a method of producing 4 -- 1 -- 2 , 9 - dimethyl - 4 , 7 - dioxo - 8 - octahydro - 1h - pyrazinotriazin - 6 - yl ) methyl ) phenyl dihydrogen phosphate , including a step of adding a reaction mixture 1 containing - n - benzyl - 6 -- 2 , 9 - dimethyl - 4 , 7 - dioxo - 8 - octahydro - 1h - pyrazinotriazine - 1 - carboxamide , triethylamine and a solvent to a reaction mixture 2 containing a phosphorylating agent and a solvent .