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Here, we synthesized three acetogenin analogs containing pyrimidine moieties linked by amine bonds, which represent the skeleton structure of pyrimidifen, a mitochondrial complex I-inhibiting insecticide. Replacing the pyrimidine moiety linked by the amine bond remarkably enhanced growth-inhibitory activity of the analogs against several human cancer cell lines. Moreover, these analogs selectively and potently inhibited the growth of these human cancer cell lines regardless of the pyrimidine substituents. Furthermore, COMPARE analyses suggested that these analogs inhibited cancer growth by inhibiting mitochondrial complex I. Our study provides insights into the design of acetogenin analogs as novel antitumor agents.

Ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes using dimethylsulfoxonium methylide proceeded regioselectively to produce 2,3,4,6,7,8-hexahydro-5H-1-benzopyran-5-ones in good to high yields. The reactions of cycloheptane- and cyclopentane-1,3-dione-2-spirocyclopropanes could construct [7.6]- and [5.6]-fused ring systems. This reaction was also carried out using sulfoxonium ethylide, butylide, and benzylide, resulting in the formation of the corresponding 2,3-trans-disubstituted products in good to high yields, and it was shown that the dimethyl group can act as a dummy substituent. It was found that the 2- and 3-phenyhexahydrobenzopyran-5-ones can be readily converted into 5-hydroxyflavan and 5-hydroxyisoflavan, respectively.

Regioselective ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes with stabilized sulfonium ylides provided 2,3-trans-disubstituted 2,3,4,6,7,8-hexahydro-5H-1-benzopyran-5-ones in high yields without the formation of any isomers. The obtained product was readily converted into highly substituted chromane.

Dirhodium(II)-catalyzed oxonium ylide formation–[2,3]-sigmatropic rearrangement of 6-allyloxy-2-diazo-3-ketoesters possessing a C-6 substituent is described. The reaction of 6-alkyl- or 6-aryl-substituted 6-allyloxy-2-diazo-3-ketoesters with a catalytic amount of Rh2(S-PTTL)4 proceeded in a chemoselective and stereoselective manner to provide 6-substituted 2-allyl-3-oxotetrahydropyran-2-carboxylates in good yields and with high diastereoselectivities. To demonstrate the utility of this sequential reaction, we conducted the total synthesis of (+)-tanikolide, in which the construction of the δ-lactone skeleton was achieved by employing a 2-iodobenzamide-catalyzed oxidative cleavage of tetrahydropyran-2-methanol.

The divergent synthesis of penaresidin B and its straight side chain analogue was accomplished by constructing an azetidine ring via SN2 type cyclization of protected 2,3-syn-3,4-syn-4-amino-1,3-dihydroxyhept-6-en-2-yl mesylate and late-stage introduction of an alkyl side chain via olefin cross-metathesis of 4-allylazetidine with readily available terminal alkenes. This synthetic route would be useful to synthesize penaresidin side chain analogues.

The ring-opening cyclization of 2',3'-nonsubstituted and 2'-electron-withdrawing group (EWG)-substituted cyclohexane-1,3-dione-2-spirocyclopropanes was accomplished using iodide as a catalyst. The nonsubstituted derivatives afforded 3,5,6,7-tetrahydro-1-benzofuran-4(2H)-ones in high yields in the presence of trimethylsilyl iodide at room temperature. The EWG-substituted spirocyclopropanes, in turn, underwent regioselective ring opening followed by cyclization, which gave rise to 2-substituted tetrahydrobenzofuran-4-ones when a combination of tetrabutylammonium iodide catalyst and trifluoromethanesulfonic acid was used, whereas calcium iodide afforded the 3-substituted derivatives.

Several N-isopropyliodobenzamides were evaluated as catalysts for the oxidation of benzhydrol to benzophenone in the presence of Oxone® (2KHSO5·KHSO4·K2SO4) as a co-oxidant at room temperature. A study on the substituent effect of the benzene ring of N-isopropyl-2-iodobenzamide on the oxidation revealed that its reactivity increased in the following order of substitution: 5-NO2 < 5-CO2Me, 3-OMe < 5-OAc < 5-Cl < H, 4-OMe < 5-Me < 5-OMe. The oxidation of various benzylic and aliphatic alcohols using a catalytic amount of the most reactive 5-methoxy derivative successfully resulted in moderate to excellent yields of the corresponding carbonyl compounds. The high reactivity of the 5-methoxy derivative at room temperature is a result of the rapid generation of the pentavalent species from the trivalent species during the reaction. 5-Methoxy-2-iodobenzamide would be an efficient and environmentally benign catalyst for the oxidation of alcohols, especially benzylic alcohols.

In this study, we described the divergent synthesis of (+)-tanikolide and its analogues, such as (4S)- and (4R)-hydroxytanikolides, and nortanikolide, employing a stereoselective dirhodium(II)-catalyzed reaction to construct the quaternary chiral center of tanokolides. The key steps involve (a) a dirhodium(II)-catalyzed oxonium ylide formation–[2,3]-sigmatropic rearrangement, (b) an N-heterocyclic carbene-catalyzed ring-expansion lactonization of tetrahydrofurfural, or (c) an oxidative cleavage of tetrahydrofuran-5-methanol to γ-lactone using a 2-iodobenzamide catalyst. This route would provide high flexibility for analogue synthesis because the long side chain can be introduced at a later stage in the synthesis.

Summary Direct C H bond functionalization reactions promise to be among the most efficient methods for the synthesis of organic molecules. This review focuses on recent syntheses of natural products via chemo-, regio-, and stereoselective Rh-catalyzed C H insertion reactions.

金沢大学医薬保健研究域薬学系, Before entering host cells, herpes simplex virus-1 uses its envelope glycoprotein B to bind paired immunoglobulin-like type 2 receptor α (PILRα) on immune cells. PILRα belongs to the Siglec (sialic acid (SA)-binding immunoglobulin-like lectin)- like family, members of which bind SA. PILRα is the only Siglec member to recognize not only the sialylated O-linked sugar T antigen (sTn) but also its attached peptide region. We previously determined the crystal structure of PILRα complexed with the sTn-linked glycopeptide of glycoprotein B, revealing the simultaneous recognition of sTn and peptide by the receptor. However, the contribution of each glycopeptide component to PILRα binding was largely unclear. Here, we chemically synthesized glycopeptide derivatives and determined the thermodynamic parameters of their interaction with PILRα. We show that glycopeptides with different sugar units linking SA and peptides (i.e. "GlcNAc-Type" and "deoxy- GlcNAc-Type" glycopeptides) have lower affinity and more enthalpy-driven binding than the wild type (i.e. GalNAc-Type glycopeptide). The crystal structures of PILRα complexed with these glycopeptides highlighted the importance of stereochemical positioning of the O4 atom of the sugar moiety. These results provide insights both for understanding the unique O-glycosylated peptide recognition by the PILRα and for the rational design of herpes simplex virus-1 entry inhibitors. © 2017 by The American Society for Biochemistry and Molecular Biology, Inc.

An efficient route to highly substituted indoles was developed. It included regioselective functionalization of tetrahydroindol-4(5H)-ones, prepared by ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes with primary amines, and subsequent oxidation. The 6-substituted indoles were synthesized from a readily available 5-substituted cyclohexane-1,3-dione-2-spirocyclopropane. The synthesis of 5- and 7-substituted indoles was achieved by regioselective electrophilic alkylation of tetrahydroindol-4(5H)-one, followed by oxidation. The 4-substituted indoles were synthesized by nucleophilic alkylation of the corresponding pyrrole derivative, which was prepared by partial oxidation of tetrahydroindol-4(5H)-one, and sequential oxidation. The synthesis of 4-substituted indoles was also accomplished by palladium-catalyzed coupling of 4-hydroxyindole-derived triflates. Furthermore, the synthesis of 4,5,6,7-tetrasubstituted indoles was achieved by using these regioselective alkylations.

A concise formal synthesis of (±)-aspidospermidine via Stork's intermediate, which could be used as a divergent synthesis of Aspidosperma alkaloids, was achieved by employing a ring-opening cyclization of spirocyclopropane with amine followed by a regioselective intramolecular/intermolecular alkylation sequence. Stork's intermediate was synthesized in only six steps from a simple starting material, 1,3-cyclohexanedione, and was converted into (±)-aspidospermidine. To the best of our knowledge, this synthesis of Stork's intermediate involves the least number of steps to date. Furthermore, no protecting groups were used during this synthesis.

Total synthesis of myriocin and mycestericin D was achieved using the Du Bois Rh(II)-catalyzed C H amination of a sulfamate and subsequent alkylation as a key step. The reaction of a sulfamate with PhI(OAc)2 and MgO in the presence of Rh2(OAc)4 gave oxathiazinane N,O-acetal as the sole product in high yield. Alkylation of N,O-acetal using vinylmagnesium bromide in the presence of ZnCl2 proceeded stereoselectively to provide an oxathiazinane bearing a quaternary chiral center in high yield. Myriocin and mycestericin D were synthesized from a common synthetic intermediate. This route includes the first application of the Du Bois procedure for constructing a quaternary chiral center.

Total synthesis of sphingofungin E and 4,5-di-epi-sphingofungin E was achieved from an intermediate same as that of myriocin and mycestericin D via antipodal stereoselective dihydroxylations.

Acid-catalyzed ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes under metal-free conditions proceeded smoothly at room temperature to provide 2-aryl-3,5,6,7-tetrahydro-1-benzofuran-4(2H)-ones in excellent yields without the formation of 3-substituted isomers. The obtained product was converted into a 2-arylbenzofuran derivative via a synthetically useful 2-aryl-2,3-dihydrobenzofuran intermediate. Furthermore, the first total synthesis of cuspidan B was achieved by using the present method.

Iodoarene catalysts immobilized on magnetite (Fe3O4) were developed. The reactivity of catalysts 4 a and 4 b were similar to that of 4-iodophenoxyacetic acid (3) for the oxidation of 4-alkoxyphenols in the presence of Oxone® as a co-oxidant to give the corresponding p-quinones in high yields. In addition, 4 b was readily recovered by the use of an external magnet and was recycled up to eight times. This is the first example of a magnetic, nanoparticle-supported iodoarene catalyst.

An environmentally friendly oxidative cleavage of tetrahydrofuran-2-methanols to the corresponding γ-lactones using a catalytic amount of 2-iodo-N-isopropylbenzamide has been developed. The reaction of various tetrahydrofuran-2-methanols with the catalyst in the presence of Oxone® (2 KHSO5⋅KHSO4⋅K2SO4) as a co-oxidant in DMF at room temperature successfully affords the corresponding lactones in good to high yields, and recovery of the catalyst is readily accomplished using a reductive work-up. This method is notable because it enables the transformation of tetrahydrofuran-2-methanols to γ-lactones under mild conditions without the use of any toxic heavy metals.

An efficient and practical synthesis of 2',3'-nonsubstituted cyclohexane-1,3-dione-2-spirocyclopropanes using a sulfonium salt was achieved. The reaction of 1,3-cyclohexanediones and (2-bromoethyl)diphenylsulfonium trifluoromethanesulfonate with powdered K2CO3 in EtOAc at room temperature (r.t.) provided the corresponding spirocyclopropanes in high yields. The synthetic method was also applied to 1,3-cyclopentanedione, 1,3-cycloheptanedione, 1,3-indanedione, acyclic 1,3-diones, ethyl acetoacetate, and Meldrum's acid.

The practical syntheses of pachastrissamine (jaspine B), 2-epi-pachastrissamine, and the 2-epimer of the pyrrolidine analogue were accomplished via the stereoselective reduction of an allylketone derived from commercially available diethyl D-tartrate and the cross-metathesis of an allyltetrahydrofuran or allypyrrolidine with 1-tridecene as key steps.

A versatile synthetic procedure for a sulfur analogue of pachastrissamine (jaspine B), which involves the tandem thiolation-cyclization of a 1,4-ditosylate to construct a tetrahydrothiophene ring, was developed. Nucleophilic amino substitution of a tetrahydrothiophene-4-sulfonate with unexpected retention of the configuration afforded the sulfur analogue of 4-epi-pachastrissamine.

The stereoselective total synthesis of myriocin was achieved by using the Du Bois RhII-catalyzed C−H amination of sulfamate 6, derived from diethyl l-tartrate, and a subsequent alkylation. The reaction of sulfamate 6 with PhI(OAc)2 and MgO in the presence of Rh2(OAc)4 gave an oxathiazinane N,O-acetal as the sole product in high yield. Alkylation of the N,O-acetal with vinylmagnesium bromide in the presence of ZnCl2 proceeded stereoselectively to provide an oxathiazinane bearing a quaternary chiral center in high yield. This route includes the first application of the Du Bois procedure for the construction of a quaternary chiral center.

An efficient synthesis of cyclohexane- and cyclopentane-1,3-dione-2-spirocyclopropanes from 1,3-cycloalkanediones using sulfonium salts was achieved. The reaction of 1,3-cycloalkanediones with (1-aryl-2-bromoethyl)-dimethylsulfonium bromides and powdered K 2 CO 3 in EtOAc provided the corresponding spirocyclopropanes in high yields. Furthermore, a one-pot synthesis of tetrahydroindol-4(5 H )-one from 1,3-cyclohexanedione was achieved using the present protocol and a sequential ring-opening cyclization of spirocyclopropane with a primary amine.

An asymmetric synthesis of (−)-E-δ-viniferin, a trans-resveratrol dimer natural product containing a dihydrobenzofuran ring, has been achieved by exploiting a Rh(II)-catalyzed intramolecular C–H insertion reaction of a diaryldiazomethane derivative as a key step. The C–H insertion under catalysis by dirhodium(II) tetrakis[N-tetrafluorophthaloyl-(S)-tert-leucinate], Rh2(S-TFPTTL)4, provided the 2,3-diaryl-2,3-dihydrobenzofuran core structure with perfect cis diastereoselectivity and 96% ee.

This review describes our recent efforts to develop efficient methods for the synthesis of heterocyclic compounds, such as indoles and benzofurans, employing ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes, which were prepared by the reaction of 1,3-cyclohexanediones with sulfonium salts. Ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes with primary amines proceeded at room temperature to provide 2-substituted tetrahydroindol-4(5H)-ones in good to excellent yield. The obtained product was readily converted into a 2-substituted 4-hydroxyindole derivative. Furthermore, acid-catalyzed ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes proceeded smoothly at room temperature to provide 2-substituted tetrahydrobenzofuran-4(2H)-ones in excellent yield. The obtained product was converted into a 2-substituted 4-hydroxybenzofuran derivative. The synthetic utility of this catalytic protocol was demonstrated by the total synthesis of cuspidan B.

The first catalytic asymmetric intramolecular 1,6-C–H insertion reaction of α-diazo esters is described. With dirhodium(II) tetrakis[N-phthaloyl-(S)-tert-leucinate], Rh2(S-PTTL)4, the C–H insertion proceeded in a chemoselective manner to give 2-alkenyltetrahydropyran-3-carboxylates with up to 95% ee and perfect cis diastereoselectivity.

The synthesis of 10b-fluorinated analogues of protubonine A and its 11a-epimer was accomplished using fluorocyclisation of tryptophan-containing dipeptides with N-fluoro-2,4,6-trimethylpyridinium triflate to 3a-fluoropyrrolo[2,3-b]indoles as a key step. Acetylation of the indole nitrogen and the following diketopiperazine formation gave the 10b-fluorinated analogues of protubonine A and its 11a-epimer.

A 2-iodobenzamide catalyst for the oxidation of alcohols was developed. Reaction of alcohols with a catalytic amount of 2-iodobenzamide in the presence of oxone as a co-oxidant at room temperature gave the corresponding carbonyl compounds in excellent yields. Primary alcohols were directly converted into the corresponding carboxylic acids, whereas secondary alcohols were oxidized to ketones. A 2-iodobenzoic acid-type catalyst for alcohol oxidation at room temperature has not been reported before.

An asymmetric total synthesis of (–)-trans-blechnic acid, a dihydrobenzofuran neolignan, has been achieved. The key steps involve an elaboration of the cis-2,3-dihydrobenzofuran core structure by enantio- and diastereoselective intramolecular C–H insertion using dirhodium(II) tetrakis[N-phthaloyl-(R)-tert-leucinate] [Rh2(R-PTTL)4] and a direct coupling of an acrylate unit with the core structure employing Yu’s palladium(II)-catalyzed intermolecular C–H olefination.

Transformation of oxacycle-2-methanols to the corresponding lactones is often applied to total synthesis of natural products. It is accomplished by either direct oxidative cleavage reaction using a heavy metal oxidant or multistep conversion. As a practical and environmentally friendly procedure, a direct oxidative cleavage reaction of tetrahydrofuran-2-methanols to the corresponding γ-lactones was developed. This new catalytic reaction proceeds with a catalytic amount of 2-iodobenzamide and stoichiometric Oxone (2KHSO5·KHSO4·K2SO4) at room temperature.

Iodoarene oxidative catalysts immobilized on magnetite (Fe3O4) were developed. The catalysts showed reactivities similar to that of 4-iodophenoxyacetic acid for the oxidation of 4-alkoxyphenols in the presence of Oxone® as a co-oxidant. In addition, they were easily recovered by the use of an external magnet and could be recycled up to eight times. The oxidation of various 4-alkoxyphenols with the catalyst proceeded smoothly at room temperature to give the corresponding p-quinones in good to high yields. This is the first example of a magnetic nanoparticle-supported iodoarene catalyst.

Acid-catalyzed ring-opening cyclization of cyclohexane-1,3-dione-2-spirocyclopropanes under metal-free conditions proceeded smoothly at room temperature to provide 2-aryl-3,5,6,7-tetrahydro-1-benzofuran-4(2H)-ones in excellent yields without the formation of 3-substituted isomers. The obtained product was converted into a 2-arylbenzofuran derivative via a synthetically useful 2-aryl-2,3-dihydrobenzofuran intermediate. Furthermore, the first total synthesis of cuspidan B was achieved by using the present method.

The total synthesis of (+)-tanikolide was accomplished by a traceless stereoinduction method using the key steps of a Rh(II)-catalyzed oxonium ylide formation–[2,3]-sigmatropic rearrangement and an N-heterocyclic carbene-catalyzed ring-expansion lactonization of tetrahydrofurfural. This synthetic route is applicable to the divergent synthesis of tanikolide analogues.

Iodoarene catalysts immobilized on magnetite (Fe3O4) were developed. The reactivity of catalysts 4 a and 4 b were similar to that of 4-iodophenoxyacetic acid (3) for the oxidation of 4-alkoxyphenols in the presence of Oxone® as a co-oxidant to give the corresponding p-quinones in high yields. In addition, 4 b was readily recovered by the use of an external magnet and was recycled up to eight times. This is the first example of a magnetic, nanoparticle-supported iodoarene catalyst.

An environmentally friendly oxidative cleavage of tetrahydrofuran-2-methanols to the corresponding γ-lactones using a catalytic amount of 2-iodo-N-isopropylbenzamide has been developed. The reaction of various tetrahydrofuran-2-methanols with the catalyst in the presence of Oxone® (2 KHSO5⋅KHSO4⋅K2SO4) as a co-oxidant in DMF at room temperature successfully affords the corresponding lactones in good to high yields, and recovery of the catalyst is readily accomplished using a reductive work-up. This method is notable because it enables the transformation of tetrahydrofuran-2-methanols to γ-lactones under mild conditions without the use of any toxic heavy metals.

The immobilization of dirhodium(II) tetrakis[N-tetrafluorophthaloyl-(S)-tert-leucinate], [Rh2(S-TFPTTL)4], has been accomplished by copolymerization of a dirhodium(II) complex-containing monomer with styrene and 1,6-bis(4-vinylbenzyloxy)hexane as a flexible cross-linker. The polymer-supported chiral fluorinated dirhodium(II) complex catalyzed the amination of silyl enol ethers with [N-(2-nitrophenylsulfonyl)imino]phenyliodinane (NsNIPh) to provide α-amino ketones in high yields with high levels of enantioselectivity and could be used up to 20 times as the catalyst readily withstood stirring in the presence of the solid reactant.

TMSOTf-promoted glycosidation of 2-azido-4,6-O-benzylidene-2-deoxygalactosyl diphenyl phosphates with fluorenylmethoxycarbonyl (Fmoc)-protected serine and threonine derivatives in THF/Et2O (1:1) gave glycosyl amino acids in high yields and with excellent levels of α-selectivity (α/β=94:6–95:5). The synthetic utility of the present glycosidation method was demonstrated by a stereoselective synthesis of mucin-type glycopeptide core 5 and core 7 building blocks, which are suitable for Fmoc-based solid-phase synthesis of O-glycopeptides.

The first example of enantioselective intermolecular cycloaddition of carbonyl ylides with indoles is described. The cycloaddition of five- and six-membered carbonyl ylides derived from diazodiketoesters with N-methylindoles under catalysis by dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(S)-tert-leucinate], Rh(2)(S-TCPTTL)(4), gave cycloadducts in high yields and with high levels of enantioselectivity (up to 99% ee) as well as excellent exo diastereoselectivity.

Tetrabromophthaloyl-protected tert-leucine represents an efficient ligand for the Rh-catalyzed asymmetric cyclopropenation of α-alkyl-α-diazoacetates with terminal alkynes.

An enantioselective C–H functionalization of N -methoxymethyl (MOM)-protected 2,3-unsubstituted indoles with α-diazopropionates has been effected via catalysis by dirhodium(II) tetrakis[ N -phthaloyl-( S )-triethylalaninate], Rh 2 (( S )-PTTEA) 4 , providing α-methyl-3-indolylacetates in high yields and with enantioselectivities of up to 86% ee. The effectiveness of this protocol was demonstrated by the first catalytic asymmetric synthesis of the (+)-α-methyl-3-indolylacetic acid fragment of acremoauxin A, a potent plant-growth inhibitor. Furthermore, the Fujioka protocol using a combination of TMSOTf and 2,2′-bipyridyl was shown to be superior for the removal of the N -MOM group.

A catalytic asymmetric synthesis of descurainin has been achieved by incorporating an enantioselective 1,3-dipolar cycloaddition, a stereoselective alkene hydrogenation, an oxidation with Fremy’s salt and a regioselective demethylation with NbCl5 as the key step. The 1,3-dipolar cycloaddition of a carbonyl ylide derived from tert-butyl 2-diazo-5-formyl-3-oxopetanoate with 4-hydroxy-3-methoxyphenylacetylene in the presence of dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(R)-tert-leucinate], Rh2(R-TCPTTL)4, provided an 8-oxabicyclo[3.2.1]octane skeleton in 95% ee.