2H- and 3H-Pyrroles

Polyalkylated 2- and 3-pyrrolenines (2H- and 3H-pyrroles) and 3-indolenines (3H-indoles) are formed by methylation of the parent 1H heterocycles (see Section 3.05.1.2.7), but the acid-catalyzed or thermal rearrangement of 2,2,5-trisubstituted 2H-pyrroles and of 3,3-disubstituted 3//-indoles yields the corresponding 2,3,5-trisubstituted IH-pyrroles and the 2,3-disubstituted l//-indoles, respectively (see Section 3.05.1.1). 

The 2H - and 3H -pyrroles and -indoles are considerably more basic than the corresponding 1//-isomers and they readily undergo alkylation to give the quaternary salts, which are then highly susceptible to nucleophilic attack. Thus, for example, methylation of the trialkyl-3//-indole (507), followed by reaction with aqueous base, yields the indolin-2-ol (508). A similar reaction sequence results from the acylation of (507) under Schotten-Baumann conditions to give (509 X = OH). When the benzoylation is conducted in benzene in the absence of the base, the 2-chloroindoline (509 X = C1) is formed and acylation of (507) with a carboxylic acid anhydride produces the ester (509 X = OCOR) (79HC(25-3)l). 

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