4-Heterocyclyloxy-2H-1-benzopyran potassium channel activators

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Abstract
The reaction of 2,4-dihydroxypyridine (2) with 3,4-epoxy-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-6-carbonitrile (1) yielded the 4-[(1,2-dihydro-2-oxo-4-pyridyl)oxy] compound 3a, accompanied by small amounts of the isomeric 4-(1,2-dihydro-4-hydroxy-2-oxo-1-pyridyl) compound 4. This could also be prepared by hydrogenation of the benzyloxy derivative 5. Reaction of 3,6-pyridazinediol (10) with 1 (R = CN) gave the 4-[(1,6-dihydro-6-oxo-3-pyridazinyl)oxy] compound 11a, which in turn rearranged on heating with NaH in DMSO in the 4-(1,6-dihydro-3-hyroxy-6-oxo-1-pyridazinyl) compound 12. A series of 6-substituted analogues (R = CO2Me, CSNH2, NO2, Br) of 3a and 11a were synthesized. N-Alkylation led to compounds 14a-c (R = Me, Et, CHMe2). The 4-heterocyclyloxychromenes 9 and 16a were obtained by alkaline hydrolysis of their c-camphorsulfonates. The racemic pyridazinyloxy compounds 11a and 14a could be resolved via their diastereomeric camphorsulfonates or camphanates. The differences between the 4-heterocyclyloxychromanols and the isomeric N-substituted compounds 4 and 12 were elucidated in the course of extensive NMR investigations. While in DMSO the former appeared to be conformationally flexible molecules the latter were rigid. All compounds were tested for oral antihypertensive activity in spontaneously hypertensive rats, using doses of 1 mg/kg. High and long lasting activities were found for the pyridyloxy compounds 3a and 3d, the pyridazinyloxy compound 11a, and its N-alkylation products, as well as for the 3S,4R-enantiomers 20a and 22a. (-)-(3S,4R)-3,4-Dihydro-4-[(1,6-dihydro-1-methyl-6-oxo-3-pyridazinyl)oxy]-3-hydroxy-2,2-dimethyl-2H-1-benzopyran-6-carbonitrile (22a) was selected for further development.