Formation Of O-quinone Methide Research Articles

Unique assembly of carbonylpyridinium and chromene reveals mitochondrial thiol starvation under ferroptosis and novel ferroptosis inducer.

To reveal the delicate function of mitochondria, spatiotemporally precise detection tools remain highly desirable. However, current probes with positively charged warheads for targeting mitochondria diffuse out of the mitochondria as the potential of the mitochondrial membrane changes, which directly influences the accuracy of the detection. Herein, we assembled carbonylpyridinium and chromene to afford the probe CM-Mit. Following the ultrafast response to thiol and the dissociation of carbonylpyridinium, the formation of o-quinone methide from CM-Mit was proposed to label proteins, thus avoiding diffusion out of the mitochondria. Therefore, the accurate spatiotemporal detection of thiol in mitochondria was realized. With this excellent probe, ferroptosis inducers were proved to stimulate thiol starvation in mitochondria for the first time in cancer cells. Moreover, CM-Mit was used to screen a compound library developed in-house and the stemona alkaloid analog SA-11 was shown to induce ferroptosis in various cancer cell lines, including a drug-resistant one.

NaHSO4/SiO2 catalyzed generation of o-quinone/ o-thioquinone methides: synthesis of arylxanthenes/ arylthioxanthenes via oxa-6π-electrocyclization.

ortho-Quinone methides, very reactive transient intermediates, are utilized successfully in synthesizing complex organic molecules of natural and biological significance. Among several synthetic protocols, the acid catalyzed generation of ortho-quinone methides from suitably substituted phenols is a promising method for further exploitation in organic synthesis. Such an interesting reactive species is conveniently employed in the synthesis of conformationally restricted triarylmethane derivatives such as 12/9-arylxanthenes/arylthioxanthenes starting from symmetrical/unsymmetrical 2-(hydroxydiarylmethyl)phenol/thiophenol, respectively, using SiO2-NaHSO4. Conformationally restricted 12/9-arylxanthenes/arylthioxanthenes were obtained in 52 to 96% yields using this protocol, which is believed to involve the formation of o-quinone methides followed by electrocyclic ring closure and isomerization at elevated temperature. Photophysical studies of selected examples in acidic media showed turn-on fluorescence by hydride ion transfer mediated π-conjugated xanthylium salt formation and suggested the application potential in bio-imaging and fluorescent sensors.

6-Hydroxyaurone aminomethyl derivatives in the inverse electron-demand Diels–Alder reaction

The transformations of 7-dimethylaminomethyl-6-hydroxyaurone and 5-dimethylaminomethyl-6-hydroxy-7-methylurone in the inversed electron-demand hetero-Diels–Alder reaction were studied. As a result of in situ thermal formation of o-quinone methides containing the benzofuranone moiety and cycloaddition of cyclic vinyl esters, 2-benzylidene derivatives of difuro[2,3-b:2',3'-f]chromen-3(2H)-one, furo[2,3-f]pyrano[2,3-b]chromen-3(2H)-one, difuro[2,3-b:3',2'-g]chromen-3(2H)-one, and furo[3,2-g]pyrano[2,3-b]chromen-3(2H)-one heterocyclic systems were synthesized. A similar reaction with cyclic enamines, accompanied by subsequent transformation of hemi-aminals, led to the formation of partially hydrogenated derivatives of furo[2,3-a]xanthen-3(2H)-one and furo[3,2-b]xanthen-3(2H)-one.

Catalytic Asymmetric Conjugate Addition of Indoles to para-Quinone Methide Derivatives.

A catalytic asymmetric conjugate addition of indoles to o-hydroxyphenyl substituted p-quinone methides has been established in the presence of chiral phosphoric acid, which afforded chiral indole-containing triarylmethanes in generally high yields (54-98%) and good enantioselectivities (90:10-96:4 enantiomeric ratio). The control experiments indicated that o-hydroxyphenyl substituted p-quinone methides had a high possibility to transform into o-quinone methides in the presence of chiral phosphoric acid, and the formation of o-quinone methides might be a necessity for the reaction. This reaction will not only contribute to the research field of catalytic asymmetric transformations of p-quinone methides and o-quinone methides but also provide a useful method for the construction of enantioenriched triarylmethane frameworks.

Microwave-assisted One-pot Synthesis of 7-Dimethylamino-4-Aryl-2- methylamino-3-nitro-4H-chromenes

4-Aryl-2-amino-4H-chromenes possessing N,N-dimethylamino group have been reported as potential anticancer drugs. Despite few synthetic methods reported in the literature for their synthesis, there appear to be no reports on the direct use of N,N-dimethyl-3-aminophenol for the synthesis of 4- aryl-2-methylamino-3-nitro-4H-chromenes. One-pot condensation of N,N-dimethyl-3-aminophenol, aromatic aldehydes and (E)-N-methyl-1-(methylthio)-2-nitro-ethenamine was carried out using MW irradiation to get the 4-aryl-2-methylamino-3-nitro-4H-chromenes under catalyst-free conditions. This transformation presumably occurs via o-quinone methide formation, Michael addition-intramolecular O-cyclization-elimination sequence of reactions creating new two C-C bonds and one C-O bond. Various substituted aromatic aldehydes reacted smoothly with N,N-dimethyl-3-aminophenol and (E)-Nmethyl- 1-(methylthio)-2-nitro-ethenamine to give the corresponding 4-aryl-2-methylamino-3-nitro-4Hchromenes in good yields. We have developed a one-pot three component condensation of N,Ndimethyl- 3-aminophenol, aromatic aldehyde and NMSM for the synthesis of N,N-dimethylamino substituted 4-aryl-2-methylamino-3-nitro-4H-chromenes in good yields. The significant features of this reaction include catalyst-free, solvent free, no column chromatographic purification, short reaction time and good yields.

Total Synthesis and Structure Revision of (-)-Siphonodictyal B and Its Biomimetic Conversion into (+)-Liphagal.

The structure of siphonodictyal B has been reassigned on the basis of the total synthesis of both possible C-8 epimers. The revised structure of siphonodictyal B was converted into liphagal by acid catalyzed rearrangement of a proposed epoxide intermediate. This biomimetic cascade features a succession of four distinct reactions (epoxidation, o-quinone methide formation, ring expansion, and benzofuran formation) that occur in a one-pot operation under mild conditions. During these studies we also isolated a surprisingly stable o-quinone methide that supports our mechanistic proposal for liphagal biosynthesis.

Toxic Metabolite Formation from Troglitazone (TGZ): New Insights from a DFT Study

The hepatotoxicity of Troglitazone (TGZ) has been ascribed to the formation of reactive metabolites, and the primary reactive metabolite of TGZ has been confirmed to be an o-quinone methide. Oxidation of the chromane moiety is also known to produce quinone containing metabolites. Quantum chemical studies have been performed to analyze the possible reaction pathways for the metabolism of the TGZ side chain, 6-hydroxy-2,2,5,7,8-pentamethylchromane (HPMC). From this analysis, a new pathway including oxidation at the C13 and C14 atoms of HPMC has been proposed for the formation of o-quinone methide (M2), while oxidation at the hydroxyl group leads to the formation of the quinone metabolite (M7). o-Quinone methide reactive metabolites have been shown to be more electrophilic at the reactive methylene center using quantum chemically estimated parameters.

Enantiopure 2,6-disubstituted piperidines bearing one alkene- or alkyne-containing substituent: preparation and application to total syntheses of indolizidine-alkaloids

A general and efficient procedure for the preparation of 2,6-disubstituted piperidines bearing one alkene- or alkyne-containing substituent was developed by using non-racemic Betti base as a chiral auxiliary. Many chiral benzylamines are excellent auxiliaries, but they were rarely used for this purpose because of the inefficient removal of the N-benzyl auxiliary residue under non-hydrogenative conditions. We found that N,N-disubstituted Betti base derivative has a typical Mannich structure of o-naphthol. When it carried out a base-catalyzed formation of o-quinone methide, an efficient non-hydrogenative N-debenzylation was achieved, and the alkene and alkyne groups survived. To demonstrate the efficiency of the method and the versatility of the products, asymmetric total syntheses of indolizidine-alkaloids (-)-167B, (-)-195H, (-)-209D and (-)-223AB were accomplished.

Photogeneration of o-quinone methides from o-cycloalkenylphenols.

6-Alkylidenecyclohexa-2,4-dienones (o-quinone methides II) have been generated by photolysis of 2-(2'-cycloalkenyl)phenols 1 and trapped by methanol to give the ring-opened products 2. The best results have been obtained with the cyclohexenyl derivatives 1a, 1e, and 1f. In the case of the cyclopentenyl derivative 1b, photoproduct 2b was not observed, whereas only small amounts of 2c and 2d were formed from the seven- and eight-membered ring analogues 1c and 1d. Thus, ring size appears to be a key factor in the formation of o-quinone methides. This experimental result has been rationalized by means of density-functional theory (DFT) calculations. On the other hand, phenol substitution also appears to play a role in the process. Thus, electron-withdrawing groups such as CF(3) (1f) accelerate the reaction, whereas the opposite is true for electron-donating groups such as OCH(3) (1e). This is explained by an excited-state intramolecular proton transfer (ESIPT) mechanism, as the above results are consistent with the excited-state acidities of the different phenols. The lack of reactivity in the case of ketone 1g, where the intersystem crossing quantum yield is close to unity, allows us to rule out a mechanism involving the triplet state.

O-Quinone methide formation from excited state intramolecular proton transfer (ESIPT) in an o-hydroxystyrene

The photohydration of o-hydroxy-α-phenylstyrene ( 7) has been studied using product studies, laser flash photolysis (LFP), and fluorescence measurements, and shown to react via excited state intramolecular proton transfer (ESIPT), to give the corresponding o-quinone methide ( 13) ( λ max = 310, 420 nm in 1 : 1 H 2O–CH 3CN). The same intermediate was also observed on LFP of o-hydroxybenzyl alcohol ( 10), a system known to give o-quinone methides efficiently, but via a different mechanism. ESIPT of 7 to give 13 is not significantly enhanced by water as the species was also observed in neat CH 3CN; addition of water (up to 1 : 1 H 2O–CH 3CN) did not increase its yield substantially. Efficient ESIPT of 7 in neat CH 3CN is corroborated by its low fluorescence emission quantum yield and subnanosecond fluorescence lifetime. These results provide new insights into ESIPT from phenol to alkene carbon, a process that has not received much attention although 7 is a close relative of o-hydroxybenzophenone ( 8), an archetypical system for the illustration of ESIPT.

Synthesis and Thermolysis of Enediynyl Ethyl Ethers as Precursors of Enyne−Ketenes

Enediynyl ethyl ethers 14/17 were synthesized by using the Pd(PPh3)4-catalyzed cross-coupling reactions between enynyl iodides 13/16 and (2-ethoxyethynyl)zinc chloride. Thermolysis of these enediynyl ethyl ethers in refluxing chlorobenzene (132 °C) promoted a retro-ene reaction to produce enyne−ketenes, which underwent the Moore cyclization reactions to form the biradicals having a phenyl radical center and a phenoxy radical center. The presence of two radical centers in the same molecule simultaneously provided many opportunities for intramolecular decay through disproportionation, radical−radical combination, and the formation of o-quinone methide.

Novel Generation of o-Quinone Methides on the Basis of Lewis Acid Catalyzed 1,4-Dethiolation of o-(1-(Alkylthio)alkyl)phenols

Abstract The treatment of o-(1-(alkylthio)alkyl)phenols with Lewis acid resulted in the formation of o-quinone methides by the novel 1,4-dethiolation process, which, being activated with Lewis acid, reacted in situ with various dienophiles in the manner of Diels–Alder reaction in high efficiency. o-Quinone allide, provided from o-(1-(alkylthio)allyl)phenol, also behaved as heterodienes.

Reactions of aryl chromium carbene complexes with alkoxalkyne: O-Quinonemethide formation and unusual diels-alder dimerization

Reactions of a pyrrole- and a phenyl-carbene chromium complexes with 3-alkoxy-1-ethoxy-1-butyne produced dimers through o-quinone-methide formation from the alkyne-carbene cycloaddition products and subsequent Diels-Alder dimerization.