Ring compounds, structure five-membered

Table 1 X-Ray Structures of Compounds with Five-membered Rings and Two, Three or Four Heteroatoms ...

Polycyclic compounds are common in nature, and many valuable substances have fused-ring structures. For example, steroids, such as the male hormone testosterone, have 3 six-membered rings and 1 five-membered ring fused together. Although steroids look complicated compared with cyclohexane or decalin, the same principles that apply to the conformational analysis of simple cyclohexane lings apply equally well (and often better) to steroids. 

This hexaamine cage ligand is derived from sar (see Section 17.16) but the three annular chelate rings are not five-membered but six-membered chelate rings and the central carbon has two methyl substituents. These chelate rings have skew-boat conformations, and their orientation is lel3 for one and ob3 for the other of the two conformers. Build these two structures in HyperChem and refine them as cobalt(III) and cobalt(II) compounds. Compute the reduction potentials and compare them with the data in Table 17.18.2. 

The best known mesoionic compounds have five-membered rings, and initially it was advocated by Baker, Ollis, Ramsden and other authors that only five-membered heterocycles which cannot be satisfactorily represented by any one covalent or ionic structure possessing a sextet of TT-electrons in association with the five atoms comprising the ring may be called "mesoionic". Here, following Katritzky, mesoionic means a mesomeric betaine. The first such compounds to be discovered were sydnones, followed by miinchnones and then by diazolones. In all these compounds the Z-type atom is part of a carbonyl group, and two Y-type atom chains separate two odd-numbered chains of X- and Z-atom chains. Only the main resonance structures are displayed in formulas (Figure 11). 

Heterocyclic compounds are ubiquitous in biology, medicine, and biochemistry, and the azoles form an important class of nitrogen heterocyclic compounds with five-membered aromatic rings that can contain one to five nitrogen atoms (other heteroatoms can also be present). Many drugs and pharmaceuticals are derived from azole structures. The structures of representative azoles are shown in Figure 6.1. Note that some of these compounds exist in tautomeric equilibrium in solution. 

The relationship between these compounds is stereoisomeric (as in the previous section). These two compounds are not interconvertible by ring flipping. They are two diflferent compounds with different physical properties. Many naturally occurring compoimds, such as steroids, incorporate decalin systems into their structures. Steroids are a class of compoimds comprised of four fused rings (three six-membered rings and one five-membered ring). Below are two examples of steroids ... 

In spite of the diverse nature of alkaloid structures, two structural units, i.e. fused pyrrolidine and piperidine rings in different oxidation states, appear as rather common denominators. We therefore chose to give several examples for four types of synthetic reactions which have frequently been used in alkaloid total synthesis and which provide generally useful routes to polycyclic compounds with five- or six-membered rings containing one nitrogen atom. These are ... 

A compound (CgHjo) contains a five membered ring When Bt2 adds to it two diaste reomenc dibromides are formed Suggest reasonable structures for the compound and the two dibromides... 

Firefly lucifenn is an example of an azole that contains a benzene ring fused to the five membered ring Such structures are fairly common Another example is benzimidazole present as a structural unit m vitamin B12 Some compounds related to benzimidazole include purine and its ammo substituted derivative adenine one of the so called het erocychc bases found m DNA and RNA (Chapter 28)... 

Details of bond lengths and bond angles for all the X-ray structures of heterocyclic compounds through 1970 are listed in Physical Methods in Heterocyclic Chemistry , volume 5. This compilation contains many examples for five-membered rings containing two heteroatoms, particularly pyrazoles, imidazoles, Isoxazoles, oxazoles, isothlazoles, thlazoles, 1,2-dlthloles and 1,3-dlthloles. Further examples of more recent measurements on these heterocyclic compounds can be found in the monograph chapters. 

Ollis and Ramsden state that A compound may be appropriately called mesoionic if it is a five-membered heterocycle which cannot be represented satisfactorily by any one covalent or polar structure and possesses a sextet of electrons in association with the five atoms comprising the ring . From the point of view of systematic nomenclature, compounds of this type are difficult to deal with, since most available nomenclature systems are designed so as to name one particular bond- and charge-localized canonical form. 

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