E. Schmidt, I. Bidusenko, N. Protsuk
Sep 2, 2012
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Influential Citations
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Chemistry of Heterocyclic Compounds
Abstract
The current chemistry of pyrrolecarbaldehydes is one of the fundamental areas of fine organic synthesis. Pyrrolecarbaldehydes are used as building blocks in the preparation of porphyrins [1, 2], ligands for metal complexes [3, 4], medications [5], and precursors of optoelectronic materials [6-8]. Transformations of pyrrolecarbaldehydes give carbolines [9], cyanopyrroles [10], and divinylpyrroles [11]. The high synthetic potential of a carbonyl group in combination with the biological importance of pyrroles ensures a steady interest in this class of compounds. There has recently been developed a preparative method for the preparation of 1-vinylpyrrole2-carbaldehydes by the formylation of 1-vinylpyrroles [12, 13], which are readily obtained from ketones (via ketoximes) and acetylene [14, 15]. It is known that aldehydes react with acetylenes in the presence of a base, giving secondary acetylenic alcohols (the Favorsky reaction) [16, 17]. However, examples of pyrrolecarbaldehyde ethynylation using acetylenes have not been reported in the literature. The use of 1-vinylpyrrole-2-carbaldehydes as starting material in the synthesis of acetylenic alcohols could even further broaden their synthetic potential, thanks to the combination of ethynylcarbinol and vinylpyrrole fragments in a single molecule. Continuing our studies in this area, we have found that 1-vinyl-4,5-dihydro-1H-benzo[g]indole-2-carbaldehyde (1) reacts with phenylacetylene (2) in a suspension of KOH–DMSO (20oC, 1.5 h), forming the E-configured ,-ethylenic ketone 5 in a non-optimized yield of 24%, rather than the expected acetylenic alcohol 3. It should be noted that the ketone 5 was obtained exclusively as the E-isomer. The analogous Z-configured product could not be identified by NMR spectroscopy or by GLC, even in the reaction mixture. The formation of ketone 5 evidently begins with a nucleophilic addition of phenylacetylene 2 to the carbonyl group of the pyrrolecarbaldehyde 1, to give the secondary acetylenic alcohol 3, which prototropically rearranges via the allenic alcohol 4 into the ,-ethylenic ketone 5.