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

DOI: 10.1055/sos-SD-110-00001

Joule, J. A.Science of Synthesis Knowledge Updates, (20102114.

A Leimgruber–Batcho-type intermediate is almost certainly involved in a sequence (Scheme 152) which begins with the ready base-catalyzed condensation of an o-nitroaraldehyde 459 with nitromethane to give a 1-nitro-2-(o-nitroaryl)ethene 460. Commonly, sodium hydroxide or ammonium acetate/acetic acid are used to promote the condensation; potassium fluoride is also used.[‌398‌] The 1-nitro-2-(o-nitroaryl)ethenes can also be obtained by nitration of 1-aryl-2-nitroethenes.[‌399‌,‌400‌] Reduction, traditionally with metal/acid combinations,[‌399‌‌402‌] then leads to a 2,3-unsubstituted indole 464, the simplest interpretation for which being the formation of an intermediate arylamino-enamine 461 (not isolated) comparable to that involved in the Leimgruber–Batcho sequence. As in that sequence, protonation of 461 gives 462, heterocyclic ring closure to 463 being followed by final loss of ammonia producing the indole 464. Alternative methods for the reductive cyclization of 1-nitro-2-(o-nitroaryl)ethenes include (see Table 7): catalytic transfer hydrogenation,[‌403‌] titanium(III) chloride,[‌404‌] sodium borohydride/palladium/carbon,[‌405‌] hydrogen/palladium(II) hydroxide/carbon,[‌406‌] palladium/carbon/ammonium formate/formic acid,[‌403‌] and iron/acetic acid/silica gel.[‌400‌] In the very special case of the synthesis of the sensitive 1H-indole-5,6-diol, the precursor, 2-(4,5-dihydroxy-2-nitrophenyl)-1-nitroethene was made by nitration of 2-(4,5-dihydroxyphenyl)-1-nitroethene with tetranitromethane in the presence of zinc sulfate, and the reductive ring closure was achieved with sodium dithionite, again in the presence of zinc sulfate.[‌407‌]

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