原田 慎吾

C-H insertion and amide insertion reactions using metal-carbene species provide a powerful synthetic method for direct functionalization of kinetically inert or thermodynamically stable chemical bonds. Our group previously developed an amide insertion reaction using a rhodium-dimer complex, constructing an array of nitrogen-bridged heterocycles. Another research group reported C-H insertion reactions using structurally related substrates and rhodium catalysts. Detailed mechanistic studies were not provided, however, and therefore, the origin of the chemoselectivity was ambiguous. Here we describe our theoretical investigation of the chemoselectivity between the amide insertion reaction and C-H functionalization. An energy gap of the identified transition states in the reaction coordinates could support the reported experimental results and the observed chemoselectivity. Moreover, frontier molecular orbital analysis revealed that functionalities adjacent to the metal-carbene species could affect orbital populations and their energy levels, resulting in the construction of a completely distinctive ring system.

Hydrocarbazolones are a key structural unit in many natural bioactive compounds and are thus a valuable synthetic target. A rhodium-catalyzed C−H functionalization reaction with acceptor/acceptor diazo compounds was developed for synthesizing 2,3-fused indole variants. An α,β-unsaturated enone was utilized as a directing group for site-selective C−H activation and as an electrophile for the subsequent cyclization. Overall, the developed annulation enabled the rapid assembly of synthetically valuable hydrocarbazolones from readily available indoles. Additionally, computational investigations were conducted to elucidate the reaction pathway and rationalize the experimentally observed site-selectivity. (Figure presented.).

Herein, we report the development of an asymmetric intramolecular cyclopropanation reaction under silver catalysis. The strategy is based on diazo-free silver–carbene generation, providing γ-lactam fused cyclopropane in an enantioenriched form. The ring system of the product is frequently encountered in bioactive molecules and pharmaceuticals. To highlight the utility of this method, the fused cyclopropane was converted into drug candidate molecules in short steps. In addition, computational investigations were performed to elucidate the reaction pathway. The computation based on density functional theory rationalized the experimentally observed chemoselectivity between the desired cyclopropanation and overoxidation process.

© 2020 American Chemical Society. Arene dearomatization is a straightforward method for converting an aromatic feedstock into functionalized carbocycles. Enantioselective dearomatizations of chemically inert arenes, however, are quite limited and underexplored relative to those of phenols and indoles. We developed a method for diazo-free generation of silver-carbene species from an ynamide and applied it to the dearomatization of nonactivated arenes. Transiently generated norcaradiene could be trapped by intermolecular [4 + 2] cycloaddition, synthesizing polycycles with five consecutive stereogenic centers. This protocol constitutes the first highly enantioselective reaction based on the diazo-free generation of silver-carbene species. Mechanistic investigations revealed a dearomatization followed by two different classes of pericyclic reactions, as well as the origin of the chemo- and enantioselectivity.

© 2020 American Chemical Society. All rights reserved. Although intense research over the last 2 decades revealed a diverse proficiency of nitrene species in chemistry disciplines, control of the selectivity in the reaction manifold has remained a challenge. We report herein the development of site-selective and chemoselective C-H functionalization involving nitrene species to synthesize densely functionalized spiroaminals. A rhodium catalyst generally used in nitrene chemistry gave amide C-N insertion products and/or the corresponding ketones, whereas a silver catalyst with an achiral bisoxazoline (BOX) ligand provided C-H insertion products. Mechanistic analysis based on integrated experimental and computational studies indicated that the nitrene transfer occurred through an asynchronous concerted process involving triplet spin-correlated radical pairs, affording the corresponding stereodefined spiromolecules.

© 2020 The Pharmaceutical Society of Japan Catalytic dearomative transformations of phenol variants via an ipso-Friedel-Crafts reaction could provide a straightforward method for the rapid assembly of functionalized spiromolecules as versatile synthetic scaffolds. We previously reported a dearomative spirocyclization reaction by merging Brønsted acid and hydrogen-bonding catalysis. However, it was unclear how the reaction proceeded and how the synergic effect was triggered. Described herein are the computational studies used to elucidate the reaction mechanism. Such calculations indicated that the applied catalysts, maleic acid and Schreiner's thiourea, work cooperatively. The synergic effect enabled the chemoselectivity to interconvert between phenol dearomatization and O-H insertion, which is a major side reaction. This investigation also revealed that not only does the Schreiner's thiourea catalyst serve as a hydrogen bonding donor, but the sulfur atom in thiourea possesses a general base function. The dual functional support of the thiourea along with maleic acid would thus realize the chemoselective prioritization of dearomatization over the O-H insertion reaction under mild conditions.

© 2020 American Chemical Society. A chemoselective dearomatization of the less reactive benzenoid unit in β-naphthol was developed. Spirocyclization with a reductant constructs a pivotal structure for drug candidates. One-pot oxidative conversion enabled the tandem dearomatization of β-naphthol, producing conjugated tetraenone variants. The potential utility of the product as an F-selective anion sensor was also demonstrated. Theoretical studies revealed the intermediacy of silver-carbenoid species leading to chemoselective spirocyclization over arene cyclopropanation.

Copyright © 2020 American Chemical Society. Despite a growing body of studies on directing-group (DG)-assisted C-H activation strategies, efficient exploitation of the used DG remains underexplored. We developed a rhodium-catalyzed C-H functionalization of indoles at the C4 position using α,β-unsaturated enones as versatile DGs. Combined experimental and theoretical analyses revealed that the C-H activation process was reversible and the course of Rh-carbene generation controlled the overall site-selectivity of the C-H functionalization. The introduced malonate unit and the used enone DG were cyclized in a further C-C bond forming process to assemble 3,4-fused tricyclic indoles in an asymmetric manner. Telescoping the two reaction sequences provided rapid entry into this densely functionalized indole architecture from readily available chemical feedstock.

Copyright © 2020 American Chemical Society. An asymmetric dearomatization of indoles bearing α-diazoacetamide functionalities was developed for synthesizing high-value spiro scaffolds. A silver phosphate chemoselectively catalyzed the sterically challenging dearomatization, whereas more typically used metal catalysts for carbene transfer reactions, such as a rhodium complex, were not effective and instead resulted in a Büchner ring expansion or cyclopropanation. Mechanistic studies indicated that the spirocyclization occurred through a silver-assisted asynchronous concerted process and not via a silver-carbene intermediate. Analyses based on natural bond orbital population and a distortion/interaction model indicated that the degree of C-Ag mutual interaction is crucial for achieving a high level of enantiocontrol. In addition, an oxidative disconnection of a C(sp3)-C(sp2) bond in the product provided unconventional access to the corresponding chiral spirooxindole.

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim An enantioselective insertion reaction of silver carbenes generated from donor–acceptor-substituted diazo compounds into the O−H bond of phenols was developed. A homobinuclear silver complex with a chiral phosphorous ligand was created in situ from AgNTf2 and (S)-XylylBINAP (in a 2:1 mole ratio). Detailed mechanistic studies using combined experimental and computational techniques revealed that one silver atom center of the catalyst forms a silver carbene and another one works as a Lewis acid for the nucleophilic addition of a phenol. Two counter-anions, two water molecules, and two silver atoms cooperatively mediate the subsequent protonation event to lower the activation energy and control enantioselectivity, affording an array of valuable α-aryl-α-aryloxy esters.

© 2019 Elsevier Ltd A chemoselective domino annulation reaction of β-naphthols with methyl aryldiazoacetate is described. The gold catalyst promoted C–H functionalization of β-naphthols, whereas a rhodium or copper complex led to O–H insertion reactions. Consecutive intramolecular lactonization occurred after site-selective alkylation at the 1-position of β-naphthol, providing functionalized naphthofuranone derivatives. The product was transformed into a chiral molecule bearing an all-carbon quaternary stereogenic center with high enantioselectivity.

© 2019 American Chemical Society. Although (+)-catharanthine is an attractive alkaloid for both clinical research and organic synthetic chemistry, only a limited number of approaches for its catalytic asymmetric synthesis exist. Herein, we describe a novel strategy for synthesizing a chiral intermediate of (+)-catharanthine via phosphoric acid-catalyzed asymmetric desymmetrization of a meso-isoquinuclidine possessing a 1,3-diol unit that was synthesized by a formal amide insertion reaction.

© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim The past few decades have witnessed extensive efforts to disclose the unique reactivity of metal–nitrenes, because they could be a powerful synthetic tool for introducing the amine functionality into unactivated chemical bonds. The reactivity of metal–nitrenes, however, is currently mainly confined to aziridination (an insertion into a C=C bond) and C−H amination (an insertion into a C−H bond). Nitrene insertion into an amide C−N bond, however, has not been reported so far. In this work we have developed a rhodium-catalyzed one-nitrogen insertion into amide C−N and sulfonamide S−N bonds. Experimental and theoretical analyses based on density functional theory indicate that the formal amide insertion proceeds via a rhodium-coordinated ammonium ylide formed between the nitrene and the amide nitrogen, followed by acyl group transfer concomitant with C−N bond cleavage. Mechanistic studies have allowed rationalization of the origin of the chemoselectivity observed between the C−H and amide insertion reactions. The methodology presented herein is the first example of an insertion of nitrene into amide bonds and provides facile access to unique diazacyclic systems with an N−N bond linkage.

© Society of Synthetic Organic Chemistry. All rights reserved. Metal carbenoids exhibit unique reactivities towards stable chemical bonds. Such reactive species are recognized as powerful tools for direct functionalization of unactivated compounds. Meanwhile, the high and versatile reactivity of metal carbenoids sometimes become problematic in the development of chemoselective reactions, because organic molecules often include some reactive sites to metal carbenoids such as C-H bonds, multiple bonds, and heteroatoms. Therefore, considerable efforts have focused on the control of chemoselectivity of metal carbenoid-mediated reactions in recent years. We have been interested in the potential of metal carbenoids to develop an unusual reaction and succeeded in developing catalyst-controlled highly chemoselective reactions for the synthesis of complex molecules. Herein we summarized our achievements; an insertion reaction of Rh-carbenoid into an amide C-N bond and a dearomative ipso-Friedel-Crafts (DIFC) reaction mediated by chiral Ag-carbenoid species. An application of the amide insertion reaction to the natural product synthesis is also described.

RATIONALE: (R,S)-ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, exhibits rapid and long-lasting antidepressant effects and anti-suicidal ideation in treatment-resistant patients with depression. However, the precise mechanisms underlying the antidepressant actions of (R,S)-ketamine are unknown. Although the previous report demonstrated the deuterium isotope effects in the antidepressant actions of (R,S)-ketamine, the deuterium isotope effects in the antidepressant actions of (R)-ketamine, which is more potent than (S)-ketamine, are unknown. METHODS: We examined whether deuterium substitution at the C6 position could affect antidepressant effects of (R)-ketamine in a chronic social defeat stress (CSDS) model. RESULTS: Pharmacokinetic studies showed that levels of (2R,6R)-d1-hydroxynorketamine [(2R,6R)-d1-HNK], a final metabolite of (R)-d2-ketamine, in the plasma and brain after administration of (R)-d2-ketamine (10 mg/kg) were lower than those of (2R,6R)-HNK from (R)-ketamine (10 mg/kg), indicating deuterium isotope effects in the production of (2R,6R)-HNK. In contrast, levels of (R)-ketamine and its metabolite (R)-norketamine in the plasma and brain were the same for both compounds. In a CSDS model, both (R)-ketamine (10 mg/kg) and (R)-d2-ketamine (10 mg/kg) showed rapid and long-lasting (7 days) antidepressant effects, indicating no deuterium isotope effect in the antidepressant effects of (R)-ketamine. CONCLUSIONS: The present study suggests that deuterium substitution of hydrogen at the C6 position slows the metabolism from (R)-ketamine to (2R,6R)-HNK in mice. In contrast, we did not find the deuterium isotope effects in terms of the rapid and long-lasting antidepressant effects of (R)-ketamine in a CSDS model. Therefore, it is unlikely that (2R,6R)-HNK is essential for antidepressant effects of (R)-ketamine.

p-Boronophenylalanine (L-BPA) is applied in clinical settings as a boron carrier for boron neutron capture therapy (BNCT) to cure malignant melanomas. Structural modification or derivatization of L-BPA, however, to improve its uptake efficiency into tumor cells has scarcely been investigated. We successfully synthesized (S)-2-amino-3-(4-boronophenyl)-2-methylpropanoic acid in enantioenriched form as a novel candidate molecule for BNCT. Key steps to enhance the efficiency of this synthesis were enantioselective alkylation of N-protected alanine tert-butyl ester with a Maruoka catalyst and Miyaura borylation reaction to install the boron functionality.

We previously developed a chemoselective asymmetric phenol dearomatization using a silver catalyst and benzoic acid as an additive to enhance the reaction efficiency. The mechanistic role of the additive, however, remained unclear. Herein we describe detailed studies to elucidate the additive effect, which revealed that benzoic acid plays two supporting roles in the silver-catalyzed reaction. First, it promotes protonation of a silver enolate intermediate to improve the chemical yield of a spirolactam. Second, it dissociates a homochiral dimer of silver phosphate to generate a monomeric species.

Diazo compounds are frequently used as precursors of metal carbenoids and act as soft nucleophiles even without the use of metal catalysts. The resulting diazonium species may also be trapped by various nucleophiles. The introduction of an iodine functionality applicable for the coupling reaction into an alkenyl diazo compound, however, has not been reported. We developed iodoalkoxylation reactions of alkenyl diazoacetates using an electrophilic iodinating reagent and oxygen nucleophile. This catalyst-free multicomponent reaction proceeded regioselectively, furnishing trisubstituted vinyl iodides in 31%–71% yield. The synthesized iodoalkenes were converted into the corresponding olefins with various functionalities in good yield via Suzuki-Miyaura coupling reactions with arylboronic acids under palladium catalysis to demonstrate the synthetic utility of the developed reaction sequence.

Intramolecular dearomative cyclization of phenols with α-diazoamide units for synthesizing functionalized spirolactams was developed by merging Brønsted acid and hydrogen-bonding catalysis as an advantageous alternative to transition metal catalysis. This metal carbenoid-free strategy enables high chemoselectivity by suppressing potentially competing C−H insertion reactions and a Büchner reaction. Preliminary mechanistic studies were performed to elucidate the positive effect of the combined use of the catalysts, and extension to an asymmetric reaction was achieved. (Figure presented.).

A simple protocol to directly access γ-amino acid derivatives by intermolecular regioselective hydroamination of trichloroethyl alkenyldiazoacetates with carbamate using a silver tetrafluoroborate catalyst is described. Density functional theory (DFT) calculations to analyze the reaction mechanism revealed that multiple attractive interactions occur in a transition state to promote the vinylogous addition of nitrogen nucleophiles.

We report asymmetric dearomatization of phenols using Ag carbenoids from α-diazoacetamides. The Ag catalyst promoted intramolecular dearomatization of phenols, whereas a Rh or Cu catalyst caused C-H insertion and a Büchner reaction. Studies indicated Ag carbenoids have a carbocation-like character, making their behavior and properties unique. Highly enantioselective transformations using Ag carbenoids have not been reported. We achieved a Ag carbenoid-mediated chemo- and highly enantioselective phenol dearomatization with substrate generality for the first time.

We developed a novel synthetic method of the core structure of dragmacidin E bearing a 7-membered ring-fused bis(indolyl)pyrazinone skeleton. Formation of the 7-membered ring-fused tricyclic indole skeleton was accomplished using a palladium-catalyzed Heck insertion-allylic amination cascade. Vicinal difunctionalization of the 7-membered ring was realized via a rhodium-catalyzed aminoacetoxylation.

Metal nitrenoids and metal carbenoids exhibit similar reactivity for insertion into a C−H bond and a C=C double bond. These reactions have attracted the attention of organic chemists due to their unusual bond-forming ability, but the reactivity difference between these chemical species has not been studied. In this paper, we examined the reactivity difference using the corresponding Rh nitrenoid and Rh carbenoid precursors. The Rh nitrenoid inserted into an intramolecular C(sp3)−H bond adjacent to an amide nitrogen, affording functionalized spiroaminals that are ubiquitous in natural products, while the Rh carbenoid inserted into an amide C−N bond. The totally different reactivity was rationalized by the relatively low energy barrier for the C−H insertion reaction of the Rh nitrenoid. Computational analysis suggests that the origin of the discrepancy is the electrophilicity of the coordinating atoms to the Rh complex.

A catalytic strategy was developed for asymmetric substitution reactions at sp3-hybridized carbon atoms by using a chiral alkylating agent generated in situ from trichloroacetimidate and a chiral phosphoric acid. The resulting chiral p-methoxybenzyl phosphate selectively reacts with β-amino alcohols rather than those without a β-NH functionality. The use of an electronically and sterically tuned chiral phosphoric acid enables the kinetic resolution of amino alcohols through p-methoxybenzylation with good enantioselectivity.

A formal synthesis of anatoxin-a was accomplished by using rhodium-catalyzed formal amide insertion reaction. The key reaction was performed on a gram scale using 0.4 mol % of Rh2(NHCOtBu)4catalyst, affording a 9-azabicyclo[4.2.1]nonane derivative in good yield. The nitrogen-bridged molecule was converted to Wiseman's intermediate through diastereoselective reduction, site-selective deoxygenation and functional group interconversions.

Asymmetric desymmetrization of σ-symmetric acid anhydrides was achieved with chiral phosphoric acid as a Brønsted acid catalyst. The key of success was finding of benzhydrol and 2,2-diphenylethanol as the nucleophiles of choice. The corresponding half esters were obtained in good yields with high selectivity.

A formal amide insertion reaction for the synthesis of nitrogen-bridged polycyclic frameworks with diverse functionalities was developed using a sustainable copper catalyst as an advantageous alternative to precious rhodium catalysts. The remarkable feature of this methodology is the amount of catalyst loading (0.05 mol%). The optimized reaction conditions enable access to aromatic ring-fused 8-azabicyclo[3.2.1]octane, 9-azabicyclo[3.3.1]nonane, and 6-azabicyclo[3.2.2]nonane derivatives in moderate to excellent yields. (Figure presented.).

Various nitrogen-bridged bicyclic skeletons are found in bioactive natural products and pharmaceuticals. The development of a new reaction to construct these molecular frameworks has attracted considerable attention in synthetic organic chemistry. We developed a novel synthetic method for obtaining a wide variety of nitrogen-bridged bicyclic compounds with a catalytic process, Rh-catalyzed formal carbenoid insertion into an amide C-N bond. Using 0.1-0.4 mol % Rh2(NHCOtBu)4 catalyst, various azabicyclo[X.Y.Z]alkane derivatives were obtained in good to excellent yield, successfully demonstrating the broad substrate scope of the developed process. Experimental and computational studies to elucidate the reaction mechanism revealed that the formal insertion reaction of a carbenoid into an amide C-N bond proceeded via the formation of Rh-associated N-ylides, followed by an acyl group-selective Stevens [1,2]-shift through a concerted addition/elimination process on the sp2-hybridized carbon.

Functionalized quinolizidin-4-ones and indolizidin-3-ones were synthesized from mixed N,O-acetals with a terminal alkyne unit and nucleophiles with two reactive sites by a gold(I)/Brønsted acid relay catalysis with 40-90% yield. The reactions are proposed to proceed through gold-catalyzed carboalkoxylation of the alkynes with the internal mixed N,O-acetals, giving the corresponding cyclic enols, which are transformed to spirocyclic compounds by a Brønsted acid catalyst. The latter spirocyclization process occurs under thermodynamic control, providing polycyclic quinolizidin-4-one and indolizidin-3-one derivatives diastereoselectively.

Gephyrotoxin 287C, a bioactive alkaloid bearing a perhydropyrrolo[1,2-a]quinoline skeleton with five stereocenters, is an attractive target for synthetic organic chemistry. We achieved an enantioselective total synthesis of (+)-gephyrotoxin 287C, for which the key steps were palladium-catalyzed asymmetric allylic amination using a chiral diaminophosphine oxide (DIAPHOX) preligand, diastereoselective intramolecular Mannich reaction, and tin tetrachloride-catalyzed diastereoselective conjugate addition/protonation.

A Brønsted-acid-catalyzed intramolecular enantioselective SN2′ reaction was developed utilizing trichloroacetimidate as a leaving group. The findings indicated that dual activation of the substrates is operative. This metal-free allylic alkylation allows highly enantioselective access to 2-vinylpyrrolidines bearing various substituents. Leaving-group activation: A Brønsted acid catalyzed intramolecular enantioselective SN2′ reaction was developed utilizing trichloroacetimidate as a leaving group. The findings indicated that dual activation of the substrates is operative. This metal-free allylic alkylation allows highly enantioselective access to 2-vinylpyrrolidines bearing various substituents.

The rate and enantioselectivity of chiral NHC-catalyzed asymmetric acylation of alcohols with an adjacent H-bond donor functionality are remarkably enhanced in the presence of a carboxylate cocatalyst. The degree of the enhancement is correlated with the basicity of the carboxylate. With a cocatalyst and a newly developed electron-deficient chiral NHC, kinetic resolution and desymmetrization of cyclic diols and amino alcohols were achieved with extremely high selectivity (up to s = 218 and 99% ee, respectively) at a low catalyst loading (0.5 mol %). This asymmetric acylation is characterized by a unique preference for alcohols over amines, which are not converted into amides under the reaction conditions.

Chiral diether-mediated asymmetric aminolithiation of indolylpropenoate with lithium amide in toluene at -78 °C for 15 min gave, after aqueous ammonium chloride quench, the corresponding conjugate addition product with 97% ee in 89% yield. If hydrogen chloride in methanol was selected as a quencher, however, aminolithiation at -78 °C for 3 h gave the corresponding adduct with 97% ee in 54% yield, along with recovery of the starting enoate in 39% yield. Based on this finding of an incomplete and slow reaction at -78 °C, the aminolithiation conditions were optimized to be at -60 °C for 15 h and subsequent enolate trap with alkyl halide upon an addition of DMPU afforded the desired aminoalkylation product with 98% ee in 89% yield. Further approach toward total synthesis of (-)-kopsinine was carried out by examining asymmetric aminolithiation with N-hydroxyethylamine equivalent, one-pot piperidine formation, and Claisen condensation. © 2013 Elsevier Ltd. All rights reserved.

Enantio and diastereoselective one-pot synthesis of three- to seven-membered cis-azaheterocycles was achieved using a triggered asymmetric conjugate addition reaction of lithium amide with an enoate, followed by C- and N-alkylation of the resulting lithium 3-aminoenolate with α, ω-dihaloalkane, which can be regarded as a [1+2+n] cyclization. The usefulness of the developed methodology was clearly demonstrated by the short-step asymmetric syntheses of azirinomycin ester, nemonapride, and kopsinine. © The Pharmaceutical Society of Japan.

All together now: A common intermediate of Kopsia alkaloids was synthesized from cis-2,3-disubstituted piperidine, which was obtained with 98 % ee in 85 % yield using chiral diether-controlled asymmetric conjugate addition of lithium amide to indolepropenoate, followed by C,N dual alkylation with 1-chloro-3-iodopropane. The synthesis of (-)-kopsinine was accomplished via the intermediate in 9.0 % overall yield in 13 steps. Copyright © 2012 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.