Bischler synthesis of indoles

The Bischler-Napieralski (BN) method has demonstrated great utility for fully aromatic P-carboline rings. Substitution on carbocyclic ring A of the p-carboline system requires suitably substituted tryptamine which can be prepared via the Fischer indole synthesis or one of several methods for synthesis of indole rings (28, 128). 

Intramolecular acylations of the Bischler-Napieralski type have been used as crucial steps in the synthesis of indole alkaloids of the yohimbine class <94JCS(Pl)299, 94TL1071>. Also, the aluminum chloride-catalysed reaction of the indolosuccinic anhydride (46) gave the bridged cyclic product (47) rather than the cyclopentanone (49), presiunably because the reaction involves initial acylation at C-3 and reaction via the less strained intermediate is favored. Formation of the cyclopentanone (49) can be achieved using the polyphosphate ester cyclodehydration of the modified intermediate (48) (Scheme 13) <90JCS(Pi)2487>. 

Wender and Aube have independently described the use of the Bischler-Napieralski reaction in the synthesis of Yohimban alkaloids. Aube s approach involved the cyclization of indole 50 followed by reduction of the resulting dihydroisoquinoline... 

A synthetic approach to the same tetracyclic y-carboline nucleus (235) is the consecutive Fischer indole and Bischler-Napieralski ring closure of o-acetamidoacetophenone methylphenyUiydrazone (234). The Bischler-Napieralski reaction has also been used in the synthesis of 3,4-benz-j8-carbolines (236) and 3,4-benz-3-carbolines, e.g. 237... 

A large number of carbazole syntheses have involved the preparation and dehydrogenation of hydrocarbazoles, mainly 1,2,3,4-tetrahydrocarbazoles, which are 2,3-disubstituted indoles. These, in turn, are usually prepared by the Fischer indole synthesis or the Bischler synthesis. This section will not deal with the preparation of the tetrahydrocarbazole, because... 

One known application of AP-methy1ani1inomethy1 ketones is their use in the Bischler indole synthesisJ4 We have found that the a-aminomethyldiphenylphosphine oxides with R3= phenyl and R4= methyl or benzyl, are excellent reagents for the synthesis of 3-substitu-... 

In the second classical method (Bischler synthesis), an aromatic primary or secondary (arylalkyl, but not diaryl) amine is condensed with an obromo (or better, hydroxy) ketone to give a 2,3-dialkylindole. This in turn is alkylated directly to the 1,2,3,3-tetraalkylindoleninium salt. Use of acetoin gives 2,3-dimethylindole, which is easily isolated and purified. The Bischler synthesis from a substitued aniline, and especially from anilines bearing one or more alkoxy groups, is much preferred in practice for indoles substitued in the 4-7-positions, since the substituted aiylhy-drazines needed for the Fischer synthesis are costly, unstable, or unavailable. 

A Bischler synthesis can be monitored best by IR,following the disappearance of the sharp, single N-H absorption of the indole, or also by the appearance of N-alkyl peaks in the NMR spectrum. 

Catalytic hydrogenation of a nitro-(formyimethyl)pyrimidine is accompanied by cyciization and formation of a fused pyrrole ring. The report on page 8 of Part I of a paper which describes the cyciization of 2-(2-aminophenyl)acetalde-hyde acetals is not cited in a 1986 paper [3189] which extends the method to the synthesis of 6-substituted indoles in which the substituent is electron-withdrawing this type of indole is not easily accessible by the Fischer or Bischler synthesis. 

Acylation of 3-substituted indoles is more difficult, however 2-acetylation can be effected with the aid of boron trifluoride catalysis." " Indoles, with a carboxyl-containing side-chain acid at C-3, undergo intramolecular acylation forming cyclic 2-acylindoles." Intramolecular Vilsmeier processes, using tryptamine amides, have been used extensively for the synthesis of 3,4-dihydro-p-carbolines, a sub-structure found in many indole alkaloids (P-carboline is the widely used, trivial name for pyrido[3,4-fc]indole). Note that it is the imine, rather than a ketone, that is the final product the cyclic nature of the imine favours its retention rather than hydrolysis to amine plus ketone as in the standard Vilsmeier sequence " this ring closure is analogous to the Bischler-Napieralski synthesis of 3,4-dihydro-isoquinolines (9.15.1.7). 

In the original method, the Bischler synthesis, harsh acidic treatment of a-arylaminoketones (produced from the 2-haloketone and an arylamine) was used to bring about electrophilic cyclisation onto the aromatic ring these conditions often resulted in mixtures of products via rearrangements. It is now known that N-acylated-Q-arylaminoketones can be cyclised under much more controlled conditions, and in contrast to early work, this approach to indoles can even be used to produce hetero-ring-unsubstituted indoles. ... 

Synthesis of left-hand segment began with 7-benzyloxyindole 197. A Vilsmeier-Haack formylation followed by condensation afforded nitroalkene 198. Reduction, acylation with succinic anhydride, and subsequent Bischler-Napieralski cyclization provided dihydro-p-carboline 199. Noyori asymmetric reduction of 199, further treatment with A-iodosuccinimide, followed by activation with silver triflate in the presence of dimethoxy-N,N-diallylaniline furnished the desired coupling product 200. Subsequent saponification and cyclization via a ketene intermediate gave the rearrangement precursor 201. Oxidative skeletal rearrangement initiated by m-CPBA followed by removal of the Fmoc group and conversion of the aniline to the hydrazine furnished Fischer indole precursor 202 (Scheme 35). 

The susceptibility of the pyrrole ring to electrophilic attack has been used in the synthesis of pyrrolo-fused 1,4-diazepines, for example, A-(3-acetylaminopropyI)-3-methyl indole underwent a Bischler-Napierelski-type cyclodehydration on treatment with phosphorous oxychloride in benzene to give ll-methyl-4,5-dihydro-3ff-l,4-diazepine[l,2-a]indole <91IJC(B)1018>. In another example the acyl azide (143) cyclized to the furo[2,3-e]pyrrolo[l,2-a]-l,4-diazepine-9-one (144) (Scheme 25) <92JHC1507>. 

Among other classic but useful indole syntheses are those of Bis-chler (1892) and Madelung (1912). In the Bischler synthesis, an aniline derivative is alkylated by an alpha-haloketone. Heating the product (9.81) effects the electrophilic attack of the carbonyl group on the benzene ring the loss of water from the product gives the indole structure (Scheme 9.38). 

The most extensively used approach is route A (Scheme 1). Some of the more commonly employed indole syntheses, such as the Fischer, Bischler, Nenitzescu and the Gassman procedures, also fall into this category.Of these, the Fischer indole synthesis (Scheme 2) has been most widely applied. However, the requirement of properly substituted starting materials, which can often be difficult to fulfil, and/or the lack of regio-selectivity in the cyclizative step, limit its utility. An example of the use of the Fischer indole synthesis in the synthesis of a carbazole alkaloid is given in Scheme 3. 

The Bischler-Mohlau indole synthesis, also known as the Bischler indole synthesis, refers to the synthesis of 3-arylindoles from the cyclization of ca-arylamino-ketones and excess anilines. 

Some other modifications of the Bischler indole synthesis have been developed. One such modification is that of Moody and coworkers, who employed rhodium to effect N-H insertion of an a-diazo-p-ketoester to an aniline (Scheme 5, equations 1 and 2) [58-60]. The so-formed arylamino P-ketoester 16 was smoothly converted to indole 17 with the ion-exchange resin Amberlyst 15 or in somewhat lower yield with boron trifluoride etherate [59]. These workers extended the method to the synthesis of A-unsubstituted indoles using the novel A-protecting groups, A-(2-ethoxycarbonylethyl)... 

In 1881, Mohlau first disclosed this reaction, which was followed by further study carried out by Bischler in 1892-1893. Typically, this indole synthesis of heating excess anilines with a-haloketones is referred to as the Bischler reaction, however naming the transformation the Bischler-Mdhlau indole synthesis is more appropriate. Despite its long history, this classical reaction... 

The Bischler-Mohlau indole synthesis was applied in the synthesis of fluvastatin sodium (Lescol) to assemble its indole core. As shown below, reaction of a-chloroketone with iV-/-Pr-aniline at elevated temperature generated a tertiary amine. The resulting A-/-Pr-aniline tertiary amine... 

The plan was to disconnect reserpine into indole 80 and tri-O-methylgallic acid (81). Indole 80 was to be converted to reserpine using a variant of the well-known Bischler-Napieralski synthesis of isoquinoline derivatives. This transformation (80 79) generates a stereogenic center and thus, there... 

A. Bunescu, C. Piemontesi, Q. Wang, J. Zhu, Heteroannulation of arynes with N-aryl a-aminoketones for the synthesis of unsymmetrical N-aryl-2,3-disubstituted indoles an aryne twist of Bischler-Mohlau indole synthesis, Chem. Commun. 49 (2013) 10284-10286. 

Alkaloids with polycyclic skdetal frameworks are, when it comes to their synthesis, excellent candidates for RCM. Illustrative are the indolizidines rhynchophylline (43) and its C(7)-epimer ixo-rhynchophylhne (44), both isolated from the plant Uncaria rhynchophytta (Rubiaceae) [26]. Deiters total synthesis of 43 and 44 started with the efficient construction of diallylamine 39 via amide formation between indole-3-acetic acid (38) and diallylamine (Scheme 2.10). One-pot RCM-carbomagnesation of 39 was smoothly achieved with only 1 mol% of [Ruj-I catalyst and 4 equiv. of EtMgCl to afford the 2-ethyl-3-butene-amine derivative in 71% yield. It appeared that the electron-withdrawing carbonyl moiety was critical to the success of the RCM-carbomagnesation steps. Amide reduction and subsequent treatment with acryloyl chloride dehvered the second metathesis precursor 40. Cyclization with [Ruj-I (5 mol%) then furnished the a,j8-unsaturated lactam 41 in a high yield of 91%. Continuation of the total synthesis of alkaloids 43 and 44 included a Bischler-Napieralski cychzation (42) and subsequent rearrangement into the oxindole framework. 

The Yao group has made use of a Ic type intramolecular Heck reaction to prepare the C2-symmetric dimeric indole core of chloptosin <06OL4919>. A solvent-free variation of the Bischler indole synthesis, electrophilic cyclization of a-arylamino imine tautomers prepared from aniline derived a-arylamino ketones, has been used by Menendez and co-workers for the preparation of 2-arylindoles <06SL91>. 

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