Pyrrole chemistry XXV: A simplified synthesis of some 3-substituted pyrroles

Positional selectivity in reactions of pyrrole and its N-substituted derivatives with electrophiles

“…It is important to note that 1-(phenylsulfonyl)pyrrole gives only 2-substituted derivatives on both Rieche formylation with alkyl dichloromethyl ethers 8,9 and Vilsmeier formylation in the DMF−POCl 3 system, 8 as well as on cyanation with cyanogen bromide in the presence of AlCl 3 or with chlorosulfonyl isocyanate. 8,11 In contrast, nitration (HNO 3 −Ac 2 O) proceeds almost exclusively in the 3-position.…”

“…8,11 In contrast, nitration (HNO 3 −Ac 2 O) proceeds almost exclusively in the 3-position. 10 As was established recently, the same orientation applies to the sulfonation of unsubstituted pyrrole and N-methylpyrrole with pyridine sulfotrioxide.…”

“…The effect of the N-phenylsulfonyl substituent was studied in detail. This substituent was simultaneously offered by two groups of researchers [8][9][10][11] as an original protecting group that owing to its electron-withdrawing effect deactivates preferably the α-position and allows one to obtain β-substituted derivatives, the PhSO 2 group being readily removed on alkaline hydrolysis. 10 The effect of the PhSO 2 group was studied in most detail for Friedel−Crafts acylation.…”

“…10 The effect of the PhSO 2 group was studied in most detail for Friedel−Crafts acylation. [8][9][10][11][12][13][14][15] The role of the nature of the reagent reveals itself most distinctly in the fact that in the presence of aluminum chloride the 3-acylsubstituted derivatives are obtained, while reaction in the presence of boron trifluoride etherate gives the 2-isomers 10 (Scheme 2).…”

3-(η6-Benzoyltricarbonylchromium)pyrrole. An example of how substitution isomerism in the ligand affects the type (σ or π) of complex formed

“…At the time of our earlier study, it was not possible to examine the effect of changing the substitution position of the benzoyl group because no readily available route to 3-benzoylpyrrole existed. Since then, Anderson, Loader and co-workers have developed successively better synthetic routes to 3-substituted pyrroles and have recently developed a good method for preparing 3-benzoylpyrroles (2). We now wish to report the result of the reaction of 3-benzoylpyrrole with Cr(C0)6 and contrast the result with that obtained earlier for 2-benzoylpyrrole.…”