A compound of formula (I) ##STR6## in which X represents a group >O, >S, >C.dbd.O or >NR wherein R is hydrogen, C.sub.1-6 alkyl, phenyl or C.sub.7-12 phenalkyl, R.sub.1 and R.sub.2 each represent hydrogen or together represent an oxo group and R.sub.3, R.sub.4 and R.sub.5 each represent hydrogen or R.sub.1 represents hydrogen and two of R.sub.2, R.sub.3, R.sub.4 and R.sub.5 together represent the second bond of a double bond joining positions 7 and 8, 8 and 9 or 9 and 10 with the remaining two of R.sub.2, R.sub.3, R.sub.4 and R.sub.5 representing hydrogen, but with the proviso that when each of R.sub.1 to R.sub.5 is hydrogen X is not >O, the compound optionally being in the form of a salt thereof formed with a physiologically acceptable inorganic or organic acid.