Novel compounds of the formula (I), processes for their preparation and their use as collagenase inhibitors are described: ##STR1## in which R.sub.1 is hydroxy; alkoxy; aryloxy; aralkyloxy; --NR.sub.6 R.sub.7 where R.sub.6 and R.sub.7 are hydrogen or alkyl, or R.sub.6 and R.sub.7 together with the N-atom to which they are bonded form a 5- to 7-membered ring with an optional heteroatom; --NHCH(R.sub.8)COR.sub.9 is hydrogen; alkyl optionally substituted by hydroxy, alkoxy, --NR.sub.6 R.sub.7, quanidine; CO.sub.2 H, CONH.sub.2, SH, or S-alkyl; or CH.sub.2 --AR where Ar is optionally substituted aryl; and R.sub.9 is hydroxy, alkoxy or --NR.sub.6 R.sub.7.R.sub.2 is hydrogen or acyl.R.sub.3 is C.sub.3-6 alkyl.R.sub.4 is hydrogen; alkyl; --CH.sub.2 R.sub.10 where R.sub.10 is optionally substituted phenyl or heteroaryl; or --CH(R.sub.12) O--R.sub.11 where R.sub.11 is hydrogen; alkyl; or --CH.sub.2 Ph where Ph is optionally substituted phenyl; and R.sub.12 is hydrogen or alkyl.R.sub.5 is hydrogen; alkyl; or --CH(R.sub.13) COR.sub.14 where R.sub.13 is hydrogen or alkyl; and R.sub.14 is hydroxy, alkoxy or --NR.sub.6 R.sub.7.