Common skeleton

A basic assumption in such additivity schemes is that the interactions between the atoms of a molecule are of a rather short-range nature. This fact can be expressed in a more precise manner The law of additivity can be expressed in a chemical equation [1]. Let us consider the atoms (or groups) X and Y attached to a common skeleton, S, and also the redistribution of these atoms on that skeleton as ejqjressed by Eq. (1). 

The E-state indices are computed for each atom in a molecule and these topological indices are best suited for applications to datasets in which all molecules have a common skeleton. For example, in the case of a series of chemical compounds based on the general structure 1, the QSAR descriptors may be the E-state indices Si, S2, S3, S4, S5 and Sis. Similar descriptors may be computed from HS-state values. However, this approach limits the application of the E-state indices only to series of compounds that have a common skeleton. 

Flavonoids constitute a large class of polyphenols found in fruits and vegetables that share a common skeleton of phenylchromane. This basic structure allows a large number of substitution patterns leading to several subclasses of flavonoids, such as flavonols, flavones, flavanones, flavanols, anthocyanidins, isoflavones, dihydroflavonols, and chalcones. Among the diverse flavonoid subclasses, flavonols (especially quercetin) and flavanols (catechins) are the most abundant in our food. Flavonols are present in foods as diverse glycosides, whereas flavanols are usually found as aglycones. 

Classical QSAR models were focused on a rather small number of compounds. The basic assumption was that a certain model is specific for a certain chemical class, sharing a common skeleton. Modifications of some simple chemical features were reflected in the property. Thus, for instance, a series of linear alcohols present aquatic toxicity, which varies in a rather linear way with the chain length [3],... 

Tetrapyrroles are macrocychc ligands that provide a common skeleton to hemes (that contains iron), chlorophylls (that contains magnesium), corrinoids (that contains cobalt), siroheme (that contains iron), and methanogenesis factor F430 (that contains nickel). 

Excellent reviews on the use of this metal as catalyst in cross-coupling reactions are now available [35]. The mild reaction conditions, as well as the wide scope of compatible functionalities, characterize palladium eouplings, leading to a considerable simplification of the retrosyntheses—henee its increasing popularity. Examples are shown in Figure 11. Among the most common skeletons encountered in molecular wires, we have selected two representative examples now easily available by palladium chemistry tolanes and biaryls. 

Once a common skeleton for the chemical analogues is defined, regression analysis is performed, considering a number S of substitution sites ( = 1, 5), and for each site a number N. of different substituents. Flydrogen atoms are also considered as substituents if present in a substitution site of some compounds. 

Many drugs which have a common skeleton are grouped together, e.g. penicillins, barbiturates, opiates, steroids, catecholamines, etc. 

There are bicyclic monoterpenes too - a-pinene 89 and p-pinene 91 share a common skeleton with four- and six-membered rings but have the alkene in different places. There is a discussion in chapter 24 on the variable ee of a-pinene and it is better to make the ( )-enantiomer from the more reliable P-pinene 91 (99% ee) that can be isomerised with strong base ( KAPA ) 92 in 93% yield to ( -)-90 without loss of ee. Many asymmetric reagents for reduction (chapter 24) and chiral auxiliaries for asymmetric aldol reactions (chapters 27 and 30) are based on a-pinene.25... 

Further examination of Lythrum anceps has yielded seven new alkaloids, all exhibiting part quinolizidine skeletons (27).25 That the alkaloids possessed a common skeleton was shown by simple chemical interrelationships. The n.m.r. 

Cydopentadietie (11) is also readily available so that the Diels-Alder approach is particularly suitable for compounds with the skeleton (12)—a common skeleton in many natural products. 

Oleanes comform the other group of triterpenes found in Celastraceae that- with friedelanes- are the most common skeletons isolated from the family. 

However, the traditional approach may be considered to have some limitations. Besides the obvious requirements of the additional thermodynamic relationship, where only series of compounds with a common skeleton framework should be considered, conformational equilibria are not taken into account, and, in general, information on the 3D stmcture is not employed at all. 

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