Rings With More Than One Heteroatom

Starting with the heteroatom as number 1. The functional groups are placed alphabetically in the name. Some examples are as follows  

Punctuation is important in the examples to follow, a comma separates the numbers and a dash separates the numbers from the heteroatom prefixes. 

A slight modification is used when two vowels adjoin one is deleted, as in the listing for oxaaza, which becomes simply oxaza.  

As for monohetero systems, substituents on the ring are listed alphabetically with a ring atom number for each (not grouped together). 

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This chapter reviews the systems where account is taken of the size of the central carbocyclic ring, the relative orientation of the fused five-membered heterocyclic rings, the types of heteroatoms, their number, and, in rings with more than one heteroatom, their relative situations. 

The concept of nucleus-independent chemical shifts (NICS) as a quantitative magnetic measure of aromaticity is discussed in Sections 2.2.2.2.2 and 2.3.2.2.1, where NICS values for six-membered rings and five-membered rings with one heteroatom are tabulated. Table 4 shows NICS values calculated for five-membered rings with more than one heteroatom <2002JOC1333>. Since NICS values are theoretical parameters, they depend on the computational method used. In Table 4 NICS is the value at the center of the ring and NICS(l) is the value 1 A above the center. 

Three-membered Rings with More than One Heteroatom... 

With more than one heteroatom in a nine-membered fully conjugated ring system in principle there are a number of possibilities when nitrogen is present, whereas with oxygen and sulfur solely 1,4,7-triheteronine systems 1 and 3, respectively, are conceivable. Furthermore there arc mixed systems possible with different heteroatoms, i.e. 4, 5. 

Fusion of a heterocycle having an oxygen or sulfur atom onto the triazine ring could occur only at its face e, otherwise it will be categorized as a heterocycle with more than one heteroatom. Three combinations are possible for that type of fusion. However, only two of them were reported during the period covered by the review. 

Systems with more than one heteroatom per ring 937... 

Systems with more than one heteroatom per ring 12.17.4.1,2(i) Systems with only nitrogen heteroatoms 12.17.4.1.2(i)(a) /3-Carboline derivatives... 

Polycyclic aromatics with more than two aromatic rings, or more than one heteroatom are relatively easy to reduce and several reviews have summarized works on their electrochemical behavior. Bicyclic heteroaromatics with one heteroatom are reduced close to or beyond the decomposition of the electrolyte unless acidic solutions are used. Very few compounds of this kind have been preparatively reduced in neutral media. Their cathodic reduction could be carried out at mercury cathodes with TA A+ electrolytes. Depending on the heteroatom and the amount of charge transferred, hydrogenated and/or reductive cleavage products were obtained. 

Many benzo-fused heterocycles are obtained by elaboration of substituted benzenes, especially useful for benzobis- 1,3-types. The strategy of photocyclization of 1,2-diheterocyclic substituted ethenes is a versatile approach to angular fused systems, and, while it has some limitations, it may yet be further exploited for systems with more than one heteroatom per heterocyclic ring. Otherwise a linking of diheterocycles by a two-carbon bridge is necessary. For compounds with indole moieties the Fischer approach is often successful. 

C. Molecules with More Than One Heteroatom in the Five-Membered Ring... 

Section 3.1 is a brief overview Section 3.2 deals with six-membered heterocycles, including those with more than one heteroatom in the ring Section 3.3 deals with five-membered heterocycles with one heteroatom Section 3.4 deals with five-membered heterocycles with more than one heteroatom in the ring Section 3.5 covers small (three- and four-membered) and large (>six) ring heterocycles. 

Reduction of one or two double bonds in the azole ring gives nonaromatic systems. Historically, these derivatives have been described as azolines and azolidines, as illustrated by the pyrazole derivatives 25 and 26. In 3-pyrazoline 25 the 3 indicates the first ring atom associated with the double bond . In the older literature the position of the double bond is indicated by An, where n is the number of the first ring atom associated with the double bond, e.g., A3-pyrazoline 25. Alternatively, they are named as dihydro or tetrahydro derivatives of the parent azole (e.g., 2,5-dihydropyrazole 25) and this nomenclature is IUPAC-preferred nomenclature for rings with more than two heteroatoms . 

Space restrictions mean that the reactivity of multiheteroatom systems or fused systems where both rings are heterocyclic cannot be covered in this section - the reader is referred to the relevant CHEC volumes. Space again dictates that the chemistries of oxygen- and sulfur-containing six-membered heterocycles, and the chemistry of monocyclic six-membered heterocycles with more than one heteroatom, are only briefly indicated alongside the description of pyridine/quinoline/isoquinoline chemistry, but especially where these are not shown by the pyridine prototypes, but again the reader should study the CHEC volumes for a full discussion. The inclusion of an extra heteroatom in a six-membered system exaggerates the effect of the first and so often it is possible to predict properties by extrapolation however, the same is not true for the five-membered systems, so these heterocycles with more than one heteroatom are considered in detail and separately. 

In Section 3.4.1.1 the reactivities of the major types of azole aromatic rings are briefly considered in comparison with those which would be expected on the basis of electronic theory, and the reactions of these heteroaromatic systems are compared among themselves and with similar reactions of aliphatic and benzenoid compounds. Later, in Sections 3.4.1.23.4.1.10 these reactions are reconsidered in more detail. Reactions of nonaromatic five-membered compounds with more than one heteroatom are considered in Section 3.4.2 and reactions of substituents attached to aromatic azoles are covered in Section 3.4.3. The reactions of azoles can only be rationalized and understood with reference to the complex tautomeric and acidbase equilibria shown by these systems. Tautomeric equilibria are discussed in Chapter 2.4. Acidbase equilibria are considered in Section 3.4.1.3 of the present chapter. 

Five-membered rings containing more than one heteroatom behave in a more complex way as regards aminomethylation. Imidazoles and thiazoles 85 react at 4 and 5 posi-tions, " - although reactions of thiazole with arylamines occurring at position 2 are reported. ... 

The heterocyclic six-membered ring halogen derivatives with more than one heteroatom in the ring have a higher degree of imidoyl halide... 

Large systems contain ring atoms enough to allow more than one way of bonding between a given sequence of atoms, and a wealth of valence isomerizations becomes possible. Already for seven ring members, two types of bicyclic isomer are observed, namely the [4.1.0] and the [3.2.0] systems, each with positional ring isomers if more than one heteroatom is present. 

This chapter provides an update of Chapter 8.33 in CHEC-II(1996) <1996CHEC-II(8)863>. The work carried out on the bicyclic ring systems with ring junction P, As, Sb, or Bi has focused primarily on phosphorus. Very little work has been done on the other heteroatoms and as such the synthesis and reactivity of these compounds have been reviewed as one section, Section 12.12.7. Most of the compounds in this class contain more than one heteroatom, the additional atoms usually being oxygen and nitrogen. 

Volumes 5 and 6 cover six-membered rings with one, or more than one, heteroatom, respectively, again with fused carbocyclic compounds. 

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