Naming the Heterocycles

Heterocyclic amines—compounds in which the nitrogen atom occurs as part of a ring—are also common, and each different heterocyclic ring system has its own parent name. The heterocyclic nitrogen atom is always numbered as position 1. 

There are three types of nomenclature used for heterocyclic compounds. Many heterocycles have trivial names, which are based on their occurrence, special properties, or historical reasons such as discovery of particular material. Systematic names of heterocyclic compounds derived from the structure of the compound are governed by lUPAC rules, which are divided into two groups the Hantzsch-Widman and replacement nomenclatures. In this book, we were trying to follow the guidelines for naming the heterocyclic compounds, which are summarized in Chapter 2 of the excellent book The Chemistry of Heterocycles Structures, Reactions, and Applications ... 

It should be pointed out that currently both trivial and systematic names are commonly used for naming the heterocyclic compounds. For example, an organic chemist will recognize without any difficulty the structures connected to names such as furane, pyrrole, pyrrolidine, pyrazole, imidazole, pyridine, or piperidine, despite the fact that all these names are trivial. On the other hand, the complex heterocycles require more sophisticated approaches in order to avoid ambiguity and correctly translate the chemical strucmre into the name. For these, compound names are often made using either trivial name (e.g., indazole for benzopyrazole, benzimidazole, indole, and isoindole) or the Hantzsch-Widman nomenclature, for example, 1,2,3- or 1,2,5-oxadiazoles, 1,3-dioxolane, 1,2- or 1,3-dithiolane, and 1,3- or 1,4-dioxane. It should be noted that the Hantzsch-Widman nomenclature treats the unsaturated heterocycle with maximum number of conjugated double bonds as parent compound. This adds another layer of complexity, giving rise to names such as... 

The synthetic application of reactions based upon the intramolecular addition of a carbanion or its enamine equivalent to a carbonyl or nitrile group has been explored extensively. One class of such reactions, namely the Dieckman, has already been discussed in Section 3.03.2.2, since ring closure can often occur so as to form either the C(2)—C(3) or C(3)—C(4) bond of the heterocyclic ring. Some illustrative examples of the application of this type of ring closure are presented in Scheme 46. 

As described in Section 1.02.2.3, any heterocycle can be named by indicating (with appropriate prefixes) the positions of heteroatoms in the corresponding carbocycle. The carbocycle can be named systematically, as described in later Sections however, the replacement procedure can also be applied to naming heterocyclic analogues of trivially named carbocycles. A list of polycarbocycles with their trivial names is given in Table 5 (taken from Rule A-21.2 in the lUPAC Blue Book (B-79MI10200)) replacement names may be derived as shown in examples (27) and (28). As demonstrated by example (28), it is unnecessary to add hydro prefixes if the heterocycle cannot accommodate as many double bonds as the original carbocycle. 

Various methods have been employed to overcome this difficulty. For example, a method used by Chemical Abstracts involves naming the dihydro derivative of the heterocyclic ketone (or imine or exocyclic alkene) form, and adding the words mesoionic didehydro derivative (example 131). A similar approach, favoured by Ollis (76AHQ19)1), involves naming the corresponding cation hydroxide, with the prefix anhydro (indicating removal of the elements of water) (example 132). 

When an exocycllc function has a higher priority, it may be necessary to name a cationic heterocyclic substituent group. The most important case is that in which the heterocyclic substituent is bonded through its cationic center. Such cases may be named in two ways, as in (182) and (183). The simplest is to use the suffix -io , as used for the H3N— substituent, ammonio terminal e is elided. More generally, however, an -yl suffix is appended after -I um , as shown in the second names given for the examples. This method applies equally well to situations with other sites of attachment, and also allows one to name divalent substituents, e.g. (184) and (185). 

The naming of heterocyclic ring carbonyl compounds always seems to present difficulties the following account of the procedures used may provide some clarification. 

Formation of five-membered ring systems (1,2-addition) can compete with formation of the seven-membered heterocycles (1,4-addition). If the first step of the reaction sequence, namely the nucleophilic attack of the terminal heteroatoin of the diene, is hindered by steric or electronic effects, the five-membered ring product is formed exclusively. 

The tetracyano derivative of 2.49, 5-diazo-l,2,3,4-tetracyanocyclopentadiene (2.52), and its heterocyclic analogue, 2-diazo-4,5-dicyano-imidazole (2.53) can be obtained by diazotization of the corresponding amines, namely the (anionic) 1-amino-... 

The use of microwaves for the preparation of aromatic five-membered heterocycles has been intensely investigated with excellent results in terms of yields and purities of the products prepared. The Paal-Knorr reaction, namely the cyclocondensation of a 1,4-dicarbonyl compound to give furans, pyrroles and thiophenes has been successfully carried out with the aid of microwaves. 

The crown ether here was named by its decoverer Pedersen51 dicyclohexyl-18-crown-6(18 = number of atoms in the heterocyclic ring, 6 = number of oxygen atoms in the ring) its membrane shows an appreciably higher K+ selectivity with respect to the other alkali metal ions. There is still much research being carried out on the synthesis and practical use of crown ethers. 

In summary, we may add that bacterial utilization of quinoline and its derivatives as a rule depends on the availability of traces of molybdate in the culture medium [363], In contrast, growth of the bacterial strains on the first intermediate of each catabolic pathway, namely, the lH-2-oxo or 1 II-4-oxo derivatives of the quinoline compound was not affected by the availability of molybdate. This observation indicated a possible role of the trace element molybdenum in the initial hydroxylation at C2. In enzymes, Mo occurs as part of the redox-active co-factor, and all the Mo-enzymes involved in N-heteroatomic compound metabolism, contain a pterin Mo co-factor. The catalyzed reaction involves the transfer of an oxygen atom to or from a substrate molecule in a two-electron redox reaction. The oxygen is supplied by the aqueous solvent. Certainly, the Mo-enzymes play an important role in the initial steps of N-containing heterocycles degradation. 

In contrast to the Simmons-Smith reagent and similar carbenoids, which are reactive and therefore difficult to characterize, adducts of the fV-heterocyclic l,3-diorganylimidazol-2-ylidenes are remarkably stable. The first iV-heterocyclic carbene complex of zinc, namely the l,3-di(l-adamantyl)imidazol-2-ylidene diethylzinc complex 48 (Figure 22), was reported by Arduengo et al. in 1993.98 Because of the general utility of these iV-heterocyclic... 

The reduction of furoxans and benzofuroxans has been reviewed in detail in CHEC(1984) and CHEC-II(1996). The main reducing agents to remove oxygen atoms from nitrogen without cleavage of the heterocyclic ring are phosphines and phosphites. These reactions, namely deoxygenation of furoxans and benzofuroxans, are discussed in Section 5.05.4.2.2. 

---