Qianghui Zhou

Direct Synthesis of Fluorinated Heteroarylether Bioisosteres

This work delineates a method for the modular synthesis of reagents that are capable of direct incorporation of difluoroalkyl groups onto heterocycles. The scope and generality of this method is exemplified with the difluoroethyl group (along with the introduction of a new reagent for difluoroethylation, DFES-Na) and a proof of principle is shown for a general synthesis of fluorinated heteroarylether bioisosteres.

C–H Methylation of Heteroarenes Inspired by Radical SAM Methyl Transferase

A practical C–H functionalization method for the methylation of heteroarenes is presented. Inspiration from Nature’s methylating agent, S-adenosylmethionine (SAM), allowed for the design and development of zinc bis(phenylsulfonylmethanesulfinate), or PSMS. The action of PSMS on a heteroarene generates a (phenylsulfonyl)methylated intermediate that can be easily separated from unreacted starting material. This intermediate can then be desulfonylated to the methylated product or elaborated to a deuteriomethylated product, and can divergently access medicinally important motifs. This mild, operationally simple protocol that can be conducted in open air at room temperature is compatible with sensitive functional groups for the late-stage functionalization of pharmacologically relevant substrates.

Strategic Redox Relay Enables A Scalable Synthesis of Ouabagenin, A Bioactive Cardenolide

Placing the Os in Ouabagenin Whereas enzymes are remarkably adept at selectively oxidizing saturated carbon centers, these reactions seriously challenge chemists. In a 19-step synthesis of ouabagenin, Renata et al. (p. 59) showcase a range of creative indirect methods to install the six hydroxyl groups in the steroids framework. These include a solid-state photochemical transformation, as well as dehydrogenation sequences that place olefins in proper position for oxygenation. The route also yields several intermediates poised for elaboration to distinct analogs for exploratory medicinal chemistry. A synthesis showcases multiple creative indirect methods of selectively hydroxylating saturated carbon centers. Here, we report on a scalable route to the polyhydroxylated steroid ouabagenin with an unusual take on the age-old practice of steroid semisynthesis. The incorporation of both redox and stereochemical relays during the design of this synthesis resulted in efficient access to more than 500 milligrams of a key precursor toward ouabagenin—and ultimately ouabagenin itself—and the discovery of innovative methods for carbon-hydrogen (C-H) and carbon-carbon activation and carbon-oxygen bond homolysis. Given the medicinal relevance of the cardenolides in the treatment of congestive heart failure, a variety of ouabagenin analogs could potentially be generated from the key intermediate as a means of addressing the narrow therapeutic index of these molecules. This synthesis also showcases an approach to bypass the historically challenging problem of selective C-H oxidation of saturated carbon centers in a controlled fashion.

Development of a Concise Synthesis of Ouabagenin and Hydroxylated Corticosteroid Analogues

The natural product ouabagenin is a complex cardiotonic steroid with a highly oxygenated skeleton. This full account describes the development of a concise synthesis of ouabagenin, including the evolution of synthetic strategy to access hydroxylation at the C19 position of a steroid skeleton. In addition, approaches to install the requisite butenolide moiety at the C17 position are discussed. Lastly, methodology developed in this synthesis has been applied in the generation of novel analogues of corticosteroid drugs bearing a hydroxyl group at the C19 position.

Epoxides as Alkylating Reagents for the Catellani Reaction

We report a cooperative catalytic system comprising a PdII complex, XPhos, and the potassium salt of 5-norbornene-2-carboxylic acid that enables the use of epoxides as alkylating reagents in the Catellani reaction, thereby expanding the existing paradigm of this powerful transformation. The potassium salt of inexpensive 5-norbornene-2-carboxylic acid acts as both mediator and base in the process. This mild, chemoselective, scalable, and atom-economical protocol is compatible with a wide variety of readily available functionalized aryl iodides and epoxides, as well as terminating olefins. The resulting products undergo facile oxa-Michael addition to furnish ubiquitous isochroman scaffolds.

Metal-catalyzed enyne cycloisomerization in natural product total synthesis

Since the seminal work of B. M. Trost et al. in 1985, metal catalyzed enyne cycloisomerization has become a fast growing and fascinating field. Due to its diversified chemistry and ability to increase molecular complexity in an efficient, atom economical, step economic and redox economic way, enyne cycloisomerization has become a powerful and attractive strategy for the construction of cyclic compounds, and thus has great potential for applications in total synthesis of natural products and pharmaceuticals. In this review, a brief summary of the state-of-the-art mechanism studies, the classification of structures accessed through enyne cycloisomerizations, and recent completed total synthesis of representative natural products showcasing creative and ingenious incorporation of enyne cycloisomerization as a strategic manoeuver are included, with the aim of providing a complement to the existing reviews to inspire future developments.

USP2a Supports Metastasis by Tuning TGF-β Signaling

TGF-β has been demonstrated to promote tumor metastasis, and the regulatory mechanisms are poorly understood. Here, we report the role of USP2a in promoting metastasis by facilitating TGF-β-triggered signaling. USP2a interacts with TGFBR1 and TGFBR2 upon TGF-β stimulation and removes K33-linked polyubiquitin chains from Lys502 of TGFBR1, promoting the recruitment of SMAD2/3. Simultaneously, TGFBR2 phosphorylates Ser207/Ser225 of USP2a, leading to the disassociation of SMAD2/3 from TGFBR1. The phosphorylation of USP2a and SMAD2 is positively correlated in human tumor biopsies, and USP2a is hyper-phosphorylated in lung adenocarcinomas with lymph node invasion. Depletion or pharmacologic inhibition of USP2a dampens TGF-β-triggered signaling and metastasis. Our findings have characterized an essential role of USP2a as a potential target for treatment of metastatic cancers.

The Discovery of a Palladium(II)‐Initiated Borono‐Catellani Reaction

Reported is a novel palladium(II)-initiated Catellani-type reaction that utilizes widely accessible aryl boronic acids as the substrates instead of aryl halides, thereby greatly expanding the existing scope of this powerful transformation. This borono-Catellani reaction was promoted by cooperative catalysis between Pd(OAc)2 and the inexpensive 5-norbornene-2-carbonitrile. Practicality is the striking feature of the reaction: it is run open to air at ambient temperature and no phosphine ligand is needed. This mild, chemoselective, and scalable protocol is compatible with a large range of readily available functionalized aryl boronic acids and bromides, as well as terminating olefins (50 examples, 39-97 % yields). Moreover, the orthogonal reactivity between the borono-Catellani and classical Catellani reaction was demonstrated. This work is expected to open new avenues for developing novel Catellani-type reactions.

Convergent syntheses of 2,3-dihydrobenzofurans via a Catellani strategy

A cooperative catalytic system comprising a Pd/XPhos complex and a potassium salt of 5-norbornene-2-carboxylic acid to promote the annulation between aryl iodides and epoxides was developed, thereby providing highly convergent access to valuable 2,3-dihydrobenzofuran (DHBF) scaffolds. The unique potassium salt of the inexpensive 5-norbornene-2-carboxylic acid serves as a highly efficient catalytic mediator (10 mol%), which leads to fewer side reactions. The salient features of the reaction include its broad substrate scope (with respect to both aryl iodides and epoxides), its high atom economy and good chemo-selectivity. Furthermore, no extra base is needed for the process.

Modular One‐Step Three‐Component Synthesis of Tetrahydroisoquinolines Using a Catellani Strategy

Reported is a modular one-step three-component synthesis of tetrahydroisoquinolines using a Catellani strategy. This process exploits aziridines as the alkylating reagents, through palladium/norbornene cooperative catalysis, to enable a Catellani/Heck/aza-Michael addition cascade. This mild, chemoselective, and scalable protocol has broad substrate scope (43 examples, up to 90 % yield). The most striking feature of this protocol is the excellent regioselectivity and diastereoselectivity observed for 2-alkyl- and 2-aryl-substituted aziridines to access 1,3-cis-substituted and 1,4-cis-substituted tetrahydroisoquinolines, respectively. Moreover, this is a versatile process with high step and atom economy.