Esters nomenclatural priority

Deprotonation of a-alkylated acetic acid esters (e.g., the propionic acid ester of Figure 10.13) with LDA at —78°C selectively yields the -enolatcs.Thc quotation marks indicate that this application of the term is based on an extension of the E/Z-nomenclature here, the Cahn-Ingold-Prelog priority of the O I, i1 substituent is considered to be higher than the priority of the OR group. The deprotonation of the ester shown in Figure 10.13 occurs via the strain-free transition state A. The alternative transition state B is destabilized by a 1,3-diaxial interaction. 

In this context, the E- and Z-nomenclature of ester enolates and silyl ketene acetals refers to their geometries where the carbonyl oxygen given highest priority irrespective of priority by CIP Cahn-Ingold-Prelog) rules. 

One special application comes when silyl enol ethers of allylic esters provide the starting material this is known as the Claisen-Ireland reaction. Esters normally form E-enolates 181 with LDA at low temperature. Because the E/Z nomenclature depends on the hierarchy of the substituents it is particularly ridiculous when applied to enol derivatives of esters. A lithium enolate (Li < C) would have the opposite stereochemical label from a silyl enol ether (Si > C) so a uniform scheme is adopted whereby the metal - O bond always has priority over the other. The mechanism 180 may remind you of the reasons for this - they are discussed in more detail in chapter 4. 

Because of the higher priority give to sulfur, the Z monothioketene acetal derived from the corresponding monothioester is mechanistically equivalent to the ketene acetal derived from an ester. Care must be taken when comparing the results from monothioketene acetals with the results from ketene acetals to avoid confusion as a result of the nomenclature inversion. 

For the sake of simplicity, the nomenclature of ester enolates will be independent of the associated cation OHM+ always takes priority over OR. 

At a glance, the descriptors Z and E might seem to be appropriate for O - metal-bound enolates like 6. Indeed, E/Z nomenclature causes no problems when the configuration of preformed enolates derived from aldehydes, ketones, and amides has to be assigned, because the O-metal residue at the enolate double bond has the higher priority. However, application of the E/Z descriptors to ester enolates leads to the dilemma that enolates with different metals but otherwise identical structures will be classified by opposite descriptors, as illustrated by lithium and magnesium enolates 9 and 10, respectively the former would have to be termed Z, and the latter E (Scheme 1.4). 

---