Abstract: A process for producing an optically active form of a compound of formula (I) ##STR1## wherein R.sub.1 represents hydrogen or a hydrocarbon group that may be substituted; R.sub.2 and R.sub.3 independently represent hydrogen, a hydrocarbon group that may be substituted, or a heteroaromatic group that may be substituted; X' represents a substituent comprising an esterified carboxyl group or an acylated hydroxyl group; ring A represents a benzene ring that may be substituted or a heteroaromatic ring that may be substituted; ring J' represents a 7- or 8-membered heterocyclic ring containing at most 3 hetero-atoms as ring-constituent members, which may have a further substituent or substituents in addition to R.sub.1, R.sub.2, R.sub.

Abstract: A process is disclosed for producing a 4-O-sulfonyluracil derivative of the formula ##STR1## [wherein R.sup.1 is hydrogen, halogen, alkyl and alkoxy, R.sup.2 is glycosyl having protected hydroxyls; and --SO.sub.2 R.sup.3 is an organic sulfonyl] which comprises reacting an uracil derivative of the formula ##STR2## [wherein R.sup.1 and R.sup.2 are as defined above] with an organic sulfonylating agent of the formula R.sup.3 SO.sub.2 X [wherein R.sup.3 SO.sub.2 is an organic sulfonyl and X is halogen] in the presence of potassium carbonate. The obtained 4-O-sulfonyluracil is further subjected to amination. The processes are industrially adavantageous and produce cytosine derivatives such as cytidine in high yield.

Abstract: A cytosine nucleoside is produced by reacting a peracylcytosine with a hydroxy-protected monosaccharide or a hydroxy-protected nucleoside whose base moiety is other than cytosine and, if necessary, removing the hydroxy-protecting group.