Heumann-Pfleger synthesis

In the industrially used Heumann-Pfleger synthesis, N-phenylglycine 1, readily accessible from aniline, is transformed through indoxyl 2 into indigo 3 in a targeted fashion (Scheme 1-1). 

Scheme 1-1 Industrial production of indigo 3 by the Heumann-Pfleger synthesis [15] from 1 via 2 to 3.

Heumann-Pfleger synthesis (indigo) 145 Hinsberg synthesis (thiophene) 98 Hoffmann-Loffler reaction 158 Huisgen synthesis... 

By far the most important member of these vat dye families is C.I. Vat Blue 1 (indigo). Its synthesis can be achieved via the four-step method shown in Fig. 13.129. The method shown is known as the Heumann-Pfleger... 

Heumann-Pfleger indigo synthesis. Cycli-zation of phenylglycine to indoxyl followed by oxidation by air or oxidizing agents, such as ferric chloride, to yield indigo. 

In the synthesis due to Heumann and Pfleger the fusion yields the potassium derivative of indoxyl, which is dehydrogenated to indigo even by atmospheric oxygen at the same time hydrogen peroxide is produced (see p. 175). 

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