Acyclic systems allylic strain

An instructive example of a 7i-facial, homoallylic OH-directed stereoselective epoxidation in an acyclic system, used for the construction of the natural product monensin, is depicted belowC As expected, the more electron-rich trisubstituted double bond in A would be more susceptible to epoxidation than the terminal double bond. To minimize allylic 1,3-strain between the ethyl group and the CH2CH=CH2 appendage, A should preferentially adopt the conformation B, in which the smallest substituent H (hydrogen) is now in the same plane as the ethyl group. This places the hydroxymethyl moiety (CH2OH) in proximity to the P-face of the double bond, leading, after treatment with mCPBA, to the formation of epoxide diastereomer C. 

Allylic strain The destabilization of a molecule, or an individual conformation, by van der Waals repulsion between substituents on a double bond and those in an allylic position [66]. Two types have been identified (see bold bond in figure) Al 2 strain occurs between substituents on an allylic carbon and the adjacent sp2 carbon. A 1 3 strain occurs between substituents on an allylic carbon and the distal sp2 carbon. The latter effect can be quite strong [67]. Originally [66], the terms were defined in the context of cyclohexane derivatives, but more recently the effects have been recognized as important factors in conformational dynamics of acyclic systems [67]. 

Next, enantiopure silicon allylation reagent will be presented, which already inherits Lewis acidity. It is accepted that Lewis acidity of silicon, as well as its high tendency to expand valence shell, increases [59, 60] if it is tetravalent and incorporated into strained four- or five-membered ring systems (strain-release Lewis acidity) [61]. This corresponds to smaller energy gaps between sp and dsp orbitals of a strained system as compared to an acyclic species. 

Another bond type with a marked conformational preference is the vinylic bond. Eclipsing interactions between vinyl and allylic substituents result in allylic 1,2-strain (A strain) and allylic 1,3-strain (A strain). Both the A and the A strain play an important role in determining the stereochemical outcome of many important reactions involving acyclic and cyclic systems (e.g., alkylation of enamines, alkylation and protonation of enolates). 

---