Total Syntheses of Scaparvins B, C, and D Enabled by a Key C-H Functionalization

Intramolecular C-H Oxidation in Iron(V)-oxo-carboxylato Species Relevant in the γ-Lactonization of Alkyl Carboxylic Acids

High-valent oxoiron species have been invoked as oxidizing agents in a variety of iron-dependent oxygenases. Taking inspiration from nature, selected nonheme iron complexes have been developed as catalysts to elicit C-H oxidation through the mediation of putative oxoiron(V) species, akin to those proposed for Rieske oxygenases. The addition of carboxylic acids in these iron-catalyzed C-H oxidations has proved highly beneficial in terms of product yields and selectivities, suggesting the direct involvement of iron(V)-oxo-carboxylato species. When the carboxylic acid functionality is present in the alkane substrate, it acts as a directing group, enabling the selective intramolecular γ-C-H hydroxylation that eventually affords γ-lactones. While this mechanistic frame is solidly supported by previous mechanistic studies, direct spectroscopic detection of the key iron(V)-oxo-carboxylato intermediate and its competence for engaging in the selective γ-C-H oxidation leading to lactonization have not been accomplished. In this work, we generate a series of well-defined iron(V)-oxo-carboxylato species (2c-2f) differing in the nature of the bound carboxylate ligand. Species 2c-2f are characterized by a set of spectroscopic techniques, including UV-vis spectroscopy, cold-spray ionization mass spectrometry (CSI-MS), and, in selected cases, EPR and Mössbauer spectroscopies. We demonstrate that 2c-2f undergo site-selective γ-lactonization of the carboxylate ligand in a stereoretentive manner, thus unequivocally identifying metal-oxo-carboxylato species as the powerful yet selective C-H cleaving species in catalytic γ-lactonization reactions of carboxylic acids. Reactivity experiments confirm that the intramolecular formation of γ-lactones is in competition with the intermolecular oxidation of external alkanes and olefins. Finally, mechanistic studies, together with DFT calculations, support a mechanism involving a site-selective C-H cleavage in the γ-position of the carboxylate ligand by the oxo moiety, followed by a fast carboxylate rebound, eventually leading to the selective formation of γ-lactones.

Unique Reactivity of the 1,4-Bis(silyloxy)-1,3-cyclopentadiene Moiety: Application to the Synthesis of 7-Norbornanone Derivatives

We describe a method for the synthesis of various 2-silyloxy-2-norbornen-7-ones by exploiting the specific reactivity of the 1,4-bis(silyloxy)-1,3-cyclopentadiene framework, which is generated by the silylation of a 2,2-disubstituted-1,3-cyclopentanedione bearing a picolinoyloxy group at the 2' position of its C-2 side chain. The release of the acyloxy group during the reaction generates carbocations that are then attacked by silyloxy-substituted carbons in the 1,4-bis(silyloxy)-1,3-cyclopentadiene moiety skeleton, forming a 4,5-cis-fused ring skeleton. Skeletal rearrangement of the bicyclic core results in the formation of the corresponding 2-silyloxy-2-norbornen-7-one. This novel transformation of 1,3-cyclopentanedione moieties can be used to synthesise other cyclopentenone-fused bicyclic frameworks.

Neurotrophic Natural Products

Neurotrophins (NGF, BDNF, NT3, NT4) can decrease cell death, induce differentiation, as well as sustain the structure and function of neurons, which make them promising therapeutic agents for the treatment of neurodegenerative disorders. However, neurotrophins have not been very effective in clinical trials mostly because they cannot pass through the blood-brain barrier owing to being high-molecular-weight proteins. Thus, neurotrophin-mimic small molecules, which stimulate the synthesis of endogenous neurotrophins or enhance neurotrophic actions, may serve as promising alternatives to neurotrophins. Small-molecular-weight natural products, which have been used in dietary functional foods or in traditional medicines over the course of human history, have a great potential for the development of new therapeutic agents against neurodegenerative diseases such as Alzheimer's disease. In this contribution, a variety of natural products possessing neurotrophic properties such as neurogenesis, neurite outgrowth promotion (neuritogenesis), and neuroprotection are described, and a focus is made on the chemistry and biology of several neurotrophic natural products.

Enantioselective and collective total synthesis of pentacyclic 19-nor-clerodanes

We report herein the collective asymmetric total synthesis of seven pentacyclic 19-nor-clerodane diterpenoids, namely (+)-teucvin (+)-cracroson A, (+)-cracroson E, (+)-montanin A, (+)-teucvisin C, (+)-teucrin A, and (+)-2-hydroxyteuscorolide. An ytterbium-catalyzed asymmetric inverse-electron-demand Diels-Alder reaction of 4-methyl-2-pyrone with a chiral C5-substituted cyclohexa-1,3-dienol silyl ether is the key feature of the synthesis, which provides the common cis-decalin intermediate with five continuous stereocenters in excellent yield and stereoselectivity. From this diversifiable intermediate, the total synthesis of (+)-teucvin and (+)-2-hydroxyteuscorolide was realized in thirteen and eighteen steps, respectively. From (+)-teucvin, five other pentacyclic 19-nor-clerodanes were divergently and concisely generated through late-stage oxidation state adjustments.

Robust Synthesis of Terpenoid Scaffolds under Mn(I)-Catalysis

The 6/6/5-fused tricyclic scaffold is a central feature of structurally complex terpenoid natural products. A step-economical cascade transformation that leads to a complex molecular skeleton is regarded as a sustainable methodology. Therefore, we report the first Mn(I)-catalyzed C(sp²)-H chemoselective in situ dienylation and diastereoselective intramolecular Diels-Alder reaction using iso-pentadienyl carbonate to access 6/6/5-fused tricyclic scaffolds. To the best of our knowledge, there is no such report thus far to utilize iso-pentadienyl carbonate as a substrate in C-H activation catalysis. Extensive mechanistic studies, such as the isolation of catalytically active organo-manganese(I) complexes, 1,3-dienyl-intermediates, and isotopic labeling experiments have supported the proposed mechanism of this cascade reaction.

Total Synthesis of Mollanol A

Mollanol A is the first isolated member of the mollane-type grayanoids which possesses an unprecedented C-nor-D-homograyanane carbon skeleton and an 5,8-epoxide. Due to its transcriptional activation effects on the Xbp1 upstream promoters in different cell types, it has a potential therapeutic effect on inflammatory bowel disease. Here we report the first total synthesis of mollanol A, which constitutes a 15-step synthesis from commercially available materials via a convergent strategy. The synthesis involves an InCl₃-catalyzed Conia-ene cyclization reaction to construct the bicyclo[3.2.1]octane moiety and a vinylogous aldol reaction/intramolecular oxa-Michael addition sequence to rapidly assemble the oxa-bicyclo[3.2.1] core.

Selective Oxidations in the Synthesis of Complex Natural ent-Kauranes and ent-Beyeranes

Syntheses of two natural products derived from the ent-kaurene kaurenoic acid are described for the first time using regio- and diastereoselective oxidations. Palladium- and manganese-mediated oxidations were used to accomplish the syntheses of two ent-beyerane metabolites. The use of the White-Gormisky-Zhao catalyst Mn(CF₃-PDP) enabled the first application of a nondirected metal-catalyzed oxidation in an unactivated C-H bond in a total synthesis.

Biogenesis-Inspired, Divergent Synthesis of Spirochensilide A, Spirochensilide B, and Abifarine B Employing a Radical-Polar Crossover Rearrangement Strategy

Triterpenoids and related abeo-steroids are of interest to the scientific community for their potent and varied biological activities as well as their unique structures. Within this large and diverse family of natural products, the fir metabolites (-)-spirochensilide A and B are particularly noteworthy for their controversial biogenesis. We herein report the chemical synthesis of the spirochensilides, which involves a concerted sequence of bioinspired rearrangements contributing to its resolution. Points of divergence after each rearrangement step also allow for an approach to the abifarine family of natural products with abifarine B as a synthetic target. Key to this strategy is a radical-polar crossover event to initiate the first rearrangement without the need for a sacrificial functionality to be introduced beforehand.

Synthesis of Cyclopentenes and Cyclohexenes Via Cobalt(II)-Catalyzed Oxidative Cyclization

A unique method for the synthesis of cyclopentenes and cyclohexenes has been achieved by the coupling of diketones and alkenes under cobalt(II) catalysis and dimethyl sulfoxide involvement. Under optimal conditions, the formation of five- and six-membered rings can be readily controlled by the α-position substitution of styrenes. This process is proposed to proceed through a reaction sequence of oxidative coupling (mediated by K₂S₂O₈), regioselective alkene insertion (promoted by cobalt), and intramolecular attack of the resulting allylcobalt species on the carbonyl group or methyl group in the reactive methylene process.

Contiguous Quaternary Carbons: A Selection of Total Syntheses

Contiguous quaternary carbons in terpene natural products remain a major challenge in total synthesis. Synthetic strategies to overcome this challenge will be a pivotal prerequisite to the medicinal application of natural products and their analogs or derivatives. In this review, we cover syntheses of natural products that exhibit a dense assembly of quaternary carbons and whose syntheses were uncompleted until recently. While discussing their syntheses, we not only cover the most recent total syntheses but also provide an update on the status quo of modern syntheses of complex natural products. Herein, we review (±)-canataxpropellane, (+)-waihoensene, (–)-illisimonin A and (±)-11-O-debenzoyltashironin as prominent examples of natural products bearing contiguous quaternary carbons.

A Concise Total Synthesis of (+)-Waihoensene Guided by Quaternary Center Analysis

The four contiguous all-carbon quaternary centers of waihoensene, coupled with the absence of any traditional reactive functional groups other than a single alkene, render it a particularly challenging synthetic target among angular triquinane natural products. Here, we show that its polycyclic frame can be assembled concisely by using a strategically chosen quaternary center to guide the formation of the other three through judiciously selected C-C bond formation reactions. Those events, which included a unique Conia-ene cyclization and a challenging Pauson-Khand reaction, afforded a 17-step synthesis of the molecule in enantioenriched form.

Chemoselective Tertiary C-H Hydroxylation for Late-Stage Functionalization with Mn(PDP)/Chloroacetic Acid Catalysis

Aromatic and heterocyclic functionality are ubiquitous in pharmaceuticals. Herein, we disclose a new Mn(PDP)catalyst system using chloroacetic acid additive capable of chemoselectively oxidizing remote tertiary C(sp ³)-H bonds in the presence of a broad range of aromatic and heterocyclic moieties. Although catalyst loadings can be lowered to 0.1 mol% under a Mn(PDP)/acetic acid system for aromatic and non-basic nitrogen heterocycle substrates, the Mn(PDP)/chloroacetic acid system generally affords 10-15% higher isolated yields on these substrates and is uniquely effective for remote C(sp ³)-H hydroxylations in substrates housing basic nitrogen heterocycles. The demonstrated ability to perform Mn(PDP)/chloroacetic acid C(sp ³)-H oxidations in pharmaceutically relevant complex molecules on multi-gram scales will facilitate drug discovery processes via late-stage functionalization.

Iron- and cobalt-catalyzed C(sp3)–H bond functionalization reactions and their application in organic synthesis

This review highlights the developments in iron and cobalt catalyzed C(sp³)–H bond functionalization reactions with emphasis on their applications in organic synthesis, i.e. natural products and pharmaceuticals synthesis and/or modification.

Three new bioactive diterpenoids from the roots of Croton crassifolius

Phytochemical investigation of Croton crassifolius led to the isolation of two new halimane diterpenoids (1 and 2), a new nor-clerodane diterpenoid (3), along with three known analogues (4-6). Their structures including absolute configurations were elucidated by spectroscopic analysis, single-crystal X-ray diffraction, and CD analysis. All isolates were evaluated for their inhibitory effects on the nitric oxide (NO) production induced by lipopolysaccharide (LPS) in RAW264.7 macrophage cells, and compound 1 exhibited moderate inhibition of NO production with an IC₅₀ value of 25.8 ± 0.9 μM.

Total Synthesis and Structure Revision of (-)-Illisimonin A, a Neuroprotective Sesquiterpenoid from the Fruits of Illicium simonsii

Illisimonin A was isolated from Illicium simonsii and has a previously unreported tricyclic carbon framework. It displayed neuroprotective effects against oxygen-glucose deprivation-induced cell injury in SH-SY5Y cells. It incorporates a highly strained trans-pentalene ring system. We report the first synthesis of (±)-illisimonin A. Notable steps in the route include a 1,3-dioxa-2-silacyclohexene templated Diels-Alder cycloaddition and type-3 semipinacol rearrangement to generate the trans-pentalene. The final step is an iron-catalyzed C-H oxidation. The synthetic route is robust, with 94 mg of racemic material prepared in a single pass. Resolving an intermediate enabled the synthesis of natural (-)-illisimonin A. The absolute configuration of (-)-illisimonin A was revised to 1S,4S,5S,6S,7R,9R,10R based on the X-ray structure of a heavy-atom analogue.

Total Syntheses of Clerodane Diterpenoids—A Review

Total syntheses of clerodane diterpenoids have been reviewed from the literature since 2000.

Convergent Total Synthesis of Principinol D, a Rearranged Kaurane Diterpenoid

The total synthesis of principinol D, a rearranged kaurane diterpenoid, is reported. This grayanane natural product is constructed via a convergent fragment coupling approach, wherein the central seven-membered ring is synthesized at a late stage. The bicyclo[3.2.1]octane fragment is accessed by a Ni-catalyzed α-vinylation reaction. Strategic reductions include a diastereoselective SmI2-mediated ketone reduction with PhSH and a new protocol for selective ester reduction in the presence of ketones. The convergent strategy reported herein may be an entry point to the larger class of kaurane diterpenoids.

Enantioselective Conia-Ene-Type Cyclizations of Alkynyl Ketones through Cooperative Action of B(C6F5)3, N-Alkylamine and a Zn-Based Catalyst

An efficient and highly enantioselective Conia-ene-type process has been developed. Reactions are catalyzed by a combination of B(C₆F₅)₃, an N-alkylamine and a BOX-ZnI₂ complex. Specifically, through cooperative action of B(C₆F₅)₃ and amine, ketones with poorly acidic α-C-H bonds can be converted in situ to the corresponding enolates. Subsequent enantioselective cyclization involving a BOX-ZnI₂-activated alkyne leads to the formation of various cyclopentenes in up to 99% yield and 99:1 er.

Development of a Terpene Feedstock-Based Oxidative Synthetic Approach to the Illicium Sesquiterpenes

The Illicium sesquiterpenes are a family of natural products containing over 100 highly oxidized and structurally complex members, many of which display interesting biological activities. This comprehensive account chronicles the evolution of a semisynthetic strategy toward these molecules from (+)-cedrol, seeking to emulate key aspects of their presumed biosynthesis. An initial route generated lower oxidation state analogs but failed in delivering a crucial hydroxy group in the final step. Insight gathered during these studies, however, ultimately led to a synthesis of the pseudoanisatinoids along with the allo-cedrane natural product 11- O-debenzoyltashironin. A second-generation strategy was then developed to access the more highly oxidized majucinoid compounds including jiadifenolide and majucin itself. Overall, one dozen natural products can be accessed from an abundant and inexpensive terpene feedstock. A multitude of general observations regarding site-selective C(sp³)-H bond functionalization reactions in complex polycyclic architectures are reported.

Contemporary Synthetic Strategies toward seco-Prezizaane Sesquiterpenes from Illicium Species

Since the elucidation of the structure of anisatin in the late 1960s, sesquiterpene lactones from various Illicium species of plants have captivated synthetic chemists worldwide, resulting in a large body of synthetic work. In particular, Illicium sesquiterpenes containing the seco-prezizaane carbon framework have seen immense interest in recent years owing to desirable structural and medicinal attributes. This synopsis will focus on recently developed synthetic strategies to access these compact, highly oxidized terpenoids.

Recent applications of C-H functionalization in complex natural product synthesis

In this review, recent examples featuring C-H functionalization in the synthesis of complex natural products are discussed. A focus is given to the way in which C-H functionalization can influence the logical process of retrosynthesis, and the review is organized by the type and method of C-H functionalization.

Aliphatic C-H Oxidations for Late-Stage Functionalization

The atomistic change of C( sp3)-H to C( sp3)-O can have a profound impact on the physical and biological properties of small molecules. Traditionally, chemical synthesis has relied on pre-existing functionality to install new functionality, and directed approaches to C-H oxidation are an extension of this logic. The impact of developing undirected C-H oxidation reactions with controlled site-selectivity is that scientists gain the ability to diversify complex structures at sites remote from existing functionality, without having to carry out individual de novo syntheses. This Perspective offers a historical view of why, as recently as 2007, it was thought that the differences between aliphatic C-H bonds of the same bond type (for example, 2° aliphatic) were not large enough to distinguish them preparatively with small-molecule catalysis in the absence of directing groups or molecular recognition elements. We give an account of the discovery of Fe(PDP)-catalyzed non-directed aliphatic C-H hydroxylations and how the electronic, steric, and stereoelectronic rules for predicting site-selectivity that emerged have affected a shift in how the chemical community views the reactivity among these bonds. The discovery that site-selectivity could be altered by tuning the catalyst [i.e., Fe(CF3-PDP)] with no changes to the substrate or reaction now gives scientists the ability to exert control on the site of oxidation on a range of functionally and topologically diverse compounds. Collectively, these findings have made possible the emerging area of late-stage C-H functionalizations for streamlining synthesis and derivatizing complex molecules.