9H-Purin-6-amine, 1-[2-deoxy-2-fluoro-3,5-bis-O-(hydroxymercaptophosphinyl)-β-D-arabinofuranosyl]-, cyclic diester with 9-[(1R,2R,3S)-2,3-bis(hydroxymethyl)cyclobutyl]-9H-purin-6-amine