when R20 is other than H, then said R20 group is optionally substituted with one or more (e.g., 1-3) substituents selected from: halo, alkyl, aryl, ???OR18 or ???N(R18)2, wherein each R18 group is the same or different, and wherein R18 is as defined above, provided that said optional substituent is not bound to a carbon atom that is adjacent to an oxygen or nitrogen atom;