Stereocontrolled Radical Bicyclizations of Oxygenated Precursors Enable Short Syntheses of Oxidized Abietane Diterpenoids

Convergent and Scalable Second-Generation Synthesis of the Fully Functionalized HIJKLMN-Ring Segment of Caribbean Ciguatoxin C-CTX-1

A highly convergent and scalable second-generation synthesis of the fully functionalized HIJKLMN-ring segment of Caribbean ciguatoxin C-CTX-1, the primary toxin responsible for ciguatera fish poisoning in the Caribbean Sea and the Northeast Atlantic regions, has been accomplished. Key aspects of the synthetic approach include the efficient syntheses of the HI- and KLM-ring fragments on gram scales, a convergent fragment coupling toward the HIJKLM-ring skeleton based on the Suzuki-Miyaura coupling strategy, and optimized iron hydride-catalyzed hydrogen atom transfer-mediated olefin coupling conditions for constructing the N-ring.

Unlocking Oxetane Potential: Modular Synthetic Platform for the Concise Synthesis of Acyclic Oligo-Isoprenoids and Terpenoids

Terpenes occupy a unique place among the secondary metabolites due to their broad utility and extraordinary structural diversity. Their synthesis via polyene cyclization, either biomimetic or enzymatic, represents the cutting edge of modern synthetic chemistry. However, these endeavors have been inherently tied to the availability of natural and non-natural acyclic polyene starting materials. Herein, we report an oxetane-based platform for the modular construction of oxygenated polyolefins with precise geometric control. This "tail-to head" iterative method leverages the site-selective cross-metathesis of terminal olefins to form an alkylidene oxetane moiety and the regioselective ring opening of alkenyl-oxetanes for chain elongation. In addition, the unique and peculiar propensity of alkylidene oxetane fragment in various reactions was also revealed and exploited for site-selective functionalization, cyclization, and as a protecting group in polyenes.

Directed Hydrogen Atom Transfer for Selective Reactions of Polyenols

Directed hydrogen atom transfer to alkenes is described. The process is catalyzed by iron complexes and allows for the site-selective hydrofunctionalization of polyenols. Experimental data suggest that coordination of the hydroxy group to the iron hydride intermediate plays an important role in preferential engagement of the allylic alcohol motif and provides a new basis for selectivity in radical hydrofunctionalization events. As a proof of concept, β- and γ-amino alcohols are prepared from the corresponding polyenols in a selective manner.

Total Synthesis of Diterpenoid Quinone Methide Tumor Inhibitor, (+)-Taxodione

An asymmetric polyene cyclization (92% ee) strategy has been successfully applied for the first asymmetric total synthesis of oxidized abietane, anticancer agent, taxodione (1) sharing a trans-decalin system. Additionally, the total syntheses of pomiferin B (2) and gaultheric acid (3) (a nor-abietane) were achieved utilizing this unified approach.

Catalytic Asymmetric Total Synthesis of (-)-Garryine via an Enantioselective Heck Reaction

The first asymmetric total synthesis of the hexacyclic veatchine-type C20-diterpenoid alkaloid (-)-garryine is presented. Key steps include a Pd-catalyzed enantioselective Heck reaction, a radical cyclization, and a photoinduced C-H activation/oxazolidine formation sequence. Of note, a highly enantioselective Heck reaction developed in this work provides efficient access to 6/6/6 tricyclic compounds, in particular, containing a C19-functionalitiy, which is useful for diverse transformations.

Unified short syntheses of oxygenated tricyclic aromatic diterpenes by radical cyclization with a photoredox catalyst

The biomimetic two-phase strategy employing polyene cyclization and subsequent oxidation/substitution is an effective approach for divergent syntheses of [6-6-6]-tricyclic diterpenes. However, this strategy requires lengthy sequences for syntheses of oxygenated tricyclic aromatic abietane/podocarpane diterpenes owing to the many linear oxidation/substitution steps after cyclization. Here, we present a new synthetic route based on a convergent reverse two-phase strategy employing a reverse radical cyclization approach, which enabled the unified short syntheses of four aromatic abietane/podocarpane diterpenes and the divergent short syntheses of other related diterpenes. Oxygenated and substituted precursors for cyclization were convergently prepared through Friedel-Crafts acylation and rhodium-catalyzed site-selective iodination. Radical redox cyclization using an iridium photoredox catalyst involving neophyl rearrangement furnished the thermodynamically favored 6-membered ring preferentially. (±)-5,6-Dehydrosugiol, salvinolone, crossogumerin A, and Δ5-nimbidiol were synthesized in only 8 steps. An oxygenated cyclized intermediate was also useful for divergent derivatization to sugiol, ferruginol, saprorthoquinone, cryptomeriololide, and salvinolone.

Synthesis of a Complex Brasilicardin Analogue Utilizing a Cobalt-Catalyzed MHAT-Induced Radical Bicyclization Reaction

We designed and executed an expedient synthesis of a complex analogue of the potent immunosuppressive natural product brasilicardin A. Our successful synthesis featured application of our recently developed MHAT-initiated radical bicyclization, which delivered the targeted, complex analogue in 17 steps in the longest linear sequence. Unfortunately, this analogue showed no observable immunosuppressive activity, which speaks to the importance of the structural and stereochemical elements of the natural core scaffold.

Hydrogen-Atom-Transfer-Initiated Radical/Polar Crossover Annulation Cascade for Expedient Access to Complex Tetralins

A radical/polar crossover annulation between allyl-substituted arenes and electron-deficient alkenes is described. Cobalt-catalyzed hydrogen atom transfer (HAT) facilitates tandem radical C-C bond formation that generates functionalized tetralin products in the face of potentially problematic hydrofluorination, hydroalkoxylation, hydrogenation, alkene isomerization, and radical polymerization reactions. The reactions proceed under mild conditions that tolerate many functional groups, leading to a broad substrate scope. This powerful ring-forming reaction very quickly assembles complex tetralins that are the formal products of the largely infeasible Diels-Alder cycloadditions of styrenes.

Photocatalytic Charge-Transfer Complex Enables Hydroarylation of Alkenes for Heterocycle Synthesis

Here, we report a photocatalytic charge-transfer complex (CTC) strategy for one electron reduction of alkenes using thiolate as a catalytic electron donor. This catalytic CTC system could engage hydroarylation of both activated and unactivated alkenes for the synthesis of various heterocycles. The reactions do not require any photocatalysts or acids and are easy to perform. Mechanistic studies revealed the formation of a CTC between catalytic thiolate and alkene.

Asymmetric Divergent Synthesis of ent-Kaurane-, ent-Atisane-, ent-Beyerane-, ent-Trachylobane-, and ent-Gibberellane-type Diterpenoids

A unified strategy toward asymmetric divergent syntheses of nine C8-ethano-bridged diterpenoids A1-A9 (candol A, powerol, sicanadiol, epi-candol A, atisirene, ent-atisan-16α-ol, 4-decarboxy-4-methyl-GA₁₂, trachinol, and ent-beyerane) has been developed based on late-stage transformations of common synthons having ent-kaurane and ent-trachylobane cores. The expeditious assembly of crucial advanced ent-kaurane- and ent-trachylobane-type building blocks is strategically explored through a regioselective and diastereoselective Fe-mediated hydrogen atom transfer (HAT) 6-exo-trig cyclization of the alkene/enone and 3-exo-trig cyclization of the alkene/ketone, showing the multi-reactivity of densely functionalized polycyclic substrates with π_C═C and π_C═O systems in HAT-initiated reactions. Following the rapid construction of five major structural skeletons (ent-kaurane-, ent-atisane-, ent-beyerane-, ent-trachylobane-, and ent-gibberellane-type), nine C8-ethano-bridged diterpenoids A1-A9 could be accessed in the longest linear 8 to 11 steps starting from readily available chiral γ-cyclogeraniol 1 and known chiral γ-substituted cyclohexenone 2, in which enantioselective total syntheses of candol A (A1, 8 steps), powerol (A2, 9 steps), sicanadiol (A3, 10 steps), epi-candol A (A4, 8 steps), ent-atisan-16α-ol (A6, 11 steps), and trachinol (A8, 10 steps) are achieved for the first time.

Toward the Asymmetric de Novo Synthesis of Lanostanes: Construction of 7,11-Dideoxy-Δ5-lucidadone H

Efforts to establish an asymmetric entry to hexanorlanostanes has resulted in a concise synthesis of 7,11-dideoxy-Δ5-lucidadone H from epichlorohydrin. By exploiting metallacycle-mediated annulative cross-coupling (to establish a functionalized hydrindane) and stereoselective formation of the steroidal C9-C10 bond to establish a stereodefined 9-alkyl estrane, 14 subsequent steps have been established to generate a hexanorlanostane system. Key transformations include formal inversion of the C13 quaternary center, oxidative dearomatization/group-selective Wagner-Meerwein rearrangement, and Lewis acid mediated semi-Pinacol rearrangement.

Concise Total Synthesis of Salimabromide

We achieved a concise total synthesis of salimabromide by using a novel intramolecular radical cyclization to simultaneously construct the unique benzo-fused [4.3.1] carbon skeleton and the vicinal quaternary stereocenters. Other notable transformations include a tandem Michael/Mukaiyama aldol reaction to introduce most of the molecule's structural elements, along with hidden information for late-stage transformations, an intriguing tandem oxidative cyclization of a diene to form the bridged butyrolactone and enone moieties spontaneously, and a highly enantioselective hydrogenation of a cycloheptenone derivative (97% ee) that paved the way for the asymmetric synthesis of salimabromide.

Total Synthesis of Natural Terpenoids Enabled by Cobalt Catalysis

Transition metal catalysis plays an essential role in the total synthesis of natural products. Cobalt-mediated asymmetric catalysis has successfully been used as a primary or a secondary step in the total synthesis of natural products, especially terpenoids. Terpenoids represent one of the most prominent families among various categories of natural products, attracting immense attention due to their promising physiological activities. This review summarizes the recent advances toward the total synthesis of terpenoids by cobalt-mediated asymmetric catalysis, which may shed some light on their future synthetic efforts toward natural pesticides such as celanguline, azadirachtin, etc.

Catalytic Enantioselective Construction of Decalin Derivatives by Dynamic Kinetic Desymmetrization of C2-Symmetric Derivatives through Aldol-Aldol Annulation

We have developed and investigated a catalytic desymmetrization reaction strategy that affords functionalized decalin derivatives with high enantioselectivities from C2-symmetric derivatives through aldol-aldol annulation. We identified the structural moieties of the catalyst necessary for the formation of the decalin derivative with high enantioselectivity. We elucidated the mechanisms of the catalyzed reactions: the first aldol reaction step was reversible, and the second aldol step was rate-limiting and stereochemistry-determining and was enantioselective. Using theoretical calculations guided by the experimental results, we identified the interactions between the catalyst and the transition state that led to the major enantiomer. The information obtained in this study will be useful for the development of catalysts and chemical transformations.

Highly (E)-Selective Trisubstituted Alkene Synthesis by Low-Valent Titanium-Mediated Homolytic Cleavage of Alcohol C-O Bond

Ti-mediated homolytic C-O bond cleavage was useful for cascade radical-ionic reactions. Benzyl alcohols treated with TiCl₄(col) (col = 2,4,6-collidine) and Mn powder generated the corresponding benzyl radicals; in addition, their reaction with 2-carboxyallyl acetates and the subsequent elimination of the acetoxy group yielded α,β-unsaturated carbonyl compounds with exclusive (E)-stereoselectivity. The simplicity of the procedure and its wide substrate scope represent a solution to the drawbacks associated with the reactions.

Total Synthesis of the Alleged Structure of (+)-Fimbricalyxoid A

An enantioselective total synthesis of the alleged structure of (+)-fimbricalyxoid A is reported. The synthetic strategy features a pyridine-N-oxidate-mediated S_N2' reaction to introduce an oxygen functionality at position C3 of the A-ring and a sequential three-step process via the cleavage of the C-O bonds and hemiketalization to form the 3,20-oxybridge. With this strategy, the target molecule was synthesized in 19% overall yield and 12 steps from our previously synthesized cis-fused octahydrophenanthrene (+)-6.

Programmed Polyene Cyclization Enabled by Chromophore Disruption

A new polyene cyclization strategy exploiting β-ionyl derivatives was developed. Photoinduced deconjugation of the extended π-system within these chromophores unveils a contrathermodynamic polyene that engages in a Heck bicyclization to afford [4.4.1]-propellanes. This cascade improves upon the limited regioselectivity achieved using existing biomimetic tactics and tolerates both electron-rich and electron-deficient (hetero)aryl groups. The utility of this approach was demonstrated with the diverted total synthesis of taxodione and salviasperanol, two isomeric abietane diterpenes that were previously inaccessible along the same synthetic pathway.

Divergent Total Syntheses of (-)-Crinipellins Facilitated by a HAT-Initiated Dowd-Beckwith Rearrangement

A hydrogen atom transfer (HAT)-initiated Dowd-Beckwith rearrangement reaction was developed, which enables the efficient assembly of diversely functionalized polyquinane frameworks. By incorporation of an iridium-catalyzed regio- and enantioselective hydrogenation and a diastereocontrolled ODI-[5+2] cycloaddition/pinacol rearrangement cascade reaction, the asymmetric total syntheses of eight tetraquinane natural products, including (-)-crinipellins A-F and (-)-dihydrocrinipellins A and B, have been achieved in a concise and divergent manner.

Scalable Total Synthesis of (-)-Triptonide: Serendipitous Discovery of a Visible-Light-Promoted Olefin Coupling Initiated by Metal-Catalyzed Hydrogen Atom Transfer (MHAT)

An efficient and scalable total synthesis of (-)-triptonide is accomplished based on a metal-catalyzed hydrogen atom transfer (MHAT)-initiated radical cyclization. During the optimization of the key step, we discovered that blue LEDs significantly promoted the efficiency of reaction initiated by Co(TPP)-catalyzed MHAT. Further exploration and optimization of this catalytic system led to development of a dehydrogenative MHAT-initiated Giese reaction.

Formal total synthesis of dankasterone B

A formal total synthesis of dankasterone B was achieved in 15 steps.

Stereocontrolled Synthesis and Structural Revision of Plebeianiol A

We report the structural revision via synthesis of the abietane diterpenoid plebeianiol A. The synthesis was accomplished by a short and convergent sequence that featured our previously established cobalt-catalyzed hydrogen-atom-transfer-induced radical bicyclization. We further connected plebeianiol A as the likely biogenetic precursor to another previously reported ether-bridged abietane. Finally, we demonstrated that the key cyclization event is efficient with the A-ring diol protected as two different cyclic acetals or in unprotected form.

Asymmetric Total Syntheses of (+)-Davisinol and (+)-18-Benzoyldavisinol: A HAT-Initiated Transannular Redox Radical Approach

The first and asymmetric total syntheses of two C11-oxygenated hetisine-type diterpenoid alkaloids, namely, (+)-davisinol and (+)-18-benzoyldavisinol, is described. The concise synthetic approach features a HAT-initiated transannular redox radical cyclization, an ODI-Diels-Alder cycloaddition, and an acylative kinetic resolution. By incorporating an efficient late-stage assembly of the azabicycle, our strategy would streamline the synthetic design of C₂₀-diterpenoid alkaloids and pave the way for their modular syntheses.

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.

Catalytic radical cascade cyclization of alkene-tethered enones to fused bicyclic cyclopropanols

Fused bicyclic cyclopropanols were achieved via an unprecedented HAT-triggered radical cascade reaction of alkene-tethered enones in the presence of an iron catalyst.

Metal-hydride hydrogen atom transfer (MHAT) reactions in natural product synthesis

Functionalization of olefins has been an important transformation in synthetic chemistry. This review will focus on the natural product synthesis employing the MHAT reaction as the key strategy.