A Configurational Switch Based on Iridium‐Catalyzed Allylic Cyclization: Application in Asymmetric Total Syntheses of Prosopis,Dendrobate, and Spruce Alkaloids

A method for the stereoselective synthesis of 2,6‐disubstituted piperidines has been developed that is based on the use of an intramolecular iridium‐catalyzed allylic substitution as a configurational switch. The procedure allows the preparation of 2‐vinylpiperidines with enantiomeric excesses (ee) of greater than 99 %. As applications, total syntheses of piperidine alkaloids have been elaborated, most often by using Ru‐catalyzed cross‐metatheses as a key step for introduction of a side chain. Asymmetric total syntheses of the prosopis alkaloids (+)‐prosopinine, (+)‐prosophylline, (+)‐prosopine, and of the dendrobate alkaloid (+)‐241D and its C6 epimer are described.     

Iridium‐Catalyzed Asymmetric Hydrogenation of 3,3‐Disubstituted Allylic Alcohols in Ethereal Solvents

Ir‐phosphinomethyl‐oxazoline complexes have been identified as efficient, highly enantioselective catalysts for the asymmetric hydrogenation of 3,3‐disubstituted allylic alcohols and related homoallylic alcohols. In contrast to other N,P ligand complexes, which require weakly coordinating solvents, such as dichloromethane, these catalysts perform well in more ecofriendly THF or 2‐MeTHF. Their synthetic potential was demonstrated with the formal total synthesis of four bisabolane sesquiterpenes.     

Iridium‐Catalyzed Stereoselective Allylic Alkylation Reactions with Crotyl Chloride

The development of the first enantio‐, diastereo‐, and regioselective iridium‐catalyzed allylic alkylation reaction of prochiral enolates to form an all‐carbon quaternary stereogenic center with an aliphatic‐substituted allylic electrophile is disclosed. The reaction proceeds with good to excellent selectivity with a range of substituted tetralone‐derived nucleophiles furnishing products bearing a newly formed vicinal tertiary and all‐carbon quaternary stereodyad. The utility of this protocol is further demonstrated via a number of synthetically diverse product transformations.     

Highly Regio‐ and Enantioselective Synthesis of N‐Substituted 2‐Pyridones: Iridium‐Catalyzed Intermolecular Asymmetric Allylic Amination

The first iridium‐catalyzed intermolecular asymmetric allylic amination reaction with 2‐hydroxypyridines has been developed, thus providing a highly efficient synthesis of enantioenriched N‐substituted 2‐pyridone derivatives from readily available starting materials. This protocol features a good tolerance of functional groups in both the allylic carbonates and 2‐hydroxypyridines, thereby delivering multifunctionalized heterocyclic products with up to 98 % yield and 99 % ee.     

Direct Asymmetric Dearomatization of Pyridines and Pyrazines by Iridium‐Catalyzed Allylic Amination Reactions

The first iridium‐catalyzed intramolecular asymmetric allylic dearomatization reaction of pyridines and pyrazines has been realized. 2,3‐Dihydroindolizine and 6,7‐dihydropyrrolo[1,2‐a]pyrazine derivatives were obtained with excellent yields and enantioselectivity. This methodology features dearomatization by direct N‐allylic alkylation of pyridines or pyrazines under mild reaction conditions.     

Enantioselective Synthesis of Tertiary Allylic Fluorides by Iridium‐Catalyzed Allylic Fluoroalkylation

Few allylic electrophiles containing two different substituents at a single allyl terminus and none in which one of the two substituents is a heteroatom, have been shown previously to react with iridium catalysts to form substitution products. We report that iridium‐catalysts are uniquely suited to form tertiary allylic fluorides enantioselectively by the addition of a diverse range of carbon‐centered nucleophiles at the fluorine‐containing terminus of 3‐fluoro‐substituted allylic esters. The products contain tertiary stereogenic centers bearing a single fluorine, which are isosteric with common tertiary stereocenters containing a single hydrogen. Computational studies reveal the principal steric interactions influencing the stability of endo and exo π‐allyl intermediates formed from 3,3‐disubstituted allylic electrophiles.     

Total Synthesis of (+)‐Madangamine D

Madangamines are a group of bioactive marine sponge alkaloids, embodying an unprecedented diazapentacyclic skeletal type. The enantioselective total synthesis of madangamine D has been accomplished, and represents the first total synthesis of an alkaloid of the madangamine group. It involves the stereoselective construction of the diazatricyclic ABC core using a phenylglycinol‐derived lactam as the starting enantiomeric scaffold and the subsequent assembly of the peripheral macrocyclic rings. The synthesis provides, for the first time, a pure sample of madangamine D and confirms the absolute configuration of this alkaloid family.     

Iridium‐Catalyzed Asymmetric Isomerization of Primary Allylic Alcohols

Nothing to sm(Ir)k at : Under appropriate reaction conditions, iridium hydride catalysts promote the isomerization of primary allylic alcohols. The best catalysts, like (R)‐ 1 (P green, O red, N blue, Ir yellow), deliver the desired chiral aldehydes with excellent enantioselectivity and good yields. Mechanistic hypotheses have been developed on the basis of preliminary investigations.

     

Immobilized Catalysts for Iridium‐Catalyzed Allylic Amination: Rate Enhancement by Immobilization

The first immobilized catalyst for Ir‐catalyzed asymmetric allylic aminations is described. The catalyst is a cationic (π‐allyl)Ir complex bound by cation exchange to an anionic silica gel support. Preparation of the catalyst is facile, and the supported catalyst displayed considerably enhanced activity compared with the parent homogeneous catalyst. Up to 43 consecutive amination runs were possible in recycling experiments.     

Enantioselective Synthesis of Vicinal All‐Carbon Quaternary Centers via Iridium‐Catalyzed Allylic Alkylation

The development of the first enantioselective transition‐metal‐catalyzed allylic alkylation providing access to acyclic products bearing vicinal all‐carbon quaternary centers is disclosed. The iridium‐catalyzed allylic alkylation reaction proceeds with excellent yields and selectivities for a range of malononitrile‐derived nucleophiles and trisubstituted allylic electrophiles. The utility of these sterically congested products is explored through a series of diverse chemo‐ and diastereoselective product transformations to afford a number of highly valuable, densely functionalized building blocks, including those containing vicinal all‐carbon quaternary stereocenters.     

10‐Step Asymmetric Total Synthesis and Stereochemical Elucidation of (+)‐Dragmacidin D

The asymmetric synthesis of dragmacidin D ( 1 ) was completed in 10 steps. Its sole stereocenter was set by using direct asymmetric alkylation enabled by a C₂‐symmetric tetramine and lithium N‐(trimethylsilyl)‐tert‐butylamide as the enolization reagent. A central Larock indole synthesis was employed in a convergent assembly of the heterocyclic subunits. The stereochemical evidence from this work strongly supports the predicted S configuration at the 6′′′ position, which is consistent with other members of the dragmacidin family of natural products.     

Regio‐ and Enantioselective Synthesis of N‐Allylindoles by Iridium‐Catalyzed Allylic Amination/Transition‐Metal‐Catalyzed Cyclization Reactions

Regio‐ and enantioselective synthesis of N‐allylindoles was realized through an iridium‐catalyzed asymmetric allylic amination reaction with 2‐alkynylanilines and subsequent transition‐metal‐catalyzed cyclization reactions. The highly enantioenriched allylic amines prepared from Ir‐catalysis were treated with catalytic amount of NaAuCl₄ ? 2 H₂O or PdCl₂ providing various substituted N‐allylindoles in excellent yields and enantioselectivities.     

Iridium‐Catalyzed Enantioselective Indole Cyclization: Application to the Total Synthesis and Absolute Stereochemical Assignment of (−)‐Aspidophylline A

The first enantioselective total synthesis of (?)‐aspidophylline A, including assignment of its absolute configuration has been accomplished. A key element of the synthesis is a highly enantioselective indole allylic alkylation/iminium cyclization cascade which was developed by employing a combination of Lewis acid activation and an iridium/ligand catalyst. This strategy relies on the direct use of 2,3‐disubstituted indoles with secondary allylic alcohols appended at C2 and heteronucleophiles appended at C3, indoles which are easily prepared from simple starting materials under C?H activation conditions.     

A Short Scalable Route to (−)‐α‐Kainic Acid Using Pt‐Catalyzed Direct Allylic Amination

An increased supply of scarce or inaccessible natural products is essential for the development of more sophisticated pharmaceutical agents and biological tools, and thus the development of atom‐economical, step‐economical and scalable processes to access these natural products is in high demand. Herein we report the development of a short, scalable total synthesis of (?)‐α‐kainic acid, a useful compound in neuropharmacology that is, however, limited in supply from natural resources. The synthesis features sequential platinum‐catalyzed direct allylic aminations and thermal ene‐cyclization, enabling the gram‐scale synthesis of (?)‐α‐kainic acid in six steps and 34 % overall yield.     

Bioinspired Synthesis of (+)‐Cinchonidine Using Cascade Reactions

The development of efficient syntheses of complex natural products has long been a major challenge in synthetic chemistry. Designing cascade reactions and employing bioinspired transformations are an important and reliable means of achieving this goal. Presented here is a combination of these two strategies, which allow efficient asymmetric synthesis of the cinchona alkaloid (+)‐cinchonidine. The key steps of this synthesis are a controllable, visible‐light‐induced photoredox radical cascade reaction to efficiently access the tetracyclic monoterpenoid indole alkaloid core, as well as a practical biomimetic cascade rearrangement for the indole to quinoline transformation. The use of stereoselective chemical transformations in this work makes it an efficient synthesis of (+)‐cinchonidine.