A compound of formula: or a pharmaceutically acceptable salt thereof, wherein: R 1 is an aryl group optionally substituted with one or more substituents independently selected from halo, hydroxyl, alkoxyl, alkylthio, OCF 3, alkyl, cycloalkyl, alkyl-R 8, -O-alkyl-R 8, -NR 9 R 10, -CO-NR 9 R 10, oxo, thioxo and phenyl; R 2 is an alkyl optionally substituted by one or two R 8 or -aryl-R 8; R 3, R 4, R 5, R 6 are each independently H, halo, hydroxyl, alkyl, alkoxyl, alkoxyl substituted with boronic acid or a boronic acid derivative, hydroxycarbonyl, aryl, -O-aryl, boronic acid or a boronic acid derivative, phosphonic acid or phosphonic acid derivative, alkyl-R 8, -O-alkyl-R 8, -COOR 11, -NO 2, -NR 9 R 10 or -CO-NR 9 R 10; R 7 is H or alkyl; or R 2 and R 7 are bound together to form a 5 or 6-membered heterocycle; R 8 is halo, hydroxyl, alkoxyl, -NR 9 R 10, -CO-X-R 11, -CN, -CF 3, aryl, cycloalkyl, -O-alkyl-alkoxyl, -O-CO-alkyl, -CO-NR 9 R 10 or -O-CO-NR 9 R 10; R 9 and R 10 are each independently H, alkyl or -CO-alkyl, or form together with the nitrogen atom a 3 to 6-membered cyclic group; X is -O- or -NR 12 -; R 11 is H or alkyl; R 12 is H, alkyl or hydroxyalkyl; and when X is -NR 12 -, R 11 and R 12 can be bound together to form a 4- to 7-membered cyclic group; wherein aryl is an aromatic or partially unsaturated (hetero)cycle comprising 1, 2 or 3 rings, which can be fused; with the proviso that R 1 is not 3,4-dimethoxyphenyl or 3-methoxy-4-hydroxyphenyl, and with the proviso that R 1 is a substituted aryl group when R 2 and R 7 are bound together to form a 5 or 6-member heterocycle and R 3, R 4, R 5 and R 6 are each H. R 1 is an aryl group optionally substituted with one or more substituents independently selected from halo, hydroxyl, alkoxyl, alkylthio, OCF 3, alkyl, cycloalkyl, alkyl-R 8, -O-alkyl-R 8, -NR 9 R 10, -CO-NR 9 R 10, oxo, thioxo and phenyl; R 2 is an alkyl optionally substituted by one or two R 8 or -aryl-R 8; R 3, R 4, R 5, R 6 are each independently H, halo, hydroxyl, alkyl, alkoxyl, alkoxyl substituted with boronic acid or a boronic acid derivative, hydroxycarbonyl, aryl, -O-aryl, boronic acid or a boronic acid derivative, phosphonic acid or phosphonic acid derivative, alkyl-R 8, -O-alkyl-R 8, -COOR 11, -NO 2, -NR 9 R 10 or -CO-NR 9 R 10; R 7 is H or alkyl; or R 2 and R 7 are bound together to form a 5 or 6-membered heterocycle; R 8 is halo, hydroxyl, alkoxyl, -NR 9 R 10, -CO-X-R 11, -CN, -CF 3, aryl, cycloalkyl, -O-alkyl-alkoxyl, -O-CO-alkyl, -CO-NR 9 R 10 or -O-CO-NR 9 R 10; R 9 and R 10 are each independently H, alkyl or -CO-alkyl, or form together with the nitrogen atom a 3 to 6-membered cyclic group; X is -O- or -NR 12 -; R 11 is H or alkyl; R 12 is H, alkyl or hydroxyalkyl; and when X is -NR 12 -, R 11 and R 12 can be bound together to form a 4- to 7-membered cyclic group; wherein aryl is an aromatic or partially unsaturated (hetero)cycle comprising 1, 2 or 3 rings, which can be fused; with the proviso that R 1 is not 3,4-dimethoxyphenyl or 3-methoxy-4-hydroxyphenyl, and with the proviso that R 1 is a substituted aryl group when R 2 and R 7 are bound together to form a 5 or 6-member heterocycle and R 3, R 4, R 5 and R 6 are each H.