Scalable procedure for the fragmentation of hydroperoxides mediated by copper and iron tetrafluoroborate salts

Dealkenylative Functionalizations: Conversion of Alkene C(sp3)-C(sp2) Bonds into C(sp3)-X Bonds via Redox-Based Radical Processes

This review highlights the history and recent advances in dealkenylative functionalization. Through this deconstructive strategy, radical functionalizations occur under mild, robust conditions. The reactions described proceed with high efficiency, good stereoselectivity, tolerate many functional groups, and are completed within a matter of minutes. By cleaving the C(sp3)-C(sp2) bond of terpenes and terpenoid-derived precursors, rapid diversification of natural products is possible.

Carbon-Carbon Bond Cleavage for Late-Stage Functionalization

Late-stage functionalization (LSF) introduces functional group or structural modification at the final stage of the synthesis of natural products, drugs, and complex compounds. It is anticipated that late-stage functionalization would improve drug discovery's effectiveness and efficiency and hasten the creation of various chemical libraries. Consequently, late-stage functionalization of natural products is a productive technique to produce natural product derivatives, which significantly impacts chemical biology and drug development. Carbon-carbon bonds make up the fundamental framework of organic molecules. Compared with the carbon-carbon bond construction, the carbon-carbon bond activation can directly enable molecular editing (deletion, insertion, or modification of atoms or groups of atoms) and provide a more efficient and accurate synthetic strategy. However, the efficient and selective activation of unstrained carbon-carbon bonds is still one of the most challenging projects in organic synthesis. This review encompasses the strategies employed in recent years for carbon-carbon bond cleavage by explicitly focusing on their applicability in late-stage functionalization. This review expands the current discourse on carbon-carbon bond cleavage in late-stage functionalization reactions by providing a comprehensive overview of the selective cleavage of various types of carbon-carbon bonds. This includes C-C(sp), C-C(sp2), and C-C(sp3) single bonds; carbon-carbon double bonds; and carbon-carbon triple bonds, with a focus on catalysis by transition metals or organocatalysts. Additionally, specific topics, such as ring-opening processes involving carbon-carbon bond cleavage in three-, four-, five-, and six-membered rings, are discussed, and exemplar applications of these techniques are showcased in the context of complex bioactive molecules or drug discovery. This review aims to shed light on recent advancements in the field and propose potential avenues for future research in the realm of late-stage carbon-carbon bond functionalization.

Capturing primary ozonides for a syn-dihydroxylation of olefins

Ozonolysis is a widely used and practical synthetic technique for the deconstructive oxidation of olefins using ozone. While there are numerous ozonolysis reactions that give a myriad of products and functionalities, almost all of them involve scission at the olefin double bond. Using ozone as a constructive reagent rather than a deconstructive one would open new domains of chemical reactivity and amplify molecular complexity in synthetic methodology. Here we report the use of primary ozonides as preparative synthetic intermediates for a safe and green olefin syn-dihydroxylation reaction. Furthermore, we have demonstrated this method using a continuous flow reactor that virtually eliminates peroxide accumulation and extended these applications towards the synthesis of pharmaceutically relevant small molecules such as guaifenesin, the active ingredient in Mucinex, and a precursor to ponesimod, a drug to treat multiple sclerosis.

Aminodealkenylation: Ozonolysis and copper catalysis convert C(sp3)-C(sp2) bonds to C(sp3)-N bonds

Great efforts have been directed toward alkene π bond amination. In contrast, analogous functionalization of the adjacent C(sp3)-C(sp2) σ bonds is much rarer. Here we report how ozonolysis and copper catalysis under mild reaction conditions enable alkene C(sp3)-C(sp2) σ bond-rupturing cross-coupling reactions for the construction of new C(sp3)-N bonds. We have used this unconventional transformation for late-stage modification of hormones, pharmaceutical reagents, peptides, and nucleosides. Furthermore, we have coupled abundantly available terpenes and terpenoids with nitrogen nucleophiles to access artificial terpenoid alkaloids and complex chiral amines. In addition, we applied a commodity chemical, α-methylstyrene, as a methylation reagent to prepare methylated nucleosides directly from canonical nucleosides in one synthetic step. Our mechanistic investigation implicates an unusual copper ion pair cooperative process.

Total Synthesis of (+)-Shearilicine

Herein we report the first total synthesis of the indole diterpenoid natural product shearilicine by an 11-step sequence via a generalizable precursor to the highly oxidized subclass of indole diterpenoids. A native chiral auxiliary strategy was employed to access the target molecule in an enantiospecific fashion. The formation of the key carbazole substructure was achieved through a mild intramolecular Heck cyclization, wherein a computational study revealed noncovalent substrate-ligand and ligand-ligand interactions that promoted migratory insertion.

Fragment Coupling Approach to Diaporthein B

Pimarane diterpenes are produced by a diverse array of plants, fungi, and bacteria. Many members of this family possess antimicrobial and antiproliferative activities. The pimarane diterpenes are characterized by a tricyclic carbon scaffold comprising three fused six-membered rings and at least three quaternary centers. Here, we describe two convergent, fragment-based strategies toward the synthesis of diaporthein B (3), one of the most highly oxidized pimarane diterpenes. The first approach provided access to the tricyclic carbon scaffold of the target and featured a highly diastereoselective fragment coupling, a novel carbonylative Stille cross-coupling to directly access an α-hydroxyketone from a vinyl iodide, and a tandem aldol cyclization-deprotection cascade. The second route utilized a diastereoselective 1,4-addition of a silyloxyfuran to an unsaturated ketone, followed by an epoxidation-ring opening sequence, to access a highly oxidized intermediate containing two elaborated cyclohexane rings. The chemistry developed herein may ultimately be useful in an eventual synthesis of this class of natural products.

Total Synthesis of Vilmoraconitine

Vilmoraconitine belongs to one of the most complex skeleton types in the C19-diterpenoid alkaloids, which architecturally features an unprecedented heptacyclic core possessing a rigid cyclopropane unit. Here, we report the first total synthesis of vilmoraconitine relying on strategic use of efficient ring-forming reactions. Key steps include an oxidative dearomatization-induced Diels-Alder cycloaddition, a hydrodealkenylative fragmentation/Mannich sequence, and an intramolecular Diels-Alder cycloaddition.

Unified Total Synthesis of the Limonoid Alkaloids: Strategies for the De Novo Synthesis of Highly Substituted Pyridine Scaffolds

Highly substituted pyridine scaffolds are found in many biologically active natural products and therapeutics. Accordingly, numerous complementary de novo approaches to obtain differentially substituted pyridines have been disclosed. This article delineates the evolution of the synthetic strategies designed to assemble the demanding tetrasubstituted pyridine core present in the limonoid alkaloids isolated from Xylocarpus granatum, including xylogranatopyridine B, granatumine A and related congeners. In addition, NMR calculations suggested structural misassignment of several limonoid alkaloids, and predicted their C3-epimers as the correct structures, which was further validated unequivocally through chemical synthesis. The materials produced in this study were evaluated for cytotoxicity, anti-oxidant effects, anti-inflammatory action, PTP1B and Nlrp3 inflammasome inhibition, which led to compelling anti-inflammatory activity and anti-oxidant effects being discovered.

Recent Achievements in Total Synthesis for Integral Structural Revisions of Marine Natural Products

A great effort to discover new therapeutic ingredients is often initiated through the discovery of the existence of novel marine natural products. Since substances produced by the marine environment might be structurally more complex and unique than terrestrial natural products, there have been cases of misassignments of their structures despite the availability of modern spectroscopic and computational chemistry techniques. When it comes to refutation to erroneously or tentatively proposed structures empirical preparations through organic chemical synthesis has the greatest contribution along with close and sophiscated inspection of spectroscopic data. Herein, we analyzed the total synthetic studies that have decisively achieved in revelation of errors, ambiguities, or incompleteness of the isolated structures of marine natural products covering the period from 2018 to 2021.

Scalable Birch reduction with lithium and ethylenediamine in tetrahydrofuran

[Figure: see text].

Asymmetric Total Synthesis of Norzoanthamine

We report herein the asymmetric total synthesis of norzoanthamine using radical reactions as key steps for rapid access to the congested carbocyclic core, which is the major synthetic challenge for most zoanthamine alkaloids. (1) The Ueno-Stork radical cyclization was applied to construct the adjacent quaternary centers at the C-9 and C-22 positions; (2) a Co-catalyzed HAT radical reaction was successfully applied to construct the quaternary center at C-12 via Csp³ -Csp² bond formation; (3) a Mn-catalyzed HAT radical reaction was used to stereospecifically reduce the tetra-substituted olefin (C13=C18) and install the contiguous stereocenters in proximity to the quaternary center. A one-pot bio-inspired cyclization step was finally applied to forge the unstable bis-amino acetal skeleton. Our approach can precisely control the stereochemistry of seven vicinal stereocenters and effectively construct the highly congested heptacyclic skeleton.

Dealkenylative Alkenylation: Formal σ-Bond Metathesis of Olefins

The dealkenylative alkenylation of alkene C(sp3 )-C(sp2 ) bonds has been an unexplored area for C-C bond formation. Herein 64 examples of β-alkylated styrene derivatives, synthesized through the reactions of readily accessible feedstock olefins with β-nitrostyrenes by ozone/FeII -mediated radical substitutions, are reported. These reactions proceed with good efficiencies and high stereoselectivities under mild reaction conditions and tolerate an array of functional groups. Also demonstrated is the applicability of the strategy through several synthetic transformations of the products, as well as the syntheses of the natural product iso-moracin and the drug (E)-metanicotine.

Oxodealkenylative Cleavage of Alkene C(sp3 )-C(sp2 ) Bonds: A Practical Method for Introducing Carbonyls into Chiral Pool Materials

Reported herein is a one-pot protocol for the oxodealkenylative introduction of carbonyl functionalities into terpenes and terpene-derived compounds. This transformation proceeds by Criegee ozonolysis of an alkene, reductive cleavage of the resulting α-alkoxy hydroperoxide, trapping of the generated alkyl radical with 2,2,6,6-tetramethylpiperidin-1-yl (TEMPO), and subsequent oxidative fragmentation with MMPP. Using readily available starting materials from chiral pool, a variety of carbonyl-containing products have been accessed rapidly in good yields.

Recent progress in the synthesis of limonoids and limonoid-like natural products

Recent progress in syntheses of limonoids and limonoid-like natural products is reviewed. The current “state-of-art” advance on novel synthetic strategy are summarized and future outlook will be presented.

Hydrodealkenylative C(sp3)-C(sp2) bond fragmentation

Chemical synthesis typically relies on reactions that generate complexity through elaboration of simple starting materials. Less common are deconstructive strategies toward complexity-particularly those involving carbon-carbon bond scission. Here, we introduce one such transformation: the hydrodealkenylative cleavage of C(sp³)-C(sp²) bonds, conducted below room temperature, using ozone, an iron salt, and a hydrogen atom donor. These reactions are performed in nonanhydrous solvents and open to the air; reach completion within 30 minutes; and deliver their products in high yields, even on decagram scales. We have used this broadly functionality tolerant transformation to produce desirable synthetic intermediates, many of which are optically active, from abundantly available terpenes and terpenoid-derived precursors. We have also applied it in the formal total syntheses of complex molecules.

Dealkenylative Thiylation of C(sp3)-C(sp2) Bonds

Carbon-carbon bond fragmentations are useful methods for the functionalization of molecules. The value of such cleavage events is maximized when paired with subsequent bond formation. Herein we report a protocol for the cleavage of an alkene C(sp³)-C(sp²) bond, followed by the formation of a new C(sp³)-S bond. This reaction is performed in nonanhydrous solvent and open to the air, employs common starting materials, and can be used to rapidly diversify natural products.

Carvone-Derived P-Stereogenic Phosphines: Design, Synthesis, and Use in Allene-Imine [3 + 2] Annulation

We have prepared a previously unreported family of P-stereogenic [2.2.1] bicyclic chiral phosphines through straightforward syntheses starting from the natural product carvone. This design rationale prompted the development of an unforeseen C-dealkenylation reaction. We have applied these organocatalysts in the asymmetric syntheses of a bevy of pyrrolines, obtained in high yields and enantioselectivities, including a biologically active small molecule, efsevin.

Total Synthesis of (-)-Xylogranatopyridine B via a Palladium-Catalyzed Oxidative Stannylation of Enones

We report a total synthesis of the pyridine-containing limonoid alkaloid (-)-xylogranatopyridine B in 11 steps from commercially available dihydrocarvone. The central pyridine ring was assembled by a late-stage fragment coupling approach employing a modified Liebeskind pyridine synthesis. One fragment was prepared by an allyl-palladium catalyzed oxidative enone β-stannylation, in which the key bimetallic β-stannyl palladium enolate intermediate undergoes a β-hydride elimination. This methodology also allowed introduction of alkyl and silyl groups to the β-position of enones.

Assembly of the Limonoid Architecture by a Divergent Approach: Total Synthesis of (±)-Andirolide N via (±)-8α-Hydroxycarapin

We report the first total synthesis of the limonoid andirolide N using a 12-step sequence from commercially available materials. The final step of this route demonstrates the chemical feasibility of our biosynthetic proposal that andirolide N arises from 8α-hydroxycarapin. The strategic use of a degraded limonoid as a platform for the synthesis of more structurally complex congeners may be a general approach to obtain limonoids with diverse functional properties.