WB 4101-related compounds. 2. Role of the ethylene chain separating amine and phenoxy units on the affinity for α1-adrenoreceptor subtypes and 5-HT(1A) receptors

WB 4101 (1)-related benzodioxanes were synthesized by replacing the ethylene chain separating the amine and the phenoxy units of 1 with a cyclopentanol moiety, a feature of 6,7-dihydro-5-[[(cis-2-hydroxy-trans-3- phenoxycyclopentyl)amino]methyl]-2-methylbenzo[b]thiophen-4(5H)- one that was reported to display an intriguing selectivity profile at α1- adrenoreceptors. This synthesis strategy led to 4 out of 16 possible stereoisomers, which were isolated in the case of (-)-3, (+)-3, (-)-4, and (+)-4 and whose absolute configuration was assigned using a chiral building block for the synthesis of (-)-3 starting from (+)-(2R)-2,3-dihydro-1,4- benzodioxine-2-carboxylic acid ((+)-9) and (1S,2S,5S)-2-amino-5- phenoxycyclopentan-1-ol ((+)-10). The aim of this project was to further investigate whether it is possible to differentiate between these compounds with respect to their affinity for α1-adrenoreceptor subtypes and the affinity for 5-HT(1A) receptors, as 1 binds with high affinity at both receptor systems. The biological profiles of reported compounds at α1- adrenoreceptor subtypes were assessed by functional experiments in isolated rat vas deferens (α(1A)), spleen (α(1B)), and aorta (α(1D)) and by binding assays in CHO and HeLa cells membranes expressing the human cloned α1- adrenoreceptor subtypes and 5-HT(1A) receptors, respectively. Furthermore, the functional activity of (-)-3, (+)-3, (-)-4, and (+)-4 toward 5-HT(1A) receptors was evaluated by determining the induced stimulation of [35S]- GTPγS binding in cell membranes from HeLa cells transfected with human cloned 5-HT(1A) receptors. The configuration of the cyclopentane unit determined the affinity profile: a 1R configuration, as in (+)-3 and (-)-4, conferred higher affinity at α1-adrenoreceptors, whereas a 1S configuration, as in (-)-3 and (+)-4, produced higher affinity for 5-HT(1A) receptors. For the enantiomers (+)-4 and (-)-4 also a remarkable selectivity was achieved. Functionally, the stereoisomers displayed a similar α1- selectivity profile, that is α(1D) > α(1B) > α(1A), which is different from that exhibited by the reference compound 1. The epimers (-)-3 and (+)-4 proved to be agonists at the 5-HT(1A) receptors, with a potency comparable to that of 5-hydroxytryptamine.