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

step 1 So for this problem, we are going to draw a protein synthesis reaction, and we're going to assig

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 in double-bond stereochemistry:

Key Concepts

Substituent Priority Changes Upon Reaction

During chemical reactions, particularly those involving functional group transformations such as deprotection or cleavage reactions, the substituents attached to a double bond may change their chemical identity. Even if the geometry of the double bond does not change physically, the priorities of the substituents can shift, leading to a re-assignment of the E/Z designation. This apparent change in stereochemistry is due to the change in the CIP priorities after the chemical transformation.

Acid-Catalyzed Deprotection Mechanism

Acid-catalyzed deprotection reactions involve the protonation of a functional group, leading to its cleavage or transformation under acidic conditions. In the context of protein synthesis, protecting groups like tert-butyl esters are removed using acids such as trifluoroacetic acid. This mechanism typically proceeds via the formation of an intermediate that ultimately changes the attached substituents, thereby modifying the priority order used in the stereochemical assignment without necessarily altering the double bond’s intrinsic geometry.

Cahn–Ingold–Prelog (CIP) Priority Rules

These rules are used to assign priorities to groups attached to stereocenters or double bonds by comparing atomic numbers and bonding situations. It is the basis for determining the configuration (E or Z) of alkenes. In alkenes, the two substituents on each sp² carbon are ranked, and the spatial arrangement of the highest priority groups is used to assign the configuration.

Alkene Stereochemistry (E/Z Designation)

Alkene stereochemistry is described using the E/Z system, where 'E' (from the German 'entgegen', meaning 'opposite') indicates that the highest priority substituents on the alkenic carbons are on opposite sides, and 'Z' (from 'zusammen', meaning 'together') indicates they are on the same side. This assignment relies on the CIP priority rules and is crucial for understanding the stereochemical outcome of reactions involving double bonds.

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