Tetrapyrroles template cyclization

Presumably the biosynthetic pathways of the porphyrins and corrins are related to some extent, and the direct template cyclization of a tetrapyrrole has now been described to give a mixture of 10 diastereoisomers of tetradehydrocorrin complexes (equation 54).271 In the pursuit of chemical and biosynthetic similarities and differences between corrins and porphyrins, it is necessary to... 

The yield of the cyclization step under the influence of a metal template can be increased when the corresponding dialdehyde 19 of the tetrapyrrole 16 is used. The reaction sequence is initiated by insertion of palladium(II) or nickel(II) into the tetrapyrrole to give 20 followed by Michael addition of one acrylaldehyde side chain to the other yielding the macrotetracycle 21 from which in a retro-Michael reaction acetaldehyde is eliminated to give 22. 

Reaction with f-butyl alkoxide in the presence of zinc (II), which acts as a template and can be readily removed with acid from the cyclized ligand system that is formed, finally results in the formation of bonellin dimethylester (85) from the linear tetrapyrrole (84). 

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