Oxetane Synthesis via Alcohol C-H Functionalization

Biocatalytic enantioselective formation and ring-opening of oxetanes

Although biocatalysis offers complementary or alternative approaches to traditional synthetic methods, the limited range of available enzymatic reactions currently poses challenges in synthesizing a diverse array of desired compounds. Consequently, there is a significant demand for developing novel biocatalytic processes to enable reactions that were previously unattainable. Herein, we report the discovery and subsequent protein engineering of a unique halohydrin dehalogenase to develop a biocatalytic platform for enantioselective formation and ring-opening of oxetanes. This biocatalytic platform, exhibiting high efficiency, excellent enantioselectivity, and broad scopes, facilitates the preparative-scale synthesis of chiral oxetanes and a variety of chiral γ-substituted alcohols. Additionally, both the enantioselective oxetane formation and ring-opening processes are proven scalable for large-scale transformations at high substrate concentrations, and can be integrated efficiently in a one-pot, one-catalyst cascade system. This work expands the enzymatic toolbox for non-natural reactions and will promote further exploration of the catalytic repertoire of halohydrin dehalogenases in synthetic and pharmaceutical chemistry.

Enhancing Electron Donor-Acceptor Complex Photoactivation with a Stable Perylene Diimide Metal-Organic Framework

Electron donor-acceptor complexes are commonly employed to facilitate photoinduced radical-mediated organic reactions. However, achieving these photochemical processes with catalytic amounts of donors or acceptors can be challenging, especially when aiming to reduce catalyst loadings. Herein, we have unveiled a framework-based heterogenization approach that significantly enhances the photoredox activity of perylene diimide species in radical addition reactions with alkyl silicates by promoting faster and more efficient electron donor-acceptor complex formation. Besides offering broad substrate scope in alkene hydroalkylation, the newly developed heterogeneous photocatalysis substantially improves the catalyst turnover numbers in comparison to previous homogeneous photocatalytic systems and demonstrates outstanding catalyst recyclability. These research findings pave the way for the advancement of various efficient and practical organic transformations using framework-supported organocatalysts.

Harnessing Oxetane and Azetidine Sulfonyl Fluorides for Opportunities in Drug Discovery

Four-membered heterocycles such as oxetanes and azetidines represent attractive and emergent design options in medicinal chemistry due to their small and polar nature and potential to significantly impact the physiochemical properties of drug molecules. The challenging preparation of these derivatives, especially in a divergent manner, has severely limited their combination with other medicinally and biologically important groups. Consequently, there is a substantial demand for mild and effective synthetic strategies to access new oxetane and azetidine derivatives and molecular scaffolds. Here, we report the development and use of oxetane sulfonyl fluorides (OSFs) and azetidine sulfonyl fluorides (ASFs), which behave as precursors to carbocations in an unusual defluorosulfonylation reaction pathway (deFS). The small-ring sulfonyl fluorides are activated under mild thermal conditions (60 °C), and the generated reactive intermediates couple with a broad range of nucleophiles. Oxetane and azetidine heterocyclic, -sulfoximine, and -phosphonate derivatives are prepared, several of which do not have comparable carbonyl analogs, providing new chemical motifs and design elements for drug discovery. Alternatively, a SuFEx pathway under anionic conditions accesses oxetane-sulfur(VI) derivatives. We demonstrate the synthetic utility of novel OSF and ASF reagents through the synthesis of 11 drug analogs, showcasing their potential for subsequent diversification and facile inclusion into medicinal chemistry programs. Moreover, we propose the application of the OSF and ASF reagents as linker motifs and demonstrate the incorporation of pendant groups suitable for common conjugation reactions. Productive deFS reactions with E3 ligase recruiters such as pomalidomide and related derivatives provide new degrader motifs and potential PROTAC linkers.

Intermolecular (4 + 3) Cycloadditions of Oxetanyl and Azetidinyl Enolsilanes

We report the intermolecular (4 + 3) cycloadditions of oxetanyl and azetidinyl enolsilanes with different dienes by silylium catalysis to generate bicyclic scaffolds. These enolsilanes were competent to react with furans, cyclopentadiene, and 1,3-cyclohexadiene to deliver cycloadducts in up to 96% yields.

High-throughput optimization of the C-H arylation of oxetanes via Ni/aldehyde photocatalysis

Oxetanes are under-explored in medicinal chemistry, despite their favorable physicochemical properties, in part, because of the challenges associated with their syntheses. High-throughput experimentation (HTE) enables the rapid screening of reaction variables, accelerating the reaction development process. Herein we report the use of HTE in the optimization of a mild C-H arylation reaction of oxetanes, and other ethers, using p-cyanobenzaldehyde as a cheap and effective photoexcited hydrogen-atom transfer catalyst, in conjunction with a Ni catalyst. Our optimized conditions enable the use of a modern, reproducible light source as well as sub-solvent quantity oxetane, while eliminating the need for toxic co-solvents and dangerous sources of UV light.

Stereoselective Synthesis of Oxetanes Catalyzed by an Engineered Halohydrin Dehalogenase

Although biocatalysis has garnered widespread attention in both industrial and academic realms, the enzymatic synthesis of chiral oxetanes remains an underdeveloped field. Halohydrin dehalogenases (HHDHs) are industrially relevant enzymes that have been engineered to accomplish the reversible transformation of epoxides. In this study, a biocatalytic platform was constructed for the stereoselective kinetic resolution of chiral oxetanes and formation of 1,3-disubstituted alcohols. HheC from Agrobacterium radiobacter AD1 was engineered to identify key variants capable of catalyzing the dehalogenation of γ-haloalcohols (via HheC M1-M3) and ring opening of oxetanes (via HheC M4-M5) to access both (R)- and (S)-configured products with high stereoselectivity and remarkable catalytic activity, yielding up to 49 % with enantioselectivities exceeding 99 % ee and E>200. The current strategy is broadly applicable as demonstrated by expansion of the substrate scope to include up to 18 examples for dehalogenation and 16 examples for ring opening. Additionally, the functionalized products are versatile building blocks for pharmaceutical applications. To shed light on the molecular recognition mechanisms for the relevant variants, molecular dynamic (MD) simulations were performed. The current strategy expands the scope of HHDH-catalyzed chiral oxetane ring construction, offering efficient access to both enantiomers of chiral oxetanes and 1,3-disubstituted alcohols.

Synthesis of alcohols: streamlined C1 to Cn hydroxyalkylation through photoredox catalysis

Naturally occurring and readily available α-hydroxy carboxylic acids (AHAs) are utilized as platforms for visible light-mediated oxidative CO2-extrusion furnishing α-hydroxy radicals proved to be versatile C1 to Cn hydroxyalkylating agents. The direct decarboxylative Giese reaction (DDGR) is operationally simple, not requiring activator or sacrificial oxidants, and enables the synthesis of a diverse range of hydroxylated products, introducing connectivity typically precluded from conventional polar domains. Notably, the methodology has been extended to widely used glycolic acid resulting in a highly efficient and unprecedented C1 hydroxyhomologation tactic. The use of flow technology further facilitates scalability and adds green credentials to this synthetic methodology.

Shedding light on thermally-activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) is a hot research topic in view of its impressive applications in a wide variety of fields from organic LEDs to photodynamic therapy and metal-free photocatalysis. TADF is a rare and fragile phenomenon that requires a delicate equilibrium between tiny singlet-triplet gaps, sizable spin-orbit couplings, conformational flexibility and a balanced contribution of charge transfer and local excited states. To make the picture more complex, this precarious equilibrium is non-trivially affected by the interaction of the TADF dye with its local environment. The concurrent optimization of the dye and of the embedding medium is therefore of paramount importance to boost practical applications of TADF. Towards this aim, refined theoretical and computational approaches must be cleverly exploited, paying attention to the reliability of adopted approximations. In this perspective, we will address some of the most important issues in the field. Specifically, we will critically review theoretical and computational approaches to TADF rates, highlighting the limits of widespread approaches. Environmental effects on the TADF photophysics are discussed in detail, focusing on the major role played by dielectric and conformational disorder in liquid solutions and amorphous matrices.

Photoredox-Catalyzed α-C-H Monoalkylation of Symmetric Polyols in the Presence of CO2

Achieving the selective modification of symmetric poly-hydroxylated compounds presents a significant challenge due to the presence of identical active sites. Herein, we address this challenge through the design of a ternary catalytic system that includes a photoredox catalyst, a hydrogen atom transfer promotor and a carbonation catalyst. This catalytic system enables the reversible carbonation of acyclic polyols under CO2 atmosphere, which modulates the reactivity of its distinct C-H bonds toward hydrogen atom transfers. An exquisite selectivity for the monoalkylation is achieved in a variety of unprotected light polyols, yielding valuable building blocks in short reaction times. Mechanistic and computational studies demonstrate that the formation of an intramolecular hydrogen bond between the transient carbonate and the free alcohol is pivotal for the kinetic and thermodynamic activation of a specific alcohol.

Diastereoselective Synthesis of Pyrroloquinolines via N-H Functionalization of Indoles with Vinyl Sulfonium Salts

A [2 + 4]/[1 + 2] annulation approach was successfully established to construct pyrroloquinoline-fused cyclopropane in a highly diastereoselective fashion (>20:1 dr). The tetracyclic 1,7-fused indoles were efficiently obtained from readily available starting materials under mild conditions. This methodology displays impressive substrate generality with two reaction components. The products resulting from this doubly annulative strategy are useful synthetic intermediates.