Nitric Acid → 5-(Thiophen-2-ylmethyl)imidazole-1-sulfonic acid dimethylamide

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Reaction Steps

To 2-amino-3-methylphenol (1) (14.72 g, 0.120 mol), triethylamine (35.0 mL, 0.251 mol) and 4-dimethylaminopyridine (0.29 g, 2.39 mmol) in anhydrous CH2Cl2 (100 mL) at 0° C. under argon was added chloroacetyl chloride (10.0 mL, 0.126 mol) dropwise via syringe. After the addition was complete the resulting solution was refluxed for 24 h. The organics were washed successively with phosphoric acid (0.5M), saturated sodium bicarbonate, water and brine and then dried over MgSO4. The resulting solution was concentrated and taken up in THF to which ether was added. The resulting crystals were filtered off to give pure 5-methyl-4H-benzo[1,4]oxazin-3-one (2) (12.30 g) in 63% yield. To (2) (14.64 g, 89.72 mmol) dissolved in concentrated H2SO4 (65 mL) at −10° C. was added 70% concentrated HNO3 (8.08 g, 89.72 mmol) in concentrated H2SO4 (25 mL) with rapid mechanical stirring at a rate whereby the internal temperature was maintained below −5° C. As soon as the addition was complete the mixture was poured onto crushed ice (500 mL) and the resultant solids were filtered off and slurried in cold water (300 mL) while sufficient NaOH was added to adjust the pH to 7. The recovered yellow powder was dissolved in THF, adhered to silica and purified by flash chromatography with 60% hexane and ethyl acetate to give the nitrated product as a mixture of two regioisomers, i.e. the desired 6-substituted aromatic comprising 6-nitro-5-methyl-4H-benzo[1,4]oxazin-3-one (3) (55%) and the 8-substituted by-product comprising 8-nitro-5-methyl-4H-benzo[1,4]oxazin-3-one (4) (22%). These isomers are separated with difficulty at this point and were carried on to the next step as a mixture. To a mixture of (3) (1.93 g, 9.27 mmol) and (4) (0.48 g, 2.32 mmol) dissolved in a solution of MeOH (300 mL) and THF (300 mL) under argon was added 10% palladium on carbon (1.20 g). The resulting solution was subjected to H2 at one atmosphere pressure. After 16 h the catalyst was filtered off and the resulting solution was concentrated in vacuo and purified by flash chromatography on silica with 50% hexane and ethyl acetate to give 6-amino-5-methyl-4H-benzo[1,4]oxazin-3-one (5) (0.96 g) in 46% yield and 8-amino-5-methyl-4H-benzo[1,4]oxazin-3-one (6) (0.17 g) in 8% yield. (5) (1.20 g, 6.74 mmol), imidazoline-2-sulfonic acid (2.02 g, 13.48 mmol) and triethylamine (2.35 mL, 16.85 mmol) were heated at reflux in anhydrous acetonitrile (50 mL) under argon for 48 h. At that time an additional amount of imidazoline-2-sulfonic acid (1.01 g, 6.74 mmol) and triethylamine (1.41 mL, 10.12 mmol) were added and the resulting mixture was stirred an additional 24 h. This solution was concentrated in vacuo and the residues were taken up in a solution of CHCl3/isopropyl alcohol (3:1) and washed successively with NaOH (1N) and brine, dried over MgSO4 and concentrated in vacuo. The resulting foam was purified by flash chromatography on silica with 20% methanol saturated with ammonia in chloroform to give 6-(imidazolidin-2-ylideneamino)-5-methyl-4H-benzo[1,4]oxazin-3-one (7) (0.42 g) as a foam in 27% yield along with 55% recovered starting material. The HCl salt was recrystallized from a mixture of ethanol and diethyl ether (EtOH/Et2O) to give fine white needles.

Provenance & Evidence