Comparative Diels−Alder Reactivities within a Family of Valence Bond Isomers: A Biomimetic Total Synthesis of (±)-UCS1025A

A synthesis of the structurally fascinating fungal metabolite UCS1025A (1) was accomplished. It features a likely biomimetic approach to the octalin subunit via an intramolecular Diels-Alder (IMDA) reaction of a putative triene precursor (2), preceded by an efficient construction of the bisheteratriquinane subunit within that compound. Specifically, an intramolecular silyl triflate-induced cyclization of an in situ-generated silyl ketene acetal onto an imide carbonyl group (e.g., 7 to 8) was developed. The IMDA relative reactivities of a family of valence bond isomers, each differing in the precise nature of the dienophilic subunit, were determined. Under biologically relevant conditions (D2O, pH 7.2 buffer, ca. 25 degrees C), triene 2, via its lactone ring-opened congener, underwent very fast (t1/2 = 10 min) conversion to the ring-opened forms of 1 (i.e., 5a) and the tetraepimeric, alternative endo-adduct, 3 [i.e., (tetraepi)-5a].