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

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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.

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Journal of the Chemical Society, Perkin Transactions 1

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