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.

Construction of Seven-Membered Oxacycles Using a Rh(I)-Catalyzed Cascade C-C Formation/Cleavage of Cyclobutenol Derivatives

Herein, we describe the synthesis of substituted oxepane derivatives through the skeletal remodeling of 4-hydroxy-2-cyclobutenones, which are readily prepared from commercially available dialkyl squarates upon their reaction with acrylonitrile. Mechanistically, a Rh(I)-catalyzed C-C bond formation and cleavage cascade is proposed. Specifically, a fused [3.2.0] bicycle is proposed to form from dialkyl squarate-derived cyclobutenols via an unusual Rh(I)-catalyzed intermolecular oxa-Michael addition of a tertiary alcohol with acrylonitrile, followed by an intramolecular conjugate addition/migratory insertion. Subsequent C(sp3)-C(sp3) bond cleavage through a Rh-catalyzed β-carbon elimination is then theorized to furnish the oxepane scaffold. Computational studies support the formation of an intermediate [3.2.0] bicycle but also point to an alternative pathway for the formation of the oxepane products involving a Rh(III) intermediate. Additional studies have shown the overall process to be stereoretentive. The functional groups that are introduced in this process can be leveraged to form fused or bridged ring systems.

Remote C-H Amination and Alkylation of Camphor at C8 through Hydrogen-Atom Abstraction

Camphor continues to serve as a versatile chiral building block for chemical synthesis. We have developed a novel method to functionalize the camphor skeleton at C8 using an intramolecular hydrogen atom abstraction. The key advance involves the use of a camphor-derived aminonitrile, which is converted to the corresponding nitrogen-centered radical under photoredox conditions to effect the 1,5-hydrogen atom transfer at C8. The resulting carbon-centered radical at C8 was utilized in a C-H amination to access topologically complex proline derivatives. Furthermore, the total synthesis of several sesquiterpenoids was accomplished by engaging the radical generated at C8 in alkylation reactions.

Mechanistic Investigation, Wavelength-Dependent Reactivity, and Expanded Reactivity of N-Aryl Azacycle Photomediated Ring Contractions

Under mild blue-light irradiation, α-acylated saturated heterocycles undergo a photomediated one-atom ring contraction that extrudes a heteroatom from the cyclic core. However, for nitrogenous heterocycles, this powerful skeletal edit has been limited to substrates bearing electron-withdrawing substituents on nitrogen. Moreover, the mechanism and wavelength-dependent efficiency of this transformation have remained unclear. In this work, we increased the electron richness of nitrogen in saturated azacycles to improve light absorption and strengthen critical intramolecular hydrogen bonding while enabling the direct installation of the photoreactive handle. As a result, a broadly expanded substrate scope, including underexplored electron-rich substrates and previously unsuccessful heterocycles, has now been achieved. The significantly improved yields and diastereoselectivities have facilitated reaction rate, kinetic isotope effect (KIE), and quenching studies, in addition to the determination of quantum yields. Guided by these studies, we propose a revised ET/PT mechanism for the ring contraction, which is additionally corroborated by computational characterization of the lowest-energy excited states of α-acylated substrates through time-dependent DFT. The efficiency of the ring contraction at wavelengths longer than those strongly absorbed by the substrates was investigated through wavelength-dependent rate measurements, which revealed a red shift of the photochemical action plot relative to substrate absorbance. The elucidated mechanistic and photophysical details effectively rationalize empirical observations, including additive effects, that were previously poorly understood. Our findings not only demonstrate enhanced synthetic utility of the photomediated ring contraction and shed light on mechanistic details but may also offer valuable guidance for understanding wavelength-dependent reactivity for related photochemical systems.

14N to 15N Isotopic Exchange of Nitrogen Heteroaromatics through Skeletal Editing

The selective modification of nitrogen heteroaromatics enables the development of new chemical tools and accelerates drug discovery. While methods that focus on expanding or contracting the skeletal structures of heteroaromatics are emerging, methods for the direct exchange of single core atoms remain limited. Here, we present a method for 14N → 15N isotopic exchange for several aromatic nitrogen heterocycles. This nitrogen isotope transmutation occurs through activation of the heteroaromatic substrate by triflylation of a nitrogen atom, followed by a ring-opening/ring-closure sequence mediated by 15N-aspartate to effect the isotopic exchange of the nitrogen atom. Key to the success of this transformation is the formation of an isolable 15N-succinyl intermediate, which undergoes elimination to give the isotopically labeled heterocycle. These transformations occur under mild conditions in high chemical and isotopic yields.

Navigating Excess Complexity: Total Synthesis of Daphenylline

Retrosynthetic analysis is a framework for designing synthetic routes to complex molecules that generally prioritizes disconnections which reduce molecular complexity. However, strict adherence to this principle can overlook pathways involving highly complex intermediates that can be easily prepared through powerful bond-forming transformations. Herein, we demonstrate this tactic of generating excess complexity, followed by strategic bond-cleavage, as a highly effective approach for the 11-step total synthesis of the Daphniphyllum alkaloid daphenylline. To implement this strategy, we accessed a bicyclo[4.1.0]heptane core through a dearomative Buchner cycloaddition, which enabled construction of the seven-membered ring after C-C bond cleavage. Installation of the synthetically challenging quaternary stereocenter methyl group was achieved through a thia-Paternò-Büchi [2 + 2] photocycloaddition followed by stereospecific thietane reduction, further illustrating how building excess complexity can enable desired synthetic outcomes after strategic bond-breaking events. This strategy leveraging bond cleavage transformations should serve as a complement to traditional bond-forming, complexity-generating synthetic strategies.

Unified Synthesis of 2-Isocyanoallopupukeanane and 9-Isocyanopupukeanane through a "Contra-biosynthetic" Rearrangement

Herein, we describe our synthetic efforts toward the pupukeanane natural products, in which we have completed the first enantiospecific route to 2-isocyanoallopupukeanane in 10 steps (formal synthesis), enabled by a key Pd-mediated cyclization cascade. This subsequently facilitated an unprecedented bio-inspired "contra-biosynthetic" rearrangement, providing divergent access to 9-isocyanopupukeanane in 15 steps (formal synthesis). Computational studies provide insight into the nature of this rearrangement.

Convergent synthesis of thiodiazole dioxides from simple ketones and amines through an unusual nitrogen-migration mechanism

We report the modular preparation of dihydro-1,2,5-thiodiazole dioxide heterocycles starting from methyl ketones and primary amines. This one-pot, three-component coupling employs 2,3-dimethylimidazole-1-sulfonyl azide triflate as a coupling reagent and oxidant. The transformation is scalable and various ketones and amines can be used, yielding thiodiazole dioxide products in up to 89% yield. In addition, 15N- and 13C-labeling studies suggest a mechanism involving a 1,2-nitrogen migration. Together with the mechanistic studies, DFT calculations provide insight into the reaction pathway and set the stage for further exploration of the mechanistic nuances of reactions that use sulfamoyl azides. In combination with the demonstrated modularity of the approach reported herein, the derivatization of the thiodiazole dioxide products highlights the potential of this methodology to rapidly access diverse chemical structures.

Oxidative cleavage of ketoximes to ketones using photoexcited nitroarenes

The methoxime group has emerged as a versatile directing group for a variety of C-H functionalizations. Despite its importance as a powerful functional handle, conversion of methoximes to the parent ketone, which is often desired, usually requires harsh and functional group intolerant reaction conditions. Therefore, the application of methoximes and their subsequent conversion to the corresponding ketone in a late-stage context can be problematic. Here, we present an alternative set of conditions to achieve mild and functional group tolerant conversion of methoximes to the parent ketones using photoexcited nitroarenes. The utility of this methodology is showcased in its application in the total synthesis of cephanolide D. Furthermore, mechanistic insight into this transformation obtained using isotope labeling studies as well as the analysis of reaction byproducts is provided.

Strategic application of C-H oxidation in natural product total synthesis

The oxidation of unactivated C-H bonds has emerged as an effective tactic in natural product synthesis and has altered how chemists approach the synthesis of complex molecules. The use of C-H oxidation methods has simplified the process of synthesis planning by expanding the choice of starting materials, limiting functional group interconversion and protecting group manipulations, and enabling late-stage diversification. In this Review, we propose classifications for C-H oxidations on the basis of their strategic purpose: type 1, which installs functionality that is used to establish the carbon skeleton of the target; type 2, which is used to construct a heterocyclic ring; and type 3, which installs peripheral functional groups. The reactions are further divided based on whether they are directed or undirected. For each classification, examples from recent literature are analysed. Finally, we provide two case studies of syntheses from our laboratory that were streamlined by the judicious use of C-H oxidation reactions.

Rearrangement of a carboxy-substituted spiro[4.4]nonatriene to annulated fulvenes through a Pd(ii)-mediated 1,5-vinyl shift

A novel synthesis of aryl-substituted, enantioenriched fulvenes from an oxidative Heck cascade and rearrangement of a carboxy-substituted spiro[4.4]nonatriene is disclosed. Mechanistic investigations with density functional theory (DFT) calculations and empirical results support the net transformation occurring through a novel Pd(ii)-mediated 1,5-vinyl shift from a vinyl-palladium intermediate that terminates with protodepalladation. This spiro-to-fused bicycle conversion tolerates a range of electron-rich and deficient arylboronic acids to give a range of mono- and diaryl substituted annulated fulvenes in moderate to good yields and enantiomeric ratios. Overall, this work connects two classes of molecules with a rich history in physical organic chemistry.

Interconverting mirror-image molecules

A light-driven multitasking catalyst enhances chirality in molecular mixtures.

Access to Naphthoic Acid Derivatives through an Oxabenzonorbornadiene Rearrangement

Herein, the synthesis of 1-hydroxy-2-naphthoic acid esters through an unexpected Lewis-acid-mediated 1,2-acyl shift of oxabenzonorbornadienes is reported. Using this methodology, novel substitution patterns for 1-hydroxy-2-naphtoic acid esters can be obtained. A mechanistic proposal and rationale for this transformation, the products of which had been previously incorrectly characterized, is given.

Methylglyoxal-derived hydroimidazolone, MG-H1, increases food intake by altering tyramine signaling via the GATA transcription factor ELT-3 in Caenorhabditis elegans

The Maillard reaction, a chemical reaction between amino acids and sugars, is exploited to produce flavorful food ubiquitously, from the baking industry to our everyday lives. However, the Maillard reaction also occurs in all cells, from prokaryotes to eukaryotes, forming advanced glycation end-products (AGEs). AGEs are a heterogeneous group of compounds resulting from the irreversible reaction between biomolecules and α-dicarbonyls (α-DCs), including methylglyoxal (MGO), an unavoidable byproduct of anaerobic glycolysis and lipid peroxidation. We previously demonstrated that Caenorhabditis elegans mutants lacking the glod-4 glyoxalase enzyme displayed enhanced accumulation of α-DCs, reduced lifespan, increased neuronal damage, and touch hypersensitivity. Here, we demonstrate that glod-4 mutation increased food intake and identify that MGO-derived hydroimidazolone, MG-H1, is a mediator of the observed increase in food intake. RNAseq analysis in glod-4 knockdown worms identified upregulation of several neurotransmitters and feeding genes. Suppressor screening of the overfeeding phenotype identified the tdc-1-tyramine-tyra-2/ser-2 signaling as an essential pathway mediating AGE (MG-H1)-induced feeding in glod-4 mutants. We also identified the elt-3 GATA transcription factor as an essential upstream regulator for increased feeding upon accumulation of AGEs by partially controlling the expression of tdc-1 gene. Furthermore, the lack of either tdc-1 or tyra-2/ser-2 receptors suppresses the reduced lifespan and rescues neuronal damage observed in glod-4 mutants. Thus, in C. elegans, we identified an elt-3 regulated tyramine-dependent pathway mediating the toxic effects of MG-H1 AGE. Understanding this signaling pathway may help understand hedonistic overfeeding behavior observed due to modern AGE-rich diets.

Skeletal Editing Approach to Bridge-Functionalized Bicyclo[1.1.1]pentanes from Azabicyclo[2.1.1]hexanes

Azabicyclo[2.1.1]hexanes (aza-BCHs) and bicyclo[1.1.1]pentanes (BCPs) have emerged as attractive classes of sp3-rich cores for replacing flat, aromatic groups with metabolically resistant, three-dimensional frameworks in drug scaffolds. Strategies to directly convert, or "scaffold hop", between these bioisosteric subclasses through single-atom skeletal editing would enable efficient interpolation within this valuable chemical space. Herein, we describe a strategy to "scaffold hop" between aza-BCH and BCP cores through a nitrogen-deleting skeletal edit. Photochemical [2+2] cycloadditions, used to prepare multifunctionalized aza-BCH frameworks, are coupled with a subsequent deamination step to afford bridge-functionalized BCPs, for which few synthetic solutions currently exist. The modular sequence provides access to various privileged bridged bicycles of pharmaceutical relevance.

Photo- and Metal-Mediated Deconstructive Approaches to Cyclic Aliphatic Amine Diversification

Described herein are studies toward the core modification of cyclic aliphatic amines using either a riboflavin/photo-irradiation approach or Cu(I) and Ag(I) to mediate the process. Structural remodeling of cyclic amines is explored through oxidative C-N and C-C bond cleavage using peroxydisulfate (persulfate) as an oxidant. Ring-opening reactions to access linear aldehydes or carboxylic acids with flavin-derived photocatalysis or Cu salts, respectively, are demonstrated. A complementary ring-opening process mediated by Ag(I) facilitates decarboxylative Csp³-Csp² coupling in Minisci-type reactions through a key alkyl radical intermediate. Heterocycle interconversion is demonstrated through the transformation of N-acyl cyclic amines to oxazines using Cu(II) oxidation of the alkyl radical. These transformations are investigated by computation to inform the proposed mechanistic pathways. Computational studies indicate that persulfate mediates oxidation of cyclic amines with concomitant reduction of riboflavin. Persulfate is subsequently reduced by formal hydride transfer from the reduced riboflavin catalyst. Oxidation of the cyclic aliphatic amines with a Cu(I) salt is proposed to be initiated by homolysis of the peroxy bond of persulfate followed by α-HAT from the cyclic amine and radical recombination to form an α-sulfate adduct, which is hydrolyzed to the hemiaminal. Investigation of the pathway to form oxazines indicates a kinetic preference for cyclization over more typical elimination pathways to form olefins through Cu(II) oxidation of alkyl radicals.

Skeletal diversification by C-C cleavage to access bicyclic frameworks from a common tricyclooctane intermediate

Herein, the diversification of tricyclo[3.2.1.03,6]octane scaffolds to afford diverse bicyclic scaffolds is described. The strained tricyclooctanes are prepared in two steps featuring a blue light-mediated [2+2] cycloaddition. Strategies for the cleavage of this scaffold were then explored resulting in the selective syntheses of the bicyclo[3.1.1]heptane, bicyclo[3.2.1]octane, and bicyclo[3.2.0]heptane cores. These findings may guide future studies of C-C cleavage reactions in strained carbon frameworks and their application in complex molecule synthesis.

Skeletal Editing of Pyrimidines to Pyrazoles by Formal Carbon Deletion

A method for the conversion of pyrimidines into pyrazoles by a formal carbon deletion has been achieved guided by computational analysis. The pyrimidine heterocycle is the most common diazine in FDA-approved drugs, and pyrazoles are the most common diazole. An efficient method to convert pyrimidines into pyrazoles would therefore be valuable by leveraging the chemistries unique to pyrimidines to access diversified pyrazoles. One method for the conversion of pyrimidines into pyrazoles is known, though it proceeds in low yields and requires harsh conditions. The transformation reported here proceeds under milder conditions, tolerates a wide range of functional groups, and enables the simultaneous regioselective introduction of N-substitution on the resulting pyrazole. Key to the success of this formal one-carbon deletion method is a room-temperature triflylation of the pyrimidine core, followed by hydrazine-mediated skeletal remodeling.

General Synthetic Approach to Diverse Taxane Cores

Chemical synthesis of natural products is typically inspired by the structure and function of a target molecule. When both factors are of interest, such as in the case of taxane diterpenoids, a synthesis can both serve as a platform for synthetic strategy development and enable new biological exploration. Guided by this paradigm, we present here a unified enantiospecific approach to diverse taxane cores from the feedstock monoterpenoid (S)-carvone. Key to the success of our approach was the use of a skeletal remodeling strategy which began with the divergent reorganization and convergent coupling of two carvone-derived fragments, facilitated by Pd-catalyzed C-C bond cleavage tactics. This coupling was followed by additional restructuring using a Sm(II)-mediated rearrangement and a bioinspired, visible-light induced, transannular [2 + 2] photocycloaddition. Overall, this divergent monoterpenoid remodeling/convergent fragment coupling approach to complex diterpenoid synthesis provides access to structurally disparate taxane cores which have set the stage for the preparation of a wide range of taxanes.

Development of a C-C Bond Cleavage/Vinylation/Mizoroki-Heck Cascade Reaction: Application to the Total Synthesis of 14- and 15-Hydroxypatchoulol

A C-C bond cleavage/vinylation/Mizoroki-Heck cascade reaction has been developed to provide access to densely functionalized bicyclo[2.2.2]octane frameworks. The sequence proceeds through the coupling of dihydroxylated pinene derivatives, prepared from carvone, with gem-dichloroalkenes. The method was applied to 12-step total syntheses of both 14- and 15-hydroxypatchoulol, which provided unambiguous support for the structure of the natural products and corrects a misassignment in the isolation report.

Unified Total Syntheses of Benzenoid Cephalotane-Type Norditerpenoids: Cephanolides and Ceforalides

Detailed herein are our synthetic studies toward the preparation of the C₁₈- and C₁₉-benzenoid cephalotane-type norditerpenoids. Guided by chemical network analysis, the core structure of this natural product family was constructed in a concise manner using an iterative cross-coupling, followed by a formal inverse-electron-demand [4 + 2] cycloaddition. Initial efforts to functionalize an alkene group in the [4 + 2] cycloadduct using a Mukaiyama hydration and a subsequent olefination led to the complete C₁₈-carbon framework. While effective, this approach proved lengthy and prompted the development of a direct alkene difunctionalization that relies on borocupration to advance the cycloadduct to the natural products. Late-stage peripheral C-H functionalization facilitated access to all of the known cephanolides in 6-10 steps as well as five recently isolated ceforalides in 8-13 steps.

Strategy Evolution in a Skeletal Remodeling and C-H Functionalization-Based Synthesis of the Longiborneol Sesquiterpenoids

Detailed herein are our synthesis studies of longiborneol and related natural products. Our overarching goals of utilizing a "camphor first" strategy enabled by skeletal remodeling of carvone, and late-stage diversification using C-H functionalizations, led to divergent syntheses of the target natural products. Our initial approach proposed a lithiate addition to unite two fragments followed by a Conia-ene or Pd-mediated cycloalkylation reaction sequence to install the seven-membered ring emblematic of the longibornane core. This approach was unsuccessful and evolved into a revised plan that employed a Wittig coupling and a radical cyclization to establish the core. A reductive radical cyclization, which was explored first, led to a synthesis of copaborneol, a structural isomer of longiborneol. Alternatively, a metal-hydride hydrogen atom transfer-initiated cyclization was effective for a synthesis of longiborneol. Late-stage C-H functionalization of the longibornane core led to a number of hydroxylated longiborneol congeners. The need for significant optimization of the strategies that were employed as well as the methods for C-H functionalization to implement these strategies highlights the ongoing challenges in applying these powerful reactions. Nevertheless, the reported approach enables functionalization of every natural product-relevant C-H bond in the longibornane skeleton.

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.

C-C Bond Cleavage of α-Pinene Derivatives Prepared from Carvone as a General Strategy for Complex Molecule Synthesis

The preparation of complex molecules (e.g., biologically active secondary metabolites) remains an important pursuit in chemical synthesis. By virtue of their sophisticated architectures, complex natural products inspire total synthesis campaigns that can lead to completely new ways of building molecules. In the twentieth century, one such paradigm which emerged was the use of naturally occurring "chiral pool terpenes" as starting materials for total synthesis. These inexpensive and naturally abundant molecules provide an easily accessed source of enantioenriched material for the enantiospecific preparation of natural products. The most common applications of chiral pool terpenes are in syntheses where their structure can, entirely or largely, be superimposed directly onto a portion of the target structure. Less straightforward uses, where the structure of the starting chiral pool terpene is not immediately evident in the structure of the target, can be more challenging to implement. Nevertheless, these "nonintuitive" approaches illustrate the ultimate promise of chiral pool-based strategies: that any single chiral pool terpene could be applied to syntheses of an indefinite number of structurally diverse complex synthetic targets.By definition, such strategies require carefully orchestrated sequences of C-C bond forming and C-C cleaving reactions which result in remodeling of the terpene architecture. The combination of traditional rearrangement chemistry and transition-metal-catalyzed C-C cleavage methods, the latter of which were primarily developed in the early twenty-first century, provide a rich and powerful toolbox for implementing this remodeling approach. In this Account, we detail our efforts to use a variety of C-C cleavage tactics in the skeletal remodeling of carvone, a chiral pool terpene. This skeletal remodeling strategy enabled the reorganization of the carvone scaffold into synthetic intermediates with a variety of carboskeletons, which we, then, leveraged for the total syntheses of structurally disparate terpene natural products.We begin by describing our initial investigations into various, mechanistically distinct C-C cleavage processes involving cyclobutanols synthesized from carvone. These initial studies showcased how electrophile-mediated semipinacol rearrangements of these cyclobutanols can lead to [2.2.1]bicyclic intermediates, and how Rh- and Pd-catalyzed C-C cleavage can lead to a variety of densely functionalized cyclohexenes pertinent to natural product synthesis. We, then, present several total syntheses using these synthetic intermediates, beginning with the bridged, polycyclic sesquiterpenoid longiborneol, which was synthesized from a carvone-derived [2.2.1]bicycle following a key semipinacol rearrangement. Next, we discuss how several members of the macrocyclic phomactin family were synthesized from a cyclohexene derivative prepared through a Rh-catalyzed C-C cleavage reaction. Finally, we describe our synthesis of the marine diterpene xishacorene B, which was prepared using a key Pd-catalyzed C-C cleavage/cross-coupling that facilitated the assembly of the core [3.3.1]bicycle that is resident in the natural product structure.

Strategic elements in computer-assisted retrosynthesis: A case study of the pupukeanane natural products

Computer-assisted synthesis planning represents a growing area of research, especially for complex molecule synthesis. Here, we present a case study involving the pupukeanane natural products, which are complex, marine-derived, natural products with unique tricyclic scaffolds. Proposed routes to members of each skeletal class informed by pathways generated using the program Synthia^™ are compared to previous syntheses of these molecules. In addition, novel synthesis routes are proposed to pupukeanane congeners that have not been prepared previously.

Rearrangements of the Chrysanthenol Core: Application to a Formal Synthesis of Xishacorene B

Reported here are substrate-dictated rearrangements of chrysanthenol derivatives prepared from verbenone to access complex bicyclic frameworks. These rearrangements set the stage for a 10-step formal synthesis of the natural product xishacorene B. Key steps include an anionic allenol oxy-Cope rearrangement and a Suárez directed C-H functionalization. The success of this work was guided by extensive computational calculations which provided invaluable insight into the reactivity of the chrysanthenol-derived systems, especially in the key oxy-Cope rearrangement.

Sequential Norrish-Yang Cyclization and C-C Cleavage/Cross-Coupling of a [4.1.0] Fused Saturated Azacycle

Methods that functionalize the periphery of azacylic scaffolds have garnered increasing interest in recent years. Herein, we investigate the selectivity of a solid-state Norrish-Yang cyclization (NYC) and subsequent C-C cleavage/cross-coupling reaction of a strained cyclopropane-fused azacyclic system. Surprisingly, the NYC primarily furnished a single lactam constitutional and diastereo-isomer. The regioselectivity of the C-C cleavage of the α-hydroxy-β-lactam moiety could be varied by altering the ligand set used in the coupling chemistry. Experimental and computational observations are discussed.

Photomediated ring contraction of saturated heterocycles

Saturated heterocycles are found in numerous therapeutics and bioactive natural products and are abundant in many medicinal and agrochemical compound libraries. To access new chemical space and function, many methods for functionalization on the periphery of these structures have been developed. Comparatively fewer methods are known for restructuring their core framework. Herein, we describe a visible light-mediated ring contraction of α-acylated saturated heterocycles. This unconventional transformation is orthogonal to traditional ring contractions, challenging the paradigm for diversification of heterocycles including piperidine, morpholine, thiane, tetrahydropyran, and tetrahydroisoquinoline derivatives. The success of this Norrish type II variant rests on reactivity differences between photoreactive ketone groups in specific chemical environments. This strategy was applied to late-stage remodeling of pharmaceutical derivatives, peptides, and sugars.

Site-Selective Cross-Coupling of Polyhalogenated Arenes and Heteroarenes with Identical Halogen Groups

Methods to functionalize arenes and heteroarenes in a site-selective manner are highly sought after for rapidly constructing value-added molecules of medicinal, agrochemical, and materials interest. One effective approach is the site-selective cross-coupling of polyhalogenated arenes bearing multiple, but identical, halogen groups. Such cross-coupling reactions have proven to be incredibly effective for site-selective functionalization. However, they also present formidable challenges due to the inherent similarities in the reactivities of the halogen substituents. In this Review, we discuss strategies for site-selective cross-couplings of polyhalogenated arenes and heteroarenes bearing identical halogens, beginning first with an overview of the reaction types that are more traditional in nature, such as electronically, sterically, and directing-group-controlled processes. Following these examples is a description of emerging strategies, which includes ligand- and additive/solvent-controlled reactions as well as photochemically initiated processes.

Bioinspired Diversification Approach Toward the Total Synthesis of Lycodine-Type Alkaloids

Nitrogen heterocycles (azacycles) are common structural motifs in numerous pharmaceuticals, agrochemicals, and natural products. Many powerful methods have been developed and continue to be advanced for the selective installation and modification of nitrogen heterocycles through C-H functionalization and C-C cleavage approaches, revealing new strategies for the synthesis of targets containing these structural entities. Here, we report the first total syntheses of the lycodine-type Lycopodium alkaloids casuarinine H, lycoplatyrine B, lycoplatyrine A, and lycopladine F as well as the total synthesis of 8,15-dihydrohuperzine A through bioinspired late-stage diversification of a readily accessible common precursor, N-desmethyl-β-obscurine. Key steps in the syntheses include oxidative C-C bond cleavage of a piperidine ring in the core structure of the obscurine intermediate and site-selective C-H borylation of a pyridine nucleus to enable cross-coupling reactions.

Metabolomics Reveals Minor Tambjamines in a Marine Invertebrate Food Chain

Metabolomics analysis detected tambjamine alkaloids in aqueous and EtOAc extracts of the marine invertebrates Virididentula dentata, Tambja stegosauriformis, Tambja brasiliensis, and Roboastra ernsti. Among several tambjamines, the new amino acid derivatives tambjamines M-O (17-19) were identified by Marfey's advanced analysis, UPLC-MS/MS analyses, and total synthesis. The tambjamine diversity increased from the bryozoan V. dentata to its nudibranch predators T. stegosauriformis and T. brasiliensis and attained a higher diversity in R. ernsti, the nudibranch that preys upon T. stegosauriformis and T. brasiliensis. The total tambjamine content also increases among the trophic levels, probably due to biomagnification. Tambjamines A (1), C (3), and D (4) are the major metabolites in the tissues of V. dentata, T. stegosauriformis, T. brasiliensis, and R. ernsti and are likely the main chemical defenses of these marine invertebrates.

Total Synthesis of the Cephalotaxus Norditerpenoids (±)-Cephanolides A-D

Concise syntheses of the Cephalotaxus norditerpenoids cephanolides A-D (8-14 steps from commercial material) using a common late-stage synthetic intermediate are described. The success of our approach rested on an early decision to apply chemical network analysis to identify the strategic bonds that needed to be forged, as well as the efficient construction of the carbon framework through iterative Csp²-Csp³ cross-coupling, followed by an intramolecular inverse-demand Diels-Alder cycloaddition. Strategic late-stage oxidations facilitated access to all congeners of the benzenoid cephanolides isolated to date.

A pyrone remodeling strategy to access diverse heterocycles: application to the synthesis of fascaplysin natural products

The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described. The pyrido[1,2-a]indole core was demonstrated to be a versatile scaffold that can be site-selectively functionalized. The utility of this novel annulation strategy was showcased in a concise formal synthesis of three fascaplysin congeners.

The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described.

Cyanoamidine Cyclization Approach to Remdesivir's Nucleobase

We report an alternative approach to the unnatural nucleobase fragment seen in remdesivir (Veklury). Remdesivir displays broad-spectrum antiviral activity and is currently being evaluated in Phase III clinical trials to treat patients with COVID-19. Our route relies on the formation of a cyanoamidine intermediate, which undergoes Lewis acid-mediated cyclization to yield the desired nucleobase. The approach is strategically distinct from prior routes and could further enable the synthesis of remdesivir and other small-molecule therapeutics.

Transition Metal-Mediated C-C Single Bond Cleavage: Making the Cut in Total Synthesis

Transition-metal-mediated cleavage of C-C single bonds can enable entirely new retrosynthetic disconnections in the total synthesis of natural products. Given that C-C bond cleavage inherently alters the carbon framework of a compound, and that, under transition-metal catalysis, the generated organometallic or radical intermediate is primed for further complexity-building reactivity, C-C bond-cleavage events have the potential to drastically and rapidly remodel skeletal frameworks. The recent acceleration of the use of transition-metal-mediated cleavage of C-C single bonds in total synthesis can be ascribed to a communal recognition of this fact. In this Review, we highlight ten selected total syntheses from 2014 to 2019 that illustrate how transition-metal-mediated cleavage of C-C single bonds at either the core or the periphery of synthetic intermediates can streamline synthetic efforts.

C-C Bond Cleavage Approach to Complex Terpenoids: Development of a Unified Total Synthesis of the Phomactins

The rearrangement of carbon-carbon (C-C) single bonds in readily available carbocyclic scaffolds can yield uniquely substituted carbocycles that would be challenging to construct otherwise. This is a powerful and often non-intuitive approach for complex molecule synthesis. The transition-metal-mediated cleavage of C-C bonds has the potential to broaden the scope of this type of skeletal remodeling by providing orthogonal selectivities compared to more traditional pericyclic and carbocation-based rearrangements. To highlight this emerging technology, a unified, asymmetric, total synthesis of the phomactin terpenoids was developed, enabled by the selective C-C bond cleavage of hydroxylated pinene derivatives obtained from carvone. In this full account, the challenges, solutions, and intricacies of Rh(I)-catalyzed cyclobutanol C-C cleavage in a complex molecule setting are described. In addition, details of the evolution of strategies that ultimately led to the total synthesis of phomactins A, K, P, R, and T, as well as the synthesis and structural reassignment of Sch 49027, are given.

C-H/C-C Functionalization Approach to N-Fused Heterocycles from Saturated Azacycles

Herein we report the synthesis of substituted indolizidines and related N-fused bicycles from simple saturated cyclic amines through sequential C-H and C-C bond functionalizations. Inspired by the Norrish-Yang Type II reaction, C-H functionalization of azacycles is achieved by forming α-hydroxy-β-lactams from precursor α-ketoamide derivatives under mild, visible light conditions. Selective cleavage of the distal C(sp2)-C(sp3) bond in α-hydroxy-β-lactams using a Rh-complex leads to α-acyl intermediates which undergo sequential Rh-catalyzed decarbonylation, 1,4-addition to an electrophile, and aldol cyclization, to afford N-fused bicycles including indolizidines. Computational studies provide mechanistic insight into the observed positional selectivity of C-C cleavage, which depends strongly on the groups bound to Rh trans to the phosphine ligand.

Treating a Global Health Crisis with a Dose of Synthetic Chemistry

The SARS-CoV-2 pandemic has prompted scientists from many disciplines to work collaboratively toward an effective response. As academic synthetic chemists, we examine how best to contribute to this ongoing effort.

A Case for Bond-Network Analysis in the Synthesis of Bridged Polycyclic Complex Molecules: Hetidine and Hetisine Diterpenoid Alkaloids

A key challenge in the synthesis of diterpenoid alkaloids lies in identifying strategies that rapidly construct their multiply bridged polycyclic skeletons. Existing approaches to these structurally intricate secondary metabolites are discussed in the context of a "bond-network analysis" of molecular frameworks, which was originally devised by Corey some 40 years ago. The retrosynthesis plans that emerge from a topological analysis of the highly bridged frameworks of the diterpenoid alkaloids are discussed in the context of eight recent syntheses of hetidine and hetisine natural products and their derivatives. This Minireview highlights the extent to which network analyses of the type described here sufficed for designing synthesis plans, as well as areas where they had to be amalgamated with functional group oriented synthetic planning considerations.