Process for preparing optically active 4-mercapto-2-pyrrolidone derivative and intermediate therefor

A process for preparing a 1-methylcarbapenem derivative of the formula [VIII]:

• ##STR9## wherein R.sup.6 is a hydrogen atom or a protecting group, R.sup.7 is a hydrogen atom or an ester residue, and X is an oxygen atom or a sulfur atom, which comprises the steps of:
• a) preparing optically active 4-amino-3-mercaptobutyric acid, or a salt thereof, by means of a process which comprises reacting racemic 4-amino-3-mercaptobutyric acid or a salt thereof with optically active 1-(trichlorophenyl)ethanesulfonic acid or a salt thereof to give two diastereomeric salts of 4-amino-3-mercaptobutyric acid 1-(trichlorophenyl)ethanesulfonate, separating a less-soluble optically active diastereomer salt by using the difference of the solubility of two diastereomer salts, and if necessary, decomposing the separated salt;
• b) optionally reacting the product of step a) to introduce a protecting group into the amino group and/or to introduce a protecting group into the mercapto group, and/or to introduce a reactive derivative at the carboxyl group thereof;
• c) subjecting the product of said steps a) and b) to a cyclization reaction to give a compound of the formula [II]:
• ##STR10## wherein R.sup.11 is a hydrogen atom or a protecting group and R.sup.21 is a hydrogen atom or a protecting group;
• d) when R.sup.11 and/or R.sup.21 are a protecting group, optionally removing said protecting groups from the product to give a compound of the formula [I]:
• ##STR11## wherein R.sup.1 is a hydrogen atom or a protecting group and R.sup.2 is a hydrogen atom or a protecting group;
• and
• e) converting the compound obtained in step c) or d) to the corresponding 1-methylcarbapenem derivative [VIII] or a salt thereof.