Six-membered ring heterocycles

Some of the partially and fully reduced heterocyclic six-membered rings are sufficiently important to have trivial names with which the reader should be familiar. Thus hexahy-dropyridine (38) is known as piperidine, and tetrahydro-l,4-oxazine (39) is morpholine. Tetrahydropyridines are also sometimes referred to as piperideines, with the position of the double bond denoted by a A, but this system is obsolescent (at the least). 

Many studies have been made of substituent effects in saturated heterocyclic six-membered rings. For a detailed discussion the review of Eliel and Pietrusziewicz should be consulted [Pg.15]

PE spectroscopy has also been applied to the study of the conformational equilibria of saturated heterocyclic six-membered rings, and in particular of hexahydropyridazines. The... 

Other forms of tautomerism are much less commonly encountered in heterocyclic six-membered rings. Like protons, acyl groups can occupy alternative positions, on a ring atom or a substituent (e.g. 92 93), and their migration from one position to another is frequently... 

The /zetero-Diels-Alder reaction permits heterocyclic-six- membered rings to be constructed by the interaction of heterodienes and/or heterodienophiles. Both the intermolecular and intramolecular versions of the /zctcro-Diels Alder reaction are, therefore, very important methods for synthesizing heterocyclic compounds. 

The reactions by which we have exemplified this brief survey of heterocyclic six-membered ring syntheses were in the main selected for their appropriateness to a discussion of general principles. Many important, interesting and unusual methods have been omitted, either because the point is already adequately illustrated, or because the examples are isolated and lacking in general applicability. 

Pyrimidine. A heterocyclic six-member ring structure. Cytosine and uracil are the main pyrimidines found in RNA, and cytosine and thymine are the main pyrimidines found in DNA. 

Tetiazines are heterocyclic, six-membered ring compds contg 4 N atoms that are remarkable for their high nitrogen content. They are characterized by their red color... 

The R group possesses Jt-electrons or lone pairs of electrons that can interact with the rest of the Jt-electron system. The most important electron donor is the amino group. Triarylmethine dyes are usually divided into mono-, di-, and triami-notriarylmethine dyes. In some di- and triarylmethine dyes, the ring carbon atoms ortho to the central methine carbon atom are bonded via a heteroatom to form a heterocyclic six-membered ring. These include the acridine, xanthene, and thiox-anthene dyes. 

Conformational Analysis of Saturated Heterocyclic Six-Membered Rings... 

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