9H-Purine, 6-chloro- 9-[2-deoxy-3,5-O-bis(4-methylphenyl)- .alpha.-D-ribofuranosyl]- 9H-Purine, 6-chloro- 9-[2-deoxy-3,5-O-bis(4-methylphenyl)-.alpha.-D-threopentofuranosyl]- NSC685829