Structure and mechanism of formation of the two forms of 18-hydroxy-11-deoxycorticosterone

18-Hydroxy-11-deoxycorticosterone (18-OH-DOC) has been observed to exist in two interconvertible forms of markedly different chromatographic mobility. The structure of these forms has now been established. The more polar form is the cyclic hemiketal and the less polar is a mixed ketal at C-20 derived by reaction with an alcoholic solvent. This unusually facile ketalization is catalyzed by traces of acidic impurities present in most commercial sources of reagent grade methanol or ethanol, but can be abolished by removal or neutralization of these impurities.