Catalytic enantioselective Pictet-Spengler reaction of carbonyl compounds: Development and application to the asymmetric total synthesis of indole alkaloids

This thesis consists in an extensive study about the enantioselective Pictet-Spengler reaction (EPSR) and its application to the total synthesis of monoterpene indole alkaloids (MIAs). The general introduction presents the literature background about this powerful yet still unsolved synthetic transformation. The use of ketone derivatives up to date is mainly underlined.The first chapter describes the enantioselective total synthesis of arborisidine for which an unprecedented EPSR with an Î±-diketone was developed. An oxidative cyclization allowed to build a strained bridged system which was rearranged following a one-pot aza-Cope/Mannich â oxidation â epimerization sequence to reach the natural product. We discovered an interesting epimerization phenomenon between arborisidine and its epimer which, after being conscienciously studied, raised the question of which isomer is the natural product.The development of a new class of single H-bond donor organocatalysts identified in the previous structure-enantioselectivity relationship process is then detailed. These prolinamides present the advantages to be cheap, easier to prepare and to purify than other known catalysts, and provide a new molecular scaffold for enantioselective H-bonding catalysis. They allow a general access to quaternary-center containing tetrahydro-Î²-carbolines derived from Î±-diketones, bulky Î±-ketoesters and Î±-ketoamides, the latter being extensively explored. They also compete with known methods with aldehydes as substrates. The products can all be obtained in good yields and enantioselectivities. The scope is large and shows excellent functional group tolerance to the exception of acidic protons close to the ketone center and sterically hindered ketone derivatives. In line with Jacobsenâs recent discovery, rearomatization is the rate- and enantioselectivity-determining step. The transformation of Î±-diketones is investigated by Vincent, Morgane and Julia, and that of Î±-ketoesters by Fenggang. Of interest, Fenggang showed that the use of two H-bond donor squaramide-organocatalysts was mandatory in order to make the reaction general with Î±-ketoesters.The power of this method was then further demonstrated. The stereocontrolled divergent total synthesis of voacafricines A and B thanks to an asymmetric PSR is the object of the third chapter. Its application allowed to install all the carbons contained in the natural products in a controlled manner, and a complexity-generating oxidative cleavage â cascade cyclization â selective reduction allowed to reach a key hemiacetal intermediate. On one hand this moiety was activated to generate the desired ammonium which provided voacafricine A after in situ hydrolysis. It could on the other hand be used to functionalize the adjacent carbon atom and the same synthetic sequence was applied to get voacafricine B.The enantioselective total synthesis of alstratine A is finally reported. Believing that the aminal could be made at the last step of the synthesis, we identified a precursor whose structure analysis suggested a possible application of an intramolecular inverse electron demand Diels-Alder reaction. The synthesis began with an EPSR of an Î±-ketoester. Funtional group manipulations followed by IMDA successfully built most of the contiguous stereocenters in a controlled fashion. The acid-mediated cyclization then took place without any issues. Alstratine A specific rotation reported in the isolation paper is corrected herein.

"Divergent Total Synthesis of (-)-Rhazinilam, (-)-Leucomidine B and (+)-Leuconodine F" and "Synthetic Studies Towards Sarpagine-Related Indoles"

Dylan Jean Robert Dagoneau

The work presented in this thesis focuses on the synthesis of monoterpene indole alkaloids. The first part describes the divergent total synthesis of (-)-rhazinilam, (-)-leucomidine B and (+)-leuconodine F, three structurally distinct natural products of the Aspidosperma family. The key step involves the heteroannulation of an advanced tetrahydropyridine with bromoacetaldehyde or oxalyl chloride in order to afford the corresponding tetrahydroindolizine and 2,3-dioxopyrrole moieties, respectively. By fine tuning of the reaction conditions, the former was converted into rhazinilam while the latter led to the syntheses of leucomidine B and leuconodine F. In the case of leucomidine B, the development of a substrate-directed diastereoselective reduction of a sterically unbiased double bond is also discussed. The second part details the synthetic studies towards various members of the Sarpagan family. After the exploration of several strategies and indoles alkaloids, the enantioselective total synthesis of N(1)-demethyl-3,5-diepi-alstolactone was accomplished. This synthesis features three key points: 1) the incorporation of a vinyl ketone function in the C-2 position of indole by using the Liebeskind-Srogl coupling in order to afford, after ring-closing metathesis, an advanced eight-membered cyclic enone, 2) a highly diastereoselective intramolecular Michael addition and 3) the formation of an azabicyclo[3.3.1]nonane bridged system from the corresponding eight-membered ring via a dehydration / transannular cyclization process.

EPFL2017

Development of New Strategies of Umpolung with Cyclic Hypervalent Iodine Reagents

Paola Caramenti

Organic chemistry is essential for the development of a modern society and technical progress requires the continuous development of synthetic methodologies Novel, efficient, selective and flexible protocols are employed to access complex frameworks from simple starting materials. However, classical synthetic methods often relies on the intrinsic reactivity of functional groups, reducing the portfolio of available reactions for synthetic chemists. The reversal of classical reactivity of functional groups allows alternative disconnection pathways and can improve synthetic efficiency. The research presented in this thesis was aimed at the development of new strategies of Umpolung with hypervalent iodine reagents. Three new Umpolung methods have been investigated. First a strategy for the synthesis of enantioenriched Î±-cyano Î±-allyl ketones has been developed. Second, novel indole- and pyrrole-based hypervalent iodine reagents have been synthesized and applied in the context of metal-catalysis ad metal-free transformations. Third, a new class of umpoled enamides and enol ethers have been prepared from simple starting materials. Î±-Cyano carbonyls are important structural motifs of bioactive compounds and natural products. Nitriles are also versatile building blocks that undergo a plethora of transformations, which are usually accessed by transfer of the nucleophilic cyanide anion to electrophiles. The reactivity of cyanide can be reversed by the use of hypervalent iodine reagents. Part I focus on the application of umpoled nitriles for the functionalization of carbonyls. With these reagents, a variety of Î²-keto esters were successfully cyanated in racemic fashion. Inspired by previous results obtained in our group, a decarboxylative asymmetric allylic alkylation strategy was applied for the synthesis of enantioenriched quaternary Î±-cyano Î±-allyl ketones embedded in an indanone framework. Cyclic indanone derivatives were accessed in good yields and enantioselectivities. Indole and pyrrole heterocycles are often embedded in relevant drugs, natural products and materials. Their ubiquity inspired decades of intensive research for both their synthesis and functionalizations. Especially functionalization methods mainly focused on their intrinsic nucleophilic properties. The field exploiting their electrophilic derivatization was severely underdeveloped. An alternative strategy to access electrophilic indole and pyrrole synthons takes advantage of the unique properties of hypervalent iodine. In Part II of this thesis, indole- and pyrrole-based benziodoxoles were prepared and used for the metal-catalyzed direct indole transfer on aromatic CâH bonds. A large variety of indole-aryl frameworks, previously elusive or accessed via multi-step syntheses, were accessed in a straightforward manner, starting from simple commercially available building blocks. A direct oxidative metal-free cross-coupling for the synthesis of mixed bi-(hetero)aryl-indoles was then developed. The reaction worked well with our previously synthesized reagents, and a new class of electrophilic indoles (activated at the C2 carbon) was specifically developed for this transformation, further expanding the pool of available benziodoxoles.

EPFL2019

Asymmetric Total Synthesis of Indole Alkaloids (+)-Condyfoline, (-)-Tubifoline, (+)-Dasycarpidone and (+)-Uleine

Bastien Delayre

The development of a titanium(III)-mediated cascade electrocyclisation/rearrangement for theenantioselective total synthesis of indole alkaloids (+)-condyfoline and (-)-tubifoline serve as the basisof the first chapter of this thesis. An introduction will focus on the use of domino sequencesincorporating a 1,2-shift in the context of complex natural product synthesis. The design of such adomino process is then presented, along with preliminary results originating from our research groupconcerning the titanium(III)-mediated Cadogan cyclisation. This novel domino sequence enabled thefirst enantioselective total synthesis of (+)-condyfoline. These results provided insights into themechanistic nature of the formal 1,2-shift, indicating that the formal 1,2-migration most likelyproceeds via 1,5-sigmatropic rearrangement rather than retro-Mannich/Mannich sequence. Theisolation of (+)-condyfoline enabled a re-investigation of its isomerisation process to Strychnos isomer(-)-tubifoline. The chapter concludes with synthetic studies towards (+)-tronoharine and (+)-tubotaiwine.The second chapter of this thesis deals with the development of an integratedOxidation/Reduction/Cyclisation (iORC) sequence for the enantioselective total synthesis of the Uleinefamily of monoterpene indole alkaloids. A brief introduction will review our group's previous effortsto implement iORC sequences for the total synthesis of alkaloids. The step-by-step development of theiORC sequence is described, along with the endgame strategy for the synthesis of (+)-nordasycarpidone, (+)-dasycarpidone, (-)-dasycarpidol and (+)-uleine. The chapter ends with theattempted titanium(III)-mediated cyclisation/rearrangement sequence towards (+)-uleine, andattempted iORC sequence towards (+)-condyfoline.The third chapter of this thesis reports a novel synthesis of benzofuro[3,2-b]indolines from onitrostylbene precursors using a titanium(III)-mediated interrupted Cadogan reaction. A rapidinvestigation of the substrate scope is presented. The remaining of the chapter reports our efforts toapply this newly developed transformation for the total synthesis of bis-indole alkaloid phalarine.The last chapter is dedicated to our synthetic studies towards (+)-pestalustaine A, a recently isolatedsesquiterpene possessing a unique 5/6/7-fused tricyclic system. A designed formal [3+2] reaction wasenvisioned to provide access to this novel sesquiterpene, but resulted in the isolation of a Wilson-Clokerearrangement product.

EPFL2022