Process for preparing .beta.-lactam derivative and synthetic intermediate

A 1-aza-3-thia-bicycloalkane compound represented by the formula:

• ##STR17## wherein:
• R.sup.1 represents
• a hydroxy-substituted lower alkyl group which may be protected by a group selected from group consisting of a lower alkoxycarbonyl group, a halogeno lower alkoxycarbonyl group, a phenyl lower alkyl group which may be substituted by a nitro group or a lower alkoxy group, a tri-lower alkylsilyl group, and a phenyl lower alkoxycarbonyl group which may be substituted by a nitro group or a lower alkoxy group, or
• an amino group which may be protected by a group selected from the group consisting of a lower alkanoyl group, a lower alkoxycarbonyl group, a benzoyl group, a benzenesulfonyl group, a phenyl lower alkoxycarbonyl group, a tri-lower alkylsilyl group and a trityl group:
• R.sup.2 represents a hydrogen atom, a group of the formula
• --O--OCOR, --O--OCO.sub.2 R or --O--O--R
• wherein O represents a lower alkylene group, and R represents a lower alkyl group, a cycloalkyl group of 3 to 8 carbon atoms, a lower alkenoyl group, a lower alkoxy lower alkyl group, or a lower alkanoyloxy lower alkyl group,
• a lower alkyl group, a lower alkenyl group, a halogeno lower alkyl group, a nitrobenzyl group or a lower alkoxybenzhydryl group;
• and
• X represents a methylene group substituted by a lower alkyl group;
• or an alkali metal salt thereof.