Abstract: The novel compounds of the formula ##STR1## wherein R.sup.11 signifies hydrogen, acyl or tri(lower alkyl)silyl, R.sup.2 signifies a phenyl group which is substituted in the o- and/or the p-position by halogen, lower alkyl, lower alkylthio or lower alkoxy and n signifies the number 0 or 2, and mixtures thereof with the corresponding optical antipodes are valuable intermediates for the manufacture of antimicrobially active penems and carbapenems which have as a common structural element a (R)-1-hydroxyethyl group in the 6-position. They can be manufactured by reacting a compound of the formula ##STR2## wherein R.sup.12 signifies acyl, tri(lower alkyl)silyl or a group of the formula --BR.sup.3 R.sup.4 and R.sup.3 and R.sup.4 have the same significance and each signify a branched-open chain, cyclic or bicyclic hydrocarbon residue with 5-10 carbon atoms or together signify an open chain or cyclic hydrocarbon residue with 5-10 carbon atoms and two free valencies and R.sup.