for serotonin-3 (5-HT3) receptor binding affinity. The 5-HT3receptorantagonistic activity of zacopride, a representative 5-HT3receptorantagonist, was unchanged by the replacement of the 4-amino substituent on the aromatic moiety by a 3-dimethyl-amino substituent. This finding prompted a structural modification of azasetron, another 5-HT3receptorantagonist. Consequently, a new series of 3,4-dihydro-2H-1