7-aminocephalosporanic acid derivatives

Derivatives of 7-aminocephalosporanic acid of formula where X is an azide group or the residue of a sulphur-containing nucleophile linked through sulphur, and salts thereof, are produced by reacting a compound of formula wherein X has the aforesaid meaning, or a salt thereof, with a nitrosyl halide in a liquid solvent medium comprising formic acid, and decomposing the resultant intermediate with water or a C1-C3 alkanol. Nitrosyl chloride or nitrosyl bromide are preferred and may be formed in situ from a hydrogen halide and alkyl nitrite. The solvent may be substantially anhydrous formic acid or may be formic acid and an inert solvent, e.g. a nitrohydrocarbon. Typical compounds are S-(7-amino-4-carboxyceph-3-em-3-ylmethyl) isothiourea; S-(7-amino-4-carboxyceph - 3 - em - 3 - ylmethyl) thiobenzoate; S-(7 - amino - 4 - carboxyceph - 3 - em - 3 - yl-methyl) thiopicolinate; S-(71-amino-41-carboxy-ceph -31 - em - 31 - ylmethyl) thio - 4 - methyl-pyrimidine S - (71 - amino - 41 - carboxyceph-31 - em - 31 - ylmethyl) thio - 4,6 - dimethyl-pyrimidine and 7-amino-3-azidomethylceph-3-em -4-carboxylic acid.