Compounds having one or two substituents at the 3-position, represented by the general formula ##STR1## wherein R.sub.1 is an optionally acylated or protected amino group and X is hydrogen atom or methoxy group, or a pharmaceutically acceptable salt thereof; when used either alone or in combination with a .beta.-lactam antibiotic, show excellent .beta.-lactamase inhibitory activity and can be used as drugs for use in humans and domestic animals.