
The invention relates to a compound having the formula I ##STR1## wherein R.sup.1 is hydroxy, (1-6C)alkoxy or NR.sup.4 R.sup.5, R.sup.4 and R.sup.5 being independently hydrogen, (1-6C)alkyl, (3-7C)cycloalkyl, (2-6C)alkenyl, (4-6C)aryl or (5-7C)aralkyl, the aryl groups of which may be optionally substituted with halogen, (1-6C)alkyl or (1-6C)alkoxy; R.sup.2 is a halo atom, (1-8C)acyloxy, --O--C(O)--O-(1-6C)alkyl, --O--C(O)--O-(5-7C)aralkyl, --SR.sup.6, --S(O)R.sup.6, --SeR.sup.6 or --Se(O)R.sup.6...











