1,6-Conjugate addition of C-nucleophiles to p-quinone methide surrogate: Synthesis of diarylpropanes

1,6-Conjugate addition of various carbon nucleophiles to p-quinone methide surrogate is reported. The active methylene containing C-nucleophiles such as 1,3-diketones, diesters and ketoesters underwent two consecutive additions leading to bis-addition products, diarylpropanes. Whereas, nitroalkanes proceeded to contribute mono-addition products. Surprisingly, bromo analogue of p-quinone methide surrogate underwent halophilic dimerization reaction with excellent yield. The reported strategy provides an efficient synthesis of 1,3-diarylpropane derivatives bearing pharmacophoric ortho-methoxy phenol moiety under mild reaction conditions.     

Retro-Mannich reactions of 3-alkyl-3,4-dihydro-2H-1,3-benz[e]oxazines and the synthesis of axially chiral resorcinarenes

Intermediates involved in the conversion of 3-alkyl-3,4-dihydro-2H-1,3-benz[e]oxazines into 2-N,N-dialkylaminomethylphenol derivatives, using morpholine and other high boiling secondary amines, have been identified and characterised. Additional experiments have established the involvement of o-quinone methide intermediates in the retro-Mannich reactions. Axially chiral resorcinarenes have been prepared by utilising the exchange reactions.     

4,4′-Dihydroxy-3,3′,5,5′-tetramethoxyazodioxybenzene: an unexpected dimer formed during hydroxylamine extractions of wheat flour

Neutral hydroxylamine extracts of wheat contained a product that was colourless at pH<5 (λ_max 340 nm) and yellow at pH>9 (λ_max 400 nm). ESI-MS showed a major ion m/z 184.0 and a possible parent ion m/z 367.2 (MH⁺) suggesting that the product resulted from the reaction of 2,6-dimethoxy-p-quinone with hydroxylamine. However, mass spectral and other spectroscopic data indicated that the compound was neither of the 2,6-dimethoxy-p-quinone oximes. A product with identical absorbance, mass spectrum, electrophoretic mobility and HPLC retention time as the pigment from hydroxylamine extracts of flour was observed amongst the reaction products of hydroxylamine and 1,4-dihydroxy-2,6-dimethoxybenzene. The structure of this product was identified by NMR, 2D NMR and IR as 4,4′-dihydroxy-3,3′,5,5′-tetramethoxyazodioxybenzene.     

Aza-Ortho-Quinone Methides as Reactive Intermediates: Generation and Utility in Contemporary Asymmetric Synthesis

The aza-ortho-quinone methide (aza-o-QM) chemistry has overwhelmingly progressed in the past few decades. This review aims to integrate various transition metal-catalyzed and organocatalytic strategies in taming aza-o-QM intermediates, including the aza-ortho-vinylidene quinone methide (aza-o-VQM), aza-ortho-alkynyl quinone methide (aza-o-AQM), aza-para-quinone methide (aza-p-QM), and indole-based aza-o-QM analog. These transient species are often utilized for the direct and enantioselective synthesis of complex (hetero)polycyclic or fused-ring molecular scaffolds such as tetrahydroquinoline and indoline, among others, which are abundant in many natural products, bioactive compounds, and pharmaceuticals.     

Synthesis of α-(4-hydroxyphenyl)-β,β,β-trifluoro-α-alanine derivatives

2,6-Dimethyl- and 2,6-dimethoxy-α-methoxycarbonyl-α-trifluoromethyl-p-methylenequinones have been synthesized. These compounds easily react with ammonia, morpholine, andp-toluidine to afford the corresponding α-(4-hydroxyphenyl)-β,β,β-trifluoro-α-alanine derivatives.     

Stereoselective synthesis of Heliannuol G

The first stereoselective synthesis of Heliannuol G has been achieved employing an o-quinone methide intermediate. The present synthesis triumphs over all the drawbacks associated with the reported one and reveals an impressive overall yield of 12.37%.     

Microwave-promoted solvent-free synthesis of para-quinone methides (p-QMs) derivatives

A rapid and efficient methodology for the synthesis of polysubstituted para-quinone methides (p-QMs) from aldehyde and 2,6-di-tert-butylphenol has been achieved in solvent-free and microwave irradiation condition within 33 min. This strategy displays the advantages including high atom economy, good functional group tolerance, and environmentally friendly operation.     

Synthesis of a Masked p -Quinone Methide β -Lactam as an active metabolite of nocardicins

Nocardicin A analogues 30, 34 , and 38 as well as the highly strained quinone methide 43 were synthesized. β -Lactam 34 was found biologically active against several Gram-negative microorganisms in vitro; pyridinium N-oxide derivative 38 possessed activity against Gram-positive S. aureus bacterium. Masked p-quinone methide β -lactam 43 exhibited significant antimicrobial activity in vitro. A mechanism involving an oxidation in vivo is proposed for the unprecedented biological properties of nocardicins.     

Mild and rapid method for the generation of o-quinone methide intermediates. Synthesis of puupehedione analogues

A route to simpler analogues to bioactive puupehedione derivatives involving a hetero Diels-Alder cycloaddition of a o-quinone methide is described. These intermediate species are generated via fluoride-induced desilylation of silyl derivatives of o-hydroxybenzyl iodides. Remarkable short reaction times and very mild experimental conditions are the main features of this method.     

Carbon-13 NMR of quinone methides

The ¹³C NMR spectra of four ortho- and seven para-quinone methides were assigned using chemical shift and long-range carbon-proton coupling information. The carbonyl shifts are compared with those in ortho- and para-benzoquinones. The chemical shifts of the carbonyls of the p-quinone methides are observed at δ 186.2–186.4 for the three ortho-di-tert-butyl-substituted compounds and at δ 180.7–181.5 for the four ortho-oxy-substituted compounds. In the three o-quinone methides with meta, para-dioxy substituents, the carbonyl signals are at δ 184.2–185.4. The carbonyl signal of the one o-quinone methide with no oxygen substitution is shifted downfield to δ 200.9, apparently as a result of hydrogen bonding to the nearby hydroxyl.     

A two-step biomimetic synthesis of antimalarial robustadials A and B

The antimalarial robustadials A and B have been synthesized in two steps starting from commercially available phloroglucinol comprising a key biomimetic three-component reaction that involves in situ generation of an o-quinone methide via Knoevenagel condensation and subsequent Diels-Alder cycloaddition with (−)-β-pinene.     

Synthesis of mannich bases of bioactive benzylphenols

2,4-Bis(1,1-dimethyl)-6-[(4-methoxyphenyl)methoxymethyl]phenol ( 4 ), prepared by oxidation of 2,4-bis(1,1-dimethylethyl)-6-[(4-methoxyphenyl)methyl]phenol ( 1 ) with silver oxide in methanol, reacts with secondary amines in boiling toluene to give Mannich bases ( 6 ) related to the biologically active o-benzylphenol. Mannich basis of the isomeric p-benzylphenol ( 7 ) were prepared by reaction of amines with the p-quinone methide formed by oxidation of 7 .     

Base-catalyzed Schmittel cycloisomerization of o-phenylenediyne-linked bis(arenol)s to indeno[1,2-c]chromenes

We describe the transition metal-free base-catalyzed Schmittel cycloisomerization reactions of o-phenylene-linked bis(arenol)s to indeno[1,2-c]chromene derivatives through prototropic rearrangement (tautomerization) to a putative vinylidene o-quinone methide intermediate with an enyne-allene system followed by a formal inverse-electron-demand hetero Diels–Alder cycloaddition. The preliminary results on catalytic asymmetric cycloisomerization with chiral bases are also disclosed.     

o-Quinone methide with overcrowded olefin component as a dehydridation catalyst under aerobic photoirradiation conditions

An o-quinone methide (o-QM) featuring an overcrowded olefinic framework is introduced, which exhibits dehydridation activity owing to its enhanced zwitterionic character, particularly through photoexcitation. The characteristics of this o-QM enable the operation of dehydridative catalysis in the oxidation of benzylic secondary alcohols under aerobic photoirradiation conditions. An experimental analysis and density functional theory calculations provide mechanistic insights; the ground-state zwitterionic intermediate abstracts a hydride and proton simultaneously, and the active oxygen species facilitate catalyst regeneration.

An o-quinone methide (o-QM) featuring an overcrowded olefinic framework is introduced, which exhibits dehydridation activity owing to its enhanced zwitterionic character, particularly through photoexcitation.     

Reaction of 2-chloromethylphenols with enaminones

The reaction of o-(chloromethyl)phenols with enamino ketones afforded a series of 3-acyl- and 3-formyl-4H-chromenes as a result of cascade transformation including [4 + 2]-cycloaddition of enamino ketone to o-quinone methide generated in situ and subsequent elimination of secondary amine.     

Transition metal complexes with 2,6-Di-tert-butyl-p-quinone 1′-phthalazinylhydrazone

2,6-Di-tert-butyl-p-quinone 1′-phthalazinylhydrazone (HL) was synthesized. Quantum-chemical calculations of the energy of possible tautomeric forms of the hydrazone were performed. The complexes of Zn(II), Hg(II), Ni(II), and Cu(II) of ML₂ composition were obtained and studied. The structure of the NiL₂ complex was established by XRD. It was shown by DFT-D3 calculations that the cis-structure of the complex is stabilized due to the interligand dispersion interaction.     

Diphosphination of ortho-quinone methide precursors with diphosphines

A fluorine anion-mediated diphosphination of ortho-quinone methide precursors (2-(chloromethyl)silyloxybenzenes) with diphosphines has been developed. The reaction proceeds smoothly under mild conditions (CH₂Cl₂ solvent, 0 °C) to form the corresponding 2-(phosphinomethyl)oxyphosphinobenzenes, which are potential bidentate ligands in metal catalysis. Additionally, some mechanistic investigations are also performed.     

Electrochemical Investigation of some Flavonoids in Aprotic Media

Cyclic voltammerty data for the oxidation of 15 flavonoids in acetonitrile have been presented. Up to three peaks in different potential regions appear in the CV curves, depending on the structure of a flavonoid. The peak in the first potential region (+0.79÷+1.03 V vs SCE) occurs when a flavonoid molecule has an o-dihydroxy moiety in the ring A or B. From the data in the literature, the formation of o-quinone is responsible for the occurrence of the peak. When the o-dihydroxy moiety is in ring B and the hydroxyl group is at position 3, the second peak in the potential region+1.08÷+1.32 V, is present. It is an agreement that the peak is due to oxidation of benzofuranone derivative formed in reaction of o-quinone with water. Molecules with an o-quinone moiety located in ring A seem to be less reactive, and the peak in the second potential region does not occur. However, the existence of an o-quinone in the ring B is not necessary for the peak in the second potential region to be present. The oxidation of kaempferol and morin, which have hydroxyl groups at positions 3 and 4’, takes place at 0.98 V and 0.97 V, respectively, and leads to the formation of p-quinone methide. We have shown that the peak in the second potential region also appears when the hydroxyl group in the flavonoid molecule is only at position 3 or at position 4’. For 3-hydroxyflavone and apigenin, which are oxidized at +1.10 V and 1.49 V respectively, p-quinone methide was detected after controlled potential bulk electrolysis. A mechanism which includes the reaction of the carbon radical with a trace of water has been proposed. For flavonoids that have an o-dihydroxy moiety in ring B and the hydroxyl group at position 3, two oxidation peaks are observed in the CV curves in the second potential region for low scan rates (below 0.1 V s⁻¹). An explanation of this behavior has been proposed.     

Convenient one-step synthesis of 4-unsubstituted 2-amino-4H-chromene-2-carbonitriles and 5-unsubstituted 5H-chromeno[2,3-b]pyridine-3-carbonitriles from quaternary ammonium salts

We have reported DBU catalyzed synthesis of 4-unsubstituted 2-amino-4H-chromene-2-carbonitriles in water under reflux. The attractive features of this process are mild reaction conditions, short reaction times, easy isolation of products and good yields. 5H-chromeno[2,3-b]pyridine-3-carbonitriles were obtained by refluxing excess of malononitrile and quaternary ammonium salts in ethanol in the presence of NaOH as catalyst. The mechanisms of these reactions are believed to involve the formation of the o-quinone methide intermediate.     

Facile Approach for the Synthesis of 2,3,4,9-Tetrahydro-1H-xanthen-1-ones and 8,9,10,12-Tetrahydro-11H-benzo[a]xanthen-11-ones via Trapping of o-Quinone Methides

An efficient, simple synthesis of 2,3,4,9-tetrahydro-1H-xanthene-1-ones and 8,9,10,12-tetrahydro-11H-benzo[a]xanthen-11-ones is reported by one-pot condensation of 3-dimethylamino-2-cyclohexen-1-ones with hydroxybenzyl alcohols, phenol, and 2-naphthol Mannich bases or their quaternized derivatives. The mechanism of the reaction is believed to involve the formation of the o-quinone methide intermediate.