合成路线1

Protection of 3-hydroxy-2-methylpyran-4-one (I) with PhCH2Br in the presence of K2CO3 in acetonitrile at 80 °C or in DMF yields 3-(benzyloxy)-2-methylpyran-4-one (II) , which, after deprotonation with LiHMDS in THF, is condensed with benzaldehyde (III) at –60 °C to afford 3-(benzyloxy)-2-(2-hydroxy-2-phenylethyl) pyran-4-one (IV). Sulfonylation of alcohol (IV) with MsCl and Et3N in THF followed by mesylate elimination by means of DBU in NMP gives alkene (V), which by oxidative cleavage with NaIO4 in the presence of RuCl3, optionally using H2S, in acetonitrile/AcOEt/water, and further oxidation of the obtained aldehyde with NaClO2 or NaClO in the presence of TEMPO, yields the pyrancarboxylic acid (VI). Condensation of the 4-pyranone (VI) with 3-aminopropane-1,2-diol (VII) in EtOH at 80 °C leads to the pyridone (VIII), which is then converted to the methyl ester (IX) using MeI and NaHCO3. Oxidative cleavage of the vicinal diol (IX) with NaIO4 and AcOH or H2SO4 in acetonitrile/water provides the pyridone-1-acetaldehyde hydrate (X), which by cyclization with 3(R)-amino-1-butanol (XI) by means of AcOH in MeOH at 90 °C or diglyme yields the pyrido[1’,2’:4,5]pyrazino[2,1-b][1,3]oxazine derivative (XII). Bromination of compound (XII) with NBS in CH2Cl2 or NMP furnishes the bromopyridone derivative (XIII), which undergoes carbonylation with CO in the presence of 2,4-difluorobenzylamine (XIV) and Pd(PPh3)4/DIEA or Pd(OAc)2/DIEA/DPPB in DMSO at 90 °C to afford carboxamide (XV). Finally, compound (XV) is deprotected by debenzylation with H2 over Pd/C .
The pyrancarboxylic acid (VI) can also be prepared by oxidation of 3-(benzyloxy)-2-methylpyran-4-one (XVI) with SeO2 in bromobenzene at 160 °C to give aldehyde (XVII) and further oxidation with NaClO2 in the presence of NH2SO3H in acetone/H2O .