Heterocyclic compounds Cyclic organic aromatic

HETCOR (Section 13 19) A 2D NMR technique that correlates the H chemical shift of a proton to the chemical shift of the carbon to which it is attached HETCOR stands for heteronuclear chemical shift correlation Heteroatom (Section 1 7) An atom in an organic molecule that IS neither carbon nor hydrogen Heterocyclic compound (Section 3 15) Cyclic compound in which one or more of the atoms in the nng are elements other than carbon Heterocyclic compounds may or may not be aromatic... 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

Atoms other than carbon and hydrogen that appear in organic compounds are called heteroatoms. Cyclic organic compounds that contain one or more heteroatoms are called heterocycles. Heterocyclic compounds are the largest class of organic compounds and can be either aromatic (such as pyridine, pyrrole, and furan) or nonaromatic (such as piperidine, pyrrolidine, and tetrahydrofuran). 

As noted in Section 9.4 in connection with a discussion of aromaticity, a cyclic organic compound that contains atoms of two or more elements in its ring is called a heterocycle. Heterocyclic amines are particularly common, and many have important biological properties. Pyridoxal phosphate, a coenzyme sildenafil (Viagra), a well-known pharmaceutical and heme, the oxygen carrier in blood, are examples. 

A coirpound does not have to be a hydrocarbon to be aromatic. Many heterocyclic compounds are aromatic. A heterocyclic compound is a cyclic compound in which one or more of the ring atoms is an atom other than carbon. The atom that is not carbon is called a heteroatom. The name comes from the Greek word heteros, which means different. The most common heteroatoms encountered in organic compounds are N, O, and S. 

In organic chemistry, A -heterocyclic compounds are cyclic compounds containing one or more nitrogen atoms. A -Heterocyclic compounds include aromatic A -heterocycles such as pyrrole, pyridine, and imidazole, as well as saturated A -heterocycles such as aziridine, piperidine [1]. A -heterocyclic compounds are very important motifs in biochemical compounds such as nitrogenous bases, as well as pharmaceuticals and materials (Fig. 1.1). Significant synthetic efforts had been made toward A -heterocycles with different structures and substitutents [1] however, it is still demanding to develop new synthetic methods toward A -heterocyclic compounds, especially via metallacycles such as zirconacycles. 

The aromaticity of three- and four-membered cyclic sulphur compounds has been discussed as part of an overall review of aromaticity in heterocyclic compounds. The photochemistry of organic sulphur compounds, including reactions involving small rings, has been reviewed. ... 

For example, unbranched aliphatic acids with an even number of carbon atoms will be generously represented, while one finds a nearly complete absence of odd-numbered and branched acids. Nature produces an incredible diversity of the most ingeniously constructed cyclic products containing cycloaliphatic, aromatic, or heterocyclic moieties, but such derivatives like aniline or thiophe-nol, as well as plethora of other simple representatives of these classes, are not in the list of naturally occurring substances. Such important types as alkyl halides, nitro compounds, and diazo compounds would be sparsely represented by very rare (if any) examples. Even the simplest compounds like formaldehyde, chloroform, diethyl ether, dioxane, etc., which are trivial to organic chemists, turn to be rather exotic for Nature. In the list of items provided by Nature one will notice the almost complete absence of various organometallic compounds, as well as many other classes of structures of immense scientific and practical significance. 

Oil Shale Retort Offgases. Analysis of the retort offgas for organic constituents before the offgas burner showed mostly hydrocarbons. Saturated hydrocarbons, alkenes, alkynes, cyclic alkanes, and cychc alkenes were found. Aromatic compounds, mostly alkyl-substituted benzenes, were detected. No heterocyclics were identified in the offgas. 

Dehydrogenations, which involve the elimination of hydrogen Ifom organic molecules, lead to compounds containing double bonds, multiple bonds, or aromatic rings. For practical reasons, only the formation of carbon-carbon double bonds, of carbon-nitrogen double bonds in cyclic amines, and of aromatic rings (both carbocyclic and heterocyclic) will be discussed in this chapter. The conversion of alcohols into aldehydes and ketones and of amines into imines and nitriles will be discussed in the chapter Oxidations (Chapter 3). 

N-Acyliminium ions are versatile intermediates in organic synthesis they not only react with various nucleophiles such as electron-rich aromatic and heteroaromatic compounds but also undergo cycloaddition reactions with unsaturated compounds.It is especially noteworthy that N-acyliminium ions serve as electron-deficient 4t7 components in [4 + 2] cycloaddition with alkenes and alkynes. " This reaction serves as a useful method for the construction of heterocyclic rings containing a nitrogen atom. Acyclic structures containing amino and hydroxyl groups can also be synthesized from the initially formed cyclic compounds. 

Subsequendy, the aromaticity concept built on carbocydes has been applied successfully to heterocyclic organic compounds, i.e., to a family of aromatic compounds which contain one or more heteroatoms as a part of their cyclic TC-electron system. However, some of the measures developed for aromatic hydrocarbons, particularly those based on geometrical parameters, have been modified to be used for heterocycles (1993JCI70, 2001T5715, 2010SY1485). 

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