Monocyclic Heterocyclic Compounds

Systematic and trivial names are both commonly in use for heterocyclic compounds. The systematic names consist of one or more prefixes from Table 1.2 with multipUers where needed designating the heteroatoms, followed by a suffix from Table 1.3 to give the ring size with an indication of the unsaturation. This is preceded by substituents. Thus, oxepin is a 

Atoms in the Ring Containing Nitrogen Containing no Nitrogen  

The ring is larger than 10 members therefore, the hydrocarbon ring name cyclotetradecyne was used, modified by 1,7-dioxa, which is a replacement of carbons 1 and 7 with oxygens. The a ending on oxa indicates replacement. The numbering begins at a heteroatom and proceeds to the other heteroatom by the shortest path. The stereochemistry at position 4 is unspecified. 

Many five- and six-membered rings and fused ring systems have trivial names that are preferred over the systematic names. Table 1.4 provides a selection of the more common ones. Additional names can be found online [8]. 

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Plots of Aae against the percentage of lone-pair axial conformer (estimated by dipole-moment measurements) for some monocyclic heterocyclic compounds with differing N-alkyl substitution showed no correlation.44 This is to be expected because the nature of the N-alkyl substituent also affects A,e40 Table III). 

We do not discuss in detail the cases of tautomerism of heterocycles embedded in supramolecular structures, such as crown ethers, cryptands, and heterophanes, because such tautomerism is similar in most aspects to that displayed by the analogous monocyclic heterocycles. We concentrate here on modifications that can be induced by the macrocyclic cavity. Tire so-called proton-ionizable crown ethers have been discussed in several comprehensive reviews by Bradshaw et al. [90H665 96CSC(1)35 97ACR338, 97JIP221J. Tire compounds considered include tautomerizable compounds such as 4(5)-substituted imidazoles 1///4//-1,2,4-triazoles 3-hydroxy-pyridines and 4-pyridones. 

The Boger group [64-66] has extensively studied the use of a-ketoheterocycles as FAAH inhibitors. In their initial studies, a range of a-ketoheterocycles based on oleic acid was synthesised. A range of five- and six-membered monocyclic heterocycles and three bicyclic heterocycles (benzothiazole, benzimidazole and benzoxazole) was examined. Although many of the compounds tested were found to inhibit FAAH activity with micromolar affinities, the best results were obtained with heterocycles that incorporated a weakly basic nitrogen a - to the heterocycle (Table 6.5) [64]. 

Aromaticity as a Cornerstone of Heterocyclic Chemistry Scheme 1. Monocyclic Aromatic Compounds... 

Hiickel s rule (1931) for planar species states that if the number of ir electrons is equal to 2 + 4n, where n equals zero or a whole number, the species is aromatic. The rule was first applied to carbon-containing monocyclics in which each C is capable of being 5p -hybridized to provide a p orbital for extended v bonding it has been extended to unsaturated heterocyclic compounds and fused-ring compounds. Note that benzene corresponds to n = 1. 

Coke-oven tar is an extremely complex mixture, the main components of which are aromatic hydrocarbons ranging from the monocyclics benzene and alkylbenzenes to polycyclic compounds containing as many as twenty or more rings. Heterocyclic compounds containing oxygen, nitrogen, and sulfur, but usually only one heteroatom per ring system are present. Small amounts of paraffinic, olefinic, and partly saturated aromatic compounds also occur. 

An important group of four-membered heterocyclic compounds are the derivatives of the /i-lactam [azetidin-2-one (15)] system. Many of the compounds are biologically active, for example, the monocyclic nocardicins [e.g. (16)] and the bicyclic penicillin and cephalosporin antibiotics [e.g. (17) and (18) respectively], and the -lactamase inhibitors of the clavam group (19). 

The generation and reactivity of conjugated azomethine ylides has been reviewed.148 1,3-Dipolar cycloaddition and 1,5- and 1,7-electrocyclizations have been described as powerful strategies for the synthesis of monocyclic and annulated five- and seven-membered nitrogen heterocyclic compounds. 

Table 10 Ring Syntheses of Monocyclic Five-membered Heterocycles Containing Three Oxygen or Sulfur Atoms from Non-heterocyclic Compounds... 

With many treatises about heterocyclic compounds, and with the impressive series Comprehensive Heterocyclic Chemistry in three series (84CHEC1,96CHEC2-1,08CHEC3-1), it makes little sense to deal in detail with individual compounds. Rather, this review will discuss trends observed when m increases stepwise from m = 4 to m = 9. Actually, it will be seen that there are no heterocyclic sextet-aromatic systems with eight-or nine-membered rings, but we have included such structures because it may be worthwhile to explore whether their strain-free bicyclic isomers may evidence isomerization to planar monocyclic mesoionic systems. 

References to reviews dating prior to 1982 may be found in the first edition of Comprehensive Heterocyclic Chemistry <84CHEC-I(6)177>. Since then, several comprehensive compilations of oxazole chemistry have been published, including the 45th volume in the series The Chemistry of Heterocyclic Compounds, edited by I. J. Turchi . There are chapters on monocyclic oxazoles, oxazolones, mesoionic oxazoles, the use of oxazoles and dihydrooxazoles in organic synthesis, as well as one covering the spectroscopy of oxazoles. A less extensive summary by... 

Cycloaddition of benzyne and of tetrahalogenobenzynes also occurs with simple monocyclic benzenoid compounds, and at higher temperatures (e.g., if benzyne is generated from phthalic anhydride) adducts such as 24 may rearomatize by elimination of acetylene. 1,4-Cycloadducts of benzyne and heterocyclic substrates are sometimes isolable, but frequently they too react further by a variety of pathways to give secondary products (Sections V, VI, and VII). 

The kinetics of the reactions of many xenobiotics with hydroxyl and nitrate radicals have been examined under simulated atmospheric conditions and include (1) aliphatic and aromatic hydrocarbons (Tuazon et al. 1986) and substituted monocyclic aromatic compounds (Atkinson et al. 1987c) (2) terpenes (Atkinson et al. 1985a) (3) amines (Atkinson et al. 1987a) (4) heterocyclic compounds (Atkinson et al. 1985b) and (5) chlorinated aromatic hydrocarbons (Kwok et al. 1995). For PCBs (Anderson and Hites 1996), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 days for 3-chlorobiphenyl to 34 days for 2,2, 3,5, 6-pentachlorbiphenyl. It was estimated that loss by hydroxylation in the atmosphere was a primary process for removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a... 

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