NEW MATERIAL:A compound of formula I {either one of R1s is H, the other is methyl or both are methyl or ethyl; R2 is H, lower alkyl or -S(O)n-R4[n is 0 or 1; R4 is (amino-substituted) lower alkyl, etc.]; R3 is H, alkali metal or ester residue; the dotted line is single or double bond, provided that R4 is amino lower alkyl or acetamido-substituted lower alkenyl, etc. when both R1s are methyl, etc.}.

EXAMPLE: (5RS, 6RS)-6-(1-Ethyl-1-hydroxypropyl)7-oxo-1-azabicyclo[3.2.0]heptane- 2-carboxylic acid of formula II.

USE: A remedy for bacterial infection having β-lactamase inhibitory activity.

PROCESS: A compound of formula III is reacted with a silylating agent and then a base, and the resultant reaction product is reacted with a dialkyl ketone to give a compound of formula IV, which is then reacted with a glyoxalic acid ester and then halogenated to give a phosphorus-ylide compound of formulaV (R'3 is ester residue). The resultant compound of formula V is then ozonized and cyclized to afford a compound of formula VI, which is subjected to the removal of the protective group.