Yellow Dioxobilin‐Type Tetrapyrroles from Chlorophyll Breakdown in Higher Plants—A New Class of Colored Phyllobilins.

In senescent leaves chlorophyll (Chl) catabolites typically accumulate as colorless tetrapyrroles, classified as formyloxobilin‐type (or type‐I) or dioxobilin‐type (type‐II) phyllobilins (PBs). Yellow type‐I Chl catabolites (YCCs) also occur in some senescent leaves, in which they are generated by oxidation of colorless type‐I PBs. A yellow type‐II PB was recently proposed to occur in extracts of fall leaves of grapevine (Vitis vinifera), tentatively identified by its mass and UV/Vis absorption characteristics. Here, the first synthesis of a yellow type‐II Chl catabolite (DYCC) from its presumed natural colorless type‐II precursor is reported. A homogenate of a Spatiphyllum wallisii leaf was used as "green" means of effective and selective oxidation. The synthetic DYCC was fully characterized and identified with the yellow grapevine leaf pigment. As related yellow type‐I PBs do, the DYCC functions as a reversible photoswitch by undergoing selective photo‐induced Z/E isomerization of its C15=C16 bond. A new type of a yellow chlorophyll catabolite, provisionally identified in senescent grapevine leaves, was prepared from its hypothetical natural colorless precursor and characterized as a yellow type‐II phyllobilin. The elementary photochemical behavior of this new bilirubin‐like tetrapyrrole was explored, revealing a reversible photoinduced Z/E isomerization, a common basic feature, apparently, of the yellow chlorophyll catabolites and of other bilin‐type natural products. [ABSTRACT FROM AUTHOR]