Structure Elucidation and Total Synthesis of Altenuic Acid III and Studies towards the Total Synthesis of Altenuic Acid II

Abstract

AbstractThe structure of theAlternariamycotoxin altenuic acid III was elucidated by NMR spectroscopic analysis of an authentic sample, and was confirmed by total synthesis. This compound is not a resorcylic acid lactone but a resorcylic acid substituted with a butenolide, and thus is the first member of a new class of alternaria toxins. For the total synthesis, a short and efficient access to halogenated butenolides bearing acetal‐protected side‐chains was carried out. Suzuki coupling of these butenolides with a highly functionalized boronate gave rise to a precursor of the natural product in high yield. The side‐chain was completed by deprotection and subsequent oxidation. An unexpected cascade reaction leading to tricyclic butenolides was discovered during optimization of the deprotection protocol. Cleavage of the acetal protecting group gave altenuic acid III. Furthermore, a synthetic study towards altenuic acid II, a compound with a characteristic spirolactone structure, is described. It was planned to construct the spirocyclic lactone by using an intramolecular Michael‐type addition of an aromatic carboxylate group to a butenolide moiety, but this approach was not successful. While testing the feasibility of this concept, a new and mild protocol for the well‐known Pinner reaction in the presence of Lewis acids was discovered.