合成路线1

3-Nitro-7,8-dihydro-5H-pyrano[4,3-b]pyridine (III) was prepared by heating tetrahydropyranone (I) with dinitropyridone (II) in methanolic ammonia following a reported procedure. Hydrogenation of the nitro derivative (III) and protection of the resulting amine (IV) with trichloroacetyl chloride afforded the trichloroacetamide (V). Oxidation of the pyridine ring of (V) with 3-chloroperoxybenzoic acid gave the N-oxide (VI). Nitration of the pyridine ring of (VI) with fuming nitric acid, followed by amide hydrolysis with ammonium hydroxide provided the 3-amino-4-nitropyridine N-oxide (VII). Reduction of the N-oxide and 4-nitro groups of (VII) was accomplished in a single step by hydrogenation in the presence of Raney-nickel to furnish the bicyclic diaminopyridine (VIII). Subsequent monoacylation of (VIII) with isoxazole-3-carbonyl chloride (IX) followed by thermal cyclization provided the desired imidazopyranopyridine compound (1,2), which was finally isolated as the title phosphate salt