Ring numbering scheme

The purine ring-numbering scheme, 1, and structures of some simple purines, viz. adenine 2, guanine 3, caffeine 4, theophylline 5, adenosine 6, 2 -deoxyadenosine 7, guanosine 8, 2 -deoxyguanosine 9, xanthine 10, and hypoxanthine 11, are shown. 

Fig. 2.23 Some geochemically important carotenoids (ring numbering scheme shown for fucoxanthin). 

Fig. 2.27 Some geochemically important chlorophylls (ring numbering scheme shown).The arrows from N atoms to Mg2+ ions denote dative bonds (see Section 2.1.2).

Figure 7.42 Steroids are biologically derived from cholesterol, and the ring-numbering scheme used to describe these compounds is shown. Androstane is an example of a steroid molecule with a methyl group at the Cl9 position, an important position for classification. 

We earlier recalled that one can build a huge number of molecules by simply assembling CeH rings, a scheme that was summarized in Table 1.4, and that a solid such as graphite can be obtained in this gedanken synthesis. Let us briefly discuss the case of linear acenes, which are linear polycyclic aromatic hydrocarbons (PAHs) composed of laterally fused CeHe rings. 

Figure 12.1 Numbering scheme in the l-hexyl-3-methylimidazolium cation, [CjCjImjL showing the three ring protons H2, H4, and H5.

The numberings of the ring carbons in each saturated structure happen to be identical. However, this is not always the case for corresponding pairs of unsaturated structures because of the differences in the bicyclo and the fixed numbering schemes for naming compounds. 

The numbering scheme for each ring system is implicit, whenever it can be deduced from the standard numbering convention. According to this convention, peripheral atoms of the macrocycle are first numbered in the clockwise direction, followed by the internal atoms (also clockwise). The first peripheral atom in the sequence is shown in each new structure. The first internal atom is the one adjacent to the first peripheral atom. Systems differing from the above rules are provided with explicit numbering. 

Figure 9-1. Structures and atomic numbering schemes of the five nucleobases. Numbers are given only for the ring atoms...

Another characteristic feature of MiBs is that the number of the components remained the same when any given MCR was transformed into a multiple MCR. Using Ugi-4CR as an example, there are six different random combinations to perform the MiBs, each of them will produce a macrocycle with different ring coimectivities (Scheme 19). 

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