Cyclopentane derivatives and process for preparing them

A compound of the formula ##STR13## and the physiologically tolerable metal and amine salts thereof, wherein R.sub.1 and R.sub.2, taken together, are oxygen;

• R.sub.1 and R.sub.2, taken separately, are each hydrogen or hydroxy but R.sub.1 and R.sub.2 are different;
• R.sub.3 is hydrogen or alkyl having 1 to 5 carbon atoms;
• U is --(CH.sub.2).sub.m --, where
• m is 0 to 5, or is ##STR14## where R.sub.4 and R.sub.5 are the same or different and are hydrogen or alkyl having 1 to 5 carbon atoms;
• V is a single bond, oxygen, ##STR15## R.sub.6 and R.sub.7 are the same or different and are hydrogen or alkyl having 1 to 5 carbon atoms;
• W is a single bond or ##STR16## where R.sub.8 and R.sub.9 are the same or different and are hydrogen or alkyl having 1 to 5 carbon atoms;
• and
• X is --(CH.sub.2).sub.m --,
• where m is 0 to 5.