely reported in the literature. Here, we report a N-heterocyclic editing reaction through the combination boron atom insertion and C-H borylation, accessing the fused-BN-heterocycles. The synthetic potential of this chemistry was demonstrated by substrate scope and late-stage diversification of products.

Convergent and Efficient Total Synthesis of (+)-Heilonine Enabled by C-H Functionalizations

We report a convergent and efficient total synthesis of the C-nor D-homo steroidal alkaloid (+)-heilonine with a hexacyclic ring system, nine stereocenters, and a trans-hydrindane moiety. Our synthesis features four selective C-H functionalizations to form key C-C bonds and stereocenters, a Stille carbonylative cross-coupling to connect the AB ring system with the DEF ring system, and a Nazarov cyclization to construct the five-membered C ring. These enabling transformations significantly reduced functional group manipulations and delivered (+)-heilonine in 11 or 13 longest linear sequence (LLS) steps.

Neurotrophic Natural Products

Neurotrophins (NGF, BDNF, NT3, NT4) can decrease cell death, induce differentiation, as well as sustain the structure and function of neurons, which make them promising therapeutic agents for the treatment of neurodegenerative disorders. However, neurotrophins have not been very effective in clinical trials mostly because they cannot pass through the blood-brain barrier owing to being high-molecular-weight proteins. Thus, neurotrophin-mimic small molecules, which stimulate the synthesis of endogenous neurotrophins or enhance neurotrophic actions, may serve as promising alternatives to neurotrophins. Small-molecular-weight natural products, which have been used in dietary functional foods or in traditional medicines over the course of human history, have a great potential for the development of new therapeutic agents against neurodegenerative diseases such as Alzheimer's disease. In this contribution, a variety of natural products possessing neurotrophic properties such as neurogenesis, neurite outgrowth promotion (neuritogenesis), and neuroprotection are described, and a focus is made on the chemistry and biology of several neurotrophic natural products.

One-Carbon Ring Expansion of Indoles and Pyrroles: A Straightforward Access to 3-Fluorinated Quinolines and Pyridines

3-Fluorinated quinolines and pyridines are prevalent pharmacophores, yet their synthesis is often challenging. Herein, we demonstrate that dibromofluoromethane as bromofluorocarbene source enables the one-carbon ring expansion of readily available indoles and pyrroles to structurally diverse 3-fluorinated quinolines and pyridines. This straightforward protocol requires only a short reaction time of ten minutes and can be performed under air atmosphere. Preliminary investigations reveal that this strategy can also be applied to the synthesis of other valuable azines by using different 1,1-dibromoalkanes as bromocarbene sources.

Unified Access to Pyrimidines and Quinazolines Enabled by N-N Cleaving Carbon Atom Insertion

Given the ubiquity of heterocycles in biologically active molecules, transformations with the capacity to modify such molecular skeletons with modularity remain highly desirable. Ring expansions that enable interconversion of privileged heterocyclic motifs are especially interesting in this regard. As such, the known mechanisms for ring expansion and contraction determine the classes of heterocycle amenable to skeletal editing. Herein, we report a reaction that selectively cleaves the N–N bond of pyrazole and indazole cores to afford pyrimidines and quinazolines, respectively. This chlorodiazirine-mediated reaction provides a unified route to a related pair of heterocycles that are otherwise typically prepared by divergent approaches. Mechanistic experiments and DFT calculations support a pathway involving pyrazolium ylide fragmentation followed by cyclization of the ring-opened diazahexatriene intermediate to yield the new diazine core. Beyond enabling access to valuable heteroarenes from easily prepared starting materials, we demonstrate the synthetic utility of skeletal editing in the synthesis of a Rosuvastatin analog as well as in an aryl vector-adjusting direct scaffold hop.

Copper-catalyzed [3+3] annulation of ketones with oxime acetates for the synthesis of pyridines

A novel and efficient copper-catalyzed strategy for the synthesis of diverse pyridines through the [3+3] annulation of ketones with oxime acetates has been reported. It is very convenient to obtain various unsymmetrical 2,6-diarylpyridines by changing different substrates. The rare copper-catalyzed direct difunctionalization of saturated ketones for the synthesis of nitrogen heterocycles is developed. This protocol has excellent functional group tolerance, readily available raw materials, high chemoselectivity and broad substrate scope.

Modular Regiodivergent Synthesis of Benzo-Fused Isocoumarins by a Cyclopropane Aromatization Strategy

Reported herein is an electrophile-modulated aromatization reaction of highly functionalized cyclopropanes to structurally diverse benzoisocoumarins featuring concurrent formation of the benzenoid and α-pyrone rings under mild conditions. An aromatization reaction of the proposed benzonorcaradiene intermediates prepared independently revealed a crucial role of the neighboring olefinic substituents in determining whether the cyclopropane ring expansion is followed by a 1,2-shift of the ester group.

Electrochemical collective synthesis of labeled pyrroloindoline alkaloids with Freon-type methanes as functional C1 synthons

Utilization of Freon-type methanes as functional one-carbon synthons in the synthesis of various deuterated indoline alkaloids was demonstrated here. A series of halomethyl radicals were generated from electro-reductive C-X cleavage of Freon-type methanes and captured efficiently by acrylamides to provide various halogenated oxindoles via radical cyclization. This reaction features good functional group tolerance, and deuterium and fluorine atoms could be introduced facilely from Freon-type methanes. Further transformation of halogenated oxindoles enabled the synthesis of many (labeled) bioactive drug molecules and skeletons, such as deuterated (±)-physostigmine, deuterated (±)-esermethole and deuterated (±)-lansai B.

Single-atom logic for heterocycle editing

Medicinal chemistry continues to be impacted by new synthetic methods. Particularly sought after, especially at the drug discovery stage, is the ability to enact the desired chemical transformations in a concise and chemospecific fashion. To this end, the field of organic synthesis has become captivated by the idea of 'molecular editing'-to rapidly build onto, change or prune molecules one atom at a time using transformations that are mild and selective enough to be employed at the late stages of a synthetic sequence. In this Review, the definition and categorization of a particularly promising subclass of molecular editing reactions, termed 'single-atom skeletal editing', are proposed. Although skeletal editing applies to both cyclic and acyclic compounds, this Review focuses on heterocycles, both for their centrality in medicinal chemistry and for the definitional clarity afforded by a focus on ring systems. A classification system is presented by highlighting methods (both historically important examples and recent advances) that achieve such transformations, with the goal to spark interest and inspire further development in this growing field.

Scaffold hopping by net photochemical carbon deletion of azaarenes

Discovery chemists routinely identify purpose-tailored molecules through an iterative structural optimization approach, but the preparation of each successive candidate in a compound series can rarely be conducted in a manner matching their thought process. This is because many of the necessary chemical transformations required to modify compound cores in a straightforward fashion are not applicable in complex contexts. We report a method that addresses one facet of this problem by allowing chemists to hop directly between chemically distinct heteroaromatic scaffolds. Specifically, we show that selective photolysis of quinoline N-oxides with 390-nanometer light followed by acid-promoted rearrangement affords N-acylindoles while showing broad compatibility with medicinally relevant functionality. Applications to late-stage skeletal modification of compounds of pharmaceutical interest and more complex transformations involving serial single-atom changes are demonstrated.

Total Synthesis of Huperserratines A and B

Huperserratines A (1) and B (2), two novel macrocyclic Lycopodium alkaloids, possess an aza-12-membered ring. Here, we describe the first total synthesis of these two natural products in 12 steps....

Molecular editing in natural product synthesis

Natural products have significant biological properties that arise from their fascinating molecular architectures. Despite many advances in synthetic method development, the efficient synthesis of highly-functionalized and structurally intricate natural products...

Natural product total synthesis using rearrangement reactions

The synthetic utility of rearrangement reactions in total synthesis for the rapid construction of core skeletons, the precise control of stereochemistry, and the identification of suitable synthons has been discussed.