A compound of formula I, ##STR22## in which R.sub.3 to R.sub.6 and R.sub.9 to R.sub.12, which may be the same or different, are each hydrogen, halogen, phenoxy, alkyl C.sub.1 to C.sub.6, fluoroalkyl C.sub.1 to C.sub.6, alkanoyl C.sub.1 to C.sub.6, benzoyl, RS(O).sub.n --, NO.sub.2, NR.sub.16 R.sub.17, NHCOR, --COOH or an ester derived from a C.sub.1 to C.sub.6 alcohol or an unsubstituted or a mono- or dialkyl C.sub.1 to C.sub.6 substituted amide thereof, or alkoxy C.sub.1 to C.sub.6 optionally substituted by phenyl,
- y is 0 or 1,
- p is 0 or 1,
- n is 0, 1, or 2,
- R.sub.18 is hydrogen, COR, COOR, or alkyl C.sub.1 to C.sub.6, said alkyl being optionally substituted by --OCOR or by phenyl,
- R, R.sub.16, and R.sub.17, which may be the same or different, are each hydrogen, phenyl or alkyl C.sub.1 to C.sub.6 optionally substituted by phenyl, the phenyl groups in turn optionally being substituted by alkyl C.sub.1 to C.sub.6,
- or a pharmaceutically acceptable salt thereof.