Total Synthesis of Ningalin D

Abstract

A concise (nine-step) and effective (19% overall yield) total synthesis of ningalin D (1a) is disclosed and is based on a key 1,2,4,5-tetrazine --> 1,2-diazine --> pyrrole Diels-Alder strategy to assemble a fully substituted pyrrole core central to its structure. Additional highlights of the synthesis include a double Dieckmann condensation to introduce the C and D aryl rings enlisting substituents judiciously placed on the dienophile and intrinsic to the widely used tetrazine 2, a highly effective Suzuki coupling of the resulting C and D phenol triflates for introduction of the sterically demanding F and G aryl rings, and an unusually effective formal oxidative decarboxylation reaction cascade initiated by a Curtius rearrangement to directly provide the biphenylene quinone methide found imbedded in the structure of ningalin D. The cytotoxic and multidrug resistance (MDR) reversal activity of ningalin D, its derivatives, and the key synthetic intermediates are detailed.