Palladium-catalyzed asymmetric trimethylenemethane cycloaddition [electronic resource] : applications to the synthesis of tetrahydrofurans and the total synthesis of (-)-marcfortine C
Keywords: Chemistry

Cycloadditions are among the most powerful reactions in organic chemistry due to their ability to rapidly build molecular complexity from simple, readily available precursors. In 1979, the Trost group described a novel method for the synthesis of cyclopentane rings by the in situ generation and subsequent [3+2] dipolar cycloaddition of palladium-bound trimethylenemethane (TMM). The method has proven to be a powerful approach for the synthesis of five-membered rings, including tetrahydrofurans and pyrrolidines, as well as larger rings sizes via [4+3] and [6+3] cycloaddition. In 2006, the Trost group demonstrated for the first time a general asymmetric protocol by employing phosphoramidites bearing cyclic amines as the chiral ligands. Using these ligands, highly enantioselective cycloadditions with electron-deficient olefins and imines were described. Herein, we describe the successful extension of the asymmetric methodology to include reactions with carbonyl groups and nitroalkenes, allowing for the synthesis of tetrahydrofurans and nitrocyclopentanes, respectively, with good to excellent enantioselectivity. For reactions with carbonyl groups, the development of novel C1-symmetric phosphoramidites was critical, and the optimized ligand was derived from a BINOL bearing a fused furan. The evolution of the ligand design will be discussed, beginning with the initial discovery that phosphoramidites with mono-substituted BINOL derivatives gave improved selectivity. In addition, since these phosphoramidites are chiral at phosphorus, the impact of this chirality on the TMM reaction is explored. The use of the optimized ligand allowed for reactions with both aromatic aldehydes and ketones. In addition, the asymmetric conditions did not require a Lewis acid co-catalyst, in contrast to the achiral ligands that had been previously investigated. For reactions with nitroalkenes, both [beta]-substituted and [beta], [beta]-disubstituted nitroalkenes could be employed as acceptors, where the use of the latter allowed for the synthesis of nitrocyclopentanes bearing a quaternary stereocenter. The nitrocyclopentane products were demonstrated to be versatile synthetic precursors, capable of undergoing further alkylation with excellent diastereoselectivity or converted to cyclopentylamines and cyclopentenones with little to no racemization. In one example, the asymmetric cycloaddition constitutes a formal synthesis of (+)-cephalotaxine. Finally, substituted TMM donors were explored and the use of a cyano donor was found to proceed with nearly perfect levels of selectivity and yield. Finally, the asymmetric TMM cycloaddition using a cyano-substituted donor was applied to the synthesis of (-)-marcfortine C. Notably, the reaction proceeds in nearly quantitative yield with high diastereo- and enantioselectivity, and the resulting chiral center was used to establish all remaining stereocenters in the natural product. Additional highlights include a direct allylic oxidation of the exocyclic olefin, a diastereoselective intramolecular Michael addition, and an oxidative radical cyclization. Using this route, (-)-marcfortine C was prepared in 16 steps and 2.4% overall yield.