Abstract: A process for preparing S-metoprolol of the formula ##STR1## or a salt thereof, with high enantiomeric purity, is described, whereby a (S)-5-hydroxymethyl-3-isopropyloxazolidin-2-one sulfonic acid ester of the formula ##STR2## is prepared, and further reacted with 4-[2-methoxyethyl]phenol of the formula ##STR3## and the resulting intermediate of the formula ##STR4## is hydrolysed to S-metoprolol, and whereby the (S)-5-hydroxymethyl-3-isopropyloxazolidin-2-one sulfonic acid ester of formula II is prepared by reacting (S)-3-isopropylamino-1,2-propanediol of the formula ##STR5## with a chloroformic acid ester of the formula ##STR6## wherein R' is an alkyl group having 1-3 carbon atoms or a phenyl group, to the formation of (S)-5-hydroxymethyl-3-isopropyloxazolidin-2-one of the formula ##STR7## which is reacted with an activated sulfonic acid of the formula ##STR8## wherein R" is an aryl group such as tolyl and X is a halogen such as Cl, to formation of the ester of formula II, which when required is enriched wit

Abstract: A method of controlling the epimeric distribution in the preparation of 16,17-acetals of pregnane derivatives characterized in reaction of a corresponding 16,17-acetonide with an aldehyde or ketone in (a) hydrocarbon solvent wherein the solubility of the pregnane derivative is less than 1 mg/l or in a halogenated hydrocarbon solvent together with a hydrohalogen acid or an organic sulphonic acid as catalyst and in the presence of small grains of an insert material in the reaction medium or (b) a halogenated hydrocarbon solvent together with a hydrohalogen acid an organic sulphonic acid as catalyst and in the presence of an epimeric distribution modifier.