A Dioxobilane as Product of a Divergent Path of Chlorophyll Breakdown in Norway Maple**

Chlorophyll breakdown is a hallmark of leaf senescence and a major contributor to the emergence of the fall colors. Strikingly, essential pieces of the puzzle of this biological phenomenon have been solved only within the last two decades. A breakthrough was the identification and structure elucidation of a colorless tetrapyrrolic chlorophyll catabolite, thereafter named Hv-NCC-1. The further characterization of tetrapyrrolic chlorophyll catabolites in higher plants has meanwhile provided the structural basis for detailed insight into the pathway of chlorophyll breakdown. Colorless tetrapyrroles, such asHv-NCC-1, typically accumulate in senescent leaves of higher plants and they have been classified as “nonfluorescent” chlorophyll catabolites (NCCs). Indeed, NCCs have been considered to be the “final” breakdown products of a largely common and wellcontrolled “linear” catabolic pathway in senescent leaves (see Scheme 1). Here we describe studies of chlorophyll breakdown in senescent leaves of Norway maple (Acer platano des, see Figures S1 and S2 in the Supporting Information), a deciduous tree native to Eurasia. Surprisingly, in fresh extracts from slightly degreened maple leaves no compounds with the absorption properties of the typical colorless chlorophyll catabolites could be detected by high-pressure liquid chromatography (HPLC). Extracts from further degreened leaves showed a similar analytical profile. In particular, no traces of the NCCs were found in any such senescent leaves. Instead one major compound was found that showed a UVabsorption spectrum with broad bands near 237 nm and 274 nm (see Figures 1 and 2). The same spectrum was reported for two “urobilinogenoidic” chlorophyll catabolites, which were found (besides Hv-NCC-1) in senescent leaves of barley (Hordeum vulgare) and which were suggested to be generated by oxidation of Hv-NCC-1. Secondary oxidative transformations of NCCs have also been observed recently in other senescent leaves, in which yellowand red-colored dehydrogenation products of the tetrapyrrolic NCC were identified.