Facile retro-Dieckmann cleavage of a pink phyllobilin: new type of potential downstream steps of natural chlorophyll breakdown

Chengjie Li,Bernhard Kräutler

doi:10.1007/s00706-022-02894-z

Abstract

In senescent leaves of higher plants, colourless chlorophyll (Chl) catabolites typically accumulate temporarily, and undergo natural oxidation, in part, to yellow- and pink-coloured phyllobilins (PBs). The latter, also classified as phylloroseobilins (PrBs), represent the final currently established products of Chl-breakdown, possibly playing important roles in metabolism. However, PrBs, themselves, do not accumulate in the leaves. Indeed, the original PrB identified, then classified as a pink Chl-catabolite (PiCC), is remarkably instable in methanolic solution. As reported here, PiCC readily converts at room temperature into yellow tetrapyrroles. The deduced main process, a retro-Dieckmann reaction, cleaves open its ring E moiety, the α-methoxycarbonyl-cyclopentanone unit characteristic of the Chls and of the natural Chl-derived PBs. This readily occurring reaction of the PiCC represents an unprecedented skeletal transformation of a PB, furnishing a cross-conjugated biladiene with a basic structure more similar to the heme-derived bilins.Graphical abstract

Highlights

The characteristic colourful breakdown of chlorophyll (Chl) in fall leaves, an enigma until about 30 years ago [1, 2], generates linear tetrapyrroles, named phyllobilins (PBs) [3], via the so-called pheophorbide a oxygenase (PaO)/phyllobilin pathway [4,5,6]
The pink Chl-catabolite (PiCC) 1 was prepared by air oxidation [16] of the natural yellow Chl-catabolite Cj-YCC, first isolated from leaves of C. japonicum [11]
The deduced structure of the cross-conjugated yellow biladiene 2 indicates its formation by a retro-Dieckmann reaction of the PiCC 1

Summary

Introduction

The characteristic colourful breakdown of chlorophyll (Chl) in fall leaves, an enigma until about 30 years ago [1, 2], generates linear tetrapyrroles, named phyllobilins (PBs) [3], via the so-called PaO/phyllobilin pathway [4,5,6]. We describe the discovery of the readily occurring cleavage of the natural PrB 1 at its characteristic Chl-derived ring E in methanol [21] The main such process at this sterically congested β-ketoester moiety is a retro-Dieckmann reaction, which cleaves open the peripheral C–C bond of the ring E moiety, eliminating the structural Chl-derived hallmark that distinguishes the natural PBs decisively from the related heme-derived ‘hemo’-bilins [4]. We delineate a retro-Dieckmann process at the corresponding β-ketoester functionality of the naturally occurring Chl-catabolite PiCC 1, readily occurring in methanol and opening up the typical ring E moiety of 1, furnishing the novel, cross-conjugated yellow biladiene 2 (Fig. 2). We have observed earlier the instability of the PiCC 1 in methanol at room temperature, and its decomposition to yellow products

Results and discussion

10 E 81 82 O

E MeO2C O HO O

Conclusion