acetonides

Acetonides are cyclic acetals derived from acetone and a vicinal diol. In acid-catalyzed condensations, acetone reacts with 1,2- or 1,3-diols to form a five-membered 1,3-dioxolane ring, known as an acetonide. The ring incorporates two methyl groups from acetone, giving the isopropylidene acetal protecting group.

The primary use of acetonides is as protecting groups for adjacent hydroxyl groups in carbohydrate and natural product synthesis. By converting two neighboring hydroxyls into a single acetonide, chemists can prevent those positions from participating in reactions while other parts of the molecule are modified. Acetonide protection is particularly common for sugars such as glucose, ribose, and related derivatives, where it helps control regio- and stereochemistry during multistep syntheses.

Stability and deprotection: Acetonide protections are generally stable under basic conditions and many organic reactions, but can be removed under aqueous acidic conditions, returning the original diol. Deprotection is typically achieved with mild acid hydrolysis, sometimes facilitated by water or specific acid catalysts.

Preparation and scope: The acetonide is formed by combining the diol with acetone in the presence of a catalytic acid under dehydrating conditions, driving water removal. The approach is widely applicable to a range of vicinal diols, though sterically hindered diols may react more slowly.

Variants: The term acetonide most often refers to the isopropylidene acetal protecting group, but related cyclic acetals derived from acetone can be described as acetonides in a broader sense. The isopropylidene acetal remains the dominant form in carbohydrate chemistry.