A quantitative study of substituent effects on oxidative cyclization of some 2-aryl-substituted aldehyde thiosemicarbazones induced by ferric chloride and cupric perchlorate

As a development of our previous work, we performed a kinetic study of the oxidative cyclization reaction of some 2,4-diaryl-substituted aldehyde thiosemicarbazones 1a-n induced by ferric chloride and by cupric perchlorate. The results of cyclization of 1a-n were compared to those of the corresponding 2-methyl derivatives. The kinetic data were analyzed by means of the Hammett's equation.     

Oxidative cyclization of some aldehyde semicarbazones induced by metallic salts

The oxidative cyclization of some aldehyde semicarbazones 10 with four different oxidizing agents has been effected. The structure of the semicarbazones and the nature of cyclizing agent affected the rate and yield of cyclization but they did not show any influence on the regiochemistry of reaction. In fact, 1,2,4-triazoline 20 was the only heterocyclic ring obtained by the cyclization reaction.     

Ultrasound-assisted synthesis of 2-amino-1,3,4-oxadiazoles through NBS-mediated oxidative cyclization of semicarbazones

A ultrasound-assisted oxidative cyclization of semicarbazones using N-bromosuccinimide in the presence of sodium acetate was established providing efficient and rapid access to a variety of 2-amino-1,3,4-oxadiazoles. Moreover, the new synthetic protocol provides a simple procedure utilizing a safer oxidizing system that affords the target products in high regioselectivity, satisfactory yields, and elevated purities.     

A quantitative study of substituent effects on oxidative cyclization of some 2-methylsubstituted aldehydes. Thiosemicarbazones induced by ferric chloride

In order to gain further mechanistical information about the cyclization of thiosemicarbazones and thiosemicarbazone-type substrates induced by metallic salts as oxidizing agents, we performed the synthesis of substrates 1a-s and a kinetic study of the oxidative cyclization of 1 to 5-imino-Δ²-1,3,4-thiadiazole 2 and 1,2,4-triazoline-5-thione 3 derivatives induced by methanolic ferric chloride solutions. The results of cyclization were compared to those of corresponding semicarbazones. The kinetic data were analyzed by means of the Hammett's equation and ρ values discussed.     

Thermometric titrations of amines with nitrosyl perchlorate in acetonitrile solvent

Thirteen aliphatic and four aromatic amines, namely diethylamine, triethylamine, n-propylamine, di-n-propylamine, tri-n-butylamine, isopropylamine, di-isopropylamine, n-butylamine, di-n-butylamine, tri-n-butylamine, isobutylamine, sec-butylamine, tert-butylamine, aniline, N,N-dimethylaniline, 2-nitroaniline and 4-nitroaniline were titrated thermometrically with nitrosyl perchlorate in acetonitrile solvent. All the aliphatic amines gave very well-shaped thermometric titration curves. The calculated recovery values of the amines were very good. In comparison, the aromatic amines, aniline and N,N-dimethylaniline gave rather well-shaped titration curves, but the recovery values were fairly low. 2-Nitro- and 4-nitro anilines gave no thermometric response at all. The heats of reaction of the amines with nitrosyl perchlorate are rather high. However, the average heat of reaction of the aromatic amines is approximately two-thirds that of the average heat of the aliphatic amines. To support this method all the amines were also titrated potentiometrically and very similar results to those obtained with the thermometric method are seen. The nitrosyl ion is a Lewis acid, strong enough to titrate quantitatively aliphatic amines in acetonitrile solvent, but not strong enough to titrate aromatic amines at the required level in the same solvent.     

A remarkable cyclization of TADDOL-bisthioacetate under oxidative conditions

Abstract  

Attempted oxidation of a TADDOL-derived bisthioacetate resulted in a rather unexpected and remarkable cyclization and deprotection reaction, giving a thiolane-1,1-dioxide as the main product. Systematic in situ ESI-HRMS studies revealed a bicyclic, highly acid labile key intermediate of this reaction. Supported by force field calculations, the high sensitivity of this intermediate was judged to be due to the formation of a highly strained trans-configured bicyclo[3.3.0]skeleton.     

A novel construction of dibenzofuran-1,4-diones by oxidative cyclization of quinone-arenols

A novel oxidative cyclization of quinone-arenols 5 leading to products 6 with a dibenzofuran-1,4-dione structure, which forms the core of several natural products, has been developed and applied to the synthesis of violet-quinone (4).     

Electrochemical oxidation of benzaldehyde and hydroxybenzaldehydes in acetonitrile on platinum and glassy carbon electrodes

The electrochemical oxidation of benzaldehyde and four hydroxybenzaldehydes was studied on platinum and glassy carbon electrodes in acetonitrile. A considerable difference was observed in electrooxidation performed on platinum and glassy carbon electrodes. All hydroxy derivatives fouled the glassy carbon electrode, but platinum was passivated only by the electrooxidation of 3-hydroxybenzaldehyde, highlighting the crucial role of the position of the substituent relative to the hydroxy group. On the glassy carbon electrode, the formation of the corresponding benzoyl radical could have taken place, which promoted the buildup of polymers on the electrode surface.     

Quenching of superoxide ions by curcumin. A mechanistic study in acetonitrile

The quenching of superoxide ions, O2.-, by curcumin has been studied by electrogenerating this anion radical from oxygen dissolved in acetonitrile solvent (that is, at best, a mimic of the lipofilic layer of biological membranes), containing known amounts of curcumin. Voltammetric tests, combined with coulometric and spectrophotometric measurements, pointed out that each mol of curcumin is able to react with six mols of such anion radical, through a process initiated by an acid-base step, which provides the perhydroxyl radical, HO2.; that disproportionates rapidly to the anionic form of hydrogen peroxide, HO2-, and oxygen, which is thus partially regenerated. At the same time, curcumin is converted to the corresponding three-charged anion. The strict resemblance existing between the mechanism of the rapid superoxide radical decay caused by curcumin and that involved in the presence of the superoxodismutase enzyme (SOD) is also underlined.     

Total synthesis of (±)-lantalucratins A and B by CAN-mediated oxidative cyclization

The first total synthesis of (±)-lantalucratins A and B is described. Introduction of an alkyl side chain at the 6-position was proceeded by directed ortho-lithiation and subsequent alkylation reaction to afford 6-alkyl-5,7,8-trimethoxy-1-naphthol as a synthetically important intermediate for (±)-lantalucratins A and B in excellent yield with complete regioselectivity. The 1,2-naphthoquinone fused five-membered cyclic ether framework was constructed directly from 3-hydroxyalkyl-naphthalenes by oxidative intramolecular cyclization reaction with diammonium cerium(IV) nitrate. (±)-Lantalucratins A and B were obtained in 69% and 45% overall yields, respectively.