A most preferred class of compounds of Formula I consists of compounds wherein R is a single alkoxy group selected from methoxy, ethoxy, n-propoxy, n-butoxy and n-pentoxy, attached at the 2- position phenyl ring of Formula I, that is, at the ortho position relative to attachment of the phenyl ring to the 8-azapurine-6-one nucleus, examples of which are included within the compounds listed in Table