A compound of the formula (I) ##STR1## in which: R.sub.1 represents a hydrogen atom, a lower alkyl group, a hydroxy group or a lower alkoxy group, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8 and R.sub.11 are as defined in the description. Medicaments.