‘Hybrid’ benzofuran–benzopyran congeners as rigid analogs of hallucinogenic phenethylamines

The synthesis and pharmacological characterization of a set of rigidified heterocyclic phenylalkylamine derivatives that have agonist activity at serotonin 5-HT 2A receptors is reported. Affinity and functional potency were measured, and drug discrimination in rats was assessed for two of the most potent compounds. Phenylalkylamines that possess conformationally rigidified furanyl moieties in place of alkoxy arene ring substituents have been shown previously to possess the highest affinities and agonist functional potencies at the serotonin 5-HT 2A receptor among this chemical class. Further, affinity declines when both furanyl rings are expanded to the larger dipyranyl ring system. The present paper reports the synthesis and pharmacological evaluation of a series of ‘hybrid’ benzofuranyl–benzopyranyl phenylalkylamines to probe further the sizes of the binding pockets within the serotonin 5-HT 2A agonist binding site. Thus, 4( a– b), 5( a– b), and 6 were prepared as homologs of the parent compound, 8-bromo-1-(2,3,6,7-tetrahydrobenzo[1,2- b:4,5- b′]difuran-4-yl)-2-aminopropane 2, and their affinity, functional potency, and intrinsic activity were assessed using cells stably expressing the rat 5-HT 2A receptor. The behavioral pharmacology of these new analogs was also evaluated in the two-lever drug discrimination paradigm. Although all of the hybrid isomers had similar, nanomolar range receptor affinities, those with the smaller furanyl ring at the arene 2-position ( 4a– b) displayed a 4- to 15-fold greater functional potency than those with the larger pyranyl ring at that position ( 5a– b). When the furan ring of the more potent agonist 4b was aromatized to give 6, a receptor affinity similar to the parent difuranyl compound 2 was attained, along with a functional potency equivalent to 2, 4a, and 4b. In drug discrimination experiments using rats trained to discriminate LSD from saline, 4b was more than two times more potent than 5b, with the latter having a potency similar to the classic hallucinogenic amphetamine 1 (DOB).