Numbering hetero atoms

Hetero)cyclic hydrocarbons Ln.J T.n.J L beginning of a carbocyclic ring T beginning of a heterocydic ring n number of atoms of the ring system f termination of the ring system... 

There exist a number of d -synthons, which are stabilized by the delocalization of the electron pair into orbitals of hetero atoms, although the nucleophilic centre remains at the carbon atom. From nitroalkanes anions may be formed in aqueous solutions (e.g. CHjNOj pK, = 10.2). Nitromethane and -ethane anions are particularly useful in synthesis. The cyanide anion is also a classical d -synthon (HCN pK = 9.1). 

For such purposes as locating substituents, monocyclic heterocycles having only one non-carbon atom are numbered starting with the hetero atom as 1. If there is more than one hetero atom of the same kind, the more saturated hetero atom takes precedence (cf. imidazole in Table 1). In isoxazole and isothiazole, the divalent atom takes precedence, in conformity with the pnaciples of the Hantzsch-Widman system (Section 1.02.2.2). 

Used when the ring contains the maximum number of noncumulative double bonds and when the hetero atoms have the valences shown in Table 2. 

When a ring is made up of repeating units of two or three hetero atoms, it may be named by specifying the unit and the number involved, prefixed by cyclo . This system is most appropriate for rings containing no carbon. 

Chapters 11 and 12 both deal with C-N-E (E = S, Se, Te) heterocycles. The coverage is limited to ring systems in which the number of hetero atoms exceeds the number of carbon atoms. The topics are organized according to ring size. After a brief section on four-... 

Selenazole is the selenium-containing compound in the series of heterocyclic 5-membered ring azoles with two different hetero atoms, of which the first two members are oxazole and thiazole. The numbering of the ring system is according to the scheme given (1). 

In the following sections, systems with various numbers of electrons are discussed. When we look for aromaticity we look for (1) the presence of a diamagnetic ring current (2) equal or approximately equal bond distances, except when the symmetry of the system is disturbed by a hetero atom or in some other way (3) planarity (4) chemical stability (5) the ability to undergo aromatic substitution. 

A very large number of heterocyclic compounds are known consisting solely of phosphorus and chalcogen atoms, as well as those containing phosphorus, chalcogen and carbon or other hetero atoms. Such heterocycles have been the subject of several reviews2,83,84 and the discussion here will be limited to some representative examples of these compounds. 

For the heterohelicenes, where isomers are possible, a new way for abbreviation is introduced by giving after the number in brackets the correct sequence of benzene and hetero rings, using symbols B = benzene, S = thiophene, NH = pyrrole, N = pyridine (the position of the hetero atom is indicated by the position number), O = furan etc. 

Two or more identical hetero-atoms are indicated by use of the multiplying prefixes di-, tri- or tetra-. When more than one distinct hetero-atom is present, the appropriate prefixes are cited in the name in descending order of group number in the periodic table, e.g. oxa- takes precedence over aza-. If both lie within the same group of the periodic table, then the order is determined by increasing atomic number, e.g. oxa- precedes thia-. 

Systematic bond disconnection of porantherine [151] with recognition of the double bond-carbonyl equivalence for synthesis generated a synthetic pathway which is based on two intramolecular Mannich reactions. The symmetrical nature of the amino diketone precursor identified by the retrosynthetic analysis facilitates its preparation and subsequent transformations. Moreover, all the hetero atoms (donors) are separated by odd-numbered carbon chains and such arrangements are most amenable to normal modes of assembly. 

In six-membered rings containing hetero atoms,243 the basic principles are the same i.e., there are chair, twist, and boat forms, axial and equatorial groups, etc., but in certain compounds a number of new factors enter the picture. We deal with only two of these.244... 

An alkyl group located on a carbon a to a hetero atom prefers the equatorial position, which is of course the normally expected behavior, but a polar group in such a location prefers the axial position. An example of this phenomenon, known as the anomeric effect,24 is the greater stability of a a-glucosides over p-glucosides. A number of explanations have... 

From aromaticity and elementary analysis the total number of rings per mean unit (R) can be derived. The number of nonaromatic rings is then given by R r = R — R.r and includes alicyclic and heterocyclic rings. If all hetero atoms were to form a ring, there could be a maximum of — 0.8 such rings per unit in all three macerals. Limits for the number of alicyclic rings per unit... 

These are the most recent <83PAC4iw> recommendations. The earlier recommendations are still substantially used in modern literature, including other chapters of this present work. b Used when the ring contains the maximum number of noncumulative double bonds and when the hetero atoms have the valences shown in Table 2. 

The position of a single hetero atom determines the numbering in a monocyclic compound. 

When the same hetero atom occurs more than once in a ring, the numbering is chosen to give the lowest locants to the hetero atoms. 

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