C1N2CN3CN(C2)CN1C3.N12CCCN=C1CCCCC2.[CH3:22][C:23]1[O:24][CH2:25][CH:26]([C:28]([O:30][CH3:31])=[O:29])[N:27]=1>ClCCl.[Cu](Br)Br>[CH3:22][C:23]1[O:24][CH:25]=[C:26]([C:28]([O:30][CH3:31])=[O:29])[N:27]=1Hexamethylenetetramine (11.7 g, 5.00 mmol) and 1,8-diazabicyclo(5,4,0)undec-7-ene were added to a stirred suspension of copper bromide (18.7 g, 83.85 mmol) in dichloromethane (50 mL) at 0° C. and purged with argon gas for 20 min. To the mixture was added methyl 2-methyl-4,5-dihydrooxazole-4-carboxylate (3 g, 20.97 mmol) at 0° C. The mixture was allowed to warm to ambient temperature and stirred for a further 12 h. The solvent was removed in vacuo and the resulting residue partitioned between ethyl acetate and 1:1 saturated aqueous ammonium chloride and ammonium hydroxide. The organic layer was separated and the aqueous layer extracted with ethyl acetate (2×20 mL). The combined organic layers were washed with 1:1 saturated aqueous ammonium chloride and ammonium hydroxide, 10% aqueous citric acid, saturated sodium bicarbonate solution, brine, dried over anhydrous sodium sulfate and concentrated in vacuo to afford methyl 2-methyloxazol-4-carboxylate (1.1 g, 34%) as a solid.
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