Total Synthesis of (−)-Cordycicadin D and 3,4-trans-Cordycicadins A and B: Entry to the 3,4-trans-Fused Cordycicadin Framework

Cordycicadins A−D are four C₂₀ polyketides, all containing a γ-lactone fused to a 10-membered lactone. The proposed biosynthetic pathway for the cordycicadins anticipates the formation of two more natural products which are unknown. We report the total synthesis of (–)-cordycicadin D and the two anticipated natural products 3,4-trans-cordycicadins A and B. The targets were convergently assembled, in a biomimetic fashion, via an efficient ketene trapping-intramolecular Michael addition sequence that delivered the requisite 3,4-trans-fused framework with high diastereoselectivity, enabled by the synthesis of complex dioxenones that serve as in situ ketene precursors. Recognition of the embedded polyketide symmetry enabled the use of a divergent-convergent synthetic strategy, based on the use of two products from an early-stage enzymatic resolution. The synthetic routes afforded (–)-cordycicadin D in 14 steps and 3,4-trans-cordycicadins A and B in 13 steps (longest linear sequence). This work confirms the structure of (−)-cordycicadin D and the observed instability of the anticipated natural product 3,4-trans-cordycicadin B during purification may explain why it is yet to be isolated.