Oxo-cyclo tautomerism

Ring-chain tautomerism in which a ring closure occurs via intramolecular addition of an OH group to a C=0 bond (Scheme 4) is observed widely in organic chemistry. This type of tautomerism is sometimes called ketolo-lactolic, keto-lactolic, or oxo-cyclo tautomerism. The ring tautomers are usually referred to as cyclic hemiacetals or hemiketals. 

Probing the Oxo-Cyclo Tautomerism of Natural Enantiomeric Cyclohemiacetals with 2,4-Dihydroxy-2i7-l,4-benzoxazin-3(4Ff)-one Skeleton... 

The structural lability of the cyclic hemiacetals is a consequence of their oxo-cyclo tautomerism, which allows a rapid interconversion of the (2R)-configurated enantiomer into the (2S)-enantiomer and vice versa via the ring-opened oxo form (Fig. (4)). An analogous and well investigated process is the formation of an equilibrium mixture of a-D-glucose and p-D-glucose on dissolution of the pure a- or P-isomer in water by mutarotation. 

Semiacetals are also stable if their formation is accompanied by ring closure, as with y- and 6-hydroxy carbonyl compounds904 (oxo-cyclo tautomerism of carbohydrates) [see (4)/(5)]. ... 

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