Planar chromatography of cholic acid-derived cis-trans isomeric bis-steroidal tetraoxanes
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Seven pairs of cis-trans isomers of bis-steroidal tetraoxanes have been examined by both normal-phase (NP) and reversed-phase (RP) planar chromatography. Unmodified silica gel was used with ethyl acetate-toluene and ethyl acetate-petroleum ether as mobile phases in typical normal-phase systems. CN-silica with the mobile phases methanol-water and acetone-water and RP-18 silica with water-organic modifier (methanol, acetone, or dioxane) mobile phases were used as reversed-phase systems. For the RP systems a good linear correlation was established between R M values and amount ([%] ν/ν ) of organic modifier in the mobile phase (usually r > 0.99). It was found that under both NP and RP conditions cis isomers were more weakly retained than the corresponding trans isomers. The only exception to this was the chromatographic behavior of C(24) methyl esters. It was established that increasing the polarity of substituents at C(24) and C(24a) led to stronger retention in NP systems, i.e. weake...r retention in RP systems. Highly selective separation was achieved in all the chromatographic systems investigated. Possible separation mechanisms are discussed on the basis of the results obtained.