Retrosynthetic Simplicity

Carbon-Atom Scavengers Enable Divergent, Selective Carbon Deletion of Azaarenes

Divergent synthesis is a powerful strategy that provides simultaneous access to multiple derivatives of a given substrate. However, the emerging developments in skeletal editing have largely delivered methods that lack this potential for diversification. Herein, we report the serendipitous discovery of reagent-controlled selective deletion of C3 or C2 carbon atoms of quinolines, affording indoles. An initial observation that an impurity in commercial samples of DBU promoted cyclization of a benzoxazepine-derived imidate led to the identification of indoline and aminoethanol as C3- and C2-selective carbon-atom scavengers, respectively. These two methods successfully convert a broad scope of quinolines and related azaarenes to the corresponding indoles and azaindoles, enabling divergent carbon deletion. In-depth mechanistic studies support the HFIP-promoted ring opening of 3,1-benzoxazepines to amidine intermediates as a rate-determining step, while providing insights into the selectivity afforded by indoline. These methods and their associated mechanisms offer a blueprint for the rational design of reagent-controlled, divergent skeletal edits.

Photocatalytic Boron Insertion into Thiaarenes via Boryl Radicals

Skeletal editing of aromatic heterocycles represents a straightforward approach to rapidly expand the accessible chemical space. While notable progress has been made on the direct modification of various nitrogen- and oxygen-heterocycles, editing of prevalent sulfur-containing heteroarenes, especially for single-atom insertion, remains exceedingly rare. This disparity is primarily attributed to the sulfur atom's inherent nucleophilicity and high susceptibility to oxidation. Here we present a conceptually distinct photocatalytic strategy that enables the insertion of a boron atom into a diverse range of thiaarenes, furnishing previously inaccessible cyclic thioborane scaffolds and synthetically valuable alkyl boronates in an efficient manner. Furthermore, mechanistic studies have revealed that the boron insertion step proceeds through an unprecedented mechanism.

Advancing Total Synthesis Through Skeletal Editing

ConspectusTotal synthesis has long been a proving ground for advancing chemical thought, pushing chemists to develop strategies that not only replicate nature's complexity but often surpass it. The pursuit of efficiency, practicality, and elegance continues to challenge and reshape the guiding principles of total synthesis. In recent years, skeletal editing has emerged as a powerful strategy for reconfiguring skeletal frameworks in ways that were previously difficult to imagine. Unlike conventional chemical synthesis approaches, which primarily rely on the logic of bond construction reactions and functional group manipulations, skeletal editing introduces elements that allow for atom insertion, deletion, and exchange and skeletal rearrangement/reorganization by harnessing the potential energy and reactivity of certain structural motifs and morphing them into new electronic and spatial configurations. The logic of modern skeletal editing has been fueling the development of new editing methods and advancing the fields of total synthesis, medicinal chemistry, materials science, and others.In this Account, we detail our program using skeletal editing-based retrosynthetic logic to facilitate natural product synthesis. We first highlight two one-carbon insertion editing strategies utilizing the Ciamician-Dennstedt rearrangement and the Büchner-Curtius-Schlotterbeck ring expansion to streamline the total syntheses of complanadine and phleghenrine Lycopodium alkaloids. We next present our synthesis of crinipellin and gibberellin diterpenes by leveraging the facile synthesis and intrinsic strain of cyclobutanes as precursors to challenging cyclopentanes via cut-and-insert editing (crinipellins) or C-C bond migratory ring expansion (GA18). Toward the end, we describe our early efforts in orchestrating structural rearrangement and functional group pairing reactions to access seven monoterpene indole alkaloids and highlight the divergent potential of skeletal editing. Each of the five examples follows a build-edit-decorate workflow, inspired by Schreiber's build-couple-pair in diversity-oriented synthesis. In the build stage, key scaffolds are efficiently assembled from starting materials with matched reactivity. The edit stage morphs these scaffolds to the desired but more challenging ones encoded by the target molecules, reminiscent of Corey's application of rearrangement transforms as a topological strategy. The decorate stage introduces additional functional groups and adjusts oxidation states to complete the total synthesis, similar to the oxidase phase of Baran's two-phase synthesis. The essence of skeletal editing-based retrosynthetic analysis is to identify latent structural relationships between the readily assembled key scaffolds constructed in the build stage and the desired ones encoded by the target molecules as well as proper editing methods to transform the former into the latter with precision. The build-edit-decorate approach parallels the dynamism of biosynthesis, enabling rapid building of complexity with great efficiency and step economy, as analyzed by the spacial scores (SPS) of each case. Drawing on these principles, chemists can adopt skeletal editing-based retrosynthetic logic by identifying latent intermediates and employing and developing strategic editing methods to overcome synthetic bottlenecks.

Strategic atom replacement enables regiocontrol in pyrazole alkylation

Pyrazoles are heterocycles commonly found as key substructures in agrochemicals and medicinally active compounds alike1,2. Despite their pervasiveness, established methods fall notably short in delivering complex pyrazoles selectively due to issues of differentiation during either assembly or N-functionalization3. This is a direct consequence of a dominant synthetic strategy that attempts to control selectivity-determining bonds between poorly differentiated starting materials. To overcome this longstanding challenge, we here describe a prototypical example of an alternative conceptual approach, 'strategic atom replacement', in which we synthesize N-alkyl pyrazoles from isothiazoles. The net forward transformation is a 'swap' of the isothiazole sulfur atom with a nitrogen atom and its associated alkyl fragment to deliver the alkylated pyrazole4,5. Linking the two azoles is an orphaned heterocycle class, 1,2,3-thiadiazine-S-oxides, whose synthetic potential has yet to be tapped6. By proceeding through these unusual heterocycles, the typical selectivity and separation challenges associated with exclusively bond-based pyrazole preparations are circumvented, and even minimally differentiated peripheral substituents can be discriminated to afford isomerically pure products.

Aromatic Metamorphosis: Skeletal Editing of Aromatic Rings

ConspectusAromatic rings are fundamental structural motifs found in natural products, synthetic intermediates, pharmaceuticals, agrochemicals, and functional materials. While transformations at the periphery of these rings are well-established, modifying their core frameworks has remained an underexplored frontier. Our group has pioneered the concept, termed "aromatic metamorphosis", enabling skeletal transformations of aromatic rings by replacing an endocyclic atom with a different atom or inserting an atom into aromatic rings, which leads to novel synthetic strategies and diverse molecular architectures.The concept of aromatic metamorphosis was first demonstrated in the stepwise conversion of dibenzothiophenes and dibenzofurans into triphenylenes. These transformations, facilitated by palladium and nickel catalysts, involve the strategic activation of robust C-S and C-O bonds as the key steps. Next, the approach was extended to the two-step conversions of dibenzothiophenes into carbazoles, dibenzophospholes, fluorenes, etc., which involve oxidation into the corresponding sulfones and subsequent sequential inter- and intramolecular nucleophilic aromatic substitution reactions. These new synthetic routes have provided efficient access to optoelectronic materials. Especially, the SNAr-based aromatic metamorphosis facilitated the construction of a heterohelicene library with systematic variation in endocyclic atoms. This strategy has revolutionized the way molecular libraries are constructed and enables the rapid discovery of functional molecules.In addition to the endocyclic substitutions, ring-expanding aromatic metamorphosis through atom insertion has also been explored. We developed nickel-catalyzed boron insertion into benzofurans, generating benzoxaborins, which are important scaffolds for medicinal chemistry. This novel catalytic transformation has been successfully scaled to industrial synthesis by companies, which demonstrates the practical utility of aromatic metamorphosis. Furthermore, manganese-catalyzed and lithium-metal-promoted methodologies have expanded the ranges of heteroatoms inserted and aromatic frameworks cleaved, providing methods to access heterocycles with a diversity in element compositions.Reductive dilithiation of thiophenes efficiently yields 1,4-dilithiobutadienes, which react with a variety of electrophiles to produce a series of nonbiogenic heteroles, such as boroles, phospholes, and siloles. In principle, this method should allow the sulfur atom in readily available thiophenes to be replaced with any atom and is therefore considered an ideal example of aromatic metamorphosis in terms of rapid construction of diverse chemical spaces with a variety of elements.Aromatic metamorphosis proposes many new synthons and retrosynthetic disconnections that defy the conventional wisdom of organic synthesis. By making full use of metamorphosing the aromatic skeleton, a library with skeletal diversity can be constructed directly with minimal effort and time investment. Its applications span from pharmaceuticals to materials science, paving the way for a new paradigm in molecular design as well as synthetic strategy.

Skeletal Editing of Isoindolines to Tetralins

We present a skeletal editing strategy for transforming isoindolines into tetralins via a cascade N-atom removal deconstruction followed by a Diels-Alder reaction between in situ generated o-quinodimethanes and activated alkenes. This approach features a broad substrate scope, excellent stereoselectivity, and high yields, demonstrating its applicability to complex bioactive compounds and natural products. Notably, case studies showcase the efficient construction of challenging spirocyclic and bridged systems, underscoring the method's versatility and potential for advancing applications in synthetic chemistry.

Transforming an azaarene into the spine of fusedbicyclics via cycloaddition-induced scaffold hopping of 5-Hydroxypyrazoles

Skeleton editing for heteroarenes, especially pyrazoles, is challenging and remains scarce because these non-strained aromatics exhibit inert reactivities, making them relatively inactive for performing a dearomatization/cleavage sequence. Here, we disclose a cycloaddition-induced scaffold hopping of 5-hydroxypyrazoles to access the pyrazolopyridopyridazin-6-one skeleton through a single-operation protocol. By converting a five-membered aza-arene into a five-unit spine of a 6/6 fused-bicyclic, this work unlocks a ring-opening reactivity of the pyrazole core that involves a formal C = N bond cleavage while retaining the highly reactive N-N bond in the resulting product. A [4 + 2] cycloaddition of a temporarily dearomatized 5-hydroxypyrrole with an in situ generated aza-1,3-diene, followed by oxidative C-N bond cleavage, constitutes the domino pathway. A library of pyrazolopyridopyridazin-6-ones, which are medicinally relevant nitrogen-atom-rich tricyclics, is obtained efficiently from readily available materials.

Photocatalytic furan-to-pyrrole conversion

The identity of a heteroatom within an aromatic ring influences the chemical properties of that heterocyclic compound. Systematically evaluating the effect of a single atom, however, poses synthetic challenges, primarily as a result of thermodynamic mismatches in atomic exchange processes. We present a photocatalytic strategy that swaps an oxygen atom of furan with a nitrogen group, directly converting the furan into a pyrrole analog in a single intermolecular reaction. High compatibility was observed with various furan derivatives and nitrogen nucleophiles commonly used in drug discovery, and the late-stage functionalization furnished otherwise difficult-to-access pyrroles from naturally occurring furans of high molecular complexity. Mechanistic analysis suggested that polarity inversion through single electron transfer initiates the redox-neutral atom exchange processes at room temperature.

Dearomatization of Pyridines: Photochemical Skeletal Enlargement for the Synthesis of 1,2-Diazepines

In this report, we developed a unified and standardized one-pot sequence that converts pyridine derivatives into 1,2-diazepines by inserting a nitrogen atom. This skeletal transformation capitalizes on the in situ generation of 1-aminopyridinium ylides, which rearrange under UV light irradiation. A thorough evaluation of the key parameters (wavelength, reaction conditions, activating agent) allowed us to elaborate on a simple, mild, and user-friendly protocol. The model reaction was extrapolated to more than 40 examples, including drug derivatives, affording unique 7-membered structures. Mechanistic evidence supports the transient presence of a diazanorcaradiene species. Finally, pertinent transformations of the products, including ring contraction reactions to form pyrazoles, were conducted and paved the way to a broad application of the developed protocol.