Concise synthesis of (±)-rhazinilam

Hydrazone Activation in the Aminocatalytic Cascade Reaction for the Synthesis of Tetrahydroindolizines

In this Letter, we demonstrate the usefulness of hydrazone activation for the synthesis of biologically relevant tetrahydroindolizines. A pyrrol-derived hydrazone bearing a Michael acceptor moiety in the N-alkyl side chain has been designed with the aim of participating in the aminocatalytic cascade reaction leading to the annulation of the new six-membered heterocyclic scaffold. The application of (S)-(-)-α,α-diphenyl-2-pyrrolidinemethanol trimethylsilyl ether as the aminocatalyst allows for the iminium ion-enamine-mediated cascade to proceed in a fully stereoselective manner.

A Modular Synthesis of the Rhazinilam Family of Alkaloids and Analogs Thereof

A short, modular strategy for synthesizing three representative alkaloids of the (±)-rhazinilam family and 10 non-natural analogs is described. The protocol involves a radical addition/cyclization cascade reaction that assembles the tetrahydroindolizine system decorated with appropriate groups for a subsequent Pd-mediated cyclization, which generates the nine-membered lactam.

Rhazinilam-leuconolam family of natural products: a half century of total synthesis

Covering: 1972 to 2021The rhazinilam family of natural products exhibits a main structure with a stereogenic quaternary carbon and a tetrahydroindolizine core imbedded within a 9-membered macrocycle, imposing axial chirality. This unique architecture combined with their taxol-like antimitotic activities have attracted various attention, especially from synthetic chemists, notably in the past decade. The present review describes the known total and formal syntheses of the members of the rhazinilam family (rhazinilam, rhazinal, leuconolam and kopsiyunnanines), according to the strategy developed.

Enantioselective palladium-catalyzed allylic alkylation reactions in the synthesis of Aspidosperma and structurally related monoterpene indole alkaloids

Covering: up to the end of 2017 Enantioselective Pd-catalyzed allylic alkylations of prochiral enolates represent a powerful tool for the construction of all-carbon quaternary stereocenters. This review describes the emergence of such reactions as strategic linchpins that enable efficient, stereocontrolled syntheses of Aspidosperma and related monoterpene indole alkaloids.

Short Enantioselective Total Synthesis of (-)-Rhazinilam Using a Gold(I)-Catalyzed Cyclization

(R)-(-)-Rhazinilam has been synthesized in nine steps and 20% overall yield. The key steps involve two metal-catalyzed processes: the enantioselective gold(I)-catalyzed cycloisomerization of an allene-functionalized pyrrole and the palladium-catalyzed hydrocarboxylation of a vinyl moiety with formate as a CO surrogate. This novel strategy represents the shortest and highest yielding enantioselective total synthesis of (-)-rhazinilam.

Selective syntheses of leuconolam, leuconoxine, and mersicarpine alkaloids from a common intermediate through regiocontrolled cyclizations by Staudinger reactions†Electronic supplementary information (ESI) available: Experimental details and procedures, compound characterization data, copies of 1H and 13C NMR spectra for new compounds. See DOI: 10.1039/c4qo00312hClick here for additional data file

Selective syntheses of leuconolam, leuconoxine, and mersicarpine alkaloids bearing distinctive core structures were achieved through Staudinger reactions using a common intermediate. In the key cyclization step, water functioned like a switch to control which core structure to produce. The chemistry allowed for selective syntheses of the group of alkaloids from a simple intermediate through straightforward chemical operations.

Biosynthetically inspired divergent approach to monoterpene indole alkaloids: total synthesis of mersicarpine, leuconodines B and D, leuconoxine, melodinine E, leuconolam, and rhazinilam

Inspired by their potential biosynthesis, we have developed divergent total syntheses of seven monoterpene indole alkaloids including mersicarpine, leuconodines B and D, leuconoxine, melodinine E, leuconolam, and rhazinilam, and one unnatural analogue with an unprecedented structural skeleton. The key steps involve a Witkop-Winterfeldt oxidative indole cleavage followed by transannular cyclization. The transannular cyclization product was then converted to the corresponding structural skeletons by pairing its functional groups into different reaction modes.

Formal total syntheses of classic natural product target molecules via palladium-catalyzed enantioselective alkylation

Pd-catalyzed enantioselective alkylation in conjunction with further synthetic elaboration enables the formal total syntheses of a number of "classic" natural product target molecules. This publication highlights recent methods for setting quaternary and tetrasubstituted tertiary carbon stereocenters to address the synthetic hurdles encountered over many decades across multiple compound classes spanning carbohydrate derivatives, terpenes, and alkaloids. These enantioselective methods will impact both academic and industrial settings, where the synthesis of stereogenic quaternary carbons is a continuing challenge.

Phosphoric acid catalyzed desymmetrization of bicyclic bislactones bearing an all-carbon stereogenic center: total syntheses of (-)-rhazinilam and (-)-leucomidine B

In the presence of a catalytic amount of an imidodiphosphoric acid, enantioselective desymmetrization of bicyclic bislactones by reaction with alcohols took place smoothly to afford enantiomerically enriched monoacids having an all-carbon stereogenic center. Concise catalytic enantioselective syntheses of both (-)-rhazinilam and (-)-leucomidine B were subsequently developed using (S)-methyl 4-ethyl-4-formylpimelate monoacid as a common starting material.

The Construction of All-Carbon Quaternary Stereocenters by Use of Pd-Catalyzed Asymmetric Allylic Alkylation Reactions in Total Synthesis

All-carbon quaternary stereocenters have posed significant challenges in the synthesis of complex natural products. These important structural motifs have inspired the development of broadly applicable palladium-catalyzed asymmetric allylic alkylation reactions of unstabilized non-biased enolates for the synthesis of enantioenriched α-quaternary products. This microreview outlines key considerations in the application of palladium-catalyzed asymmetric allylic alkylation reactions and presents recent total syntheses of complex natural products that have employed these powerful transformations for the direct, catalytic, enantioselective construction of all-carbon quaternary stereocenters.

Total synthesis of (±)-leuconolam: intramolecular allylic silane addition to a maleimide carbonyl group

A concise total synthesis of the plant alkaloid (±)-leuconolam (1) has been achieved. A regio- and diastereoselective Lewis-acid mediated allylative cyclization was used to establish, simultaneously, two adjacent tetrasubstituted carbon centers. Furthermore, an essential arene cross-coupling to a hindered haloalkene was enabled by the use of a novel 2-anilinostannane.

C-H activation: a complementary tool in the total synthesis of complex natural products

The recent advent of transition-metal mediated C-H activation is revolutionizing the synthetic field and gradually infusing a "C-H activation mind-set" in both students and practitioners of organic synthesis. As a powerful testament of this emerging synthetic tool, applications of C-H activation in the context of total synthesis of complex natural products are beginning to blossom. Herein, recently completed total syntheses showcasing creative and ingenious incorporation of C-H activation as a strategic manoeuver are compared with their "non-C-H activation" counterparts, illuminating a new paradigm in strategic synthetic design.

Enantioselective construction of quaternary N-heterocycles by palladium-catalysed decarboxylative allylic alkylation of lactams

The enantioselective synthesis of nitrogen-containing heterocycles (N-heterocycles) represents a substantial chemical research effort and resonates across numerous disciplines, including the total synthesis of natural products and medicinal chemistry. In this Article, we describe the highly enantioselective palladium-catalysed decarboxylative allylic alkylation of readily available lactams to form 3,3-disubstituted pyrrolidinones, piperidinones, caprolactams and structurally related lactams. Given the prevalence of quaternary N-heterocycles in biologically active alkaloids and pharmaceutical agents, we envisage that our method will provide a synthetic entry into the de novo asymmetric synthesis of such structures. As an entry for these investigations we demonstrate how the described catalysis affords enantiopure quaternary lactams that intercept synthetic intermediates previously used in the synthesis of the Aspidosperma alkaloids quebrachamine and rhazinilam, but that were previously only available by chiral auxiliary approaches or as racemic mixtures.

Total synthesis of rhazinilam: axial to point chirality transfer in an enantiospecific Pd-catalyzed transannular cyclization

A total synthesis of rhazinilam based on a transannular cyclization strategy is described. Using a Heck reaction, the axial chirality of a halogenated 13-membered lactam can be exploited to create the quaternary chiral stereogenic center in the target molecule with high enantiospecificity.

Chiral molecules containing the pyrrole framework

Synthetic routes to chiral molecules containing a pyrrolic ring are discussed: strategies include formation of the pyrrole ring using starting materials appended with chirality, as well as the attachment of chirality to a pre-formed pyrrole.

Total synthesis of (+/-)-rhazinal using novel palladium-catalyzed cyclizations

A concise synthesis of (+/-)-rhazinal that hinges on novel oxidative Heck cyclizations and palladium-catalyzed direct couplings is described. An X-ray structure of N-MOM-rhazinal, which provides insight into the conformation of the strained 9-membered lactam ring, is described.

Amino acid-azetidine chimeras: synthesis of enantiopure 3-substituted azetidine-2-carboxylic acids

Azetidine-2-carboxylic acid (Aze) analogs possessing various heteroatomic side chains at the 3-position have been synthesized by modification of 1-9-(9-phenylfluorenyl) (PhF)-3-allyl-Aze tert-butyl ester (2S,3S)-1. 3-Allyl-Aze 1 was synthesized by regioselective allylation of alpha-tert-butyl beta-methyl N-(PhF)aspartate 13, followed by selective omega-carboxylate reduction, tosylation, and intramolecular N-alkylation. Removal of the PhF group and olefin reduction by hydrogenation followed by Fmoc protection produced nor-leucine-Aze chimera 2. Regioselective olefin hydroboration of (2S,3S)-1 produced primary alcohol 23, which was protected as a silyl ether, hydrogenated and N-protected to give 1-Fmoc-3-hydroxypropyl-Aze 26. Enantiopure (2S,3S)-3-(3-azidopropyl)-1-Fmoc-azetidine-2-carboxylic acid tert-butyl ester 3 was prepared as a Lys-Aze chimera by activation of 3-hydroxypropyl-Aze 26 as a methanesulfonate and displacement with sodium azide. Moreover, orthogonally protected azetidine dicarboxylic acid 4 was synthesized as an alpha-aminoadipate-Aze chimera by oxidation of alcohol 26. These orthogonally protected amino acid-Aze chimeras are designed to serve as tools for studying the influence of conformation on peptide activity.

Enantioselective synthesis of 3,3-disubstituted piperidine derivatives by enolate dialkylation of phenylglycinol-derived oxazolopiperidone lactams

The stereochemical outcome of the enolate dialkylation of simple phenylglycinol-derived oxazolopiperidone lactams is studied. High stereoselectivities in the generation of the quaternary stereocenter are obtained by the appropriate choice of both the configuration of the starting lactam and the order of introduction of the substituents. The usefulness of the methodology is illustrated by the conversion of some of the dialkylated lactams into known synthetic precursors of alkaloids and by the total synthesis of (-)-quebrachamine.

Explorations on the Asymmetric Total Synthesis of Isoschizogamine

Two approaches to the synthesis of isoschizogamine were reported. Both routes utilized an efficient aza-Claisen rearrangement to establish the absolute stereochemistry of the all-carbon quaternary center in the natural product. In the first approach, a highly diastereoselective (10:1) hetero-Diels-Alder reaction was utilized to reach a densely functionalized tetrahydroquinoline derivative as an advanced intermediate to the targeted alkaloid. An acylamidine intermediate was prepared and studied in the intramolecular cyclization reaction under acidic conditions in our second approach.

A Review on Recent Developments in Syntheses of the post-Secodine Indole Alkaloids. Part II: Modified Alkaloid Types

The second part of the planned review on developments in the field of total and formal total synthesis of the post-secodine indole alkaloids concentrates on modified alkaloid types, i.e. those skeletons derived from primary types by formation of additional and/or rupture of existing bonds, while connectivities next to indol(e)ine moiety remain intact. It thus reviews the synthesis of alkaloids of quebrachamine/cleavamine type including VLB-bis-indoles, rhazinilam type, aspidofractinine/kopsane and kopsifoline type, as well as kopsijasminilam alkaloids, lapidilectine B and danuphylline. It covers the literature of from 1991-1992 up to approximately end 2006. A review with 174 references.

Concise Synthesis of (±)-Rhazinilam through Direct Coupling

[reaction: see text] A concise synthesis of rhazinilam through direct, palladium-catalyzed, intramolecular coupling is described.

Diversity Oriented Synthesis of Hispanane-like Terpene Derivatives from (R)-(+)-Sclareolide

(R)-(+)-Sclareolide 1 has been used as a starting material to develop a diversity oriented methodology to access hispanane 28 a, and hispanane-like derivatives 27 b-27 e. This methodology is based on the intramolecular Friedel-Crafts acylation of the corresponding 12-desoxylabdanoic-like acids 27, for the construction of the cycloheptane ring which is characteristic of the hispananes. Acids 27 are obtained from alcohols 20, available by addition of the lithium or magnesium reagents to amide 12 (followed by Luche reduction), or to aldehyde 21. This sequence has resulted in the preparation of hispanane framework 27 a. The versatility of this methodology therefore allows a structural diversity oriented synthesis, since it allows the access to a wide variety of hispanane-like derivatives.