Oxindol

Asymmetric cyclization using chiral ligands has been studied. After early attempts[142-144], satisfactory optical yields have been obtained. The hexahy-dropyrrolo[2,3-6]indole 176 has been constructed by the intramolecular Heck reaction and hydroaryiation[145]. The asymmetric cyclization of the enamide 174 using (S j-BINAP affords predominantly (98 2) the ( )-enoxysilane stereoisomer of the oxindole product, hydrolysis of which provides the ( l-oxindole aldehyde 175 in 84% yield and 95% ec. and total synthesis of (-)-physostig-mine (176) has been achieved[146]. 

The reduction of o-nitrophenyl acetic acids or esters leads to cyclization to oxindoles. Several routes to o-nitrophenylacetic acid derivatives arc available, including nitroarylation of carbanions with o-nitroaryl halides[2l,22] or trif-late[23] and acylation of o-nitrotoluenes with diethyl oxalate followed by oxidation of the resulting 3-(u-nitrophenyl)pyruvate[24 26]. 

Bromo-2-nitrophenylacetic acid (26 g, 0.10 mol) was dissolved in a mixture of 50% HjSO (400 ml) and ethanol (600 ml) and heated to 90°C. Over a period of 1 h, zinc dust (26.2 g, 0.40 mol) was added. slowly and then heating was continued for 2 h. The excess ethanol was removed by distillation. The solution was cooled and filtered. The filtrate was extracted with EtOAc. The filtered product and extract were combined, washed with 5% NaCOj and brine and then dried (MgSO ). The solvent was removed in vacuo and the residue recrystallized from methanol to give 20.5 g (97% yield) of the oxindole. 

The Tf cyclization pattern has also proved useful for the preparation of oxindoles from o-haloalkenoylanilines. Table 4.2 gives some examples. 

Besides Pd-catalysed cyclizations, both radical[12] and organolithium[13] intermediates can give oxindoles by exo-trig additions. 

Intramolecular Friedel-Crafts substitution has also figured prominently in the synthesis of oxindoles from cx-haloacelanilides. Typical reaction conditions for cyclizalion involve heating with A1CI,[13-17]. 

Another version of the o-aminobenzyl anion synthon is obtained by dilithi-ation of A-f-Boc-protected o-alkylanilines. These intermediates are C-acylated by DMF or A"-methoxy-At-melhyl carboxamides, leading to either 3- or 2,3-disubstituted indoles. In this procedure dehydration is not spontaneous but occurs on brief exposure of the cyelization product to acid[4]. Use of CO as the electrophile generates oxindoles. 

Oxindoles can be prepared from Af,p-acylphenylhydrazines by a reaction which is analogous to the Fischer cyclization. This is known as the Brunner reaction. The reaction is typically conducted under strongly basic conditions. For example, heating Af-phenylcyclopentanecarbonylhydrazide with CaO gives a 70% yield of spiro-cyclopentane oxindole[l]. 

Gassman and co-workers developed a synthetic route from anilines to indoles and oxindoles which involves [2.3]-sigmatropic rearrangement of anilinosul-fonium ylides. These can be prepared from Ai-chloroanilines and ot-thiomcthyl-ketones or from an aniline and a chlorosulfonium salt[l]. The latter sequence is preferable for anilines with ER substituents. Rearrangement and cyclizalion occurs on treatment of the anilinosulfonium salts with EtjN. The initial cyclization product is a 3-(methylthio)indole and these can be desulfurized with Raney nickel. Use of 2-(methylthio)acetaldehyde generates 2,3-unsubstituled indoles after desulfurization[2]. Treatment of 3-methylthioindoles with tri-fiuoroacetic acid/thiosalieylie acid is a possible alternative to Raney nickel for desulfurization[3]. 

The Gassman synthesis has been a particularly useful method for the synthesis of oxindolcs[lb,8]. Use of methylthioacetate esters in the reactions leads to 3-(methylthio)oxindoles which can be desulfurized with Raney nickel. Desulfurization can also be done by reduction with zinc or tin[10,ll]. 

A solution of 2-aminobenzophenone (98 g, 0.50 mol) and methyl 2-(methyl-thio)propanoate (74 g, 0,50 mol) in CH Clj (21) was cooled to —70 C and 95% 7-butyl hypochlorite (56 g, 0.5 mol) was added dropwise at such a rate that the temperature did not rise above — 65 C. One hour after the addition was complete, EtjN was added and the mixture was allowed to come to room temperature. The solution w as mixed with 3 N HCl (800 ml) and stirred for 1 h. The organic layer was separated, dried (Na2S04 ) and filtered. The solution was evaporated in vacuo and the residue triturated with ether. Filtration gave the 3-(methylthio)oxindole intermediate (92 g) in 62% yield. 

The conversion of indoles to oxindoles can be achieved in several ways. Reaction of indoles with a halogenaling agent such as NCS, NBS or pyridin-ium bromide perbromide in hydroxylic solvents leads to oxindoles[l]. The reaction proceeds by nucleophilic addition to a 3-haloindolenium intermediate. 

Use of an excess of the halogenating agent results in halogenation at the 3-position of the oxindole[3,4]. The halogenation and hydrolysis can be carried out as two separate steps. One optimized procedure of this type used NCS as the halogenating agent and it was found that 70% phosphoric acid in 2-mcthoxycthanol was the most effective medium for hydrolysis[2]. If the halogenation is carried out in pyridine, the intermediate is trapped as an... 

A -(indol-2-yl)pyridinium salt which can subsequently be hydrolysed to an oxindole[5]. 

The oxidation of 3-substituted indole to oxindoles can also be done with a mixture of DMSO and cone, hydrochloric acid[6-9]. This reaction probably involves a mechanism similar to the halogenation with a protonated DMSO molecule serving as the electrophile[10]. 

Ha.logena.tlon, 3-Chloroindole can be obtained by chlorination with either hypochlorite ion or with sulfuryl chloride. In the former case the reaction proceeds through a 1-chloroindole intermediate (13). 3-Chloroindole [16863-96-0] is quite unstable to acidic aqueous solution, in which it is hydroly2ed to oxindole. 3-Bromoindole [1484-27-1] has been obtained from indole using pytidinium tribromide as the source of electrophilic bromine. Indole reacts with iodine to give 3-iodoindole [26340-47-6]. Both the 3-bromo and 3-iodo compounds are susceptible to hydrolysis in acid but are relatively stable in base. 

Mixtures of products are frequentiy observed. Oxidation by peroxycarboxylic acids usually give similar products (22). Several chemical oxidants give good yields of specific oxidation products. Dimethyl sulfoxide in aqueous acid gives oxindoles (23). In methanol, MoO HMPA (hexamethylphosphoramide) gives 3-hydroxy-2-methoxyindolines (24). 

Another indole/oxindole synthesis achieves the critical ortho-substitution by Sommelet-Hauser rearrangement of an anilinosiilfonium ion intermediate. Use of P-thioketones (G = R, an alkyl group) generates 2-substituted indoles, whereas P-thioesters (G = OR) lead to oxindoles. In each case, a 3-thio substituent must be removed by desulfuri2ation. 

Baeyer s investigation of indigotin spanned a period of almost 20 years. In 1905, he received the Nobel Prize in recognition of his accomphshments. In 1890, it was observed that treatment of CO-bromoacetanilide with alkali produced oxindole [S9-48-3] (122) (37) ... 

An unusual case of addition of a carbanion to an unconjugated carbon-carbon double bond is shown in Scheme 47a. The subsequent transfer of the amide group is also noteworthy (80CC1042). The intramolecular addition of a carbanion to an aryne is a more widely established process. Such reactions have been applied to the synthesis of indoles (Scheme 47b) (75CC745> and oxindoles (Scheme 47c) (63JOC1,80JA3646). 

Irradiation of 4-hydroxy- and 4-alkoxy-3-pyrazolin-5-one derivatives (163 R = OH, OR) leads to ring cleavage with the formation of /3-diamides (165) 69TL271). The methylene blue sensitized rearrangement of the same pyrazolinone (R = H) to the oxindole (166) also... 

Oximes unsaturated synthesis, 1, 462 Oxindole, 3-acyl-oximes... 

Oxindole, 3-(methyIthio)-synthesis, 4, 339 Oxindole, 1-tosyIoxy-rearrangement, 4, 303 Oxindole, 3,3,5-trichloro-synthesis, 4, 213 Oxindoles... 

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