[NH:1]1[C:6](=O)[CH2:5][O:4][C:3]2[CH:8]=[N:9][CH:10]=[CH:11][C:2]1=2.B.CSC>O1CCCC1>[NH:1]1[CH2:6][CH2:5][O:4][C:3]2[CH:8]=[N:9][CH:10]=[CH:11][C:2]1=2To a stirred solution of 1H-pyrido-[3,4-b][1,4]oxazine-2-(3H)one (3 g, 21 mmol) in tetrahydrofuran (150 ml) was added rapidly a borane/dimethyl sulfide solution (52 ml of a 2M solution in THF, 5 eq) at 25° C. over 15 minutes. The reaction mixture was stirred at 25° C. for 2 hours and quenched with 5 ml of water. The solvent was removed in vacuo using a rotary evaporator and the residue treated with aqueous 1M HCl with stirring. The water was removed and the residue chromatographed on silica gel using CH2Cl2 /MeOH/NH4OH (90/10/1) to give 2 g (70%) of titled product. m.p. 167°-169° C. The structural assignment is supported by the proton NMR spectrum.
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