According to the invention there is provided a compound the formula I；wherein: A isan optionally substituted: phenyl, naphthyl, pyridyl, pyrazinyl, pyridazinyl,pyrimidyl,thienyl, thiazolyl, oxazolyl or thiadiazolyl； B is an optionally substituted:phenyl,pyridyl, thiazolyl, oxazolyl, thienyl, thiadiazolyl, isoxazole, pyrazole,furyl, pyrrolyl,imidazolyl, pyrazinyl, pyridazinyl, pyrimidyl, pyridone, pyrimidone,pyrazinone orpyridazinone； X is optionally substituted: pyridyl, pyrazinyl, pyrimidinyl,pyridazinyl, pyrrolyl, thienyl, furyl, imidazolyl, pyrazolyl, thiazolyl,isothiazolyl,oxazolyl, isoxazolyl or phenyl. R1 is CO2H, CO2R, COSO2NR2, tetrazolyl,P(O)(OR)2 or SONH2, R2 is H, C1-6 alkyl, C2-6 alkenyl, C2-6 alkynyl or C1-3alkylaryl.R3 is H or C1-5 alkyl, R4 is H or C1-5 alkyl.(see formula I)
