Quinilone disulfide as intermediates

A process of preparing rufloxacin, a compound of formula (A) ##STR9## and its pharmaceutically acceptable salts, in which is used a quinolone disulfide of the formula (I) ##STR10## wherein R represents a C.sub.1 -C.sub.4 -alkyl group, and its salts which comprises reducing a compound of formula (I) with a suitable reducing agent and cyclyzing the resultant compound with a base to afford a compound of Formula (A') ##STR11## and then hydrolyzing a compound of formula A' to afford a compound of formula (A) and, if required, preparing its pharmaceutically acceptable salts.