METHODOLOGY DEVELOPMENT OF N-SULFINYL METALLOENAMINES AND ASYMMETRIC TOTAL SYNTHESIS OF (+)-EPIIBOGAMINE

Abstract

This thesis focuses on methodology development using N-sulfinyl metallodienamines and metalloenamines derived from Davis-Ellman N-sulfinyl imines. The application of these methodologies toward the total syntheses of (+)-epiibogamine and (+)-chimonanthine is described. The vinylogous aldol reaction of N-sulfinyl metallodienamines was first reported in 2017 in the total synthesis of (−)-albocycline. However, the diastereoselectivity of this reaction was found to be incorrectly reported. Mosher ester analysis and HPLC analysis with a chiral column were employed to determine the dr. The vinylogous aldol reaction was optimized. Studies on the mechanisms affecting stereoselectivity revealed that stereochemical erosion occurred using SnCl4 combined with LiHMDS, but not with NaHMDS or KHMDS. A (Z)-metallodienamine with metal chelated transition state was proposed and supported by DFT calculations to rationalize the stereochemical course of the reaction. Iboga alkaloids have been historically used for anti-addictive properties and recently brought more attention as potential drug candidates for neurological disorders. However, the complex isoquinuclidine fused azapine scaffold of Iboga alkaloids are challenging to synthesize. To construct the key chiral 2-aminocyclohexene intermediate and isoquinuclidine ring more efficiently, three new methods were explored. These included the domino-Michael/Mannich annulation of N-sulfinyl metallodienamines, the Diels-Alder reaction of N-sulfinyl imines, and the three-component domino-Michael/Michael/Mannich (DM3) annulation of N-sulfinyl metalloenamines. The DM3 reaction provided the 1-amino-2,4-diester scaffold to access isoquinuclidine in high yield (84−94%) and excellent diastereoselectivity (up to >95:5 dr). Chiral N-sulfinyl silylenamines, were introduced to serve as a surrogate for poorly reacting N-sulfinyl aldimines in the DM3 reaction. This silyl-modified DM3 reaction enabled the shortest asymmetric total synthesis of (+)-epiibogamine in 7 steps. This isoquinuclidine intermediate would also provide access to (+)-dihydrocatharanthine and other Iboga alkaloids in future studies. Oxidative enolate coupling, via radical intermediates, was used to construct 1,4-dicarbonyl scaffold from the reaction between two enolate nucleophiles. An oxidative coupling of N-sulfinyl metalloenamines was discovered having the ability to construct vicinal quaternary stereocenters in high diastereoselectivity (16:1 dr). To showcase this new methodology, the synthesis of dimeric indole alkaloids, (+)-chimonanthine, was investigated. A new class of compounds, bis N-sulfinyl amidines, were prepared as the potential oxidative coupling monomers for the synthesis of (+)-chimonanthine in future studies.