The chemistry of heterocycles (second edition)

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[...]... marked by consecutive lettering according to the serial numbering of the base component In contrast to the example on p 9, the fused component must also be numbered, always observing the principle of assignment to the lowest possible locants The name of the fused component, by the replacement of the terminal V with 'o', is put before the name of the base component The atoms common to both rings are described... wherein the sequence of the numbers must correspond to the direction of the lettering of the base component Finally the whole system is numbered • Indicated hydrogen In some cases, heterocyclic systems occur as one or more structural isomers which differ only in the position of an H-atom These isomers are designated by indicating the number corresponding to the position of the hydrogen atom in front of the. .. - diazocine The Structure of Heterocyclic Compounds From the annulenes, one can formally derive two types of heterocycles: — systems of the same ring size, if CH is replaced by X — systems of the next lower ring size, if HC=CH is replaced by X In both cases, the resulting heterocycles are iso-^-electronic with the corresponding annulenes, i.e the number of ^-electrons in the ring is the same This... the successive numbers of the ring atoms by the letters a, b, c, etc The letter b in brackets between benzo and the name of the base component denotes the atoms of the base component which are common to both rings The letter must be as early as possible alphabetically and hence benzo[c/]furan is incorrect It is generally accepted that the numbering of the whole system in the case of bi- and also polycyclic... shown on the cover of issues of the Journal of Heterocyclic Chemistry The guiding principle is ring size (see Table 2) Heterocycles of certain ring sizes are further subdivided according to the type of heteroatoms, following the sequence shown in Table 1, starting with one heteroatom, two heteroatoms, etc The parent compound is covered first, provided it is known or of importance It is followed by the benzo-fused... give the corresponding conjugate bases This is followed by an intramolecular displacement of the halogen atom as the rate-determining step e /A + ci \ ci In spite of the ring strain in the product and the considerable activation enthalpy, the reaction occurs rapidly at room temperature owing to favourable entropy The activation entropy is affected only by the loss of the degree of freedom of the internal... systems starts at the heteroatom • Monocyclic systems, two or more identical heteroatoms The prefixes di-, tri-, tetra-, etc., are used for two or more heteroatoms of the same kind When indicating the relative positions of the heteroatoms, the principle of the lowest possible numbering is used, i.e the numbering of the system has to be carried out in such a way that the heteroatoms are given the lowest possible... 171 nm (Ig £= 3.34) The chemical shifts in the NMR spectrum are .