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9.13.4.2.1.1 Method 1: Reduction of Acyl Groups to Alkyls

DOI: 10.1055/sos-SD-009-00542

Black, D. StC.Science of Synthesis, (20019535.

Perhaps the most important and specific substituent reactions of pyrroles are the reductions of acyl groups to alkyl groups (Scheme 79). Reduction of a carboxylic ester group to methyl can be effected by sodium dihydrobis(2-methoxyethoxy)aluminate (red-Al) in benzene, as illustrated by the conversion of the pyrrole-3-carboxylic ester 542 into 3,4-dimethyl-1H-pyrrole (543).[‌464‌] A more important reaction, which has received wide application, is the reduction by hydride donors of simple α-carbonyl groups to alkyl as the result of instability of the initially formed carbinol intermediates. The elimination of a metal-complexed oxygenated species, to provide the unsaturated functionality for further reduction, is enhanced by the pyrrole nitrogen atom. For this reason the reactivity is particularly enhanced for C2 carbonyl groups, but can still apply to those at C3. Conditions can include the use of lithium aluminum hydride in tetrahydrofuran, or sodium borohydride in hot propan-2-ol, followed by hydrogenation.[‌465‌,‌466‌] For example, the former conditions have been used for the reduction of 1H-pyrrole-2-carbaldehyde (417) to 2-methyl-1H-pyrrole (544),[‌465‌] and the second are exemplified by the conversion of the benzyl pyrrolyl ketone 545 into the phenylethylpyrrole 546.[‌466‌] In this case, a mixture of alkane and alkene is formed from the borohydride step, and the yield of alkane is maximized by a hydrogenation treatment. This reductive process can be coupled with alkylation to give an effective overall reaction. For instance, 1H-pyrrole-2-carbaldehyde (417), on treatment with (2-methoxyphenyl)lithium, gives the (2-methoxybenzyl)pyrrole 547.[‌467‌] 1H-Pyrrole-3-carbaldehyde (550) can be prepared by the sodium borohydride reduction of the pyrrole glyoxylic ester 548 followed by the periodate oxidation of the resulting diol 549.[‌468‌] The related glyoxylic amides, such as 551, serve as sources of pyrrolylethylamines such as the amine 552 through reduction with excess lithium aluminum hydride in dimethoxyethane.[‌469‌]

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