Indoles, methyl substituent effects

Methyl substituent effects in indoles (ppm relative to indole)... 

Radical bromination of pyrrole and indole methyl substituents continues to be investigated, as does the substitution of the resulting bromo compound. Selective bromination of the 2-methyl group in 1-acetyl-2,3-dimethylindole can be achieved and the bromide ion displaced by malonate <85TL5253>. A recent example of an effective sequence of bromination and substitution is shown in Scheme 116 <928743, 930PP249,93TL3215>. 

In common with a number of heterocyclic iodinations, kinetic effects are found in the iodination of indole and 2-methylindole [68AC(R)1435], When the substituent effects for the reaction are examined it is clear that any resonance effects from the fused benzene ring are only poorly relayed to the reactive 3-position, and the rates appear to be controlled by inductive effects. A 5-methyl group was more activating than 5-methoxy [69AC(R)799]. 

The effects caused by replacement of the geminal dimethyl grouping in the indole part of indolidan by a spiroalkane ring system have been investigated recently [84], It has been found that ring size and inotropic potency are inversely related and that potency within this series can be further increased by incorporation of a methyl substituent into the pyridazine nucleus. The most potent compound is represented by formula (26). It has an ED50 of 1.5 //g/kg (i.v. administration, anaesthetized dogs). 

The relative reactivities of indole, 2- and N-methylindole, pyrrole, thiophene, and furan have been determined in alkylation by a benzenonium ion coordinated to iron tricarbonyl (73CC540). The effects of methyl substituents in pyrrole were determined in alkylation by 4-(7V,7V-dimethyl-amino)benzaldehyde [76JCS(P2)696]. In neither of these methods, nor in the alkylation of indole by aziridinium tetrafluoroborate [67AG(E)178], nor in self-alkylation of a X5-phosphorinyl tetrafluoroborate [73AG (E)753], is a catalyst required. 

It is well established that 3-alkyl pyridines are selectively reduced at N1-C2, to produce 3-alkyl-l,2-DHPs [58,59,60], 5-Alkyl-l,2-DHPs, which result from hydride addition at C-6, are potentially valuable synthetic intermediates [61]. Substituent effects on the regiochemistry of the reduction of A-carbalkoxypyridinium salts have been studied in detail by Sundberg [62]. These DHPs have served as useful dienes for the synthesis of ISQs. Methyl 2-[l-phenylsulfonyl-lH-indol-2-yl]-2-propenoate (62) served as dienophile in most of these reactions [61,62,63,64,65,66,67,68]. Palladium-catalyzed radical cyclization [63], photocyclization [64] or thermal cyclization [61,65,66,67,68] reactions have all been employed to furnish the Diels-Alder adducts (e.g., 63). 

A similar substituent effect operates in the reaction of indole with diethyl ketomalonate conducted by microwave irradiation oti montmorillonite K-10 clay [125]. Indole, its 1-methyl and 2-methyl derivatives, along with C-ring analogs, gave mixtures of both the 3-carbinol and te-indol-3-yl products. 

Amat di San Filippo and De Fabrizio109 studied the effects of several substituents in the benzene ring on the reactivity at C-3 of indole on iodination. The effects of a methyl group at positions 1 and 2 were also studied. The results are summarized in Table XXVII. There is no possibility of resonance interaction between a substituent in position 5 (or 6) and the reaction center in 3. Heteronuclear nonconjugative ... 

Investigation of replacement of the 5-methoxy group by substituents with different electronic and lipophilic properties and methylation of the indole nitrogen or its replacement by a sulfur atom was evidence for the shift of the 5-methoxy group to the 4-position of the indole nucleus led to the most active radical scavenger but much less effective as a cytoprotectant [135]. 5-alkoxy-2-(N-acylaminoethyl)indole (Fig. 15) appeared as the key feature to confer both antioxidant and cytoprotective activity to the structure. Antioxidant activity seems essential for cytoprotection, but it is not sufficient, and there is no statistically significant correlation between the two types of activity. 

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