9H-Purin-6-amine, 9-.beta.-D-arabinofuranosyl-2-fluoro-, mixt. with 9-(2,5-di-O-formyl-.beta.-D-arabinofuranosyl)-2-fluoro-9H-purin-6-amine, 9-(3,5-di-O-formyl-.beta.-D-arabinofuranosyl)-2-fluoro-9H-purin-6-amine and 2-fluoro-9-(5-O-formyl-.beta.-D-arabinofuranosyl)-9H-purin-6-amine NSC302067