Scheme 3. Conditions: (a) 1) POCl3, 1,8-bis(dimethylamino)naphthalene, (CH3O)3PO, 0°C, 5 min; 2) tri-n-butylamine, tris(tetrabutylammonium)hydrogen pyrophosphate, DMF, 0°C, 5 min; (b) NH4OH, aqueous AcONH4, MeOH, 60°C, 3 h, 25% (two steps).
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