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To confirm the proposed structures of the minor metabolites of a potential gastroprokinetic agent, mosapride, 4-amino-5-chloro-2-ethoxy-3-hydroxy-N-(2-morpholinylmethyl)benzamide (3) and the N-(5-oxo-2-morpholinyl)-methyl analogue 4 were prepared. As the common intermediate, 2-ethoxy-3-hydroxy-4-nitrobenzoic acid (15) was prepared via the regioselective ethylation of 2,3-dihydroxybenzaldehyde (10) and subsequent nitration of the resultant 2-ethoxy-3-hydroxybenzaldehyde (11). The key intermediate 15 was converted into the benzamides 3 and 4. After enzymatic treatment of the isolated metabolites, their structures were identified by comparison with the synthetic compounds. Serotonin-4 receptor binding affinity of these metabolites was found to be lower than that of mosapride.

A new series of 4-amino-5-chloro-2-ethoxybenzamides 3b--f and 5--8 bearing six- and seven-membered heteroalicycles was prepared and evaluated for gastroprokinetic activity. Compounds 3b--e, derived by replacement of the morpholine oxygen of 4-amino-N-[(4-benzyl-2-morpholinyl)methyl]-5- chloro-2-ethoxybenzamide (3a) with other atoms (sulfur, nitrogen and carbon), generally exhibited a potent gastric emptying activity. N-(4-Benzyl-3-morpholinyl)methylbenzamide (5a) and its analogues 5b--e had weaker activity. However, N-(4-benzyl-3-morpholinyl)ethylbenzamide 8 was as potent as 3a. Benzamides 6a--e, having seven-membered heteroalicycles, showed fairly potent activity. Molecular superimpositions of 5a, 6a and 8 upon 3a using computer graphics suggested that the direction of the N-benzyl group greatly influences the gastric emptying activity, whereas the location of the alicyclic nitrogen is less critical.

In order to confirm the proposed structures of two metabolites 3 and 4 of the gastroprokinetic agent mosapride [4-amino-5-chloro-2-ethoxy-N-([4-(4-fluorobenzyl)-2-morpholinyl] methyl)benzamide, 2], the compounds were synthesized and their biological activity was examined. The structures of the metabolites were confirmed by means of comparison with the synthetic compounds. The serotonin 5-HT4 receptor agonistic activities of the metabolites were found to be less than that of mosapride.

The enantiomers, (S)-(-)-1 and (R)-(+)-1, of (+/-)-4-amino-5-chloro-2-ethoxy- N-[[4-(4-fluorobenzyl)-2-morpholinyl]methyl]benzamide (mosapride) [(+/-)-1], a new and selective gastroprokinetic agent, were prepared from optically active [4-(4-fluorobenzyl)-2-morpholinyl]methylamines (S)-(-)-4 and (R)-(+)-4, respectively. The requisite (S)-(-)-4 and (R)-(+)-4 were prepared by optical resolution of [4-(4-fluorobenzyl)-2-morpholinyl]methyl p-toluenesulfonate [(+/-)-5] using (-)- and (+)-N-(p-toluenesulfonyl)glutamic acids, followed by amination of the tosyloxy groups of (R)-(-)-5 and (S)-(+)-5, respectively. The absolute configurations of (R)-(-)-5 and (S)-(+)-5 were determined on the basis of an asymmetric synthesis of (R)-(-)-5 from (S)-(+)-benzyl glycidyl ether [(S)-(+)-11]. Mosapride and its enantiomers, (S)-(-)-1 and (R)-(+)-1, were essentially equipotent in serotonin 5-HT4 receptor agonistic activity on the electrically evoked contractions in isolated guinea pig ileum.

The title compounds (19-55) with a 4-substituted 2-(aminomethyl)morpholine group were prepared and evaluated for the gastrokinetic activity by determining their effect on gastric emptying of phenol red semisolid meal in rats. Introduction of chloro, fluoro, and trifluoromethyl groups to the benzyl group of the parent compounds 1a and 1b enhanced the activity. Among compounds tested, 4-amino-5-chloro-2-ethoxy-N-[[4-(4-fluorobenzyl)-2-morpholinyl] methyl] benzamide (23b) showed the most potent gastric emptying activity (effects on phenol red semisolid meal in rats and mice, and on resin pellets solid meal in rats). The gastrokinetic activity of 23b citrate (AS-4370) compared very favorably with that of cisapride and was higher than that of metoclopramide. In contrast to metoclopramide and cisapride, AS-4370 was free from dopamine D2 receptor antagonistic activity in both in vitro ([3H]spiperone binding) and in vivo (apomorphine-induced emesis in dogs) tests.