Imidazo-rifamycin derivatives with antibacterial utility

A process for preparing a compound of formula ##STR9## wherein:

• A is ##STR10## --x-- is a covalent chemical bond or nil;
• R is hydrogen or acetyl;
• R.sub.1 and R.sub.2 independently represent hydrogen, (C.sub.1-4) alkyl, benzyloxy, mono- and di-(C.sub.1-3)alkylamino(C.sub.1-4)alkyl, (C.sub.1-3)alkoxy- (C.sub.1-4)alkyl, hydroxymethyl, hydroxy-(C.sub.2-4)-alkyl, nitro or R.sub.1 and R.sub.2 taken together with two consecutive carbon atoms of the pyridine nucleus form a benzene ring unsubstituted or substituted by one or two methyl or ethyl groups;
• R.sub.3 is a hydrogen atom or nil;
• with the proviso that, when A is A.sub.1, --x-- is nil and R.sub.3 is a hydrogen atom;
• with the further proviso that, when A is A.sub.2, --x-- is a covalent chemical bond and R.sub.3 is nil, which consists of reacting a molar amount of a 3-halorifamycin S of formula:
• ##STR11## wherein hal stands for a halogen atom and R is hydrogen or acetyl, with a molar excess of a compound of formula ##STR12## where R.sub.1 and R.sub.2 are as above defined, in the presence of an inert organic solvent, at room temperature, for a period of time comprised between about 1 and about 4 hours, whereby a compound of formula 1 is obtained in which A is A.sub.2, R, R.sub.1, and R.sub.2 are as above defined, --x-- is a covalent chemical bond and R.sub.3 is nil, which can be transformed into the corresponding compounds of formula 1 wherein A is A.sub.1, R, R.sub.1 and R.sub.2 are as above defined, --x-- is nil and R.sub.3 stands for a hydrogen atom, by treatment with a suitable reducing agent, in an inert organic solvent;
• said process being further characterized in that:
• (a) the compound of formula 1 wherein A identifies the structure A.sub.1, R, R.sub.1 and R.sub.2 are as above defined, --x-- is nil and R.sub.3 stands for a hydrogen atom, is transformed into the corresponding compounds of formula 1 wherein A is A.sub.2, R, R.sub.1 and R.sub.2 are as above defined, --x-- is a covalent chemical bond and R.sub.3 represents nil, by treatment with a suitable oxidizing agent selected from manganese dioxide, lead tetracetate, dichloro-dicuano-benzoquinone, 2,3,4,5-tetrachloro-benzoquinone, 2,3,5,6-tetrachloro-benzoquinone and analogs;
• (b) the compound of formula 1 wherein R is acetyl and is converted into the corresponding compound of formula 1 wherein R is hydrogen, by mild alkaline hydrolysis.