Immonium group

Indole can be nitrated with benzoyl nitrate at low temperatures to give 3-nitroindole. More vigorous conditions can be used for the nitration of 2-methylindole because of its resistance to acid-catalyzed polymerization. In nitric acid alone it is converted into the 3-nitro derivative, but in a mixture of concentrated nitric and sulfuric acids 2-methyl-5-nitroindole (47) is formed. In sulfuric acid, 2-methylindole is completely protonated. Thus it is probable that it is the conjugate acid which is undergoing nitration. 3,3-Dialkyl-3H-indolium salts similarly nitrate at the 5-position. The para directing ability of the immonium group in a benzenoid context is illustrated by the para nitration of the conjugate acid of benzylideneaniline (48). 

Work on alkaloid isolation is summarized in Table 11.3 2—396 Arnottianamide (145 R = H, R = Me) and isoarnottianamide (145 R = Me, R = H) have been isolated as minor constituents of Zanthoxylum cuspidatum. These amides can be obtained chemically by Baeyer-Villiger-type oxidation of the immonium group in chelery-thrine and nitidine respectively and a similar biogenesis is suggested. Three Argemone subfusiformis taxa and A, polyanthemos contained mainly the same... 

Under the reaction conditions, the enolate is protonated by methanol 215 to give the more favourable cis ring junction 216 and borohydride reduces the immonium group stereoselectively to give the major diastereoisomer 217 of the final product. 

Addition of hydrocyanic acid to the electron-deficient immonium group proceeds easily. Treatment of aqueous solutions of 3-imidazolinium methyl sulfates (24a,b) with a solution of an equimolecular quantity of... 

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