Ehrlich’s reaction

Fundamentally similar to pyrromethene formation are two of the classical colour reactions associated with pyrroles. The redness produced by pyrroles with an acid-moistened pine shaving is due to reaction with aldehydes in the wood, but it is not specific for pyrroles " . Ehrlich s reaction depends on the formation of coloured products by the reaction of a pyrrole with j -dimethylaminobenzaldehyde in acid solution. The nature of the reaction is indicated by the isolation of a salt of the cation (26) after treatment of (27) with the Ehrlich reagent, a carboxyl group being eliminated (26) gives red solutions, the disodium salt of the derived free base produces yellow solutions, and treatment of (26) with excess acid the colourless di-protonated salt . ... 

It can be concluded from the publications of Rohde [2] and Freund and Lebach [3, 4] that it was Ehrlich who first suggested the use of 4-(dimethylamino)-benzalde-hyde in the presence of hydrochloric acid for color reactions with methylketols that was found also to apply to indole derivatives [S]. Auterhoff [6] designated a reagent for urobilinogen, consisting of a solution of 2 g 4-(dimethylamino)-benzaldehyde in 20 percent hydrochloric acid, correctly as Ehrlich s solution. Hence, it is essentially correct to refer to all reagents which contmn these components as Ehrlich s reagent. 

The 3-vinylpyrroles (366 X = H or C02H) also react with electrophiles. No products were isolated, but it was suggested that the blue colouration produced upon the acid-catalyzed reaction with Ehrlich s reagent was due to the formation of (370) and that it was identical with that of the product obtained from the reaction of the trisubstituted pyrrole and the cinnamaldehyde. Similarly, the red colouration obtained from the reaction of (366 X = H or C02H) with diazotized sulfanilic acid was ascribed to the formation of (371) <58LA(611)205). 

Indolizine, like many of its derivatives, is air and light sensitive. It gives a colour reaction with Ehrlich s reagent and is highly fluorescent. 

In general, the azaindoles are stable, colorless, high-melting solids with little, if any, odor. They fail to give a positive test with Ehrlich s reagent or the pine-splint reaction. Also in contrast to indoles, they are stable to boiling mineral acids and darken on exposure to air very gradually. Like pyrroles and indoles, they are stable to aqueous alkaline solutions. 

Ehrlich s reagent reacts with a wide range of controlled substances and other indole alkaloids. A positive reaction does not therefore prove the presence of a specific drug. This is why it is necessary to carry out an additional confirmatory test. 

Scheme 11. Reaction of the dipyrromethane cofactor with Ehrlich s reagent...

A comparison between certain chemical characteristics of the siaUc, iV-acetylneuraminic, and methoxyneuraminic acids, especially their functional groups and the color reactions afforded with Ehrlich s and Bial s reagents, suggested a close relationship between these nonulosaminic acids and indicated that methoxyneuraminic acid might be the methyl glycoside of a completely deacylated sialic acid. This concept was substantiated with the conversion of A-acetylneuraminic acid into methoxyneuraminic... 

The traditional method of assay of 2-acetamido sugars is the Morgan-Elson reaction. This involves treatment of the sugars in base, which produces furans, and reaction of these furans in situ with Ehrlich s reagent (p-dimethylamino-benzaldehyde in acid), which generates delocalised chromophores, as shown in Figure 7.8." ... 

The tea consumed by these four women was analyzed using spectrophotometry after reaction with Ehrlich s reagent revealed unsaturated PAs that were determined to be senecionine and the corresponding TV-oxide in concentrations of 0.42 mg/g and 1.4 mg/g, respectively. The leaves were determined to be from the family Compositae, to which coltsfoot belongs, although their exact identity could not be determined because of their chopped condition. Cumulative doses of these alkaloids were calculated to be 1350 mg over 45 d for one patient, 1380 mg over 46 d for the patient who died, 570 mg over 19 d for another patient, and 630 mg over 21 d for the patient with the mildest symptoms. The cumulative dose was 15 mg/kg in the patient who exhibited mild symptoms, while the mean cumulative dose was 18 mg/kg in the other three patients. 

Isoindole reacts with ethanolic 1,3,5-trinitrobenzene to give a deep red color attributed to a 7t-complex. With Ehrlich s reagent a red-purple color, which slowly turns blue, is observed. This reaction presumably involves electrophilic substitution at C-l (Section V,D) to give charge resonance systems of the types shown in 17 and 18 (cf. Section V,F,2). 

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