Method for the preparation of certain .beta.-lactam antibiotics

A method of preparing a .beta.-lactam antibiotic comprising enzymatic acylation of a 7-amino-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene nucleus in water or in a mixture of water and one or more water-miscible organic solvents by enzymatic acylation with the acid or a derivative of the acid corresponding to the desired side chain, characterized in that naphthalene or a naphthalene derivative with the formula

• Naphthyl-R
• wherein Naphthyl- designates 1-naphthyl or 2-naphthyl and R is selected from the group consisting of hydrogen, halogen, hydroxy, nitro, cyano, branched or unbranched alkyl of 1 to 6 carbon atoms optionally substituted with cyano, branched or unbranched alkoxy of 1 to 6 carbon atoms and alkanoyl of 1 to 6 carbon atoms, where on a molar basis the ratio between the amount of naphthalene or naphthalene derivative present in the reaction mixture and the amount of .beta.-lactam antibiotic present in the reaction mixture is at least 0.45, is present in the reaction mixture during the formation of a naphthalene complex of the .beta.-lactam antibiotic which is isolated or decomposed to the .beta.-lactam antibiotic by deprotection of a protected intermediate.