A compound of formula (I) ** Formula ** in which 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- alkyl 8-aryl, -C 1-8 alkyl-heterocycle, -C 1-8 alkyl-NH 2, -aryl-NH 2, -C 1-8 alkyl-O-aryl-aryl-OH, C 1-8 alkyl-OH, -COOR, - C1-8-OR alkyl, methyloxycarbonyl-C1-8 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 (O) R, -C 1-8 alkyl-N (R) C ( O) R ', -aryl-OR, -aryl-COOR, -aryl-COR, -aryl-OC (O) R, -aryl-N (R) C (O) R', -CH (amino acid side chains ) CO2R, -CH (amino acid side chains) C (O) NR), -CH (CO2R) -amino acid side chains and CH (CONRR ') - amino acid side chains, R3 is COOH； R and R ', the same 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 alkyl-heterocycle； protective group, -C (O) CH- (amino acid side chains) -NHT, -NH-CH (amino acid side chains) COOT and -CH (amino acid side chains) COOT, in which T is selected from H and C1-8 alkyl； R and R 'combined with each other can form a cycloalkyl； and wherein the alkyl, alkenyl, alkynyl, cycloalkyl, aryl, heterocycle groups mentioned above may be substituted with one or more moieties selected from the group consisting of halogen, CN, NO2, NH2, OH, COOH, CO and C1 alkyl -6； excluding the compounds (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 (1R, 5S, 7R) -3- (p-phenyl) -benzyl-2-oxo-6,8-dioxa-3- azabicyclo [ 3.2.1] octan-7-carboxyli
