We hope that our survey of the important methods for reduction and oxidation has shown you that, by choosing the right reagent, you can often get reaction only at the functional group you want. The chemoselectivity you obtain is kinetic chemoselectivity—reaction at one functional group is simply faster than at another. Now look at the acylation of an amino alcohol (which is, in fact, a synthesis of the painkiller isobucaine) using benzoyl chloride under acid conditions. The hydroxyl group is acylated to form an ester. Yet under basic conditions, the selectivity is quite different, and an amide is formed.

A clue to why the selectivity reverses is shown below—it is, in fact, possible to interconvert the ester and the amide simply by treating either with acid or with base.

The selectivity in these reactions is thermodynamic chemoselectivity. Under conditions in which the ester and amide can equilibrate, the product obtained is the more stable of the two, not necessarily the one that is formed faster. In base the more stable amide predominates, while in acid the amine is protonated, which prevents it from acting as a nucleophile and removes it from the equilibrium, giving the ester.

مواضيع ذات صلة

The Boc protecting group—gastrin and aspartame

The Cbz protecting group—oxytocin

Peptide synthesis

A survey of protecting groups

How to react the less reactive group (II): protecting groups

Chemoselectivity in the reactions of dianions

How to react the less reactive group (I): react both then ‘unreact’ one

How to oxidize primary alcohols to aldehydes

How to oxidize secondary alcohols to ketones

Birch reduction of arenes

Dissolving metal reductions

Getting rid of functional groups