A compound having the structure of formula (IIa) wherein: R 2 and R 6 are independently selected from the group consisting of halo, hydroxyl, sulfhydryl, C 1 -C 12 alkyl, C 2 -C 12 alkenyl, C 1 -C 12 alkoxy, C 5 -C 20 aryloxy, C 2 -C 12 alkylcarbonyl, C 6 -C 20 arylcarbonyl, C 2 -C 12 acyloxy, C 2 -C 12 alkoxycarbonyl, C 6 -C 20 aryloxycarbonyl,, C 2 -C 12 alkylcarbonato, carboxy, carbamyol, mono-(C 1 -C 12 alkyl)-substituted carbamoyl, di-(C 1 -C 12 alkyl)-substituted carbamoyl, amino, mono- and di-(C 1 -C 12 alkyl)substituted amino, C 2 -C 12 alkylamido, C 1 -C 12 alkylsulfanyl, C 1 -C 12 alkylsulfinyl, and C 1 -C 12 alkylsulfonyl; R 11 and R 12 are independently selected from the group consisting of hydrogen, C 1 -C 24 alkyl, C 2 -C 24 alkoxycarbonyl, amino-substituted C 1 -C 24 alkyl, (C 1 -C 24 alkylamino)-substituted C 1 -C 24 alkyl, and di-(C 1 -C 24 alkyl)amino-substituted C 1 -C 24 alkyl; R 13 and R 14 are independently selected from the group consisting of hydrogen, C 1 -C 24 alkyl, C 2 -C 24 alkenyl, C 2 -C 24 alkynyl, C 5 -C 20 aryl, C 6 -C 24 alkylaryl, C 6 -C 24 aralkyl, halo, hydroxyl, sulfhydryl, C 1 -C 24 alkoxy, C 2 -C 24 alkenyloxy, C 2 -C 24 alkynyloxy, C 5 -C 20 aryloxy, acyl, acyloxy, C 2 -C 24 alkoxycarbonyl, C 6 -C 20 aryloxycarbonyl, halocarbonyl, C 2 -C 24 alkylcarbonato, C 6 -C 20 arylcarbonato, carboxy, carboxylato, carbamoyl, mono-(C 1 -C 24 alkyl)-substituted carbamoyl, di-(C 1 -C 24 alkyl)-substituted carbamoyl, mono-substituted arylcarbamoyl, thiocarbamoyl, carbamido, cyano, isocyano, cyanato, isocyanato, isothiocyanato, azido, formyl, thioformyl, amino, mono- and di-(C 1 -C 24 alkyl)-substituted amino, mono- and di-(C 5 -C 20 aryl)-substituted amino, C 2 -C 24 alkylamido, C 5 -C 20 arylamido, imino, alkylimino, arylimino, nitro, nitroso, sulfo, sulfonato, C 1 -C 24 alkylsulfanyl, arylsulfanyl, C 1 -C 24 alkylsulfinyl, C 5 -C 20 arylsulfinyl, C 1 -C 24 alkylsulfonyl, C 5 -C 20 arylsulfonyl, phosphono, phosphonato, phosphinato, phospho, phosphino, and combinations, with the provisio that at least one of R 13 and R 14 ist other than hydrogen; and R 15 and R 16 are hydrogen, C 1 -C 6 alkyl, or together form =CR 18 R 19 where R 18 and R 19 are hydrogen or C 1 -C 6 alkyl, or a pharmaceutically acceptable salt thereof. R 2 and R 6 are independently selected from the group consisting of halo, hydroxyl, sulfhydryl, C 1 -C 12 alkyl, C 2 -C 12 alkenyl, C 1 -C 12 alkoxy, C 5 -C 20 aryloxy, C 2 -C 12 alkylcarbonyl, C 6 -C 20 arylcarbonyl, C 2 -C 12 acyloxy, C 2 -C 12 alkoxycarbonyl, C 6 -C 20 aryloxycarbonyl,, C 2 -C 12 alkylcarbonato, carboxy, carbamyol, mono-(C 1 -C 12 alkyl)-substituted carbamoyl, di-(C 1 -C 12 alkyl)-substituted carbamoyl, amino, mono- and di-(C 1 -C 12 alkyl)substituted amino, C 2 -C 12 alkylamido, C 1 -C 12 alkylsulfanyl, C 1 -C 12 alkylsulfinyl, and C 1 -C 12 alkylsulfonyl; R 11 and R 12 are independently selected from the group consisting of hydrogen, C 1 -C 24 alkyl, C 2 -C 24 alkoxycarbonyl, amino-substituted C 1 -C 24 alkyl, (C 1 -C 24 alkylamino)-substituted C 1 -C 24 alkyl, and di-(C 1 -C 24 alkyl)amino-substituted C 1 -C 24 alkyl; R 13 and R 14 are independently selected from the group consisting of hydrogen, C 1 -C 24 alkyl, C 2 -C 24 alkenyl, C 2 -C 24 alkynyl, C 5 -C 20 aryl, C 6 -C 24 alkylaryl, C 6 -C 24 aralkyl, halo, hydroxyl, sulfhydryl, C 1 -C 24 alkoxy, C 2 -C 24 alkenyloxy, C 2 -C 24 alkynyloxy, C 5 -C 20 aryloxy, acyl, acyloxy, C 2 -C 24 alkoxycarbonyl, C 6 -C 20 aryloxycarbonyl, halocarbonyl, C 2 -C 24 alkylcarbonato, C 6 -C 20 arylcarbonato, carboxy, carboxylato, carbamoyl, mono-(C 1 -C 24 alkyl)-substituted carbamoyl, di-(C 1 -C 24 alkyl)-substituted carbamoyl, mono-substituted arylcarbamoyl, thiocarbamoyl, carbamido, cyano, isocyano, cyanato, isocyanato, isothiocyanato, azido, formyl, thioformyl, amino, mono- and di-(C 1 -C 24 alkyl)-substituted amino, mono- and di-(C 5 -C 20 aryl)-substituted amino, C 2 -C 24 alkylamido, C 5 -C 20 arylamido, imino, alkylimino, arylimino, nitro, nitroso, sulfo, sulfonato, C 1 -C 24 alkylsulfanyl, arylsulfanyl, C 1 -C 24 alkylsulfinyl, C 5 -C 20 arylsulfinyl, C 1 -C 24 alkylsulfonyl, C 5 -C 20 arylsulfonyl, phosphono, phosphonato, phosphinato, phospho, phosphino, and combinations, with the provisio that at least one of R 13 and R 14 ist other than hydrogen; and R 15 and R 16 are hydrogen, C 1 -C 6 alkyl, or together form =CR 18 R 19 where R 18 and R 19 are hydrogen or C 1 -C 6 alkyl, or a pharmaceutically acceptable salt thereof.