A compound of formula wherein R 1 is selected from the group consisting of H, C 1-8 alkyl, C 1-8 alkenyl, C 2-8 alkynyl, cycloalkyl, aryl, heterocycle -C 1-8 alkyl-aryl, -C 1-8 alkyl-heterocycle, -C 1-8 alkyl-NH2, - aryl-NH2, -C 1-8 alkyl-O-aryl, -aryl-OH, C 1-8 alkyl-OH, -COOR, -C 1-8 alkyl-OR, methyloxycarbonyl-C 1-8 alkyl, carboalkyloxy-aryl alkyl carbamoyl aryl and -(amino acid side chains); R 2 is selected from the group consisting of H, C 1-8 alkyl, C 2-8 alkenyl, C 2-8 alkynyl, cycloalkyl, aryl, -C 1-8 alkyl-aryl, -C 1-8 alkyl-heterocycle, -C 1-8 alkyl-NRR', -aryl-NRR', - C 1-8 alkyl-OR, -C 1-8 alkyl-COOR, -C 1-8 alkyl-OC R, -C 1-8 alkyl-N C R', -aryl-OR, -aryl-COOR, -aryl-COR, -aryl-OC R, -aryl-N C R', -CH(amino acid side chains)CO 2 R, -CH(amino acid side chains)C NR, -CH(CO 2 R)-amino acid side chains and CH(CONRR')-amino acid side chains, R 3 is COOH; R and R', equal or different from each other, are selected from the group consisting of H, C 1-8 alkyl, C 2-8 alkenyl, C 2-8 alkynyl, cycloalkyl, aryl, heterocycle, -C 1-8 alkyl-aryl; -C 1-8 heterocycle alkyl; protecting group, -C CH-(amino acid side chains)-NHT, -NH-CH(amino acid side chains)COOT and -CH(amino acid side chains)COOT, where T is selected from H and C 1-8 alkyl; R and R' combined with each other can form a cycloalkyl; and where the above alkyl, alkenyl, alkynyl, cycloalkyl, aryl, heterocycle groups may be substituted with one or more moieties selected from the group consisting of halogen, CN, N02, NH2, OH, COOH, CO and C 1-6 alkyl; excluding the (1S,5R,7S)-3-ethyl-2-oxo-6,8-dioxa-3-azabicyclo[3.2.1]octan-7-carboxylic acid, (1R,5S,7R)-3-(p-methoxybenzyl)-2-oxo-6,8-dioxa-3-azabicyclo[3.2.1]octan-7-carboxylic acid, (1R,5S,7R)-3-Benzyl-2-oxo-6,8-dioxa-3-azabicyclo[3.2.1]octan-7-carboxylic acid and (1R,5S,7R)-3-(p-phenyl)-benzyl-2-oxo-6,8-dioxa-3-azabicyclo[3.2.1]octan-7-carboxylic acid compounds. R 1 is selected from the group consisting of H, C 1-8 alkyl, C 1-8 alkenyl, C 2-8 alkynyl, cycloalkyl, aryl, heterocycle -C 1-8 alkyl-aryl, -C 1-8 alkyl-heterocycle, -C 1-8 alkyl-NH2, - aryl-NH2, -C 1-8 alkyl-O-aryl, -aryl-OH, C 1-8 alkyl-OH, -COOR, -C 1-8 alkyl-OR, methyloxycarbonyl-C 1-8 alkyl, carboalkyloxy-aryl alkyl carbamoyl aryl and -(amino acid side chains); R 2 is selected from the group consisting of H, C 1-8 alkyl, C 2-8 alkenyl, C 2-8 alkynyl, cycloalkyl, aryl, -C 1-8 alkyl-aryl, -C 1-8 alkyl-heterocycle, -C 1-8 alkyl-NRR', -aryl-NRR', - C 1-8 alkyl-OR, -C 1-8 alkyl-COOR, -C 1-8 alkyl-OC R, -C 1-8 alkyl-N C R', -aryl-OR, -aryl-COOR, -aryl-COR, -aryl-OC R, -aryl-N C R', -CH(amino acid side chains)CO 2 R, -CH(amino acid side chains)C NR, -CH(CO 2 R)-amino acid side chains and CH(CONRR')-amino acid side chains, R 3 is COOH; R and R', equal or different from each other, are selected from the group consisting of H, C 1-8 alkyl, C 2-8 alkenyl, C 2-8 alkynyl, cycloalkyl, aryl, heterocycle, -C 1-8 alkyl-aryl; -C 1-8 heterocycle alkyl; protecting group, -C CH-(amino acid side chains)-NHT, -NH-CH(amino acid side chains)COOT and -CH(amino acid side chains)COOT, where T is selected from H and C 1-8 alkyl; R and R' combined with each other can form a cycloalkyl; and where the above alkyl, alkenyl, alkynyl, cycloalkyl, aryl, heterocycle groups may be substituted with one or more moieties selected from the group consisting of halogen, CN, N02, NH2, OH, COOH, CO and C 1-6 alkyl; excluding the (1S,5R,7S)-3-ethyl-2-oxo-6,8-dioxa-3-azabicyclo[3.2.1]octan-7-carboxylic acid, (1R,5S,7R)-3-(p-methoxybenzyl)-2-oxo-6,8-dioxa-3-azabicyclo[3.2.1]octan-7-carboxylic acid, (1R,5S,7R)-3-Benzyl-2-oxo-6,8-dioxa-3-azabicyclo[3.2.1]octan-7-carboxylic acid and (1R,5S,7R)-3-(p-phenyl)-benzyl-2-oxo-6,8-dioxa-3-azabicyclo[3.2.1]octan-7-carboxylic acid compounds.