Issue 29, 2023

Synthesis of model southern rim structures of photosynthetic tetrapyrroles and phyllobilins

Abstract

The photosynthetic tetrapyrroles and their catabolic products (phyllobilins) share similar structural features in the southern rim encompassing rings C–E with variation in the degree of saturation including a dihydrodipyrrin (chlorophyll a and bacteriochlorophyll a), a dipyrromethane (phylloleucobilin), and a dipyrrin (chlorophyll c). Here, a direct, two-step route to southern rim structures was examined. The Knoevenagel condensation of a pyrrole bearing a β-ketoester at the 3-position with a pyrrole-2-carboxaldehyde formed the dipyrrole-substituted propenone, which upon Nazarov cyclization gave the dipyrromethane-type southern rim containing the annulated, isocyclic ring. In this manner, three southern rim structures (51–80%) were obtained that vary in the nature of the pyrrole substituents (H, methyl, carbomethoxy). Characterization by 1H NMR spectroscopy in each case revealed an expected mixture of trans/cis isomers (∼9 : 1 ratio). Several derivatization processes (pyrrole acylation with trichloroacetic anhydride, ketone reduction with borohydrides) were examined. Analysis of one southern rim compound by single-crystal X-ray diffraction showed the trans configuration for the expected pair of enantiomers in the unit cell; a second compound gave the trans enantiomers as a conglomerate. The Knoevenagel–Nazarov route from relatively accessible pyrroles may provide new scaffolds for constructing tetrapyrrole architectures.

Graphical abstract: Synthesis of model southern rim structures of photosynthetic tetrapyrroles and phyllobilins

Supplementary files

Article information

Article type
Paper
Submitted
30 may. 2023
Accepted
27 jun. 2023
First published
05 jul. 2023

New J. Chem., 2023,47, 13626-13637

Author version available

Synthesis of model southern rim structures of photosynthetic tetrapyrroles and phyllobilins

A. T. Nguyen Tran, Z. Wu, D. T. M. Chung, P. Nalaoh and J. S. Lindsey, New J. Chem., 2023, 47, 13626 DOI: 10.1039/D3NJ02515B

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