R2 is a member selected from the group consisting of ??-hydroxyalkyl having from 5 to 8 carbon atoms and in which the hydroxyl is separated from the xanthine nucleus by at least two carbon atoms, unbranched (??-1)-hydroxyalkyl having from 5 to 8 carbon atoms, branched (??-1)-hydroxyalkyl having from 5 to 8 carbon atoms and in which hydroxyl is separated from the xanthine nucleus by at least two ca