Heterocyclic compounds naturally occurring

Hantzsch 2 gave the name pseudo bases to those carbinols that gave salts with acids by the elimination of water and a simultaneous change of constitution. Such carbinols are common among the nitrogen heterocyclic compounds and the naturally occurring alkaloids e.g., berberine, sanguinarine, chelerythrine. 

Naturally occurring halo derivatives of heterocyclic compounds 98ACR141. 

Benzoxazine, an heterocycle present as structural subunit in many naturally occurring and synthetic bioactive compounds, was prepared under microwave irradiation from a mixture of 2-aminophenol 218 and an a-bromoester 219 (Scheme 80). The reaction proceeded through an initial base-catalyzed alkylation of the phenoUc OH followed by spontaneous amidation. Yields from 44 to 78% were reported for 17 different benzoxazines 220 [ 141]. 

Furans and benzofurans continue to play an important role in the field of heterocyclic chemistry because their skeletons are present in many naturally occurring molecules, and they serve also as extremely useful precursors or intermediates towards the realization of many complex molecules. The authors of the present chapter have placed their attention on the more interesting applications and syntheses of these families of compounds, instead of executing an exhaustive literature search of all the relevant papers that were recorded in 2000. 

At that time, as now, the enantiomers of many chiral amines were obtained as natural products or by synthesis from naturally occurring amines, a-amino acids and alkaloids, while others were only prepared by introduction of an amino group by appropriate reactions into substances from the chiral pool carbohydrates, hydroxy acids, terpenes and alkaloids. In this connection, a recent review10 outlines the preparation of chiral aziridines from enantiomerically pure starting materials from natural or synthetic sources and the use of these aziridines in stereoselective transformations. Another report11 gives the use of the enantiomers of the a-amino acid esters for the asymmetric synthesis of nitrogen heterocyclic compounds. 

Pyrimidines A family of 6-membered heterocyclic compounds occurring in nature in a wide variety of forms. They include several nudeic add constituents (cytosine, thymine, and uracil) and form the basic structure of the barbiturates. [NIH]... 

Barnes, C.S. Occolowitz, J.L. Mass Spectra of Some Naturally Occurring Oxygen Heterocycles and Related Compounds. Aust. J. Chem. 1964, 77,975-986. 

Many of the simple heterocycles occur naturally within human biochemistry. For example, the amino acids proline, histidine, and tryptophan contain, respectively, a pyrrolidine, an imidazole, and an indole ring. The nucleic acids contain purine and pyrimidine rings. Vitamins are heterocyclic compounds vitamin Bg (8.8) is a substituted pyridine vitamin Bj (8.9) contains a pyrimidine ring. Simple heterocycles are therefore important to human biochemistry and thus to drug design. 

The natural cofactor of the AAHs, BH4 (Scheme 2), is a heterocyclic compound chemically classified as a pteridine that includes a fused pyrimidine and pyrazine rings. As many other naturally occurring pteridines BH4 has a pterin structure, which includes an amino substituent in position 2 and an oxo group in position 4 of the pyrimidine ring. The term biopterin is reserved for pterins with a dihydroxypropyl group in position 6. 

Syntheses of the Nitrogen-containing Heterocycles Through the Reactions of Activated Amides and Other Activated Species T. Oishi etal., Heterocycles, 1977,6,1705-1710. Preparative Electrolyses of Synthetic and Naturally Occurring N-Heterocyclic Compounds ... 

Pyrethroids, e.g. cypermethrin (89) and fenvalerate (90), are being employed. This class of compound is the result of structural manipulation by medicinal chemists of the naturally occurring pyrethrins (91) to overcome the latters photoinstability and lack of persistence when in use as ectoparasiticides. There are no heterocyclic representatives of this important class of insecticide/acaricide in use though many heterocyclic examples have appeared in the patent literature. 

The scarcity of reviews1 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. Chemical Abstracts (through November, 1973) has been employed as the principal reference source and nomenclature guide for this review. 

Selenophene and tellurophene must be ranked as late developers relative to the other heterocycles covered in this volume. Some indication of the situation is provided by the fact that selenophene was first described in 1923 and tellurophene in 1966, although a few derivatives had been reported earlier. Undoubtedly an important reason for their earlier neglect is the failure of either ring system to be detected in a naturally occurring compound. Much of the stimulus for contemporary studies of these compounds has been provided by the almost unique opportunity of examining the relationship between the aromaticity of these heterocycles and the position of the heteroatom in the periodic system, a topic which has already been discussed in Section 3.01.5.2. 

The presence of a six-membered oxygen heterocyclic ring in a range of naturally occurring compounds provided the stimulus for the development of synthetic routes to these compounds. Many of the methods which evolved in the closing years of the 19th century and the earlier decades of this century are still of value today. These routes have been supplemented by modern approaches and consequently there is often a choice of methods available for the synthesis of a particular oxygen heterocyclic compound. 

Whereas the chemistry of pyrans is an immense body of knowledge, that of thiopyrans has been less extensively investigated this is probably a reflection of the widespread availability of diverse six-membered oxygen heterocycles and their congeners in Nature vis-a-vis the almost total non-existence of naturally occurring sulfur-containing analogues. Apart from a small number of compounds found in crude oil, all thiopyrans are only... 

The reader is referred to other chapters for the following heterocyclic systems ligands with mainly amine-type groups and one heterocyclic substituent (Chapter 13.1) polypyrazolyl compounds (Chapter 13.6) ligands that also contain another donor atom, such as O, S or P (Chapters 14, 20.3 and 20.4) macrocyclic ligands (Chapter 21) and naturally occurring ligands, such as histidine (Chapters 22 and 62). 

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