Heteroannulation of Nitroketene N,S-Arylaminoacetals with POCl3: A Novel Highly Regioselective Synthesis of Unsymmetrical 2,3-Substituted Quinoxalines (original) (raw)

were recorded in CDCl 3 , and TMS was used as an internal reference. Melting points were obtained on Mel-Temp melting point apparatus (capillary method) and were uncorrected. The IR spectra were recorded on Perkin Elmer 1320 spectrophotometer. The FAB mass spectra were recorded on a JEOL SX 102/DA-6000 Mass Spectrometer/Data System. Chromatographic purification was conducted by column chromatography using 100-200 mesh silica gel obtained from Acme Synthetic Chemicals. DMF, NMP and CH 2 Cl 2 were distilled over CaH 2 and stored over 4Å molecular sieves. CH 3 CN and Cl 2 CHCHCl 2 were distilled over P 2 O 5. THF was distilled over sodium/benzophenone prior to use. NaH (as 60% suspension in oil), N-benzyl piperazine and Fe(acac) 3 were purchased from the standard firms and used as such whereas NiCl 2 (PPh 3) 2 and NiCl 2 (dppp) catalysts were S2 prepared according to the literature procedure. mCPBA (50-55% suspension in water) was diluted with CH 2 Cl 2 and dried over anhydrous Na 2 SO 4. All the known 1a-b, 1f-h, 1j, 4 and unknown 1c-e, 1i, 1k, 9, 11 nitroketene N,S-arylaminoacetals were prepared by slight modification of the reported procedure. 23 General Procedure for Preparation of Nitroketene N,S-Acetals 1a-k, 4 and Bis(nitroketene-N,S-acetals) 9 and 11 In a typical experiment a solution of nitroketene S,S-acetal (5.0 g, 30.30 mmol) and corresponding aniline (30.30 mmol) in 150 mL ethanol was refluxed for 24 h with constant stirring, the reaction being monitored by TLC. The reaction mixture was then cooled by ice-water, filtered and washed with ether to give crystalline nitroketene N,Sacetals 1a-k, 4 in high yields. The bis(nitroketene-N,S-acetals) 9 and 11 were also prepared following the similar procedure from the corresponding diamine (1.1 g, 10 mmol) and nitroketene S,Sacetal (3.30 g, 20 mmol). N H SMe NO 2 MeO 1a 2-(3'-Methoxyanilino)-2-(methylthio)-1-nitroethylene (1a) Yield 88% (6.41 g); Yellow crystals; m.p. 122-123 ºC; R f 0.49 (7:3 hexanes