Why are acetals so important? Well, they’re important to both nature and chemists because many carbohydrates are acetals or hemiacetals (see the box below). One important use that chemists have put them to is as protecting groups. One synthesis of the steroid class of com pounds (about which more later) requires a Grignard reagent with an impossible structure. This compound cannot exist as the Grignard functional group would attack the ketone: it would react with itself. Instead, the protected Grignard reagent is used, made from the same bromoketone, but with an acetal-forming step.

Acetals, as we stressed, are stable to base and to basic nucleophiles such as Grignard rea gents, so we no longer have a reactivity problem. Once the Grignard reagent has reacted with an electrophile, the ketone can be recovered by hydrolysing the acetal in dilute acid. The acetal is functioning here as a protecting group because it protects the ketone from attack by the Grignard reagent. Protecting groups are extremely important in organic synthesis and we will return to them in Chapter 23.

Acetals in nature.

 We showed you glucose as on p.137 an example of a stable, cyclic hemiacetal. Glucose can, in fact, react with itself to form an acetal known as maltose. Maltose is a disaccharide (made of two sugar units) produced by the enzy hemiacetal matic hydrolysis of starch or cellulose, which are themselves polyacetals made up of a string of glucose units.

مواضيع ذات صلة

Amines react with carbonyl compounds

Cyclic acetals are more stable than acyclic acetals

Acetals hydrolyse only in the presence of acid

Amines from imines: reductive amination

Making acetals

Acetals are formed from aldehydes or ketones plus alcohols in the presence of acid

Acid or base catalysts increase the rate of equilibration of hemiacetals with their aldehyde and alcohol parents

Aldehydes can react with alcohols to form hemiacetals

Making the products less reactive

Making the starting materials more reactive

A bit of shorthand

Making alcohols instead of ketones