Overview

Betaenone B is a secondary metabolite isolated from the fungus Phoma betae Frank. Due to its phytotoxic properties, it has been shown to cause sugar beet leaf spots, which is characterized by black, pcynidia containing, concentric circles eventually leading to necrosis of the leaf tissue. Most of the major work on Betaenone B, including the initial structure elucidation of Betaenone A,B and C as well as the mechanism of synthesis, was presented in three short papers published in the 1980’s by Sadao Sakamura.

Figure 1: Structrue of Betaenone B                                                                                 Figure 2: Phoma betae Frank induced leaf spots on a

                                                                                                                                    sugar beet plant. The dark concentric circles contain

                                                                                                                                    pcynidia (fungal spores) and eventually will cause

                                                                                                                                    necrosis of the leaf tissue

Biosynthesis

The backbone carbon units of Betaenone B are synthesized via polyketide synthesis. The backbone is made 2 carbon units at a time via addition of acetate units from acetyl-CoA. The 5 methyl groups are added via S-adenosyl methionine methylation, as opposed to incorporation of propionate instead of acetate during biosynthesis, followed by an internal cyclization via a Diels-Alder mechanism catalyzed by an unknown enzyme. The origin of the subsequent oxidations at positions 1, 2 and 8 have yet to be characterized, but they have been shown not to originate from acetate. It has been theorized that cytochrome P-450 is responsible for the oxidation at these three positions since its inhibition produces Probetaenone 1, which is essentially the non-oxidized form of betaenone B.

While a complete de novo synthesis of Betaenone B has yet to be reported, Daniel Pratt and Paul Hopkins in 1987 proposed a method for synthesizing a precursor of Betaenone B from methoxybenzoquinone and 1,3-butadiene via Diels-Alder and Claisen chemistry.

Figure 3: The biosynthetic pathway of betaenone B as proposed by Sadao Sakamura and Akitami Ichihara (1,2, 3).

References

1. Akitami Ichihara, Hideaki Oikawa, Kazuko Hayashi, Sadao Sakamura, Structures of Betaenones A and B, Novel Phytotoxins from Phoma betae

    Fr., J. Am. Chem. Soc. 1983, 105, 2907-2908.

2. Hideaki Oikawa, Akitami Ichihara, Sadao Sakamura, Biosynthesis of Betaenone B, Phytotoxins of Phoma betae Fr., J. Chem. Soc., Chem.

    Commun. 1984, 814-815.

3.   Hideaki Oikawa, Akitami Ichihara, Sadao Sakamura, Biosynthetic Study of Betaenone B: Origin of the Oxygen Atoms and Accumulation of Deoxygenated Intermediate using P-450 Inhibitor, J. Chem. Soc., Chem. Commun. 1988, 600-602. 

4. Afonin, A.N.; S.L. Greene; N.I. Dzyubenko, A.N. Frolov (eds.). 2008. Interactive Agricultural Ecological Atlas of Russia and Neighboring Countries.   Economic

   Plants and their Diseases, Pests and Weeds[Online]. Available at: http://www.agroatlas.ru.