Ketals

Ketals are a class of acetal compounds formed from ketones and diols. When a ketone reacts with a diol, the carbonyl carbon is converted into a carbon bearing two alkoxy groups, giving a structure generally written as R1R2C(OR')2. If the diol is cyclic, the product is typically a cyclic ketal such as a 1,3-dioxolane or 1,3-dioxane; if the diol is acyclic, a dialkyl acetal is formed. Cyclic ketals are common protective groups in organic synthesis.

Ketals are typically formed under acid catalysis in the presence of a dehydrating agent or by removing water, since the condensation is reversible. The reaction is equilibrium-based: ketals can be hydrolyzed back to the original ketone and diol under acidic conditions in the presence of water. They are generally stable to bases and many nucleophiles, but susceptible to acid; the rate of hydrolysis depends on the ketone, the diol, and the protecting group.

In practice, ketals are widely used as carbonyl protecting groups. They are especially important in carbohydrate and natural product synthesis, where vicinal diols can be protected as isopropylidene or acetonide acetals. Examples include the 2,2-dimethyl-1,3-dioxolane ring formed from acetone and ethylene glycol (an acetonide) and various isopropylidene acetals used to shield diols. Deprotection is achieved by acid hydrolysis to reveal the original ketone and diol.