Conformations cyclic compounds

Conformational Control with Six-Membered Rings Addition of nucleophiles to cyclic ketones Reactions of exocyclic enolates Part II - Chirality in Place from the Start Using one chiral centre to control another Reactions of Cyclic Compounds Conformational Control The half-chair... 

Perhaps the most notable difference between S-N and N-O compounds is the existence of a wide range of cyclic compounds for the former. As indicated by the examples illustrated below, these range from four- to ten-membered ring systems and include cations and anions as well as neutral systems (1.14-1.18) (Sections 5.2-5.4). Interestingly, the most stable systems conform to the well known Htickel (4n -1- 2) r-electron rule. By using a simple electron-counting procedure (each S atom contributes two electrons and each N atom provides one electron to the r-system in these planar rings) it can be seen that stable entities include species with n = 1, 2 and 3. 

It is noteworthy that the value of this substrate is smaller by one order compared to non-cyclic compounds. According to the discussions proposed above, this is considered to be due to its conformation already being fixed to the one that fits to the binding site of the enzyme. This estimation was demonstrated to be true by the examination of the effect of temperature on the kinetic parameters. Arrhenius plots of the rate constants of indane dicarboxylic acid and phenyl-malonic acid showed that the activation entropies of these substrates are —27.6 and —38.5 calmol K , respectively. The smaller activation entropy for the cyclic compound demonstrates that the 5yn-periplanar conformation of the substrate resembles the one of the transition state. 

In cyclic compounds the conformation from which elimination can take place may to a considerable extent be enforced by the relative rigidity of the ring structure. Thus for a series of eliminations from different sized rings, the following degrees of stereoselectivity were observed for HY elimination from the cyclic compounds (CH2)nCHY ... 

Depending on the molar ratio, 8 reacts with SiF4 to give the acyclic or cyclic compounds 9 or 10. 10 represents the first halofunctional hydrazine ring system. The ring is far from planar and has a twist conformation [4,5]. 

The process of assessing the preferred conformation has become of importance of cyclic compounds, at least in six-membered rings because in their formation, almost all the strains are involved. We have seen that cyclohexane exists mostly in the chair conformation and the boat form occurs in negligible proportion because of higher energy which is of the order of >6 K cals/mole. 

More evidence for the existence of several conformational isomers, at least in liquid and gaseous substances comes from infrared and also Raman spectra. For example each conformer has its own I.R. spectrum, but the peak positions are often different. Thus the C-F bond in equatorial fluorocyclohexane absorbs at 1062 Cm-1, the axial C-F bonds absorbes at 1129 Cm . So the study of infrared spectrum tells, which conformation a molecule has. Not only this, it also helps to tell what percentage of each conformation is present in a mixture and since there is relationship between configuration and conformation in cyclic compounds the configuration can also be frequently determined. 

The last section is a compilation of useful empirical additivity rules for 13C chemical-shift prediction, regardless of underlying transmission mechanisms. Peculiarities of less common substituents are cited, and, finally, substituent-induced 13C signal shifts in various cyclic compounds are discussed, with special emphasis on their use in conformational analysis I do not, however, claim completeness for this latter discussion. 

The practical consequences of conformational isomerism become much more significant when we consider cyclic compounds. The smallest ring system will contain three atoms in the case of hydrocarbons this will be cyclopropane. 

However, note carefuUy that changing the conformation does not affect the spatial sequence about the chiral centres, i.e. it does not change the configuration at either chiral centre. This seems a trivial and rather obvious statement, and indeed it probably is in the case of acyclic compounds. It is when we move on to cyclic compounds that we need to remember this fundamental concept, because a common mistake is to confuse conformation and configuration (see Box 3.11). 

The same stereochemical principles are going to apply to both acyclic and cyclic compounds. With simple cyclic compounds that have little or no conformational mobility, it is easier to follow what is going on. Consider a disubstituted cyclopropane system. As in the acyclic examples, there are four different configurational stereoisomers possible, comprising two pairs of enantiomers. No conformational mobility is possible here. 

Chemical transformation of acyclic diastereomers into cyclic derivatives and subsequent determination of relative configuration is a common and secure method. In general, there are far fewer conformations in cyclic compounds and the stereochemical dependence of NMR parameters is well documented. A few representative examples are given in this section (see also Section A.4.3.). 

---