Palladium Catalyzed Kinetic and Dynamic Kinetic Asymmetric Transformations of γ-Acyloxybutenolides. Enantioselective Total Synthesis of (+)-Aflatoxin B₁ and B₂_a

The reaction of gamma-tert-butoxycarbonyloxy-2-butenolide with phenol nucleophiles in the presence of a Pd(0) complex with chiral ligands may be performed under conditions that favor either a kinetic resolution or a kinetic asymmetric transformation (KAT) or dynamic kinetic asymmetric transformation (DYKAT). Performing the reaction at high concentration (0.5 M) in the presence of a carbonate base favors the former, i.e., KAT; whereas, running the reaction at 0.1M in the presence of tetra-n-butylammonium chloride favors the DYKAT process. Syntheses of aflatoxin B(1) and B(2a) employs the DYKAT to introduce the stereochemistry. Starting with Pechmann condensation of the monomethyl ether of phloroglucinol, the requisite phenol nucleophile is constructed in two steps. The DYKAT proceeds with > 95% ee. A reductive Heck cyclization followed by a lanthanide catalyzed intramolecular acylation completes the synthesis of the pentacyclic nucleus in 3 steps. Reduction of the lactone provides aflatoxin B(2a) and its dehydration product B(1). This synthetic strategy creates an asymmetric synthesis of the former in only 7 steps and the latter in 9 steps. Thus, the ultimate synthetic sequence involves 3 + 5 --> 39 --> 40 --> 42 --> 43 --> 46 --> 47 --> 48 (aflatoxin B(2a)) --> 49 (aflatoxin B(1)).