Modification of the 1,4-benzodioxan ring present in RX 781094 has not previously been considered. This paper describes a number of analogues of this ring system, including compounds in which one of the oxygen atoms has been replaced by a methylene group and also those in which the ring size has been changed to give, for example, furan and thiophene derivatives. The dihydrobenzofuranylimidazoline compound 7 is the only analogue possessing presynaptic antagonist potency potency and selectivity comparable to that of 1. In view of this result, a number of derivatives was prepared to determine the structure-activity relationships within this series. Many derivatives, as well as the parent compound 7, were found to possess presynaptic alpha 2-adrenoreceptor antagonist and postsynaptic alpha 1-adrenoreceptor partial agonist properties. Two of the selective presynaptic antagonists, 13 and 14 possess greater potency and selectivity than that possessed by 1. The 5-chloro derivative 25 is twice as potent as after oral administration but only about half as potent when given intravenously.

The synthesis and pharmacological activity of a series of 2-substituted derivatives of the selective alpha 2-adrenoreceptor antagonist idazoxan (RX 781094) is described. Substitution in this position by alkyl, alkenyl, cycloalkenyl, and alkoxy groups in many cases gives compounds whose potencies and selectivities are significantly greater than those of the parent compound.