Compounds of formula (I) in which B is oxygen, CR.sup.17 R.sup.18 or NR.sup.19 where R.sup.17, R.sup.18 and R.sup.19 are independently hydrogen or C.sub.1-6 alkyl or B is a group S(O).sub.b where b is 1, 2, or 3; and R.sup.6 is a group --(CH.sub.2).sub.p --R.sup.15 where R.sup.15 is OR.sup.16 or SR.sup.16 is hydrogen or C.sub.1-6 alkyl or R.sup.15 is NR.sup.10 R.sup.11 where R.sup.10 and R.sup.11 are as defined for R.sup.1 ; have been found to exhibit 5TH.sub.1B antagonist activity. ##STR1##