Total Synthesis of (+)-Lyconadin A and Related Compounds via Oxidative C−N Bond Formation

Diastereoselective Hydrogenation of Tetrasubstituted Olefins using a Heterogeneous Pt-Ni Alloy Catalyst

Stereoselective hydrogenation of tetrasubstituted olefins is an attractive method to access compounds with two contiguous stereocenters. However, homogeneous catalysts for enantio- and diastereoselective hydrogenation exhibit low reactivity toward tetrasubstituted olefins due to steric crowding between the ligand scaffold and the substrate. Monometallic heterogeneous catalysts, on the other hand, provide accessible surface active sites for hindered olefins but exhibit unpredictable and inconsistent stereoinduction. In this work, we develop a Pt-Ni bimetallic alloy catalyst that can diastereoselectively hydrogenate unactivated, sterically-bulky tetrasubstituted olefins, utilizing the more oxophilic Ni atoms to adsorb a hydroxyl directing group and direct facially-selective hydrogen addition to the olefin via the Pt atoms. Structure-activity studies on several Pt-Ni compositions underscore the importance of exposing a uniform PtNi alloy surface to achieve high diastereoselectivity and minimize side reactions. The optimized Pt-Ni/SiO2 catalyst exhibits good functional group tolerance and broad scope for tetrasubstituted olefins in a cyclopentene scaffold, generating cyclopentanol products with three contiguous stereocenters. The synthetic utility of the method is demonstrated in a four-step synthesis of (1R,2S)-(+)-cis-methyldihydrojasmonate with high yield and enantiopurity.

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.

Electrochemical Oxidative C-H Amination through a Ritter-Type Reaction

A straightforward strategy for direct benzylic C-H bond amination via an electrochemical Ritter-type reaction is developed. The reaction demonstrates simpler and milder reaction conditions over the existing methods without extra mediator. Moderate to excellent yields up to 94% of the desired amide products were obtained with a broad substrate scope. The removal of the Ac group by a simple step can afford NH-free benzylic amines, providing a suitable approach for the late-stage functionalization of bioactive molecules.

Selective C-O Bond Forming Reactions at Indole-C2-Position toward Polycyclic Indolone or Indolinone Derivatives Tethered with Medium-Sized Rings

A methodology involving the chemoselective synthesis of tetracyclic [1,3]oxazino[3,2-a]indol-4-one or tetracyclic [1,3]oxazino[3,2-a]indoline-4-one tethered with a medium-sized ring by cross dehydrogenative coupling (CDC) or nucleophilic addition (NA) reaction has been developed. [1,3]Oxazino[3,2-a]indol-4-one compounds fused with a medium-sized ring were constructed through a CDC reaction in the presence of I₂ and K₂CO₃. Whereas, [1,3]oxazino[3,2-a]indoline-4-ones tethered with a medium-sized ring were obtained with a TfOH system by NA reaction.

Asymmetric Total Syntheses of (-)-Lycoramine, (-)-Lycoraminone, (-)-Narwedine, and (-)-Galanthamine

A concise asymmetric total synthesis of naturally occurring Amaryllidaceae alkaloids sharing dihydrobenzofuran scaffolds, (-)-galanthamine (1a), (-)-lycoramine (1b), (-)-narwedine (2a), and (-)-lycoraminone (2b), is reported. Orthoester Johnson-Claisen rearrangement of allyl alcohol (+)-9 (98% ee) in diisopropylethylamine furnished enantioenriched cyclohexene (+)-8 (97.4% ee) with a quaternary stereogenic center.

Total syntheses of naturally occurring antiviral indolosesquiterpene alkaloids, xiamycins C–F via Csp3–H functionalization

Concise total syntheses of naturally occurring antiviral indolosesquiterpene alkaloids, xiamycin C (2a), D (2b), E (2c) and F (2d), have been achieved via a late-stage oxidative δ-Csp³–H functionalization of an advanced pentacyclic enone intermediate 8. This strategy takes advantage of ipso-nitration of naturally occurring abietane diterpenoids to synthesize o-bromo nitroarene derivative 11. A Suzuki–Miyaura coupling of 11 with phenylboronic acid followed by Cadogan's ring closure provided a modular approach to a carbazole ring required for a functionalized pentacyclic core of indolosesquiterpene alkaloids.

Enantioenriched enone 8 was synthesized via three key transformations: ipso-nitration of abietane diterpenoids to furnish o-bromo nitroarene 11, Suzuki coupling with phenylboronic acid, and Cadogan's reductive ring closure to craft a carbazole ring.

Silver-Catalyzed N-H Functionalization of Aryl/Aryl Diazoalkanes with Anilines

Herein, we report on the N-H functionalization reaction of primary and secondary anilines with diaryldiazoalkanes using simple AgPF₆ as catalyst. We demonstrated broad applicability in the reaction of diaryldiazoalkanes with different anilines (31 examples, up to 97% yield). Furthermore, we propose a possible reaction mechanism for the N-H functionalization.

Testing the limits of radical-anionic CH-amination: a 10-million-fold decrease in basicity opens a new path to hydroxyisoindolines via a mixed C-N/C-O-forming cascade

An intramolecular C(sp3)-H amidation proceeds in the presence of t-BuOK, molecular oxygen, and DMF. This transformation is initiated by the deprotonation of an acidic N-H bond and selective radical activation of a benzylic C-H bond towards hydrogen atom transfer (HAT). Cyclization of this radical-anion intermediate en route to a two-centered/three-electron (2c,3e) C-N bond removes electron density from nitrogen. As this electronegative element resists such an "oxidation", making nitrogen more electron rich is key to overcoming this problem. This work dramatically expands the range of N-anions that can participate in this process by using amides instead of anilines. The resulting 107-fold decrease in the N-component basicity (and nucleophilicity) doubles the activation barrier for C-N bond formation and makes this process nearly thermoneutral. Remarkably, this reaction also converts a weak reductant into a much stronger reductant. Such "reductant upconversion" allows mild oxidants like molecular oxygen to complete the first part of the cascade. In contrast, the second stage of NH/CH activation forms a highly stabilized radical-anion intermediate incapable of undergoing electron transfer to oxygen. Because the oxidation is unfavored, an alternative reaction path opens via coupling between the radical anion intermediate and either superoxide or hydroperoxide radical. The hydroperoxide intermediate transforms into the final hydroxyisoindoline products under basic conditions. The use of TEMPO as an additive was found to activate less reactive amides. The combination of experimental and computational data outlines a conceptually new mechanism for conversion of unprotected amides into hydroxyisoindolines proceeding as a sequence of C-H amidation and C-H oxidation.

Synthetic Entry to the 2-Azatricyclo[4.3.2.04,9]undecane Ring System via Tropone

A synthesis of the 2-azatricyclo[4.3.2.0^4,9]undecane ring system-a hitherto unreported bridged azatricyclic ring system-beginning from tricarbonyl(tropone)iron and allylamine was accomplished in three steps: (1) aza-Michael addition of allylamine to tricarbonyl(tropone)iron; (2) Boc-protection of the resulting secondary amine; and (3) oxidative demetallation leading to a spontaneous intramolecular Diels-Alder reaction. The effect of a variety of parameters on the intramolecular Diels-Alder reaction was investigated, including diene and dienophile substitution patterns and dienophile tether length.

Total synthesis of (+)-ar-macrocarpene

This report features the first catalytic asymmetric total synthesis of a sesquiterpene, (+)-ar-macrocarpene (1), in 7 steps with 42.1% overall yields from commercially available inexpensive 5,5-dimethylcyclohexane 1,3-dione. This strategy relies on a key [3,3]-sigmatropic rearrangement effecting reductive transposition through allylic diazene rearrangement (ADR) in a single step from intermediate allylic alcohol (+)-12 under the Mitsunobu reaction conditions with o-nitrobenzenesulfonyl hydrazide (o-NBSH). Enantioselective reduction of α-bromo vinylogous ester 16 under the Corey-Bakshi-Shibata reduction conditions forges the required stereocenter in the allylic alcohol (+)-12 in a highly enantioenriched manner (95% ee).

A Practical Total Synthesis of (±)-Cermizine D and a Formal Synthesis of (±)-Cermizine C

A practical total synthesis of (±)-cermizine D and a formal synthesis of (±)-cermizine C were achieved. The four-step total synthesis comprised mainly a Michael addition, a Weinreb amide-activated substitution and a sequence of ketalation/PtO2-catalysed dearomatisation/reductive amination reactions that resulted in ring closure in 13.7% overall yield. The formal synthesis was accomplished in 29.7% overall yield.

Base-promoted synthesis of N-arylbenzamides by N-benzoylation of dimethylphenylthioureas

An efficient synthesis of 10 N-arylbenzamides was achieved by N-benzoylation of N,N-dimethyl-( N′-phenyl)thioureas with 4-substituted benzoyl chlorides in dimethylacetamide in the presence of K2CO3. The features of this method include good yields, short reaction times and broad substrate scope. This work showed that arylthiourea derivatives can be alternative and useful starting materials for the preparation of N-arylbenzamides.

Catalytic Synthesis of N-Heterocycles via Direct C(sp3)-H Amination Using an Air-Stable Iron(III) Species with a Redox-Active Ligand

Coordination of FeCl3 to the redox-active pyridine-aminophenol ligand NNOH2 in the presence of base and under aerobic conditions generates FeCl2(NNOISQ) (1), featuring high-spin FeIII and an NNOISQ radical ligand. The complex has an overall S = 2 spin state, as deduced from experimental and computational data. The ligand-centered radical couples antiferromagnetically with the Fe center. Readily available, well-defined, and air-stable 1 catalyzes the challenging intramolecular direct C(sp3)-H amination of unactivated organic azides to generate a range of saturated N-heterocycles with the highest turnover number (TON) (1 mol% of 1, 12 h, TON = 62; 0.1 mol% of 1, 7 days, TON = 620) reported to date. The catalyst is easily recycled without noticeable loss of catalytic activity. A detailed kinetic study for C(sp3)-H amination of 1-azido-4-phenylbutane (S1) revealed zero order in the azide substrate and first order in both the catalyst and Boc2O. A cationic iron complex, generated from the neutral precatalyst upon reaction with Boc2O, is proposed as the catalytically active species.

Natural product analogues: towards a blueprint for analogue-focused synthesis

A review of approaches to natural product analogues leads to the suggestion of new methods for the generation of biologically active natural product-like scaffolds.

Lyconadins G and H, Two Rare Lyconadin-Type Lycopodium Alkaloids from Lycopodium complanatum

Two rare lyconadin-type Lycopodium alkaloids, lyconadins G (1) and H (2), together with four known ones (3-6), were isolated from Lycopodium complanatum. The structures were determined on the basis of their spectroscopic analyses, and the absolute configuration of 1 was established by an X-ray crystallographic analysis. It is the first time to establish the absolute configuration of lyconadin-type Lycopodium alkaloid by an X-ray diffraction experiment. In addition, these findings may provide more information for the biosynthesis of lyconadins.

Synthesis of 2-oxindoles via 'transition-metal-free' intramolecular dehydrogenative coupling (IDC) of sp(2) C-H and sp(3) C-H bonds

The synthesis of a variety of 2-oxindoles bearing an all-carbon quaternary center at the pseudo benzylic position has been achieved via a ‘transition-metal-free’ intramolecular dehydrogenative coupling (IDC). The construction of 2-oxindole moieties was carried out through formation of carbon–carbon bonds using KOt-Bu-catalyzed one pot C-alkylation of β-N-arylamido esters with alkyl halides followed by a dehydrogenative coupling. Experimental evidences indicated toward a radical-mediated path for this reaction.

Syntheses of [1,2,4]triazolo[1,5-a]benzazoles enabled by the transition-metal-free oxidative N-N bond formation

A transition-metal-free oxidative N-N bond formation strategy was developed to generate various structurally interesting [1,2,4]triazolo[1,5-a]benzazoles efficiently. The mechanism of the key oxidative N-N bond formation was investigated by using an intramolecular competition reaction. Notably, the first single crystal structure was also obtained to confirm the structure of 2-aryl[1,2,4]triazolo[1,5-a]benzimidazole.

Concise total syntheses of (±)-mesembrane and (±)-crinane

A straightforward and unified strategy to access Amaryllidaceae alkaloids comprising a cis-3a-aryloctahydroindole scaffold has been developed. The strategy features Eschenmoser-Claisen rearrangement of allylalcohol as a key step for the installation of all-carbon quaternary stereocenters present in these alkaloids. The consequent iodolactonization-reduction-oxidation sequence beautifully assembles the advanced intermediate keto-aldehyde 10a, b in synthetically viable yields. The methodology has been successfully applied in the efficient syntheses of (±)-mesembrane (1a) and (±)-crinane (2a).

Semibullvalene and diazasemibullvalene: recent advances in the synthesis, reaction chemistry, and synthetic applications

Semibullvalene (SBV) and its aza analogue 2,6-diazasemibullvalene (NSBV) are theoretically interesting and experimentally challenging organic molecules because of four unique features: highly strained ring systems, intramolecular skeletal rearrangement, extremely rapid degenerate (aza-)Cope rearrangement, and the predicted existence of neutral homoaromatic delocalized structures. SBV has received much attention in the past 50 years. In contrast, after NSBV was predicted in 1971 and the first in situ synthesis was realized in 1982, no progress on NSBV chemistry was made until our results in 2012. We have been interested in the reaction chemistry of 1,4-dilithio-1,3-butadienes (dilithio reagents for short), especially for their applications in the synthesis of SBV and NSBV, because (i) the cyclodimerization of dilithio reagents could provide the potential eight-carbon skeleton of SBV from four-carbon butadiene units and (ii) the insertion reaction of dilithio reagents with C≡N bonds of two nitriles could provide a 6C + 2N skeleton that might be a good precursor for the synthesis of NSBV. Therefore, we initiated a journey into the synthesis and reaction chemistry of SBV and NSBV starting from dilithio reagents that has been ongoing since 2006. In this Account, we outline mainly our recent achievements in the synthesis, structural characterization, reaction chemistry, synthetic application, and theoretical/computational analysis of NSBV. Two efficient strategies for the synthesis of NSBV from dilithio reagents and nitriles via oxidant-induced C-N bond formation are described. Structural investigations of NSBV, including X-ray crystal structure analysis, determination of the activation barrier for the aza-Cope rearrangement, and theoretical analysis, show that the localized structure of NSBV is the predominant form and that the homoaromatic delocalized structure exists as a minor component in the equilibrium. We also discuss the reaction chemistry and synthetic applications of NSBV. Several novel reaction patterns have been explored, including thermolysis, C-N bond insertion, rearrangement-cycloaddition, oxidation, and nucleophilic ring-opening reactions. Diverse and interesting N-containing polycyclic skeletons can be constructed, such as nickelaazetidine, 1,5-diazatriquinacenes, and triazabrexadienes, which are not available by other means. Our results show that NSBV not only features a rapid aza-Cope rearrangement with a low activation barrier but also acts as unique synthetic reagent that is significantly different from aziridine. The strained rigid ring systems as a whole can be involved in the reactions. Our achievements highlight two significant advances: (i) the well-established efficient synthesis and isolation of NSBV has greatly accelerated the development of NSBV chemistry, and (ii) the previously unattainable molecules have become "normal" and routine starting materials for the synthesis of otherwise unavailable but interesting structures. We expect that our pursuits will inspire and help direct future chemical and physical research on NSBV.

Intramolecular dearomative oxidative coupling of indoles: a unified strategy for the total synthesis of indoline alkaloids

Indole alkaloids, one of the largest classes of alkaloids, serve as an important and rich source of pharmaceuticals and have inspired synthetic chemists to develop novel chemical transformations and synthetic strategies. Many biologically active natural products contain challenging indoline scaffolds, which feature a C3 all-carbon quaternary stereocenter that is often surrounded by a complicated polycyclic ring system. The creation of this quaternary stereocenter creates an inherent synthetic challenge because the substituents on the carbon center cause high steric repulsion. In addition, the presence of nitrogen atoms within the surrounding polycyclic rings can lead to synthetic difficulties. Oxidative coupling between two sp(3)-hybridized carbon anions provides a unique and powerful method for building C-C single bonds, especially for generating a C-C bond that joins one or two vicinal quaternary stereocenters. Although chemists have known of this transformation for a long time, they have only applied this reaction in total synthesis of complex natural products during the past decade. The progress of this class of reaction depends on the use of indole moieties as coupling partners. In this Account, we summarize our recent efforts to develop iodine-mediated intramolecular dearomative oxidative coupling (IDOC) reactions of indoles as part of a unified strategy for the total synthesis of three classes of indoline alkaloids. We categorized these IDOC reactions into three types based on their mode of connection to the indole moiety. In type I, the carboanion nucleophile was tethered to the indole at the C3 position. This reaction enabled the assembly of skeleton A, which features a spiro ring at the C3 position of the indole. We demonstrated the efficiency of this method by quickly assembling two classes of tetracyclic compounds and completing the total synthesis of (-)-communesins F, A, and B. For the type II IDOC reactions, the carboanion nucleophile residing at the C2 position of the indole formed a quaternary center at the C3 position of indole to produce skeleton B. We applied this IDOC reaction to synthesize two akuammiline alkaloids, vincorine and aspidophylline A. Type III IDOC reactions employed substrates with a preinstalled ring at the C2 and C3 positions of the indole. These transformations proceeded smoothly to afford polycyclic ring system C, which we used in the first enantioselective total synthesis of Kopsia alkaloid methyl N-decarbomethoxychanofruticosinate. These results further demonstrate how new chemical strategies and reactions facilitate both the first total syntheses of natural products and the discovery of more efficient synthetic routes.

Synthetic efforts toward the Lycopodium alkaloids inspires a hydrogen iodide mediated method for the hydroamination and hydroetherification of olefins

Progress toward the total syntheses of a diverse set of fawcettimine-type Lycopodium alkaloids via a "Heathcock-type" 6-5-9 tricycle is disclosed. This route features an intermolecular Diels-Alder cycloaddition to rapidly furnish the 6-5-fused bicycle and a highly chemoselective directed hydrogenation to build the azonane fragment. While conducting these synthetic studies, trimethylsilyl iodide was found to effect a hydroamination reaction to furnish the tetracyclic core of serratine and related natural products. This observation has been expanded into a general method for the room temperature hydroamination of unactivated olefins with tosylamides utilizing catalytic "anhydrous" HI (generated in situ from trimethylsilyl iodide and water). The presence of the iodide anion is critical to the success of this Brønsted acid catalyzed protocol, possibly due to its function as a weakly coordinating anion. These conditions also effect the analogous hydroetherification reaction of alcohols with unactivated olefins.

Total Syntheses of Lyconadins: Finding Efficiency and Diversity

Lyconadins A-C are important members of the Lycopodium alkaloid family with challenging structural features and interesting biological profile. Herein, various synthetic strategies and methods for their preparation are summarized with the focus on constructive bond formation and our efficient and divergent synthesis based on functional group pairing (FGP) strategy.

Carbamoyl radical-mediated synthesis and semipinacol rearrangement of β-lactam diols

In an approach to the biologically important 6-azabicyclo[3.2.1]octane ring system, the scope of the tandem 4-exo-trig carbamoyl radical cyclization-dithiocarbamate group transfer reaction to ring-fused β-lactams is evaluated. β-Lactams fused to five-, six-, and seven-membered rings are prepared in good to excellent yield, and with moderate to complete control at the newly formed dithiocarbamate stereocentre. No cyclization is observed with an additional methyl substituent on the terminus of the double bond. Elimination of the dithiocarbamate group gives α,β- or β,γ-unsaturated lactams depending on both the methodology employed (base-mediated or thermal) and the nature of the carbocycle fused to the β-lactam. Fused β-lactam diols, obtained from catalytic OsO4-mediated dihydroxylation of α,β-unsaturated β-lactams, undergo semipinacol rearrangement via the corresponding cyclic sulfite or phosphorane to give keto-bridged bicyclic amides by exclusive N-acyl group migration. A monocyclic β-lactam diol undergoes Appel reaction at a primary alcohol in preference to semipinacol rearrangement. Preliminary investigations into the chemo- and stereoselective manipulation of the two carbonyl groups present in a representative 7,8-dioxo-6-azabicyclo[3.2.1]octane rearrangement product are also reported.

Divergent total syntheses of lyconadins A and C

Divergent and concise total syntheses of two lycopodium alkaloids, lyconadins A and C have been developed. The synthesis of lyconadin A, having potent neurotrophic activity, features an efficient one-pot ketal removal and formal aza-[4+2] cyclization to form the cagelike core structure. A tandem ketal removal/Mannich reaction was developed to build the tricyclic structure of lyconadin C. Both lyconadins A and C were synthesized from a pivotal intermediate.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.

Pyridone annulation via tandem Curtius rearrangement/6π-electrocyclization: total synthesis of (-)-lyconadin C

A concise, enantioselective total synthesis of the Lycopodium alkaloid (-)-lyconadin C was achieved in 12 steps and high overall yield. Key features include construction of a luciduline congener through Mannich-type cyclization and a one-pot, tandem Curtius rearrangement/6π-electrocyclization to fashion the 2-pyridone system of lyconadin C.

Exploration of an epoxidation-ring-opening strategy for the synthesis of lyconadin A and discovery of an unexpected Payne rearrangement

In the context of synthetic efforts targeting the alkaloid lyconadin A, scalemic epoxide 25 was prepared by a highly stereoselective sequence involving a Myers alkylation and a Shi epoxidation. Ring-opening of this epoxide with a vinylcopper complex afforded alcohol 26 instead of the expected product 27. An unusual Lewis acid promoted Payne rearrangement of an α-trityloxy epoxide is proposed to account for this outcome.

Toward a unified approach for the lycopodines: synthesis of 10-hydroxylycopodine, deacetylpaniculine, and paniculine

The enantioselective syntheses of 10-hydroxylycopodine, deacetylpaniculine, and paniculine have been accomplished through use of a common intermediate. Key steps in the synthetic sequence toward these lycopodium alkaloids include formation of the tricyclic core via a conformationally accelerated, intramolecular Mannich cyclization and an organocatalyzed, intramolecular Michael addition to form the C(7)-C(12) linkage.