Abstract: The present invention relates to methods for the preparation of diastereomerically pure (3R,3aS,6aR)hexahydro-furo[2,3-b]furan-3-ol as well as a novel intermediate, (3aR,4S,6aS)4-methoxy-tetrahydro-furo[3,4-b]furan-2-one for use in said methods. More in particular the invention relates to a stereoselective method for the preparation of diastereomerically pure (3R,3aS,6aR)hexahydro-furo[2,3-b]furan-3-ol, as well as methods for the crystallization of (3aR,4S,6aS)4-methoxy-tetrahydro-furo[3,4-b]furan-2-one and for the epimerization of (3aR,4R,6aS)4-methoxy-tetrahydro-furo[3,4-b]-furan-2-one to (3aR,4S,6aS)4-methoxy-tetrahydro-furo[3,4-b]furan-2-one.

Abstract: The invention relates to a process for the preparation of (S)-glyceraldehyde acetonide in aqueous solution from 3,4-O-isopropylidene-L-threonic acid or a salt thereof in aqueous solution, and hypochlorite in aqueous solution wherein the aqueous hypochlorite solution has a pH >7.5 and wherein during addition of at least 0.1 molar equivalents of hypochlorite based on the amount of 3,4-O-isopropylidene-L-threonic acid, an acid solution is not simultaneously added. The invention also relates to a process according to the invention, wherein 3,4-O-isopropylidene-L-threonic acid or a salt thereof is prepared from 5,6-O-isopropylidene-L-ascorbic acid or a salt thereof in the presence of H2O2 and a base in a manner known per se, wherein excess H2O2 is optionally removed by catalase.