Hemiacetals, Lactols and Carbonyl Hydrates

Acyclic hemiacetals are in most cases unstable and exist only as transient intermediates, in contrast with cyclic hemiacetals (lactols), which are often stable, isolable compounds. Lactols may be prepared using various different methods, including the reduction of lactones, reduction of dicarbonyl compounds, or the selective oxidation of diols, as well as by addition of carbon nucleophiles to lactones or dicarbonyl compounds, deprotection of O-protected cyclic hemiacetals, or from enol ethers. Carbonyl hydrates are generally short-lived intermediates, although a limited number of stable carbonyl hydrates (generally derived from strained carbonyl compounds or carbonyl compounds bearing electron-withdrawing groups) have been documented.