Indoxyl

This hydroxy-indol occurs in the urine of herbivorous animals as indoxylsulphuric acid. Indol is converted into indoxyl-sulphuric acid in the animal organism. From this compound indoxyl is formed by warming with concentrated hydrochloric acid. It may also be obtained by heating indoxylic acid, according to the equation  

Indoxyl is an oil not volatile in a current of steam. On oxidation it gives indigo blue. 

Indoxylsulphuric acid may be obtained by heating indoxyl with potassium pyrosulphate [12]. It exists only in the form of its salts. These are eolourless and yield indigo blue on heating or on oxidation. 

The ethyl ether of this acid is formed by reduction of orthonitro-j)henylpropiolic acid ethyl ether with ammonium sulphide [13]. 

The free acid is obtained by saponification of the ether with an alkali. It forms a crystalline precipitate, sparingly soluble in water. It splits up into indoxyl and carbonic acid on heating. Oxidising agents convert it to indigo blue, W hile on heating with concentrated sulphuric acid indigo sulphonic acid is formed. 

3-Hydroxyindole certainly contribntes in the tautomeric equilibrium with the carbonyl form, though it is the minor component. Indoxyl, 10.46, ° is more acidic than oxindole, the anion produced is ambident reactions with electrophiles at both oxygen and carbon are known.  

The indoxyl anion is particularly easily autoxidised, producing the ancient blue dye, indigo. The mechanism probably involves dimerisation of a radical formed by loss of an electron from the anion. 

0-AcetylindoxyP and iV-acyl-indoxyls are more stable snbstances the latter undergo normal ketone-carbonyl reactions, such as the Wittig reaction.  

Mirroring oxindoles, aldol-type condensation at the 2-position in indoxyls can be accomplished either using the acetate of the enol form and base catalysis,or with indoxyl itself, in either acid or basic condi-tions. Borohydride redaction and dehydration allows these aUcylidene condensation prodncts to be converted into 2-substituted indoles. 

Peroxide oxidation of Af-phenylsulfonylindole-3-boronic acid gives A -phenylsulfonylindoxyl, which can be converted into the triflate of the 3-hydroxyindole tautomer. The same A -protected indoxyl can be prepared by ring synthesis, as shown below. 

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It is an important intermediate in the indigo industry. When phenylglycine is fused with sodamide a good yield of indoxyl is obtained. The indoxyl is easily oxidized to indigo. 

Supplement XXI, 2nd 1935 3032-3457 Hydroxy-pyridine, 43. Indoxyl, 69. Hydroxyquinoline, 77. Papaverine, 220. 

Obermayer s reagent (detection of indoxyl in urine) dissolved g of FeCl3 in a liter of concentrated HCl. 

The leaves of the indigo plant do not contain the dye as such, but in the form of its precursor, a glycoside known as indican (109). Indican [487-60-5] is the dextrose derivative (35) of indoxyl [480-93-3] (110). Indoxyl occurs also in the urine of humans as the potassium salt of indoxyl sulfonic acid (111). 

Based on this observation, K. Heumann treated N-phenylglycine [103-01 -5] with alkali and obtained indoxyl (38) (keto form), which on aerial oxidation converted to indigotin ... 

The greatest improvement in the manufacture of indigotin came when sodamide was used with alkali in the conversion of phenylglycine to indoxyl (125). Not only was the fusion temperature lowered from about 300°C to 200°C, but also the reaction was made practically anhydrous by the sodamide... 

Hydroxyindole (181) represents a well known example of a compound in which the hydroxyl group is to the ring heteroatom. The equilibrium mixture again contains mainly the carbonyl form (182), indoxyl. Deprotonation gives a reactive ambident anion which can be methylated either on oxygen or C-2 (Scheme 73). Indoxyl is easily oxidized to indigo (184), which may be formed by dimerization of the radical (183) produced by electron loss from the anion. 

BAEYER - DREWSON Indoxyl Synfhesis Conversion of o-nitrobenzaldehyde and ketones to indoxyls. 

The quindoline 224 may be prepared by the condensation of indoxyl-2-carboxylic acid with 6-aminopiperonaldehyde in the presence of hydrochloric acid, when decarboxylation and cyclization take place. Nitric acid oxidation of 224 gave an unstable nitrodicarboxylic acid which decarboxylated readily to a nitromonocarboxylic acid formulated as 8-nitro-8-carboline-3-carboxylic acid (225). ... 

Indoxyl 122 (prepared from the corresponding 0,A-diacetate) exists in the solid state as keto tautomer (as confirmed by IR spectrum) (65CC381). Analysis of its solution in [DgjDMSO showed the presence only of the keto form 122 which, over a period of 24 h, converts to the enol 123 (R = H) (>90%). 

In 1883, the hydroxy structure 82 was assigned to indoxyl on the basis of chemical evidence. More recently, however, the infrared spectra of 1-acetyl- and 1-methyl-indoxyl measured in chloroform indicated that the oxo form 83 (R = Ac, Me) greatly predominates, " ... 

Reaction of a Dithioacetal Derived from Indoxyl with Allyl and Stabilized Benzyl Anions... 

The a-oxoketene dithioacetal 6.40 is derived from indoxyl (l,2-dihydroindol-3-one), a heterocyclic carbonyl precursor, and its reaction with simple allyl anions will also yield the corresponding Jl-annulation product. Thus when 6.40 was reacted with allyl anions 65 the corresponding carbinol acetals 66 formed insitu underwent smooth BF3.Et20 assisted cyclization to afford the corresponding carbazoles 67 in high yields <99T11563>. 

Synthesis of this compound began with the condensation of O, N-acetyl-indoxyl 174 with 5-bromoisatin 175 to give indolo [ 3,2-fo ] quinoline 176 after stirring at room temperature for 10 days (Fig. 49) [104]. This product was then decarboxylated by heating at reflux for 6 h in diphenyl ether. Yields were... 

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