Synthesis
General procedure for the synthesis of 5-nitrothiophene-2-carbaldehyde and 4-nitrothiophene-2-carbaldehyde from 2-thiophenecarboxaldehyde:
Step 1: Synthesis of 4-nitrothiophene-2-carbaldehyde (1)
Fuming nitric acid (40 mL) was mixed with concentrated sulfuric acid (31 mL) and slowly added to a cooled solution of thiophene-2-carbaldehyde (20 g, 17.8 mmol) in concentrated sulfuric acid (4 mL). The reaction was carried out in an ice-salt bath and stirring was continued for 5 min after addition. Subsequently, the reaction was quenched by the addition of ice water and the mixture was extracted with ether. The combined ether extracts were washed sequentially with saturated sodium bicarbonate solution and brine, dried over anhydrous magnesium sulfate, filtered and concentrated to give a brown oily crude product. 1H NMR analysis of the crude product showed a mixture of 4-nitrothiophene-2-carboxaldehyde and 5-nitrothiophene-2-carboxaldehyde in the ratio of about 40:60. Separation of the mixture by column chromatography (eluent: 30-50% dichloromethane/hexanes) gave 4-nitrothiophene-2-carboxaldehyde in 40% yield as a light yellow solid.1H NMR (CDCl3) δ (ppm). 9.95 (s, 1H), 8.63 (s, 1H), 8.27 (s, 1H).
References
[1] Journal of Chemical Research, Miniprint, 1997, # 9, p. 2001 - 2013
[2] Patent: WO2008/104077, 2008, A1. Location in patent: Page/Page column 46
[3] Chemical and Pharmaceutical Bulletin, 1995, vol. 43, # 1, p. 162 - 165
[4] Bulletin de la Societe Chimique de France, 1963, p. 479 - 484
[5] Journal of the American Chemical Society, 1954, vol. 76, p. 1378
