Abstract: The process for preparing 5-deoxy-arabinose comprises reacting 5-tosyl-L-arabinose-dialkylmercaptal with NaBH.sub.4 in DMSO to give 5-deoxy-L-arabinose-dialkylmercaptal and then reacting the obtained 5-deoxy-L-arabinose-dialkylmercaptal with hydrochloric acid in DMSO. According to the process, 5-deoxy-L-arabinose can be obtained in a high yield without employing heavy metal such as mercury.

Abstract: A novel compound, 1',2'-diacyl-(6R,S)-5,6,7,8-tetrahydro-L-biopterin which is prepared by catalytical hydrogenation of a 1',2'-diacyl-L-biopterin in a solvent in the presence of a catalyst. The 1',2'-diacyl-L-biopterin is prepared from a 1,1-dialkylsulfonyl-L-rhamnose through an acyl derivative of 5-deoxy-L-arabinose and a hydrazine derivative of tetrahydro-L-biopterin without isolating the intermediate products. The 1', 2'-diacyl-(6R,S)-5,6,7,8-tetrahydro-L-biopterin can be used for treatment of atypical phenylketonuria or dihydropterin-reductase deficiency and can readily cross the blood brain barrier without neurotransmitter precursors.