Synthesis and characterization of models for the bilin catabolites of chlorophylls using metallo-β-oxochlorins and -benzo[at]chlorins: Comparison of macrocycle cleavage versus meso-oxochlorin formation
Synthetic studies on meso-substitution and subsequent macrocycle ring-opening reactions have been performed on a 3-oxochlorin 7 and benzo[at]chlorin 19 to obtain access to bilins related to the natural chlorophyll catabolites 2 and 3. The zinc oxochlorin 8 could be trifluoroacetoxylated with thallium(III) trifluoroacetate in the 5-, 15- or 20-meso-positions. The 15-trifluoroacetoxy derivative 10 spontaneously ring-opened to give a bilintrione 13, which exhibited the same basic structural characteristic found in natural chlorophyll catabolites, namely macrocycle cleavage at a meso-position distant from (rather than adjacent to) the pyrrolenine ring. A single crystal structure analysis of 13 showed a linear conformation, indicating that a Z to E isomerization had taken place. This result is in contrast to most known bilin structures derived from macrocycles, which exhibit a helical conformation. meso-Oxygenated derivatives (related to compounds formerly termed oxophlorins) were obtained upon acid hydrolysis of the 5-(11) and 20-trifluoroacetoxy-3-oxochlorin 9. Attempts to ring-open 9 or 11 under basic or photochemical reaction conditions were unsuccessful. Trifluoroacetoxylation of the nickel(II) β-benzochlorin 19 yielded regioselectively the 5-substituted derivative 20, which could be ring-opened under mildly basic conditions to give the 19-methoxybenzobilin-1 -ones 21 and 22. Here macrocycle cleavage occurred at the substituted meso-position neighbouring the reduced ring. Application of a similar strategy to the nickel(II) β-oxochlorin 23 gave similar results with formation of only the 20-trifluoroacetoxylated derivative 24, which upon basic hydrolysis reacted to give the corresponding meso-oxo-derivative 25/26. Thus, selective meso-substitution, a requirement for regioselective macrocycle cleavage, giving access to model compounds for the natural chlorophyll catabolites depends strongly on the central metal in the macrocyclic precursor.
Publication Source (Journal or Book title)
Journal of the Chemical Society, Perkin Transactions 1
Isaac, M., Senge, M., & Smith, K. (1995). Synthesis and characterization of models for the bilin catabolites of chlorophylls using metallo-β-oxochlorins and -benzo[at]chlorins: Comparison of macrocycle cleavage versus meso-oxochlorin formation. Journal of the Chemical Society, Perkin Transactions 1 (6), 705-714. https://doi.org/10.1039/p19950000705