The more usual way of reacting a less reactive group in the presence of a more reactive one is to use a protecting group. This tertiary alcohol, for example, could be made from a keto-ester if we could get phenylmagnesium bromide to react with the ester rather than with the

As you would expect, simply adding phenylmagnesium bromide to ethyl acetoacetate leads mainly to addition to the more electrophilic ketone.

One way of making the alcohol we want is to protect the ketone from attack by disguising it from the nucleophile. An acetal protecting group (shown in black) is used.

The fi rst step puts the protecting group on to the (more electrophilic) ketone carbonyl, making it no longer reactive towards nucleophilic addition. The Grignard then adds to the ester, and finally a ‘deprotection’ step, acid-catalysed hydrolysis of the acetal, gives us back the ketone. An acetal is an ideal choice here—acetals are stable to base (the conditions of the reaction we want to do), but are readily cleaved in acid.

مواضيع ذات صلة

The Boc protecting group—gastrin and aspartame

The Cbz protecting group—oxytocin

Peptide synthesis

A survey of protecting groups

Chemoselectivity in the reactions of dianions

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

Competing reactivity: choosing which group reacts

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