Showing papers in "Advances in Heterocyclic Chemistry in 1975"

Taulomerism and Electronic Structure of Biological Pyrimidines

Abstract: Publisher Summary The chapter focuses tautomerism and electronic Structure of biological pyrimidines. The understanding of the tautomeric properties of the purine and pyrimidine bases of the nucleic acids and the determination of the electronic properties of the principal tautomers are of fundamental importance in molecular biology, in particular, in connection with the theory of mutations. The principal tautomeric properties of the fundamental biological pyrimidines-cytosine, uracil, and thymine-are due to the presence in these N-heteroaromatic compounds of electron-donor substituents, such as NH and OH, and of SH in some important analogs. The chapter discusses tautomerism of cytosine and presents the essential physicochemical properties of cytosine and some of its derivatives and to the theoretical interpretation of these properties. The chapter also discusses tautomerism and physicochemical properties of uracil and thymines and to give a theoretical interpretation of these properties. The study of the optical properties of naturally occurring pyrimidines and purines is a problem of great interest in molecular biology. The knowledge of these properties provide information important for understanding energy transfer, hypochromism, photochemistry, optical rotatory dispersion, circular dichroism, magnetic optical rotator dispersion, and magnetic circular dichroism of DNA, RNA, and related model systems. Tautomerism, electronic structures, and spectra of rare pyrimidine bases of the nucleic acids have also been discussed.

Recent Advances in the Chemistry of Benzo[b]furan and Its Derivatives. Part I: Occurrence and Synthesis

Abstract: Publisher Summary The chapter focuses on the recent advances in the chemistry of benzo[b]furan and its derivatives. The chemistry of benzofuran derivatives has developed in a spectacular fashion: research in this field has reached such a scale that the period covered by this review has given rise to a larger number of papers than that between 1870 and 1950. It has proceeded along the following three main lines: (1) Extraction, determination of structures, partial or total synthesis of the simple or complex heterocyclic derivatives constituting the natural benzofuran derivatives of vegetable origin. (2) Synthesis of benzofuran derivatives with physiological, pharmacological, therapeutic, or toxic properties. (3) Studies on the reactivity of benzofuran and its derivatives from the chemical and physical point of view. Many substances with physiological, pharmacological, therapeutic, or toxic properties are to be found among natural products with a benzofuran ring. As to synthetic benzofuran derivatives prepared for the study of their physiological properties, their number is large. Benzofuran is prepared in semicommercial amounts. The chapter discusses the presence of benzofurans in coal tar and in petroleum. Natural benzofuran derivatives have also been discussed. The numerous syntheses of the benzofuran ring may be classified under four headings: (1) Formation of the heterocyclic ring from an aromatic substrate (2) formation of the heterocyclic ring from a nonaromatic substrate (3) fusion of the benzene ring to a furan substrate (4) formation of the heterocyclic ring from other heterocyclic compounds.

Thiochromanones and Related Compounds

Abstract: Publisher Summary This chapter focuses on thiochromanones and related compounds. The scarcity of reviews and recent interest in thiochromanones and related compounds calls for a comprehensive summary of this area of heterocyclic chemistry. The apparent similarity between these systems and the naturally occurring chromanones, chromones (flavones), chromenes, etc., is responsible for the continued importance of these sulfur heterocycles. The focus is directed to five ring systems: Thiochromans, thiochromones, isothiochromans, benzothiopyrans, and benzothiopyrylium salts. It is completely difficult to segregate their chemistry since they are so interrelated; cross-references will be employed as often as possible. Thiocoumarins and thioxanthones are excluded since these molecules differ considerably from those reviewed here. The chapter discusses about thiochromans, thiochroman-4-ones, and thiochromones. isothiochromans and isothiochromanones have also been discussed in the chapter. The chapter closes with a discussion on benzothiopyrans and benzothiopyrylium salts.

Thioureas in the Synthesis of Heterocycles

Abstract: Publisher Summary The chapter presents the heterocyclic systems prepared from thioureas in order of increasing ring size and in order of increasing numbers of heteroatoms. The heteroatorns have been arranged in the following sequence: Nitrogen, sulfur, oxygen, and other elements. Heterocycles prepared from selenoureas and selenopseudoureas are presented together with their sulfur analogs. Heteropolycycles are discussed in a separate section and are grouped according to general ring type, rather than strictly by ring size. Thioureas are useful in the preparation of two four-membered ring systems: Thietanes and 1,3-thiazetidin-4-ones. The chapter also discusses five-membered rings, six-membered rings, and seven-membered rings. C1yclohcxanones have been employed ill reactions with thioureas to give quinazolines with varying degrees of saturation. A simple preparation of otherwise difficulty prepared heterocyclic ring systems is the cyclization of derivatives of type, resulting from reaction of carbethoxy isotliiocyanate with 2-aminoazaheterocycles to give bridgehead-nitrogen heterobicyclic, s-triazinonethiones.

Advances in the Chemistry of Charom-3-enes

Abstract: Publisher Summary This chapter focuses on the advances in the chemistry of chrom-3-enes. No quantum mechanical calculation, or similar theoretical treatment seems to have been dedicated to chrom-3-ene or to its simple derivatives. Interatomic distances and angles of two bromo derivatives of natural chromenes have been obtained by X-ray analysis. The UV absorption, emission, fluorescence, and fluorescence excitation spectra of some 2,2-dialkylchromenes have been studied in connection with their photochromic behavior. Almost every class of natural phenolic compounds contains examples of substances with a 2,2-dialkylchromene ring, and the number of those that are discovered increases every year. It would be difficult to give an exhaustive list of these compounds. Their chemistry is discussed here only when it is related to some particular behavior of the benzopyran ring. Much progress has been made in the last, ten years in the synthesis of chrom-3-enes. Most of the activity has been stimulated by the discovery of new natural products and by the suggestions of possible biosynthetic pathways. In general, the reactions of chromenes are those expected from styrenes. Perhaps for this reason little attention has been paid to their chemical behaviour in recent years. Among chromenes, only the spiropyrans and their heterocyclic derivatives have found a wide practical application as photochromic substances. Chromenes with structure and activity similar to those of hashish constituents have been prepared. A number of chromenes, mostly with aryl substituents in positions 2,3 and 4 have been patented as biologically active substances.