A compound of formula wherein a is an integer from 0 to 2; R 1 is selected from the group consisting of halogen, hydroxy, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy, halogenated C 1-4 alkoxy, nitro, cyano, amino, C 1-4 alkylamino and di(C 1-4 alkyl)amino; R 2 is selected from the group consisting of hydrogen, -C 1-4 alkyl, -C 1-4 alkyl-OH,-C 1-4 alkyl-cycloalkyl, -C 1-4 alkyl-aryl, -C 1-4 alkyl-heterocyclyl, -C 1-4 alkyl-C OH, -C 1-4 alkyl-C O-C 1-4 alkyl, -C 1-4 alkyl-OC -C 1-4 alkyl, -CH 2 -CH -CH 2 -NR A R B, -CH 2 -CH -CH 2 -S-R C, -C 1-4 alkyl-oxiranyl and -C 1-4 alkyl-O-tetrahydro-pyran-2-yl; wherein R A and R B are each independently selected from hydrogen, C 1 - 4 alkyl,-C 1-4 alkyl-aryl or-C 1-4 alkyl-heterocycloalkyl, wherein the aryl is optionally substituted with one to two substituents independently selected from halogen, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy or halogenated C 1-4 alkoxy; alternatively, R A and R B are taken together with the nitrogen atom to which they are bound to form a 5 to 6 membered, saturated, heterocycloalkyl ring; wherein R C is selected from CH 2 -CH(CO 2 H)-NH2 and CH 2 -CH(CO 2 H)-NH-C -CH 3; wherein the cycloalkyl, aryl or heterocyclyl, whether alone or as part of a substituent group is optionally substituted with one to two substituents independently selected from halogen, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy, halogenated C 1-4 alkoxy, nitro, cyano, amino, C 1-4 alkylamino or di(C 1-4 alkyl)amino; R 3 is selected from the group consisting of -C 1-4 alkyl-cycloalkyl, -C 1-4 alkyl-aryl,-C 1-4 alkyl-biphenyl, and -C 1-4 alkyl-(phenyl)-(thienyl); wherein the cycloalkyl or aryl is optionally substituted with one to two substituents independently selected from halogen, hydroxy, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy, halogenated C 1-4 alkoxy, nitro, cyano, amino, C 1-4 alkylamino or di(C 1-4 alkyl)amino; alternatively, R 3 is selected from -A-phenyl or -A-cycloalkyl; wherein A is selected from the group consisting of-C - and -C -CH 2 CH 2 -C -; and wherein the phenyl or cycloalkyl is optionally substituted with one to two substituents independently selected from halogen, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy, halogenated C 1-4 alkoxy, nitro, cyano, amino, C 1-4 alkylamino or di(C 1-4 alkyl)amino; R 4 and R 5 are taken together as =O; provided further that when a is an integer from 0 to 1, R 1 is halogen, C 1-4 alkyl or C 1-4 alkoxy, and R 2 is hydrogen, then R 3 is other than benzyl; provided further that when a is an integer from 0 to 1, R 1 is C 1-4 alkoxy, and R 2 is C 1-4 alkyl, then R 3 is other than benzyl or phenylethyl; or a pharmaceutically acceptable salt thereof. a is an integer from 0 to 2; R 1 is selected from the group consisting of halogen, hydroxy, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy, halogenated C 1-4 alkoxy, nitro, cyano, amino, C 1-4 alkylamino and di(C 1-4 alkyl)amino; R 2 is selected from the group consisting of hydrogen, -C 1-4 alkyl, -C 1-4 alkyl-OH,-C 1-4 alkyl-cycloalkyl, -C 1-4 alkyl-aryl, -C 1-4 alkyl-heterocyclyl, -C 1-4 alkyl-C OH, -C 1-4 alkyl-C O-C 1-4 alkyl, -C 1-4 alkyl-OC -C 1-4 alkyl, -CH 2 -CH -CH 2 -NR A R B, -CH 2 -CH -CH 2 -S-R C, -C 1-4 alkyl-oxiranyl and -C 1-4 alkyl-O-tetrahydro-pyran-2-yl; wherein R A and R B are each independently selected from hydrogen, C 1 - 4 alkyl,-C 1-4 alkyl-aryl or-C 1-4 alkyl-heterocycloalkyl, wherein the aryl is optionally substituted with one to two substituents independently selected from halogen, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy or halogenated C 1-4 alkoxy; alternatively, R A and R B are taken together with the nitrogen atom to which they are bound to form a 5 to 6 membered, saturated, heterocycloalkyl ring; wherein R C is selected from CH 2 -CH(CO 2 H)-NH2 and CH 2 -CH(CO 2 H)-NH-C -CH 3; wherein the cycloalkyl, aryl or heterocyclyl, whether alone or as part of a substituent group is optionally substituted with one to two substituents independently selected from halogen, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy, halogenated C 1-4 alkoxy, nitro, cyano, amino, C 1-4 alkylamino or di(C 1-4 alkyl)amino; R 3 is selected from the group consisting of -C 1-4 alkyl-cycloalkyl, -C 1-4 alkyl-aryl,-C 1-4 alkyl-biphenyl, and -C 1-4 alkyl-(phenyl)-(thienyl); wherein the cycloalkyl or aryl is optionally substituted with one to two substituents independently selected from halogen, hydroxy, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy, halogenated C 1-4 alkoxy, nitro, cyano, amino, C 1-4 alkylamino or di(C 1-4 alkyl)amino; alternatively, R 3 is selected from -A-phenyl or -A-cycloalkyl; wherein A is selected from the group consisting of-C - and -C -CH 2 CH 2 -C -; and wherein the phenyl or cycloalkyl is optionally substituted with one to two substituents independently selected from halogen, C 1-4 alkyl, halogenated C 1-4 alkyl, C 1-4 alkoxy, halogenated C 1-4 alkoxy, nitro, cyano, amino, C 1-4 alkylamino or di(C 1-4 alkyl)amino; R 4 and R 5 are taken together as =O; provided further that when a is an integer from 0 to 1, R 1 is halogen, C 1-4 alkyl or C 1-4 alkoxy, and R 2 is hydrogen, then R 3 is other than benzyl; provided further that when a is an integer from 0 to 1, R 1 is C 1-4 alkoxy, and R 2 is C 1-4 alkyl, then R 3 is other than benzyl or phenylethyl; or a pharmaceutically acceptable salt thereof.