Saturated nitrogen-containing rings

Polyphenol adsorbents are mainly polyamides (Dadic, 1973). At one time various nylons were used, but PVPP is most frequently used today (McMurrough et ah, 1997). The structure of PVPP (see Fig. 2.23) resembles that of polyproline (Fig. 2.6) both have five-membered, saturated, nitrogen-containing rings and amide bonds. 

Aziridines (three-membered saturated nitrogen-containing rings) are also found in anti-tumour agents, such as the mitomycins. Thiiranes also occur naturally, as plant products such as thiirane-2-carboxylic acid, isolated from asparagus. 

The current volume consists of four chapters. It commences with a comprehensive survey of the conformational rates and equilibria of saturated nitrogen-containing six-membered rings, authored by T. A. Crabb and the series editor. This is a subject wherein many of the past controversies have now been resolved and where it is now possible to provide a rather satisfying account. 

The simplest alkaloids of this series are those in which the nitrogen-containing ring of the isoquinoline system is completely saturated (Fig. 32.7). Quaternary bases also are found. Tetrahydrobenzylisoquinoline alkaloids are widespread and are found in almost all the families that possess benzylisoquinoline alkaloids. About 100 alkaloids of this type are known (Guinaudeau and Bruneton, 1993). 

Some of the newer compounds may contain both saturated and unsaturated rings, heteroatoms such as oxygen, nitrogen, or sulfur, and halogen substituents. Others, such as synthetic pyrethroids, may have one or more chiral centers, often needing stereospecific methods of synthesis or resolution of isomers (42). Table 4 Hsts examples of some more complex compounds. Stmctures are shown ia Eigure 1 (25). 

The a-metalation of azoles (aromatic nitrogen-containing five-mem-bered rings) is a much more facile process than that for the analogous saturated systems, and a small number of heterocycles containing free NH groups can undergo some direct lithiation, despite the ionization of the... 

Conformational Equilibria in Nitrogen-Containing Saturated Six-Membered Rings... 

The parent saturated three-membered ring systems containing nitrogen, or oxygen, or sulphur as the heteroatom are known as aziridine (1), oxirane (2) and thiirane (3). 

Conformational equilibria in nitrogen-containing saturated six-membered rings, 36, 1... 

Diazetidines. A saturated four-membered ring containing two adjacent nitrogen atoms is named a 1,2-diazetidine 1. When the two nitrogen atoms and the two carbon atoms are alternatively present in the four-membered ring, it is named a 1,3-diazetidine 2, which is also known as uretidine. Fused multinuclear heterocyclic systems are known as diazacyclobutane derivatives. For example, 3,4-benzo-l,2-diazetidine is referred to as l,2-diaza-3,4-benzocyclobutane 3. 

Polycyclic aromatic nitrogen compounds, such as quinoline, indole, acridine, and carbazole, are the main nitrogen-containing compounds in oil fractions, and monocyclic pyridine and pyrrole as well as anilines are present in coal tar (Scheme 11). Saturated cyclic amines are found as products of ring hydrogenation of the compounds shown in Scheme 11 after HDN. Aliphatic amines are rarely found in fuels and only in low concentrations after HDN since their nitrogen atoms are easily removed. 

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