[C:1]([C:5]1[O:6][CH:7]=[C:8]([CH2:10]Cl)[N:9]=1)([CH3:4])([CH3:3])[CH3:2].[P:12]([O:19]CC)([O:16][CH2:17][CH3:18])[O:13][CH2:14][CH3:15].C(OCC)(=O)C>C(OCC)(=O)C.C(O)C>[C:1]([C:5]1[O:6][CH:7]=[C:8]([CH2:10][P:12](=[O:19])([O:16][CH2:17][CH3:18])[O:13][CH2:14][CH3:15])[N:9]=1)([CH3:4])([CH3:3])[CH3:2]A mixture of 2-tert-butyl-4-chloromethyl-1,3-oxazole (10.42 g) and triethyl phosphite (19.94 g) was stirred at 160° C. for 16 hrs. Excess triethyl phosphite was evaporated under reduced pressure. The residue was subjected to silica gel column chromatography and eluted with ethyl acetate-hexane (1:1, v/v), ethyl acetate, then ethyl acetate-ethanol (10:1, v/v) to give diethyl [(2-tert-butyl-1,3-oxazol-4-yl)methyl]phosphonate as a colorless oil (16.51 g, yield 100%).
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