Synthesis and calcium ion antagonistic activity of 2-[2-[(aminoalkyl)oxy]-5-methoxyphenyl]-3,4-dihydro-4-methyl-3-oxo-2H-1,4-benzothiazines

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Abstract
As an extension of the previous investigation (J. Med. Chem. 1988, 31, 919), we synthesized a series of 2-[2-[(aminoalkyl)oxy]-5-methoxyphenyl]-3,4-dihydro-4-methyl-3-oxo-2H-1,4-benzothiazines (3) and evaluated their Ca2+ antagonistic activities. Ca2+ antagonistic activity was measured with isolated depolarized guinea pig taenia cecum. On the basis of their potent Ca2+ antagonistic activity, six benzothiazines were selected and further evaluated for their vasocardioselectivity. Among these six compounds, the key compound 15 [3,4-dihydro-2-[5-methoxy-2[3-[N-methyl-N-[2-[3,4-(methylenedioxy)phenoxy]ethyl]amino]propoxy]phenyl]-4-methyl-3-oxo-2H-1,4-benzothiazine hydrogen fumarate] was recognized as having the lowest cardioselectivity. Following optical resolution, the absolute configuration of the compound''s optically active enantiomer was determined by means of X-ray crystallography of a synthetic precursor (+)-4a. The Ca2+ antagonistic activity of 15 was found to reside primarily in (+)-15 (which was about 7 times more potent than (-)-15). The in vitro study showed that (+)-15 had a low cardioselectivity compared to verapamil and diltiazem. This result suggests that (+)-15 would exhibit less adverse effects due to cardiac inhibition than diltiazem and verapamil in therapeutic use.

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