tert-Butyl esters [RCO2 C(CH3)3] are converted into carboxylic acids (RCO2 H) by reaction with t

step 1 Key butyl esters are hydrolyzed by trifloroacetic acid. Determine EZ configurations of the doubl

Tert-Butyl esters [RCO2 C(CH3)3] are converted into...

tert-Butyl esters $\left[\mathrm{RCO}_{2} \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\right]$ are converted into carboxylic acids $\left(\mathrm{RCO}_{2} \mathrm{H}\right)$ by reaction with trifluoroacetic acid, a reaction useful in protein synthesis (Section $26.7)$. Assign $E, Z$ designation to the double bonds of both reactant and product in the following scheme, and explain why there is an apparent change of double-bond stereochemistry:

Key Concepts

E/Z Stereochemistry

E/Z stereochemistry refers to the method of describing the relative positions of substituents around a carbon–carbon double bond. The designation is determined by comparing the priorities of the substituents on each carbon; if the higher priority groups are on opposite sides, the configuration is denoted as E (from the German 'entgegen'), and if they are on the same side, the configuration is denoted as Z (from 'zusammen').

Cahn–Ingold–Prelog Priority Rules

The Cahn–Ingold–Prelog (CIP) rules provide a systematic way to rank substituents around double bonds or stereocenters based on atomic number and the connectivity of the atoms. This ranking is essential for correctly assigning the E or Z designation to a double bond, ensuring that nomenclature reflects the actual spatial arrangement of groups.

Impact of Functional Group Transformation on Stereochemical Assignment

Chemical reactions that change a functional group, such as converting a tert-butyl ester into a carboxylic acid, can alter the set of substituents connected to a double bond. This, in turn, may change the priority order as determined by the CIP rules, leading to a different E/Z designation even though the actual three?dimensional arrangement of the atoms may not have physically changed. This apparent change arises solely from the shifting priorities of the substituents between reactant and product.

Use of Protecting Groups in Synthesis

Protecting groups like the tert-butyl ester are employed in synthesis to temporarily mask reactive functional groups. Their selective removal under specific conditions (for example, using trifluoroacetic acid) allows for controlled reactions at other parts of the molecule while preserving the rest of the molecular framework, including its stereochemistry. The design and deprotection of these groups are critical in complex syntheses, such as in protein synthesis.

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