A compound of formula (I): ##STR00001## or a salt, solvate and chemically
protected form thereof, wherein one of R.sup.2 and R.sup.5 is: (i) H or
an optionally substituted C.sub.1-4 alkyl group; or (ii)an optionally
substituted C.sub.5-7 aryl; and the other of R.sup.2 and R.sup.5 is the
other group; m and n can be 0 or 1, and m+n=1 or 2 R.sup.N is H or
optionally substituted C.sub.1-4 alkyl R.sup.3 is either: (i) carboxy;
(ii) a group of formula (II): ##STR00002## (iii) a group of formula
(III): ##STR00003## wherein R is optionally substituted C.sub.1-7
alkyl, C.sub.5-20 aryl, or NR.sup.N3R.sup.N4, where R.sup.N3 and R.sup.N4
are independently selected from optionally substituted C.sub.1-4 alkyl;
or (iv) tetrazol-5-yl.
