The chemistry of isatins: a review from 1975 to 1999

Isatina (1H-indol-2,3-diona) é um composto de grande versatilidade sintética, podendo ser utilizado na obtenção de diversos sistemas heterocíclicos, como derivados indólicos e quinolínicos, o que a torna uma importante matéria-prima na síntese de fármacos. Isatina também tem sido detectada em tecidos de mamíferos, o que tem despertado o interesse em seu estudo como modulador em diversos processos bioquímicos. Os avanços na aplicação de isatinas em síntese orgânica, bem como na compreensão de seus efeitos biológicos e farmacológicos, nos últimos vinte e cinco anos encontram-se relatados nesta revisão e seus respectivos materiais suplementares.

This review aims to document the publications concerning isatin, its synthesis,

The Sandmeyer methodology
The method developed by Sandmeyer is the oldest and the most frequently used for the synthesis of isatin. It consists in the reaction of aniline with chloral hydrate and hydroxylamine hydrochloride in aqueous sodium sulfate to form an isonitrosoacetanilide, which after isolation, when treated with concentrated sulfuric acid, furnishes isatin in >75% overall yield 17 . The method applies well to anilines with electron-withdrawing substituents, such as 2-fluoroaniline 18 , and to some heterocyclic amines, such as 2-aminophenoxathine 19 (Scheme 1). This method has some economic advantages, as the reagents are cheap and readily available, and the yields are usually high. Recently, the Sandmeyer methodology has been modified by the incorporation of ethanol as a co-solvent 20 . This modification proved to be particularly useful in cases where the aniline derivative was insoluble in the conventional reaction matrix. Application of the modified Sandmeyer methodology allowed the synthesis of 4,6-dibromoisatin, a key intermediate for the synthesis of the marine natural product convolutamydine A, in 85% yield, thus representing a greater than 700% improvement in yield over the existing published procedure. The use of microwave irradiation during both stages of the Sandmeyer procedure has been investigated, and this modified procedure was also employed for the synthesis of convolutamydine A 21 .
In addition to the use of H 2 SO 4 for the cyclization step, isonitrosoacetanilides can be heated in BF 3 .Et 2 O at 90 o C. After cooling the reaction mixture, addition of water allows isolation of the respective isatins. This methodology has proved to be particularly effective for the preparation of benzo-oxygenated isatin derivatives 22,23 The Sandmeyer synthesis has been described as being unapplicable to ortho-hydroxy or ortho-alkoxyanilines. Therefore an alternative procedure for the synthesis of the isonitrosoacetanilides was reported 24,25 (Scheme 2).

Scheme 2
On the other hand, there are some disadvantages, for instance those listed below.
a) The use of N-alkylanilines furnishes the corresponding N-alkylisatins in low yield. For example, N-methylisatin is obtained in 22% overall yield 26 .
These isomers can be separated by conversion to the corresponding sodium isatinates using 0.5N NaOH. Subsequent controlled acidification of the reaction medium leads to cyclisation of the two isomers at different pH values, regenerating the corresponding isatins, which precipitate from the reaction medium 27 (Scheme 3). c) The formation of HCN during the reaction has been detected by the formation of Prussian blue on addition of ferrous sulfate and NaOH 28 . The measured concentration of HCN in the mother liquors from the preparation of the isonitrosoacetanilides was found to be 100 to 200 ppm 29 . The mechanism informally proposed for the formation of HCN is described below (Scheme 4).
Please use CPS: orgchem/0010004 in any reference to this article

Scheme 5
A futher possibility exists. It has been shown that nitrosoalkenes decompose, with formation of HCN, via the formation of an oxazete and retro-cyclisation 31 (Scheme 6). Whatever the mechanism for formation of HCN during the Sandmeyer isonitrosoacetanilide synthesis, it is resasonable to recommend that appropriate precautions be taken during the preparation of these compounds.

Use of nitroacetanilides
Nitroacetanilides, obtained by alkaline hydrolysis of 1-arylamino-1-methylthio-2nitroethenes, are readily cyclised to isatin-3-oximes by the use of concentrated sulfuric acid or trifluoromethanesulfonic acid at room temperature; the latter giving somewhat higher yields 32 .
Although this methodology is related to the Sandmeyer methodology, it has no obvious benefit over the latter (Scheme 7).  The synthesis of 5-azaisatin was realized by ortho-lithiation of the 4-aminopyridine tbutylcarbamate followed by reaction with an excess of diethyl oxalate. Heating the glyoxylic ester under vaccum gave 5-azaisatin 50 (Scheme 13).

Scheme 13
Recently, a metal-halogen exchange method was described for the synthesis of isatins by lithiation of ortho-bromophenylureas, carbonylation and subsequent intramolecular cyclisation to give the desired products in 71-79% yield 51 (Scheme 14).

Miscellaneous procedures
These previously discussed methodologies are the most general and/or most commonly employed procedures for the synthesis of isatins. Other methodologies have been employed, but they are less general and some of them lead to the desired product in low yield.
Parrick and co-workers developed a synthetic methodology for isatins from indoles, using N-bromosuccinimide to promote their oxidation to yield 3,3-dibromooxindoles which were subsequently hydrolysed to the desired isatins 52,53 . By using this method it was possible to obtain 7-azaisatin from 7-azaindole, although in low yield. This isatin is more readily obtained by oxidation of the indolic compound using chromic anhydride in acetic acid 54 and this methodology can also be applied to the oxidation of 5-azaindole to yield 5-azaisatin 55 .
In an alternative methodology, 4-and 6-substituted-2-oxindoles, obtained from onitroarylmalonates, were converted to 3,3-dibromooxindoles by reaction with pyridinium perbromide. These intermediates were hydrolyzed to the corresponding isatins. This method, although limited to substrates with moderate to strongly electron withdrawing groups

Scheme 15
Nitrones and dichloroketene react to furnish 3,3-dichlorooxindoles, which upon hydrolysis, lead to the desired isatins 57 . N-Aryl-benzoisatins can also be obtained from napthoquinones and anilines as a result of oxidation of the cyclic anils 58

Scheme 17
Meth-Cohn and co-workers have observed that the treatment of 1,2-bis (Nmethylanilino)-1,2-dichloroethanes, obtained by the dimerisation of the Vilsmeier reagents prepared from N-methylformanilides in POCl 3 using a tertiary amine, with an eletrophilic species yielded isatins in 11 to 79% after hydrolysis. The best yields were observed when bromine was used as the electrophilic species 60

Scheme 23
The formation of isatins has been reported during decomposition studies of the structure or reactivity of natural products. In this manner, the attempted epoxidation of rutacridone led to N-methylisatin 67 (Scheme 24).

N-alkylation
Many methods have been devised for the N-alkylation of isatins. These derivatives are commonly synthesized from the reaction of the sodium salt of isatin with alkyl halides or sulphates 72,73 . Various methods for the preparation of this salt have been reported, and include the reaction of isatin with sodium hydride, either in toluene under reflux 74 or in DMF 75 . Other methods include the use of potassium carbonate in DMF 76,77 or in acetone 78 . In the latter case an aldol reaction of the solvent also occurs with the C-3 carbonyl of the isatin derivative.
Please use CPS: orgchem/0010004 in any reference to this article Heating in ortho-dichlorobenzene results in a retro-aldol reaction and the obtention of the Nalkylated isatin. More recently the use of CaH 2 in DMF has been reported 79 and this method was used for the synthesis of both mono and bis-N-alkylisatins. These latter compounds have been previously prepared using dihaloalkanes and NaH in dioxane 80 or DMF 81 or by the use of LiH 82 . Some of these alkylation methodologies were evaluated for the synthesis of isatins bearing a glycosidic residue linked to the N-1 position 83 .
An alternative method for preparing 1-alkylisatins consists in the reaction of isatin and alkyl halides in a benzene-chloroform/50% aq. KOH biphasic system, employing tetrabutylammonium hydrogensulfate as the phase transfer catalyst 84 .
N-Propargylisatins, obtained from isatin and propargyl halides 79,85 , can be converted to N-acetonylisatins through hydration with Hg(II) salts in acidic media 86 .
The synthesis of 1-methylisatin by the method of Stolle, using tris(methylphenylamino) methane instead of N-methylaniline, leads to the desired product in low yields 87 .
The reaction of isatin with vinyl acetate in the presence of Na 2 PdCl 4 yields 1vinylisatin 88 .
On the other hand, O-alkylation at position 2 has been reported, along with the N-alkyl product, using γ-butyrolactone 89 or allyl bromide 90 as alkylating agents and the sodium salt of isatin. O-Methylisatin is described as the product of the reaction of methyl iodide with the silver salt of isatin, which can be prepared from isatin and silver acetate 91 . The alkoxy group has been reported to be displaced by nucleophiles such as hydrazines 92 .

N-arylation
N-Arylisatin can be obtained from isatin in quantitative yields by reaction with Ph 3 Bi(OAc) 2 and Cu 0 under an inert atmosphere 93 or from aryl bromides and cupric oxide 94 .
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N-methyleneamino derivatives
The Mannich reaction is readily applied to isatins. The products of this reaction, the Naminomethylisatins (Mannich bases), can also be obtained from the N-hydroxymethyl derivatives by reaction with an amine 95 or by reaction with acetyl chloride to yield Nchloromethylisatin which can be further treated with potassium phthalimide or alcohols to give the corresponding N-phthalimidomethyl or N-alkoxymethyl isatins 96 . The Mannich reaction can also be performed with isatin derivatives, such as isatin-3-hydrazones 97 and isatin-3-thiosemicarbazones 98 .

N-acylation and N-sulfonylation
The synthesis of N-acylisatins under a variety of conditions has been described using acyl chlorides or anhydrides under reflux, either alone 99 or using perchloric acid in benzene, triethylamine in benzene 100 , pyridine in benzene 101 , or triethylamine in chloroform 102,103 as catalysts; or by conversion of isatin to sodium isatide using NaH in toluene under reflux and subsequent reaction with acyl chlorides 77 .

Scheme 28
N-Sulfonylisatins are obtained from the reaction of isatin and sulfonyl chlorides by applying the same methodologies as used for obtaining 1-acylisatins. For example, 1tosylisatin is formed in 71-74% yield by mixing tosyl chloride with isatin in the presence of Et 3 N or with the sodium salt of isatin 109 .

N-Haloderivatives
The treatment of isatin with sodium hypochlorite in acetic acid leads to 1-chloroisatin,

4.1.1-Synthesis of indoles
The reduction of isatins with lithium aluminum hydride in pyridine gave indoles in moderate yields. However, the use of THF as a solvent under an inert atmosphere gave greater yields (86-92%) and this procedure was applied to the synthesis of substituted ellipticine derivatives 120 .

Scheme 34
The analgesic drug pemedolac [122][123][124] , analogues of etodolac 125,126 and the synthesis of the alkaloids hobartine and aristoteline 127 were initiated by the C-3 alkylation of isatins to yield dioxindoles that were then reduced to the corresponding indoles by the use of lithium aluminum hydride (Scheme 34).

Scheme 35
As part of the synthetic methodology for the synthesis of the cytotoxic marine alkaloid, dragmacidin, 6,7-dibromo-4-methoxyisatin was reduced to the corresponding indole in 33% yield using a commercial solution of 1M BH 3 .THF 128 .
Isatins are readily converted to 3-fluoroindoles in a two step process involving firstly the reaction of an isatin derivative with DAST (diethylaminosulfur trifluoride) to yield the 3,3-difluoro-2-oxindole derivative and secondly reduction of the difluorooxindole using BH 3 .THF to give the respective 3-fluoroindole. The reaction course was shown to proceed by formation of the 3,3-difluoroindolines, which subsequently eliminated HF. The presence of electron withdrawing groups on the aromatic nucleus retarded elimination of HF resulting in the obtention of 3,3-difluoroindolines as the major product 130 .
Isatins have been used for the synthesis of fused indole derivatives. The reduction of 1-methylisatin-3-oximes, by Zn in acidic media, leads to an acetamidooxindole, which upon reaction with P 4 S 10 gives indolothiazoles in moderate to good yields 131 (Scheme 36).

-Synthesis of oxindoles and dioxindoles
The products of partial reduction of isatin, dioxindole and oxindole, have been widely used in organic synthesis, especially in the development of new drugs. Some natural products also belong to these classes of compounds, for instance dioxibrassinin 132 . There is also a medical interest, as dioxindole has been isolated from the urine of a schizophrenic patient and from suspected drug abusers 133 .
Dioxindoles can be obtained from isatins by reduction of, or by carbanion addition to the C-3 ketone functionality. Amongst the methods for the reduction of isatin to dioxindoles are the use of Zn/HgCl 2 in refluxing benzene 72  Oxindoles can be prepared by the reduction of dioxindoles or isatins: by using red phosphorous and iodic acid 134 ; by reduction of isatin with H 2 S in a pyridine/co-solvent mixture 1137 ; by reduction of the isatin-3-ethylene thioketal with Raney nickel 138 or by the Wolf-Kishner reaction 139-142 , where the use of lower molecular weight alcohols as solvent, such as EtOH or iPrOH, lead to high yields of the desired product 143 . It has however been found that isatin could be reduced to the corresponding oxindoles in high yields (76-92%) by the use of hydrazine hydrate as the solvent in the absence of any additional base 144,145 .
Please use CPS: orgchem/0010004 in any reference to this article A chromatographic method for the quality control of oxindoles, frequently used as raw materials for pharmaceutical products, using normal phase HPLC has been developed 146 .
Indigo, isoindigo and indirubin are natural pigments bearing the oxindole motif and have considerable economical importance. As a consequence synthetic methodologies have

-Reduction involving free radicals
Isatin and 1-methylisatin can be reduced by merostabilized free radicals to isatide and

Scheme 39
Isatoic anhydride can be condensed with proline in polar aprotic solvents at high temperature, or in a reaction catalyzed by the enzyme catalase, to yield a pyrrolo [1,4]benzodiazepine ring, a structural pattern found in some antineoplasic antibiotics 188 (Scheme 40).  Anthranilates can be prepared from isatins by reaction of hydrogen peroxide in an alkaline solution 24,196 , or by the use of chloroamine-T or dichloroamine-T 197 or by hydrolysis of isatoic anhydride with an aqueous alkaline solution 198 and may also be formed through oxidation of indigo carmine by hypohalides in alkaline medium 199 . Anthranilic acid is also formed in the photolysis of isatin to isatoic anhydride, which is subsequently hydrolysed 200 .
The economic importance of anthranilates resides in their well-estabilished antiinflammatory activity. Thus, many derivatives have been synthesized with the objective of discovering new pharmacological agents such as immunosupressants 202 , fungicides 203 and agents for the prevention of nerve cell damage 204 . Anthranilic acid has also been used in the synthesis of polycyclic aromatic hydrocarbons, such as dicyclooctabiphenylenes 205 ,

Scheme 45
Anthranilate, as well as isatin, isatoic anhydride, dioxindole and oxindole have been found to be products of microbial oxidation of indoles, as shown in the sequence below 214,215 (Scheme 46). Similar pathways are found in the degradation of indole-3-acetic acids 216 .

-Amines and related compounds a) Ammonia, hydroxylamine and hydrazine
Isatin reacts with ammonium hydroxide or ammonium acetate to furnish a mixture of compounds. Amongst them are isamic acid and its corresponding amide, isamide. Since 1877 there had been a discussion as to their structure, which in 1976 was finally elucidated, by Sir John Cornforth on the basis of chemical and spectroscopic data 228   Isatin-3-imines also react with hydrazine derivatives such as heteroarylhydrazines 264 , thiosemicarbazides 265 and acylhydrazides 266 , resulting in a substitution reaction at the C-3 position. Substitution reactions are also described to occur when O-methylisatin is treated with thiosemicarbazines, furnishing isatin-2-thiosemicarbazones 91 .
The stereochemistry of isatin-3-thiosemicarbazone-5-sulfonate was studied in aqueous solution, and in acidic pH the Z isomer was determined to be the most stable, but after deprotonation, the corresponding anion slowly converts to the E isomeric anion 267 (Scheme 56). The results described by Tomchin are far more complex than those described above. It has been stated that 1-acetylisatin reacts with thiosemicarbazide to furnish the corresponding isatin-3-thiosemicarbazone, together with a small portion of the ring opened product that results from attack at C-2. The yield of the latter product increases as the solvent is changed from ethanol to dimethylacetamide and to AcOH, whilst none of the ring opened product is obtained using dioxane. On the other hand, when the same solvents were used in the reaction of 1-butyrylisatin with thiosemicarbazides, the only product formed was the corresponding 3-Please use CPS: orgchem/0010004 in any reference to this article thiosemicarbazone. A further conflicting result is that of 1-tosylisatin which behaves similarly to 1-acetylisatin in its reaction with thiosemicarbazides, but when using dioxane as the solvent the major product is that due to ring opening 109 . The reaction of 1-acetyl-5-bromoisatin with thiosemicarbazine in EtOH yielded only the corresponding 3-thiosemicarbazone, while in acetic acid a mixture of products resulting from attack at C-2 and C-3 was observed, the former being favored 276  The decarboxylation of α-aminoacids catalyzed by isatin in aqueous media has been studied as a model for the enzymatic decarboxylation of these compounds. As a result, phenylglycine yields benzaldehyde and benzoic acid as products, but the efficiency of isatin is far lower than that of methoxatin (PQQ), the coenzyme of several alcohol and amine Please use CPS: orgchem/0010004 in any reference to this article dehydrogenases. The redox cycle for methoxatin is proposed to proceed as below 284 (Scheme   64):
Please use CPS: orgchem/0010004 in any reference to this article These imines have been studied as hair dyes 319 . The use of the polar aprotic solvent N,Ndimethylacetamide, and high temperatures yields spirobenzimidazoles in high yields 320
When the reaction is carried out with 1-acylisatins, ring opened products are formed using benzene, acetic acid or ethanol as the solvent 325-329 . However, it has been reported that with the latter two solvents a spiro benzimidazole derivative is also formed 330 . The formation of ring opened products has also been reported to occur when using alkyldiamines 331 (Scheme 76). The reactions of these compounds with o-aminophenol occur only at reflux. In the case of isatin, the formation of a ring opened product (25%) occurs along with the 3-imino derivative (30%), whilst 1-acetylisatin furnishes solely a ring opened product (48%). Characterisation of the products was based upon their mass, IR, 1 H and 13 C NMR spectra (Scheme 80).

-Oxygen, sulfur and phosphorous nucleophiles
Isatin 340 and 1-arylisatins 341,342,343 suffer hydrolysis in alkaline solutions, leading to isatinates. Kinetic studies have shown that this is a thermodynamically favored process, which also occurs under physiological conditions, implying that some, or all, of the biological and pharmacological activities described for isatins are indeed due to their isatinates 344,345 .
The pH profile for the hydrolysis of isatin has shown that at pH < 3, isatin is the predominant species, and at pH > 6, the ring opened isatinate is the major component. At pH values between 5 and 6, the rate of hydrolysis is first-order in hydroxide concentration or inversely proportional to the concentration of the hydronium ion, but from pH 6.5 to 10.5 it is pH independent. This result reveals the existence of a complex behaviour for the hydrolysis of isatin, with different rate limiting steps depending upon the pH of the solution. 346 . A similar profile has been observed for 1-methylisatin 347 (Scheme 82).

Scheme 82
It was also observed that in the presence of ethanol and ethylene glycol the rate of hydrolysis decreases 348 . The effects of other solvents 349 , as well as the photophysics 350 of the hydrolysis reaction of isatin have also been studied.
Isatinates can be electrochemically reduced to mandelates at different pH values using mercury electrodes 351 .
The isatinates can be converted to the corresponding oximes, which possess pharmacological interest as anti-inflammatory agents 352 .
Isatin-3-imines are hydrolyzed to isatin and the corresponding amine. A ring opened intermediate is proposed to be involved in the process as it was detected by polarography 353 .
The alkaline hydrolysis of isatin is the first step of a method for the construction of the indazolic ring system 354 . This method has been applied to the synthesis of serotonin antagonists 355 (Scheme 83).

Scheme 87
The addition of thiols to isatin anils to yield the respective thioazoketals is general 365 .
The reaction of isatin with P 4 S 10 in pyridine resulted in the obtention of The reaction of isatin with triphenylphosphine was believed to furnish indirrubin 375 , but a reinvestigation of this reaction has shown that the products formed are 3triphenylphosphoranylideneoxindole and isoindigo 376 .

-Carbon nucleophiles
Carbon nucleophiles add to isatin and derivatives at position C-3 in most cases, and the products formed have been submitted to further transformations giving rise to a variety of heterocyclic systems.  In the reaction of isatins with some cyclic ketones, such as 4-hydroxy-2H-benzopyran-2one 417 , the initial dioxindole formed reacts with a second equivalent of the ketone yielding a 3,3-disubstituted oxindole.
The addition of methyl lithium to isatin-3,3-dimethylketal (3,3-dimethoxyoxindole) at room temperature for 2.5 min lead to an indolenine derivative through addition at C-2 and substitution of one methoxy group at C-3. By extending the reaction time to 3 hours, the second methoxy group was also substituted, furnishing 2,3,3-trimethylindolenine. This same product was obtained when the reaction was carried out at 0 o C, together with 3,3dimethyloxindole 418 (Scheme 96).
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Scheme 101
Masked carbanions, such as silanes, also react with isatins at position 3 and this methodology has been applied to the synthesis of 1,3-oxathiolanes 440  The reactions of isatin and thiophene or 2,2'-bithiophene proceed similarly to those of indoles. However, in these cases mixtures of oligomeric products were obtained 448 . This reaction has been applied to the synthesis of electrically conducting polymers 449 .
Under acidic catalysis, isatin condenses with thiophene or pyrrole to give indophenine dyes. These compounds are formed as a mixture of geometric isomers 450 , and may possess one or two thiophene units; the latter being the major product 451 (Scheme 108).

Scheme 109
When isatins are used in the Friedel-Crafts alkylation of m-cresol in an acidic medium at high temperature, the adduct formed suffers dehydration, furnishing a spiro dibenzopyran derivative 458 (Scheme 110). Diazoalkanes, such as diazomethane 460 and diazoarylalkanes 461,462 add to isatin at the C-3 position, leading to a carbinol that suffers a ring expansion to give the corresponding quinolone 463 (Scheme 111). Oxindolylacrylates react with diazomethane in a regioselective manner, depending on the substituent attached to the carbon atom α to the ester carbonyl group. In non-substituted acrylates, the 1,3-dipolar cycloaddition occurs furnishing a pyrazoline which, upon heating, losses N 2 to give a spiro cyclopropane derivative 464,465 . If α-cyanoacrylates are used, the cyano group reverses the polarization of the C-C double bond, and the diazomethane addition involves initial C-C bond formation β to the ester. The adduct loses N 2 , furnishing a quinolone 465 (Scheme 112). The same reaction when carried out in the presence of triethylamine gives a furoquinoline derivative 466 .
The O-methylether of isatin reacts with diazomethane to furnish a quinoline derivative as the major product, together with a spirooxirane derivative 467 (Scheme 113). The Pftzinger reaction has been carried out with aliphatic ketones 475

Scheme 121
In a similar manner, the use of phenols 494 or dihydronaphthalenones 495  the Z isomer is more stable, but as the cyclization step from the E isomer is irreversible the equilibrium is shifted towards this isomer. The presence of electron-withdrawing groups bonded to the aromatic nucleus shifts the equilibrium in the direction of the Z isomer due to a decrease in the nucleophilicity of the carbamoyl nitrogen atom, and thus favors the cyclization product that results from the Z isomer 502 (Scheme 126). When the alkaline hydrolysis is performed with N-phenacyl 503 or Nacetonylisatins 504,505 , 2-substituted indoles are obtained after spontaneous decarboxylation of the resultant 2-acylindole-3-carboxylic acid . The mechanism of this reaction probably involves a ring opening reaction, followed by cyclization through a Knovenagel-like condensation 506 , but a very complex mechanism has also been proposed 507 . This methodology Please use CPS: orgchem/0010004 in any reference to this article has many advantages over others previously described for the obtention of these indolic derivatives, due to the readily available raw materials (Scheme 127

4 -Halogen nucleophiles
The reaction of isatin with phosphorous pentachloride led to 3,3-dichlorooxindole when the reaction was carried out in benzene at room temperature. This intermediate has been used in the synthesis of oxindoles substituted at position 3 by reaction with a diverse range of nucleophiles such as KSCN, amines and thiols 508 . When the reaction was performed with boiling benzene, a red crystalline product was obtained. This compound was originally characterized as 2-chloro-3H-indol-3-one based not on spectral data but on its reactivity. For example, 4-bromoisatin, after reaction with PCl 5 in toluene under reflux for eight hours was treated with methanethiol to furnish the corresponding 2,2-thioketal, which was decomposed to 4-bromo-2-methylthio-indolin-3-one 509 (Scheme 128).

Scheme 128
The putative 2-chloro-3H-indol-3-one was also reacted with phenols 510 and N,Ndimethylaniline 511 to give dyestuffs (Scheme 129). On the other hand, the reaction of this chloride with anilines always led to isatin-3imines. In an attempt to rationalize these contradictory results, it was proposed that 2-chloro-3H-indol-3-one was the substrate but that this compound, which reacts with nucleophiles at the C-2 position, readily hydrolyzed in solvents containing water, thus yielding isatin and products resulting from attack at C-3 512 . Sir John Cornforth revisited the chemistry of this compound recently, and elucidated the structure as being 2-(2,2-dichloro-2,3-dihydro-3oxoindol-1-yl)-3H-indol-3-one based upon 1 H, 13 C n.m.r. and X-ray crystallographic analysis.
The same authors used this compound to synthesize an indoloquinazoline structurally related to the alkaloid tryptanthrin 513 (Scheme 130). Due to its ability to bind ferric ions, isatin-3-thiosemicarbazone can be used to form magneto-polymer composites with poly (vinyl chloride) 517 .
An extensive list of metal complexes can be found in the Supplementary Material 3.

-Crystallographic data
The crystallographic data for isatin reveals that it is almost planar, with a bond length between the two carbonyls of 1.55 Å. This large value was attributed to lone pair electron repulsion between the two oxygen atoms 518,519 , though this interpretation was subsequently refuted by comparison of bond lengths of cis and trans 1,2-diketones where no systematic or substantial difference between the bond lengths was observed 520

-Infrared spectroscopy
The infrared spectrum of isatin shows two strong bands at 1740 and 1620 cm  (Table 2).  512,502 . Again, acetylation of N-1 implies an important change in the pattern of the spectra, with a deshielding effect over C-7 538 (Table 3).

-Mass spectrometry
The electron-impact mass spectra of isatin 542 , 1-alkylisatins 543 and derivatives, such as hydrazones 544 , usually show an intense molecular ion peak. In the case of 3,3-dissubstituted oxindoles 545 , the base peak corresponds to the loss of the substituents at C-3. A peak corresponding to the loss of CO (ion a) can also be observed, whose intensity decreases with Please use CPS: orgchem/0010004 in any reference to this article the increase in size of the alkyl chain of 1-alkylisatins 546 . Ion a usually looses HCN, leading to a fulvene ion (ion b). An arene aziridine is also observed (ion c), which arises from a second loss of CO [547][548][549] . The ions b and c are also observed in the gas-phase pyrolysis of isatin 550

Scheme 133
A different pattern is observed in the mass spectra of isatin-3-oximes, where a peak corresponding to the loss of CO is not found; this is attributed to a Beckmann rearrangement of the molecular ion leading to a heterocyclic ring opened ion 552 .
In the case of the acetylated derivatives, the molecular ion is usually of low intensity.
The fragmentation pattern includes loss of ketene (ion d) and of CO (ion e) (Scheme 134).

-Further spectroscopic data
The eletronic absoption spectra of isatin 556,557,558 , isatin-3-arylhydrazones 559 , isatin and 1-methylisatin anion radicals 560 were studied and correlated with theoretical calculations with Please use CPS: orgchem/0010004 in any reference to this article good results. The electron spin resonance spectra of the isatin anion radical was also recorded and revealed that the monoanion radical exists in equilibrium with the dianion radical in the solvents employed 560 . DSC thermograms of some alkylisatins were also recorded 561 .

-Organic analytical chemistry
Isatin is known to be a colour reagent for the aminoacid proline, forming a blue derivative 562 . This property has been exploited for the determination of the level of this aminoacid in pollens 563,564,565,566 and other vegetal materials 567 using paper chromatography, or for the detection of polymer bound compounds possessing proline residues 568 . It has also been used in a colourimetric screening test for human serum hyperprolinemia 569 , in a colourimetric assay of HIV-1 proteinase 570 and for the estimation of the age of bones in crime investigations 571 .
As isatin produces a fluorogenic derivative when reacted with tryptophan, it has been used for the detection of this aminoacid by thin layer chromatography 572,573 . It is also useful for the detection of 3,4-dehydroproline, which is oxidized by isatin and further reacted with pdimethylaminobenzaldehyde to give a coloured derivative 574 .
In a similar manner, isatin-3-hydrazone has been studied for the colourimetric determination of steroids 575,576 , including deoxycorticosterone 577 . A further application of isatin in steroid analysis is its use as a coloured marker in the Sephadex LH-20 chromatographic separation of steroidal blood components 580,581 .
1-Chloromethylisatin has been used as a derivatizing agent for alcohols 582  Isatin has been used in the determination of the enzymatic activity of ketopantoyllactone reductase [587][588][589][590] and other fungal carbonyl reductases [591][592][593][594] , as it is a substrate of these enzymes that is reduced to a dioxindole in a reaction that can be monitored by colourimetry.
Ketopantoyl-lactone reductase, also named as isatin hydrolase, can be used to remove unwanted isatin from the broth of the microbial production of indigo 595,596 .
Isatin serves as a substrate for the biosynthesis of violacein, a trypanocide agent, by Chromobacterium violaceum 597 .

-Pigments and dyes
Isatins, associated with other amino heterocycles, can be used for hair dyes 598-616 , while azobisisatins have been thoroughly studied as dyes for plastic materials 617 . 3-Methyleneoxindoles derived from isatins bearing a benzimidazole ring 618 , as well as thioindigoid thiazolidines 619 , have also been used for dyeing synthetic and natural fibers

-Miscellaneous applications
Please use CPS: orgchem/0010004 in any reference to this article Isatins and derivatives have been used in the development of colour photographic recording materials [620][621][622] , of blood coagulation promoters [623][624][625][626] , of liquid crystal components for display devices [627][628][629] and in the inhibition of corrosion of aluminum 630 and Fe-Ni alloys 631 and of iron 632 .
Isatin can be used as a photosensitizer, together with a photoinitiator, for methacrylate 633,634 and epoxysilicone 635 polymerization. It is also used for the synthesis of branched polycarbonate resins, improving the moldability of this polymer 636 .
The reaction of isatin with thiophene in an acidic medium, containing ferrous ion, gives rise to an intense violet color, due to the formation of indophenine dyes. Due to this phenomenon, it was proposed that isatin could be used as a revealing agent for the presence of thiophene in water-soluble organic solvents where it is used as a denaturating agent 637 .
The lithium and thallium (I) salts of isatin-3-oxime (isatin oximates) were employed in the development of ion-selective electrodes for these cations 638 . Transition metal complexes of isatin derivatives can also be employed as catalysts for the oxidative self-coupling of alkylphenols 639,640 .

Pharmacological activity
Isatin and derivatives display diverse pharmacological activities. A summary of these activities can be found in the Supplementary Material 1 and a review on the biological properties of isatin was published some years ago 641