Asymmetric Total Synthesis of Indole Alkaloids: (+)-Melonine, (-)-Ajmalicine, (+)-Mayumbine, (-)-Roxburghine C and Pd(II)-Catalyzed Wacker-Type Oxidation of Trisubstituted Alkenes

This thesis is divided into two main parts. The first two chapters focus on the enantioselective total synthesis of indole alkaloids, while the third chapter describes the development of a Wacker-type oxidation of trisubstituted alkenes involving a fluorinative ring expansion of exocyclic unsaturated carbonyl compounds into 2-fluoro-1,3-dicarbonyl derivatives under Pd(II)/Pd(IV) catalysis. Chapter 1 presents the journey towards the development of an enantioselective total synthesis of (+)-melonine, an unprecedented monoterpene indole alkaloid whose structure was revised in 2021, along with its N4-oxide derivative. After a general introduction on the indole alkaloid total synthesis, with a particular emphasis on strategies to access the 2,2,3-trisubstituted indoline moiety, three distinct synthetic approaches will be discussed. The first two failed strategies were respectively based on accessing the proposed biosynthetic iminium intermediate and on a challenging 7-exo-trig dearomative radical cyclization. The third approach, which successfully provided the natural product, features the following key steps: a) a Johnson-Claisen rearrangement, and a two-step key sequence involving b) a TFA-mediated Boc-deprotection/aziridination/indoline formation domino process, followed by c) a malonate desymmetrization through AlMe3-mediated bridge lactamization. The total synthesis of (+)-melonine was achieved in 15 linear steps, ultimately affording 165 mg of the natural product in a single batch, along with its first X-Ray crystallographic structure. Chapter 2 describes the development of the divergent and enantioselective total synthesis of three heteroyohimbines. Notably, the synthesis successfully exploited Franzéns stereodivergent organocatalyzed domino cascade, involving an enantioselective Michael addition with subsequent diastereoselective Pictet-Spengler reaction. Ley oxidation furnished a concise access to a key lactone intermediate in gram-scale, which is predominantly targeted in the heteroyohimbine total synthesis. This lactone serves as a versatile platform to access (-)-ajmalicine, (+)-mayumbine and ultimately (-)-roxburghine C. Although roxburghine isomers were isolated in the 1970s, only one previous synthetic effort had been reported towards these intriguing octacyclic structures. Remarkably, the total synthesis of (-)-roxburghine C was completed through a diastereoselective intramolecular Pictet-Spengler reaction of a methyl ketone and a chemoselective reduction of amidoester into enaminoester using Vaska's catalyst. This route ultimately provided nearly 200 mg of the bis-indole alkaloid with an overall yield of 12% over the 12-step longest linear sequence. Chapter 3 reports the first examples of a Wacker-type oxidation applied to trisubstituted alkenes. Following a review on Pd-catalyzed domino processes involving a type I dyotropic rearrangement of organo-Pd(IV) species, along with the evolution of the Wacker oxidation, the development of t