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10.13.1.1.1.2.1.1.7 Variation 7: From 1-Nitro-2-(o-nitroaryl)ethenes

DOI: 10.1055/sos-SD-010-00559

Joule, J. A.Science of Synthesis, (200110426.

A LeimgruberBatcho-type intermediate is almost certainly involved in a sequence (Scheme 103) which begins with the ready base-catalyzed condensation of an o-nitroaraldehyde 367 with nitromethane to give a 1-nitro-2-(o-nitroaryl)ethene 368. Commonly, sodium hydroxide or ammonium acetate/acetic acid are used to promote the condensation; potassium fluoride is also used.[‌258‌] The 1-nitro-2-(o-nitroaryl)ethenes can also be obtained by nitration of 1-aryl-2-nitroethenes.[‌259‌,‌260‌] Reduction, traditionally with metal/acid combinations,[‌259‌‌262‌] then leads to a 2,3-unsubstituted indole 372, the simplest interpretation for which being the formation of an intermediate arylamino-enamine 369 (not isolated) comparable to that involved in the LeimgruberBatcho sequence. As in that sequence, protonation of 369 gives 370, heterocyclic ring closure to 371 being followed by final loss of ammonia producing the indole 372. Alternative methods for the reductive cyclization of 1-nitro-2-(o-nitroaryl)ethenes include (see Table 6): catalytic transfer hydrogenation,[‌263‌] titanium(III) chloride,[‌264‌] sodium borohydride/palladium/carbon,[‌265‌] hydrogen/palladium(II) hydroxide/carbon,[‌266‌] palladium/carbon/ammonium formate/formic acid,[‌263‌] and iron/acetic acid/silica gel.[‌260‌]

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