PURPOSE: To prepare the titled compound with decreased amount of solvent and simplified procedure, in high yield, by using 1',1'-dialkylsulfonyl-L-rhamnose as a starting raw material, and carrying out the reaction of the material continuously without separating the unstable intermediate.

CONSTITUTION: The objective compound of formula III (R1 and R2 are acyl) is prepared by (1) reacting 1',1'-dialkylsulfonyl-L-rhamnose of formula I (R is alkyl) with e.g. NH4OH, (2) reacting the resultant 5-deoxy-L-arabinose with phenylhydrazine to obtain the compound of formula II, (3) acylating the product, and (4) reacting the obtained 2,3,4-triacyl-5-deoxy-L-arabinose-phenyl-hydrazone with 2,4,5-triamino-6-hydroxy-pyrimidine dihydro-chloride. The whole process is carried out continuously without separating the intermediates.

USE: Precursor of a drug for the remedy of phenylketonuria, ademonia, Parkinson's disease, etc.