Arylspiroborates Derived from 4‐tert‐Butylcatechol and 3,5‐Di‐tert‐butylcatechol and Their Antimicrobial Activities

A family of arylspiroborates has been prepared by the addition of either 4‐tert‐butylcatechol or 3,5‐di‐tert‐butylcatechol to boric acid and an alkali metal hydroxide. All compounds were characterized fully using multinuclear NMR spectroscopy and by elemental analyses. A single crystal X‐ray diffraction was carried out on potassium (bis‐(3,5‐di‐tertbutyl[1,2‐benzenediolato(2‐)‐O,O′]borate)) ( 8 ). All compounds displayed appreciable anti‐microbial activities.     

Electro‐Oxidation of 3,4‐Dihydroxybenzoic Acid in the Presence of 6‐Methyl‐1,2,4‐Triazine‐3‐Thione‐5‐One: Unique Synthesis of 7H‐Thiazolo[3,2‐b]‐1,2,4‐Triazin‐7‐One Derivative in Aqueous Media

The electrochemical oxidation of 3,4‐dihydroxy benzoic acid ( 1 ) has been studied in the presence of 6‐methyl‐1,2,4‐triazine‐3‐thione‐5‐one ( 2 ) in aqueous solution. The oxidation mechanism of 1 and its reaction in the presence of 2 was offered. It was confirmed that 1 is converted to 7H‐thiazolo[3,2‐b]‐1,2,4‐triazin‐7‐one derivative 5 through Michael addition reaction of 2 to anodically generated o‐benzoquinone. The results of the research were used for electrochemical synthesis of 5 in an undivided cell in good yield and purity.     

Crystal Structures,Magnetic Properties and Catecholase‐Like Activities of μ‐Alkoxo‐μ‐Carboxylato Double Bridged Dinuclear and Tetranuclear Copper(II) Complexes

One μ‐alkoxo‐μ‐carboxylato bridged dinuclear copper(II) complex, [Cu₂(L¹)(μ‐C₆H₅CO₂)] ( 1 )(H₃L¹ = 1,3‐bis(salicylideneamino)‐2‐propanol)), and two μ‐alkoxo‐μ‐dicarboxylato doubly‐bridged tetranuclear copper(II) complexes, [Cu₄(L¹)₂(μ‐C₈H₁₀O₄)(DMF)₂]·H₂O ( 2 ) and [Cu₄(L²)₂(μ‐C₅H₆O₄]·2H₂O·2CH₃CN ( 3 ) (H₃L² = 1,3‐bis(5‐bromo‐salicylideneamino)‐2‐propanol)) have been prepared and characterized. The single crystal X‐ray analysis shows that the structure of complex 1 is dimeric with two adjacent copper(II) atoms bridged by μ‐alkoxo‐μ‐carboxylato ligands where the Cu···Cu distances and Cu‐O(alkoxo)‐Cu angles are 3.5 11 Å and 132.8°, respectively. Complexes 2 and 3 consist of a μ‐alkoxo‐μ‐dicarboxylato doubly‐bridged tetranuclear Cu(II) complex with mean Cu‐Cu distances and Cu‐O‐Cu angles of 3.092 Å and 104.2° for 2 and 3.486 Å and 129.9° for 3 , respectively. Magnetic measurements reveal that 1 is strong antiferromagnetically coupled with 2J =‐210 cm^?1 while 2 and 3 exhibit ferromagnetic coupling with 2J = 126 cm^?1 and 82 cm^?1 (averaged), respectively. The 2J values of 1–3 are correlated to dihedral angles and the Cu‐O‐Cu angles. Dependence of the pH at 25 °C on the reaction rate of oxidation of 3,5‐di‐tert‐butylcatechol (3,5‐DTBC) to the corresponding quinone (3,5‐DTBQ) catalyzed by 1–3 was studied. Complexes 1–3 exhibit catecholase‐like active at above pH 8 and 25 °C for oxidation of 3,5‐di‐tert‐butylcatechol.     

Novel asymmetric 3,5‐bis(arylidene)piperidin‐4‐one derivatives: synthesis,crystal structures and cytotoxicity

3,5‐Bis(arylidene)piperidin‐4‐one derivatives (BAPs) display good antitumour activity because of their double α,β‐unsaturated ketone structural characteristics. Reported BAPs have generally been symmetric and asymmetric BAPs have been little documented. Three asymmetric BAPs, namely (5E)‐3‐(4‐tert‐butylbenzylidene)‐5‐(4‐fluorobenzylidene)‐1‐methylpiperidin‐4‐one, C₂₄H₂₆FNO, ( 5 ), (5E)‐3‐(4‐tert‐butylbenzylidene)‐5‐(3,5‐dimethoxybenzylidene)‐1‐methylpiperidin‐4‐one, C₂₆H₃₁NO₃, ( 6 ), and (5E)‐3‐{3‐[(E)‐(2,3‐dihydroxybenzylidene)amino]benzylidene}‐5‐(2‐fluorobenzylidene)‐1‐methylpiperidin‐4‐one, C₂₇H₂₃FN₂O₃, ( 12 ), were generated by Claisen–Schmidt condensation. They are characterized by NMR and FT–IR spectroscopies, and elemental analysis. Single‐crystal structure analysis reveals that the two arylidene rings on both sides of the BAP structures adopt an E stereochemistry of the olefinic double bonds and the compounds are E,E isomers. Molecules of ( 5 ) and ( 12 ) generate one‐dimensional chains through intermolecular hydrogen bonds, while compound ( 6 ) generates a two‐dimensional network through hydrogen bonds. Preliminary cytotoxicities toward human liver hepatocellular carcinoma cell line (HepG2), human acute mononuclear granulocyte leukaemia (THP‐1) and human normal hepatical cell line (LO2) were evaluated.     

Über die Oxo‐cyclotetraphosphane (PBut)4O1–4

Contributions to the Chemistry of Phosphorus. 243 On the Oxocyclotetraphosphanes (PBu^t)₄O_1–4 Under suitable conditions, the reaction of tetra‐tert‐butylcyclotetraphosphane, (PBu^t)₄, with dry atmospheric oxygen gives rise to the corresponding monoxide (PBu^t)₄O ( 1 ) which has been isolated by column chromatography. The reaction with hydrogen peroxide furnishes a mixture of oxocyclotetraphosphanes (PBu^t)₄O_1–4 consisting of two constitutionally isomeric dioxides (PBu^t)₄O₂ ( 2 a , 2 b ), the trioxide (PBu^t)₄O₃ ( 3 ), and the tetraoxide (PBu^t)₄O₄ ( 4 ), in addition to 1 . According to the ³¹P NMR parameters the oxygen atoms are exclusively exocyclically bonded to the phosphorus four‐membered ring. Which of the P atoms are present as λ⁵‐phosphorus follows from the different low‐field shifts of the individual P nuclei compared with the starting compound. Accordingly, 1 is 1,2,3,4‐Tetra‐tert‐butyl‐1‐oxocyclotetraphosphane, 2 a and 2 b are 1,2,3,4‐Tetra‐tert‐butyl‐1,2‐dioxo‐ and ‐1,3‐dioxocyclotetraphosphane, respectively, 3 is 1,2,3,4‐Tetra‐tert‐butyl‐1,2,3‐trioxocyclotetraphosphane, and 4 is 1,2,3,4‐Tetra‐tert‐butyl‐1,2,3,4‐tetraoxocyclotetraphosphane. When the oxidation reaction proceeds a fission of the P₄ ring takes place.     

Electrografting of 4‐tert‐Butylcatechol on GC Electrode. Selective Electrochemical Determination of Homocysteine

A new sensor based on the grafting of 4‐tert‐butylcatechol on the surface of a glassy carbon electrode (GC) was developed for the catalytic oxidation of homocysteine ( Hcy ). The GC‐modified electrode exhibited a reversible redox response at neutral pH. Under the optimum conditions cyclic voltammetric results indicated the excellent electrocatalytic activity of modified electrode toward the oxidation of Hcy at reduced over‐potential about 350 mV. A linear dynamic range of 0.01–3.0 mM and a detection limit of 1.0 µM were obtained for Hcy . The modified electrode was used as an electrochemical sensor for selective determination of Hcy in human blood.     

Synthesis and properties of fluorinated polyimides based on 1,4‐bis(4‐amino‐2‐trifluoromethylphenoxy)‐2,5‐di‐tert‐butylbenzene and various aromatic dianhydrides

A novel fluorinated diamine monomer, 1,4‐bis(4‐amino‐2‐trifluoromethylphenoxy)‐2,5‐di‐tert‐butylbenzene ( 2 ), was prepared through the nucleophilic substitution reaction of 2‐chloro‐5‐nitrobenzotrifluoride and 2,5‐di‐tert‐butylhydroquinone in the presence of potassium carbonate, followed by catalytic reduction with hydrazine and Pd/C. Fluorinated polyimides ( 5a – 5f ) were synthesized from diamine 2 and various aromatic dianhydrides ( 3a – 3f ) via thermal or chemical imidization. These polymers had inherent viscosities of 0.77–1.01 dL/g. The 5 series polyimides were soluble in N‐methyl‐2‐pyrrolidone, N,N‐dimethylacetamide, and N,N‐dimethylformamide and were even soluble in dioxane, tetrahydrofuran, and dichloromethane. 5 (C) showed cutoff wavelengths between 363 and 404 nm and yellowness index (b*) values of 6.5–40.2. The polyimide films had tensile strengths of 93–114 MPa, elongations to break of 9–12%, and initial moduli of 1.7–2.1 GPa. The glass‐transition temperatures were 255–288 °C. The temperatures of 10% weight loss were all above 460 °C in air or nitrogen atmospheres. In comparison with a nonfluorinated polyimide series based on 1,4‐bis(4‐aminophenoxy)‐2,5‐di‐tert‐butylbenzene, the 5 series showed better solubility and lower color intensity, dielectric constants, and moisture absorption. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2272–2284, 2004     

1,1‐Diethyl‐1‐germa‐2,3,4,5‐tetra‐tert‐butyl‐2,3,4,5‐tetraphospholan (C2H5)2Ge(tBuP)4, Molekül‐ und Kristallstruktur

1,1‐Diethyl‐1‐germa‐2,3,4,5‐tetra‐ tert ‐butyl‐2,3,4,5‐tetraphospholane (C₂H₅)₂Ge( t BuP)₄, Molecular and Crystal Structure The reaction of the diphosphide K₂[(tBuP)₄] · THF ( 1 ) with the germanium(IV) compound (C₂H₅)₂GeCl₂ leads via a [4 + 1]‐cyclo‐condensation reaction to 1,1‐diethyl‐1‐germa‐2,3,4,5‐tetra‐tert‐butyl‐2,3,4,5‐tetraphospholane (C₂H₅)₂Ge(tBuP)₄ ( 2 ) with the 5‐membered GeP₄ ring system. 2 could be characterized ³¹P NMR spectroscopically, mass spectrometrically and by a single crystal structure analysis.     

Microwave Induced Synthesis of 3‐Aryl‐6‐(6‐/8‐substituted 4‐chloroquinoline‐3‐yl) ‐ s ‐triazolo [3,4‐b] −1,3,4‐thiadiazoles

The condensation of 4‐amino‐3‐aryl‐5‐mercapto‐1, 2, 4‐triazoles (1a‐f) with 6‐/8‐substituted 1,4‐dihydro‐4‐oxo‐quinoline‐3‐carboxylic adds (2a‐d) in the presence of phosphorus oxychloride on refluxng or under microwave irradiation gave twenty four novel 3‐aryl‐6‐ (6‐/8‐substituted 4‐chloroquinoline‐3‐yl)‐s‐triazolo[3,4‐b]‐1, 3,4‐thiadiazoles (4a‐x), Considerable increase in the reaction rate has been observed with improved yields under microwave irradiation. The structures of the compounds synthesized were determined by elemental analyses, IR, ¹H NMR and MS spectra. Their spectral properties and the reaction mechanism were also discussed. The preliminary biological test showed that some of compounds bad moderate antibacterial activities.     

One‐Pot Syntheses of 2‐(2‐Sulfanyl‐4H‐3,1‐benzothiazin‐4‐yl)acetic Acid Derivatives via Reactions of 3‐(2‐Isothiocyanatophenyl)prop‐2‐enoic Acid Derivatives with Thiols or Sodium Sulfide

Two efficient methods for the preparation of 2‐(2‐sulfanyl‐4H‐3,1‐benzothiazin‐4‐yl)acetic acid derivatives 3 under mild conditions have been developed. The first method is based on the reaction of 3‐(2‐isothiocyanatophenyl)prop‐2‐enoates 1a – 1c with thiols in the presence of Et₃N in THF at room temperature, leading to the corresponding dithiocarbamate intermediates 2 , which underwent spontaneous cyclization at the same temperature by an attack of the S‐atom at the prop‐2‐enoyl moiety in a 1,4‐addition manner (Michael addition) to give 2‐(2‐sulfanyl‐4H‐3,1‐benzothiazin‐4‐yl)acetates in one pot. The second method involves treatment of 3‐(2‐isothiocyanatophenyl)prop‐2‐enoic acid derivatives 1b – 1d with Na₂S leading to the formation of 2‐(2‐sodiosulfanyl‐4H‐3,1‐benzothiazin‐4‐yl)acetic acid intermediates 5 by a similar addition/cyclization sequence, which are then allowed to react with alkyl or aryl halides to afford derivatives 3 . 2‐(2‐Thioxo‐4H‐3,1‐benzothiazin‐4‐yl)acetic acid derivatives 6 can be obtained by omitting the addition of halides.     

2,4‐Disila‐1,3,5‐oxadiazin,Cyclodisilazan‐Carbonsäure‐silylamid und ‐silylester

The lithium salts of the Me₃Si‐ as well as Me₃Si‐ and Me₂SiF‐substituted Cyclotrisilazanes I and II react with tert‐butylacylchloride under ring contraction and formation of the cyclodisilazane‐silylester, Me₃SiN(SiMe₂–N)₂SiMe₂–O–CO–CMe₃ ( 1 ). The lithium salt of the fluorodi‐methylsilyl‐substituted cyclotrisilazan III forms with benzoylchloride primarily in the analogous reaction the carboxy‐silyl‐amide, Me₂SiF(N–SiMe₂)₂SiMe₂–NH–CO–C₆H₅⁺ ( 2 ), which can be converted with III and benzoylchloride into the cyclodisilazane‐silylester, Me₂SiF(NSiMe₂)₂SiMe₂–O–CO–C₆H₅, ( 3 ). A silylester substituted six‐membered disila‐oxadiazine ( 4 ) is the result of the reaction of the lithiated cyclotrisilazane, (Me₂SiNH)₂, (Me₂SiNLi) with tert‐butyl‐acylchloride. The reaction includes anionic ring contraction and can be rationilized by a process analogous to keto‐enol‐tautomerism. Dilithiated octamethyl‐cyclotetrasilazane, (Me₂SiNHMe₂SiNLi)₂, reacts with tert‐butyl‐acylchloride or benzoylchloride in a molar ratio 1:2 to yield symmetrically acylestersubstituted cyclodisilazanes, (RCO–O–SiMe₂–NSiMe₂)₂, R = C₆H₅ ( 5 ), CMe₃ ( 6 ). The reaction mechanisms are discussed and the crystal structures of 2 and 6 are reported.     

固相合成甲胺基阿维菌素苯甲酸盐

To implement the solid phase synthesis of 4““-epi-methylamino-4““-deoxyavermecfin B1 benzoate, tert-butyldimethylsilylchloride was chosen for the first solution synthesis. Then a novel silyl chloride resin 1, achieved from hydroxymethyl polystyrene resin and dimethyldichlorosilane, was used successfully for the attachment of avermectin B1 2. Through oxidation, amination formation, cleavage, and benzoate formation, resin bounded avermectin B1 9 gave 4““-epi-methylamino-4““-deoxyavermectin B1 benzoate 3.     

Synthesis of dihydrofuro[2,3‐b]pyridines by the reaction of 2‐amino‐4,5‐dihydro‐3‐furancarbonitriles with α,β‐unsaturated carbonyl compounds

The reactions of 2‐amino‐4,5‐dihydro‐3‐furancarbonitriles 1a‐d with α,β‐unsaturated carbonyl compounds in the presence of sodium ethoxide (0.1 equivalent) gave the corresponding Michael adducts 2a‐d , 3a‐d and 4a‐d. Compounds 2a‐d and 3a‐c reacted with sodium alkoxide (1 equivalent) to yield the corresponding 7a‐alkoxyhexahydrofuro[2,3‐b]pyridines 5a‐d, 6a‐d, 7a‐c and 8a‐c . Treatment of 5a‐d, 6a‐d, 7a‐c and 8a‐c with potassium tert‐butoxide produced the corresponding dihydrofuro[2,3‐b]pyridines 9a‐d and 10a‐c . The reaction of 4a‐c with sodium ethoxide (1 equivalent) afforded the corresponding dihydro‐furo[2,3‐b]pyridines 11a‐c .     

Synthesis of 4‐Aryl‐1,7‐naphthyridine‐2(1H)‐thiones by the Electrocyclic Reaction of 4‐(1‐Arylalk‐1‐enyl)‐3‐isothiocyanatopyridines Generated in situ from the Corresponding Isocyanides

A convenient synthis for 4‐substituted and 3,4‐disubstituted 1,7‐naphthyridine‐2(1H)‐thiones 7 has been developed. The method is based on the electrocyclic reaction of 4‐(1‐arylalk‐1‐enyl)‐3‐isothiocyanatopyridines 6 , generated in situ by the treatment of the respective isocyanides 5 with S₈ in the presence of a catalytic amount of selenium. The isocyanides 5 can be easily prepared from commercially available pyridin‐3‐amine by conventional organic reactions.     

Reaction of the Pincer‐type Ligand {2, 6‐[P(O)(OEt)2]2‐4‐tert‐Bu‐C6H2}— with [Ph3C]+[PF6]—. Unprecedented Rearrangement and Carbon‐Carbon Bond Formation

The reaction of the organolithium derivative {2, 6‐[P(O)(OEt)₂]₂‐4‐tert‐Bu‐C₆H₂}Li ( 1 ‐Li) with [Ph₃C]⁺[PF₆]^— gave the substituted biphenyl derivative 4‐[(C₆H₅)₂CH]‐4′‐[tert‐Bu]‐2′, 6′‐[P(O)(OEt)₂]₂‐1, 1′‐biphenyl ( 5 ) which was characterized by ¹H, ¹³C and ³¹P NMR spectroscopy and single crystal X‐ray analysis. Ab initio MO‐calculations reveal the intramolecular O···C distances in 5 of 2.952(4) and 2.988(5)Å being shorter than the sum of the van der Waals radii of oxygen and carbon to be the result of crystal packing effects. Also reported are the synthesis and structure of the bromine‐substituted derivative {2, 6‐[P(O)(OEt)₂]₂‐4‐tert‐Bu]C₆H₂}Br ( 9 ) and the structure of the protonated ligand 5‐tert‐Bu‐1, 3‐[P(O)(OEt)₂]₂C₆H₃ ( 1 ‐H). The structures of 1 ‐H, 5 , and 9 are compared with those of related metal‐substituted derivatives.     

Synthesis of dihydrothieno[2,3‐b]pyridines based on titanium(IV) chloride‐mediated michael reactions of 2‐amino‐4,5‐dihydro‐3‐thiophenecarbonitriles with α,β‐unsaturated ketones

2‐Arnino‐4,5‐dihydro‐3‐thiophenecarbonitriles 1a‐c reacted with α,β‐unsaturated ketones (e.g. methyl vinyl ketone 2 and benzalacetone 3 ) in the presence of titanium(IV) chloride to give the corresponding Michael adducts 4a‐c and 5a‐c. Thermal treatment of compounds 4a‐c and 5a‐c with titanium(IV) chloride caused intramolecular cyclocondensation to yield the corresponding tetrahydrothieno[2,3‐b]pyridines 6a‐c and 7a‐c. Aromatization of 6a‐c and 7a‐c with potassium tert‐butoxide in refluxing tert‐butyl alcohol pro ceeded smoothly to afford the corresponding dihydrothieno[2,3‐b]pyridines 8a‐c and 9a‐c.     

A Simple Synthesis of 2‐{(Arylmethylidene)hydrazinylidene]‐3‐hydroxy‐4H‐furo[3,2‐c]pyran‐4(3H)‐ones

A simple synthesis of 2‐hydrazinylidene‐3‐hydroxy‐4H‐furo[3,2‐c]pyran‐4‐ones is described. A mixture of (isocyanoimino)(triphenyl)phosphorane, an aromatic aldehyde, and dehydroacetic acid (=3‐acetyl‐2‐hydroxy‐6‐methyl‐4H‐pyran‐4‐one) undergo a 1 : 1 : 1 addition reaction under mild conditions to afford the title compounds in excellent yields.     

Synthesis of Novel Pyrimido[4,5‐b]quinolin‐4‐ones with Potential Antitumor Activity

The 5,6,7,8,9,10‐hexahydro‐2‐methylthiopyrimido[4,5‐b]quinolines 4a , 4b , 4c , 4d , 5a , 5b , 5c , 5d and their oxidized forms 6a , 6b , 6c , 6d , 7a , 7b , 7c , 7d were obtained from the reaction of 6‐amino‐2‐(methylthio)pyrimidin‐4(3H)‐one 2 or 6‐amino‐3‐methyl‐2‐(methylthio)pyrimidin‐4(3H)‐one 3 and α,β‐unsaturated ketones 1a , 1b , 1c , 1d using BF₃.OEt₂ as catalyst and p‐chloranil as oxidizing agent. Some of the new compounds were evaluated in the US National Cancer Institute (NCI), where compound 5a presented remarkable activity against 46 cancer cell lines, with the most important GI₅₀ values ranging from 0.72 to 18.4 μM from in vitro assays.     

Synthesis and characterization of new soluble polyamides from 1,1‐bis[4‐(4‐carboxyphenoxy)phenyl]‐4‐tert‐butylcyclohexane and various diamines

A new dicarboxylic acid monomer, 1,1‐bis[4‐(4‐carboxyphenoxy)phenyl]‐4‐tert‐butylcyclohexane, bearing a pendent tert‐butylcyclohexylidene group was prepared in three steps from 4‐tert‐butylcyclohexanone. The monomer was reacted with various diamines to produce a series of new polyamides with triphenyl phosphite and pyridine as condensing agents. These polyamides were produced with inherent viscosities of 0.74 to 1.02 dL g⁻¹. All the polymers were characterized by X‐ray diffraction that revealed this amorphous nature. These polymers exhibited excellent solubility in a variety of solvents. Almost all the polymers could be dissolved in N‐methyl‐2‐pyrrolidinone, N,N‐dimethylacetamide (DMAc), N,N‐dimethylformamide, dimethyl sulfoxide, pyridine, and even in tetrahydrofuran and cyclohexanone. These polymers showed glass‐transition temperatures between 223 and 256 °C and decomposition temperatures at 10% weight loss ranging from 468 to 491 °C and 469 to 498 °C in nitrogen and air atmospheres, respectively. Transparent, tough, and flexible films of these polymers were cast from the DMAc solutions. These polymer films had tensile strengths ranging from 76 to 99 MPa, elongations at break from 7 to 19%, and initial moduli from 2.1 to 2.7 GPa. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 797–803, 2000     

DPPH (= 2,2‐Diphenyl‐1‐picrylhydrazyl = 2,2‐Diphenyl‐1‐(2,4,6‐trinitrophenyl)hydrazyl) Radical‐Scavenging Reaction of Protocatechuic Acid Esters (= 3,4‐Dihydroxybenzoates) in Alcohols: Formation of Bis‐alcohol Adduct

Protocatechuic acid esters (= 3,4‐dihydroxybenzoates) scavenge ca. 5 equiv. of radical in alcoholic solvents, whereas they consume only 2 equiv. of radical in nonalcoholic solvents. While the high radical‐scavenging activity of protocatechuic acid esters in alcoholic solvents as compared to that in nonalcoholic solvents is due to a nucleophilic addition of an alcohol molecule at C(2) of an intermediate o‐quinone structure, thus regenerating a catechol (= benzene‐1,2‐diol) structure, it is still unclear why protocatechuic acid esters scavenge more than 4 equiv. of radical (C(2) refers to the protocatechuic acid numbering). Therefore, to elucidate the oxidation mechanism beyond the formation of the C(2) alcohol adduct, 3,4‐dihydroxy‐2‐methoxybenzoic acid methyl ester ( 4 ), the C(2) MeOH adduct, which is an oxidation product of methyl protocatechuate ( 1 ) in MeOH, was oxidized by the DPPH radical (= 2,2‐diphenyl‐1‐picrylhydrazyl) or o‐chloranil (= 3,4,5,6‐tetrachlorocyclohexa‐3,5‐diene‐1,2‐dione) in CD₃OD/(D₆)acetone 3 : 1). The oxidation mixtures were directly analyzed by NMR. Oxidation with both the DPPH radical and o‐chloranil produced a C(2),C(6) bis‐methanol adduct ( 7 ), which could scavenge additional 2 equiv. of radical. Calculations of LUMO electron densities of o‐quinones corroborated the regioselective nucleophilic addition of alcohol molecules with o‐quinones. Our results strongly suggest that the regeneration of a catechol structure via a nucleophilic addition of an alcohol molecule with a o‐quinone is a key reaction for the high radical‐scavenging activity of protocatechuic acid esters in alcoholic solvents.