CYCLIC ACETALS AND KETALS – VALUABLE PRECURSORS IN ORGANIC SYNTHESIS

Acetals and ketals are a latent form of carbonyl compounds and important intermediates in synthetic and carbohydrate chemistry. There is probably not a single scheme for the synthesis of biologically active compounds in which the technique with diol protection of the carbonyl function has not been used at least once. This is due to the convenience of both setting up the protective group and removing it. Classical synthesis involves the addition of alcohols to aldehydes and ketones in slightly acidic or basic solutions. Before the stage of formation of hemiacetals, the reaction is catalyzed by both acids and bases. However, the conversion of a hemiacetal to an ace- tal occurs only under conditions of acid catalysis and proceeds via the SN1 mechanism through the stage of for- mation of a carbocation from the intermediate hemiacetal. Cyclic acetals and ketals can be classified according to the structure of the ketal center: external (1,3- dioxolane and 1,3-dioxane) and internal. Internal ketals and acetals are cyclic compounds formed from 1,4- and 1,5-oxyaldehydes and oxyketones in the presence of anhydrous acids. Prominent representatives of internal ketals and acetals are carbohydrates and bicyclic ketals containing an adjacent C-C bond at the site of which there is a ketal center, for example, chromanes – bicyclic ketals or hemiketals. Optically active cyclic acetals and ketals are prepared by reacting aldehydes or ketones with chiral diols in three ways. The classic method is to boil the corresponding aldehyde or ketone with alcohol, azeotropically re- moving water in a Dean-Stark apparatus. In α,-enones, to protect the keto group and eliminate the side reaction – migration of the double bond, an alternative approach is used, the Noyori method, which involves the use of disilyl ethers of chiral alcohols. In the case when the initial carbonyl compounds are unstable or are in the form of dimethoxyketals or acetals, the method of transketalization or acetalization is used. Chiral diols are used as alco- hol components, for example, 2,3-butanediols, 2,4-pentanediols, hydrobenzoins. They are capable of attaching nucleophiles and are prone to various rearrangements at acetal and ketal cen- ters. The value of bicyclic hemiketals lies in their use in the synthesis of lactones of various ring sizes. This re- view is devoted to these and some other chemical transformations of cyclic acetals and ketals.