Issue 32, 2013

Indolesvia Knoevenagel–Hemetsberger reaction sequence

Abstract

A series of substituted indoles have been synthesized by the sequential reaction of aromatic aldehydes with ethyl azidoacetate in the presence of sodium ethoxide to form the corresponding ethyl α-azido-β-arylacrylates (Knoevenagel process) followed by a solvent mediated thermolysis (Hemetsberger process). The isolated yields of the ethyl α-azido-β-arylacrylates were significantly increased when employing the sacrificial electrophile ethyl trifluoroacetate. 1H NMR and coupled 1H–13C NMR analysis of the ethyl α-azido-β-arylacrylates indicate that the condensation is stereospecific—only the Z-isomer could be detected. Solvent mediated thermal treatment of the meta-substituted ethyl α-azido-β-arylacrylates resulted in the formation of both the 5- and 7- substituted indoles—the 5-regioisomer being slightly favored over the 7-regioisomer. Analogous thermal treatment of (2Z, 2Z′)-diethyl 3,3′-(1,3-phenylene)bis(2-azidoacrylate) and (2Z, 2Z′)-diethyl 3,3′-(1,4-phenylene)bis(2-azidoacrylate) exclusively produced pyrroloindoles, diethyl 1,5-dihydropyrrolo[2,3-f]indole-2,6-dicarboxylate and diethyl 1,5-dihydropyrrolo[2,3-f]indole-2,6-dicarboxylate, respectively. Results are also reported which indicate that the α-azido-β-arylacrylates can be used in the subsequent Hemetsberger indolization process without prior purification.

Graphical abstract: Indoles via Knoevenagel–Hemetsberger reaction sequence

Supplementary files

Article information

Article type
Paper
Submitted
16 Apr 2013
Accepted
31 May 2013
First published
31 May 2013

RSC Adv., 2013,3, 13232-13242

Indoles via Knoevenagel–Hemetsberger reaction sequence

W. L. Heaner IV, C. S. Gelbaum, L. Gelbaum, P. Pollet, K. W. Richman, W. DuBay, J. D. Butler, G. Wells and C. L. Liotta, RSC Adv., 2013, 3, 13232 DOI: 10.1039/C3RA42296H

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements