Compounds of the formula I wherein R3 is CH2 O--CO--R4 and R4 is (C1-C5)-alkyl, tertiary butyl or benzyl may be prepared by reacting 1,4-dideoxy-1,4-imino-2,3-O-isopropylidene-??-D-lyxitol with a compound of the formula R6 --CO--R4 wherein R6 is a halogen preferably fluorine, chlorine, iodine, and bromine and R4 is (C1-C5)-alkyl, tertiary butyl or benzyl, under alkaline conditions.