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Semotiadil fumarate has been obtained by several related ways: 1) The reaction of 2-(2-hydroxy-5-methoxyphenyl)-4-methyl-3,4-dihydro-2H-1,4-benzothiazin-3-one (I) with 3-chloropropyl bromide (II) by means of NaH in DMF gives 2-[2-(3-bromopropoxy)-5-methoxyphenyl]-4-methyl-3,4-dihydro-2H-1,4-benzothiazin-3-one (III) , which is condensed with N-[2-(1,3-benzodioxol-5-yloxy)ethyl]-N-methylamine (IV) by means of NaI in NaHCO3 in DMF yielding 2-[2-[3-[N-[2-(1,3-benzodioxol-5-yloxy)ethyl]-N-methylamino]propoxy]-5-methoxyphenyl]-4-methyl-3,4-dihydro-2H-1,4-benzothiazin-3-one (V). Finally, this compound is submitted to optical resolution by salification with L-(+)-2-hydroxy-2-phenylacetic acid (VI), fractional crystallization of the resulting salts, elimination of the chiral acid with NaHCO3 and salification with fumaric acid in ethanol. 2) Compound (V) can also be obtained by condensation of benzothiazinone (I) with N-[2-(1,3-benzodioxol-5-yloxy)ethyl]-N-(3-bromopropyl)-N-methylamine (VII) by means of NaH in DMF. 3) Compound (V) can also be obtained by condensation of 2-[5-methoxy-2-[3-(methylamino)propoxy]phenyl]-4-methyl-3,4-dihydro-2H-1,4-benzothiazin-3-one (VIII) with 2-(1,3-benzodioxol-5-yloxy)ethyl methanesulfonate (IX) by means of triethylamine in ethanol. 4) The optical resolution of the racemic compound (V) can also be performed by chromatography over a column packed with crystalline cellulose triacetate eluted with 95% ethanol.