A beta-lactam compound of the formula: ##STR1## wherein R.sub.1 is a hydrogen atom, a lower alkyl group or a 1-hydroxy(lower)alkyl group wherein the hydroxyl group is optionally protected, R.sub.2 is a hydrogen atom or a protective group for the nitrogen atom and R.sub.3 is a methyl group, a halomethyl group, a hydroxymethyl group, a protected hydroxymethyl group, a formyl group, a carboxyl group, a lower alkoxycarbonyl group or an ar(lower)alkoxycarbonyl group wherein the aryl group is optionally substituted, or R.sub.2 and R.sub.3 are combined together to form an oxaalkylene group and, when taken together with one nitrogen atom and two carbon atoms adjacent thereto, they represent a six-membered cyclic aminoacetal group, which is useful as a valuable intermediate in the stereospecific production of 1-methylcarbapenem compounds.