Bile acid-derived mono- and diketals--synthesis, structural characterization and self-assembling properties

Three oxo-derivatives of bile acid methyl esters have been used as starting compounds in the preparation of novel bile acid monoketals with 1,2-benzenediol (catechol) and 2,3-naphthalenediol, as well as mono- and diketals with pentaerythritol. Monoketals of pentaerythritol showed a tendency to form thermoreversible gels in many aromatic solvents and the methyl lithocholate derivative proved to be a supergelator able to form a gel with t-butylbenzene at a concentration as low as 0.5% w/v. Whereas the naphthalenediol ketals formed film-type materials in the studied solvents, the catechol ketals underwent rapid crystallization into X-ray quality single crystals. Single crystal X-ray structures of the catechol ketals have been determined. The monoketal obtained from methyl-3,7,12-trioxo-5beta-cholan-24-oate (dehydrocholate) revealed to have an unusual packing pattern in its solid state compared to other bile acid derivatives reported in the literature. The synthesis of diketals from pentaerythritol furnished a mixture of two diastereomers which, in the case of the methyl lithocholate derivative, have been separated and the X-ray crystal structure of one isomer resolved.