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The art of dyeing textiles has a long history, as natural dyes have been used since prehistoric times. With the development of synthetic dyes in the 19th century, the focus shifted from natural to synthetic dyes due to their superior properties. Recently, however, interest in natural dyes has increased again due to environmental and health concerns. Among industrial dyes, heterocyclic dyes, especially azo dyes, are of great importance due to their color brilliance and fastness. This review examines the synthesis, application, and analysis of azo dyes, especially heterocyclic dyes. It deals with monoazo, diazo, and polyazo dyes and highlights their structures, synthesis methods, and fastness properties. In addition, the ecological impact of azo dyes and practical solutions for their synthesis and application are discussed. [ABSTRACT FROM AUTHOR]

DFT calculations on the photoisomerization of 2,6-dimethylpyrazine allowed us to confirm the role of benzvalene isomers in the isomerization of hexatomic heterocyclic compounds. 2,6-Dimethylpyrazine in the excited singlet states can be converted into the corresponding Dewar isomers. If the S2 state is populated, two Dewar isomers can be obtained, while the S1 state allows the formation of only one of the possible Dewar isomers. Both Dewar isomers can be converted into the benzvalene isomer, that is, the precursor of 4,5-dimethylpyrimidine, the reaction product. In fact, the benzvalene isomer can be obtained from the Dewar isomers in processes that occur without an activation energy, and it is the more stable benzvalene isomers that can be obtained from the Dewar isomers. CASSCF study indicates the presence of a conical intersection allowing the direct formation of the benzvalene isomer. [ABSTRACT FROM AUTHOR]

The comparison of D index with HOMHED gave a good correlation (R2= 0.8221). However, a modification of D index, namely D', where the number of cycles involved in the tested molecule is substituted by the ratio between the number of atoms involved in π orbitals in the tested molecule and in reference molecule, gave a better correlation with a larger number of compounds (R2=0.8819), The correlation of D' with calculated HOMHED for pentatomic heterocyclic compounds did not give a unique correlation but a series of possible correlations. This behavior depends on the parameters used for the calculation of HOMHED that allow some negative values. In fact, the comparison of D' with NICS for selected pentatomic heterocycles gave good correlation. Good correlations were observed also in the comparison of D' with HOMHED and NICS for antiaromatic compounds. [ABSTRACT FROM AUTHOR]

The Paternò-Büchi reaction is a photochemical reaction between a carbonyl compound and an alkene to give the corresponding oxetane. In this review the mechanism of the reaction is discussed. On this basis the described use in the reaction with electron rich alkenes (enolethers, enol esters, enol silyl ethers, enanines, heterocyclic compounds has been reported. The stereochemical behavior of the reaction is particularly stressed. We pointed out the reported applications of this reaction to the synthesis of naturally occuring compounds. [ABSTRACT FROM AUTHOR]

The photochemical reaction of 2-substituted heterocyclic aldehydes with furan gave the corresponding exooxetane derivatives through the excited triplet state. However, in situthe oxetane derivatives were converted through a metathesis reaction into the corresponding Z,E-butadienyl formate derivatives. On the contrary, 3-substituted heterocyclic aldehydes gave the corresponding exooxetane derivatives. The effect of 2-substituted heterocyclic ring in order to facilitate the metathesis reaction is explained considering the possible participation of the π aromatic orbitals in the oxetane C–O bond cleavage. [ABSTRACT FROM AUTHOR]

Abstract: The low reactivity of pyridine, pyridazine, pyrazine, pyrimidine, and purine toward electrophilic substitution reactions can be explained assuming a frontier orbitals control of the reaction. DFT study of the molecular orbitals of these molecules shows that the HOMOs of these substrates are not π orbitals. Furthermore, the change in the reactivity observed in pyridine-N-oxide, in activated pyrimidines, and in activated purines can be explained considering that, in this case, the HOMOs are π orbitals. [Copyright &y& Elsevier]

A theoretical study of photoinduced ring-isomerization of 3-amino-5-methyl- and 3-amino-5-phenyl-1,2,4-oxadiazoles is reported. The results well agree with the reported experimental data: in particular, they explain the ring-photoisomerization into the corresponding 2-amino-1,3,4-oxadiazoles through a ring contraction-ring expansion route; moreover, the occurrence of competing pathways involving both the ring contraction and the internal cyclization–isomerization mechanism during irradiation of the 5-alkyl substituted substrates in the presence of a base has been also substantiated. [Copyright &y& Elsevier]

Review: 263 refs. [ABSTRACT FROM AUTHOR]