Abstract: (6R)-Tetrahydro-L-biopterin of the following formula: ##STR1## is prepared by catalytically reducing L-erythrobiopterin or an acyl derivative thereof in the presence of an amine, at pH 10-13 and with a platinum-base catalyst and when one or more acyl groups still remain, then removing the acyl groups. (6R)-Tetrahydro-L-biopterin is effective for the treatment of certain serious neuroses and malignant hyperphenylalaniemia. The present invention has succeeded in preparing (6R)-tetrahydro-L-biopterin at a high asymmetric ration R/S and moreover with a high yield. Since the process of this invention makes use of an amine as a base, the process is free from admixture of any inorganic salt and can hence provide high-purity crystals with ease.

Abstract: N.sup.4 -Acyloxy-2,2'-cyclocytidines of the formula: ##STR1## and N.sup.4, 3',5'-tri-O-acyl-2,2'-cyclocytidines of the formula: ##STR2## wherein R' and R are the same and represent an acyl group; which are useful for decreasing the total packed cell volume ratio (T/C%) with respect to ascitic form of Sarcoma 180 in small animals, and thus prolonging their lives.