Involvement of Diels–Alder reactions in the biosynthesis of secondary natural products: the late stage of the biosynthesis of the phytotoxins solanapyrones

Abstract

Advanced intermediates, prosolanapyrones I (6) and II (7) have been synthesized in deuterium labelled form and administered to cultures of Alternaria solani. Incorporation of [17,17,18,18,18-²H₅]prosolanapyrone I (6b) afforded solanapyrones A (1) labelled at C-17 and C-18 with the expected integration in its ²H NMR spectrum. Subsequently, [2,3,17,18,18,18-²H₆]prosolanapyrone II (7a) was incorporated into solanapyrone A labelled, as expected, at C-2, C-3, C-17 and C-18. These results strongly support the involvement of a Diels–Alder reaction in the biosynthesis of solanapyrones. This is the first example of intact incorporation of diene-dienophile precursors into natural [4 + 2] adducts.