This compound can be prepared in two related ways: 1) The catalytic hydrogenation of biopterin (I) with H2 over PtO2 aqueous K2HPO4 at pH 11.4 or aq. (Et)4NOH at pH 12 yields a solution which is acidified with HCl. After evaporation, the residue is crystallized in ethanol - HCl. 2) The acetylation of biopterin (I) with refluxing acetic anhydride gives the triacetyl derivative (II), which is hydrogenated with H2 over PtO2 in trifluoroacetic acid, yielding the (6RS)-mixture of triacetyl derivatives (III). Acetylation of (III) with refluxing acetic anhydride affords the tetracetyl (6RS)-derivative (IV), which by fractional crystallization or column chromatography of the dihydrochloride in methanol gives the desired compound as pure (6R)-isomer.