9,10-Difluoro-3-(hydroxymethyl)-7-oxo-2,3-dihydro-7H-pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acid ethyl ester (I), a racemic intermediate in the synthesis of racemic ofloxacin, is esterified with 3,5-dinitrobenzoyl chloride (II) in the usual way to give the racemic ester (III), which is resolved into its optical isomers by HPLC over a SUMIPAX OA-4200 column, using hexane-1,2-dichloroethane-ethanol as carrier solvent. The (-)-optical isomer (IV) is partially hydrolyzed with ethanolic aqueous NaHCO3 to afford the (-)-alcohol (V), which is treated with triphenylphosphite methiodide in DMF giving the corresponding (-)-iodomethyl derivative (VI). The reduction and simultaneous hydrolysis of (VI) with tributyltin hydride in ethanol yields (-)-9,10-difluoro-3-methyl-7-oxo-2,3-dihydro-7H-pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acid (VII), which is finally treated with N-methylpiperazine (VIII) to give (-)-ofloxacin.
This compound can be obtained by two related ways: 1) The condensation of 2-O-(p-toluenesulfonyl)-D-lactoyl chloride (IV) with 1-methyl-2-oxoimidazolidine-4(S)-carboxylic acid tert-butyl ester (VIII) by means of potassium tert-butoxide in THF gives 1-methyl-2-oxo-3-[2(R)-(p-toluenesulfonyloxy)propionyl]imidazolidine-4(S)-carboxylic acid tert-butyl ester (IX), which is then condensed with 2(S)-amino-4-phenylbutyric acid ethyl ester (X) by means of triethylamine in DMSO yielding 3-[N-[1(S)-(ethoxycarbonyl)-3-phenylpropyl]-L-alanyl]-1-methyl-2-oxoimidazolidine-4(S)-carboxylic acid tert-butyl ester (XI). Finally, this compound is hydrolyzed with HCl in dioxane - water. The starting compounds (IV) and (VIII) are obtained as follows: a) The condensation of D-lactic acid methyl ester with p-toluenesulfonyl chloride and triethylamine gives the corresponding sulfonate (II), which is hydrolyzed with NaOH to the free acid (III). Finally, this compound is treated with refluxing SOCl2 to give acid chloride (IV). b) The esterification of 3-(benzyloxycarbonyl)-2-oxoimidazolidine-4(S)-carboxylic acid (V) with tert-butanol gives the corresponding ester (VI), which is methylated with methyl iodide and K2CO3 to 3-(benzyloxycarbonyl)-1-methyl-2-oxoimidazolidine-4(S)-carboxylic acid tert-butyl ester (VII). This compound is debenzylated by hydrogenation with H2 over Pd/C to afford imidazolidine ester (VIII). 2) The condensation of 2(S)-bromo-4-phenylbutyric acid ethyl ester (XII) with L-alanine benzyl ester (XIII) by means of K2CO3 in DMSO gives N-[1(S)-(ethoxycarbonyl)-3-phenylpropyl]-L-alanine benzyl ester (XIV), which is debenzylated by hydrogenation as before yielding the free acid (XV). The esterification of (XV) with N-hydroxysuccinimide gives the corresponding active ester (XVI), which is finally condensed with imidazolidine ester (VIII) by means of potassium tert-butoxide to afford the precursor (XI) already obtained.
The methylation of commercially available des-methyl levofloxacin (I) with 11C-methyl iodide and NaOH in DMF or DMA at 150?C gives the target labeled levofloxacin.