Pyrrole-2-carbodithioic esters

Pyrrole anions generated in KOH-DMSO attacked carbon disulfide either exclusively or preferably at position 2 to afford pyrrole-2-carbodithioate anions, which after alkylation gave the corresponding esters of pyrrole-2-carbodithioic acids 134 (Equation (38)) (03M1127, 05M197). 

Systematic investigations of the reaction of pyrroles with carbon disulfide in the superbase system KOH/DMSO [537-541] have shown that pyrrole anions, generated in this system, attack CS2 (20°C-25°C, 2 h) exclusively or mainly by the position 2 to afford pyrrole-2-carbodithioates. The latter, after alkylation with alkylhalides (20°C-25°C, 2 h), give the corresponding pyrrole-2-carbodithionic acid esters in 46%-75% yields (Scheme 2.71, Table 2.9) [537-540]. The only exception is unsubstituted pyrrole, which gives only pyrrole-l-carbodithioate [540]. 

With electron-deficient acetylenes, the dithioate anions can be involved in the reactions of both 1,3-anionic cyclo- and nucleophilic additions [550], The reactions of 1,3-anionic cycloaddition are typical provided that the central atom of [S-C-S] has aromatic substituents ensuring the anion stabilization [551,552], For instance, potassium pyrrole-l-carbodithioate selectively reacts (acetonitrile, -30°C, CH3COOH) with dimethyl ester of acetylenedicarboxylic acid to form rapidly polymerizing 2-(pyrrol-l-yl)-4,5-dimethoxycarbonyl-l,3-dithiol (Scheme 2.80) [552,553]. 

SCHEME 2.80 Reaction of pyrrole-l-carbodithioates with dimethyl ester of acetylenedicarboxylic acid. 

The reaction of ethylcyanoacetate, ethylacetoacetate, or diethylmalonate as CH acids with pyrrole-2-carbodithioates leads to vinylpyrroles bearing the ester fnnc-tion (Scheme 2.90) [563,564]. 

SCHEME 2.90 Synthesis of pyrrolizin-3-ones from pyrrole-2-carbodithioates and methylene active esters. 

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