Issue 30, 2021

Synthesis of model bacteriochlorophylls containing substituents of native rings A, C and E

Abstract

A route under development for the synthesis of bacteriochlorophyll a and analogues relies on joining an AD-dihydrodipyrrin (bearing a D-ring carboxaldehyde) and a BC-dihydrodipyrrin (bearing a C-ring β-ketoester group and a B-ring dimethoxymethyl group) via Knoevenagel condensation followed by double-ring closure (Nazarov cyclization, electrophilic aromatic substitution, and elimination of methanol). Prior synthetic studies afforded the bacteriochlorophyll skeleton containing a gem-dimethyl group in ring B, a trans-dialkyl group in ring D, and a carboethoxy group at the 3-position of ring A. To explore the incorporation of native substituents, the synthesis of two bacteriochlorophyll analogues thereof was pursued, one with 12-methyl and 3-carboethoxy groups and the other with 2,12-dimethyl and 3-acetyl groups. The 12-methyl group resulted in half the yield (versus the unsubstituted analogue) in the Knoevenagel reaction, but insignificant effects in all other steps including the rate and yield of double-ring closure despite the known effects of alkyl groups to facilitate electrophilic substitution of pyrroles. The 2-methyl-3-acetyl group, however, resulted in diminished yields in several steps, including the Knoevenagel reaction, but not the double-ring closure. The results point to obstacles and openings on the path to total syntheses of the native pigments.

Graphical abstract: Synthesis of model bacteriochlorophylls containing substituents of native rings A, C and E

Supplementary files

Article information

Article type
Paper
Submitted
19 May 2021
Accepted
29 Jun 2021
First published
08 Jul 2021

New J. Chem., 2021,45, 13302-13316

Synthesis of model bacteriochlorophylls containing substituents of native rings A, C and E

D. T. M. Chung, P. V. Tran, K. Chau Nguyen, P. Wang and J. S. Lindsey, New J. Chem., 2021, 45, 13302 DOI: 10.1039/D1NJ02469H

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