How are acetals used as protecting groups?

How are acetals used as protecting groups?

Acetals are commonly used to protect the carbonyl groups of aldehydes and ketones from basic, nucleophilic reagents. Once the protection is no longer needed, the acetal protect- ing group is easily removed, and the carbonyl group re-exposed, by treatment with dilute aqueous acid.

Is glucose a cyclic acetal?

They often form readily, especially when they are 5- and 6-membered rings. In this case an intramolecular OH group reacts with the carbonyl group. Glucose and many other aldoses exist as cyclic hemiacetals whereas fructose and similar ketoses exist as cyclic hemiketals.

Why are cyclic acetals more stable?

Cyclic acetals are more stable than regular acetals because of the chelate effect, which derives from having both -OH groups of the acetal connected to each other in the diol.

Can be protected as cyclic acetals and ketals?

Compounds bearing cis-1,2-diols and cis- and trans- 1,3-diols can be protected as cyclic acetals and ketals or cyclic orthoesters. As has been shown for ethers, these blocking groups are stable to bases but are labile to acids. Diol protection as cyclic esters, sensitive to basic conditions, is also possible.

Why are acetals stable to base?

In the absence of acid, acetals are not susceptible to hydrolysis i.e. the reverse reaction. They are therefore quite stable to neutral and basic reaction conditions.

What are acetals used for?

Acetals are common carbonyl compound derivatives that are often used in Organic Synthesis as protecting groups for aldehydes and ketones, as well as in many other reactions.

How does glucose become cyclic?

Linear glucose has four chiral carbons (pointed to by the red arrows). The rest is one of two cyclic forms of glucose formed when the hydroxyl group on carbon 5 (C5) bonds to the aldehyde carbon 1 (C1), as shown below.

Does glucose have a hemiacetal group?

Cyclization of glucose to its hemiacetal form Glucose has an aldehyde group and five hydroxyl groups. Does that ring a bell? Yes, glucose can form an intramolecular cyclic hemiacetal.

Are cyclic hemiacetals more stable?

Cyclic hemiacetals are more stable and the equilibrium favours their formation. This is especially true when 5- or 6-membered rings are formed. Most sugars can exist in an open-chain form and several different cyclic hemiacetal forms.

Why are hemiacetals unstable?

You can see why hemiacetals are unstable: they are essentially tetrahedral intermediates contain- ing a leaving group and, just as acid or base catalyses the formation of hemiacetals, acid or base also catalyses their decomposition back to starting aldehyde or ketone and alcohol.

What is protection and deprotection of functional groups?

A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis. This step is called deprotection.

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