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5.2.1.3 [1,5] Rearrangement

DOI: 10.1055/sos-SD-206-00325

Fringuelli, F.; Piermatti, O.Science of Synthesis: Water in Organic Synthesis, (20121495.

Pyridine 74 (R1=R2=R3=H) treated with nitric pentoxide (N2O5) in liquid sulfur dioxide at 11°C gives N-nitropyridinium nitrate (75, R1=R2=R3=H), which subsequently affords 3-nitropyridine (78, R1=R2=R3=H) in 63% yield in water (Scheme 20).[‌61‌] The reaction is not an electrophilic aromatic substitution but occurs through a [1,5]-sigmatropic rearrangement of N-nitropyridinium nitrate sulfonic acid 76 (R1=R2=R3=H) to intermediate 77 (R1=R2=R3=H). Moderate and good yields have been observed for 3- and 4-substituted pyridines, respectively (Scheme 20).

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