Enantioselective synthesis of (2S,3′R,7′Z)-N-(3′-hydroxy-7′-tetradecenoyl)-homoserine lactone

A concise enantioselective total synthesis of (2S,3′R,7′Z)-N-(3′-hydroxy-7′-tetradecenoyl)-homoserine lactone is described. Key feature of this protocol is a catalytic asymmetric hydrogenation and a prophenol-zinc-catalyzed diazo addition to imine reaction as genesis of chirality. Moreover, flexibility is built in the synthesis to generate enantioenriched analogs using catalytic amount of enantioenriched C₂-symmetric ligands.     

Direct,enantioselective synthesis of pyrroloindolines and indolines from simple indole derivatives

The (R)-BINOL·SnCl₄-catalyzed formal (3+2) cycloaddition between 3-substituted indoles and benzyl 2-trifluoroacetamidoacrylate is a direct, enantioselective method to prepare pyrroloindolines from simple starting materials. However, under the originally disclosed conditions, the pyrroloindolines are formed as mixtures of diastereomers, typically in the range of 3:1 to 5:1 favoring the exo-product. The poor diastereoselectivity detracts from the synthetic utility of the reaction. We report here that use of methyl 2-trifluoroacetamidoacrylate in conjunction with (R)-3,3′-dichloro-BINOL·SnCl₄ provides the corresponding pyrroloindolines with improved diastereoselectivity (typically ≥10:1). Guided by mechanistic studies, a one-flask synthesis of enantioenriched indolines by in situ reduction of a persistent iminium ion is also described.     

Efficient synthesis of chiral phenethylamines: preparation, asymmetric hydrogenation, and mild deprotection of ene-trifluoroacetamides

A mild and efficient route to enantioenriched aryl alkyl amines from ketones has been developed. The first successful synthesis and asymmetric hydrogenation of ene-trifluoroamides from oximes gave highly enantioenriched trifluoroacetamides (94-98% ee). The corresponding phenethyl amides are liberated under mild conditions (K₂CO₃, MeOH/H₂O). In addition, a new application of Josiphos ligands toward the asymmetric hydrogenation of both ene-acetamides and ene-trifluoroacetamides was discovered.     

Direct synthesis of methyl 2-diazo-4-aryl-3-butenoates and their application to the enantioselective synthesis of 4-aryl-4-(1-naphthyl)-2-butenoates

An improved one-flask synthesis of various methyl 2-diazo-4-aryl and 4-heteroaryl-3-butenoates, precursors to donor/acceptor substituted carbenoids, is described. Their Rh₂(S-DOSP)₄ catalyzed reaction with 1-acetoxy-3,4-dihydronaphthalene, via a combined C–H activation/Cope rearrangement pathway followed by elimination of acetic acid affords a highly enantioselective (98–99% ee) entry to methyl 4-aryl- and 4-heteroaryl-4-(1-naphthyl)-2-butenoates.     

Asymmetric de novo synthesis of fluorinated d-glucitol and d-mannitol analogues

A highly efficient anti-S_E2′ electrophilic fluorination of enantioenriched allylsilanes a subsequent dihydroxylation of the resulting allylic fluorides were used as key steps for the synthesis of three fluorinated carbohydrate analogues, 1,5-di-O-benzyl-2-deoxy-2-fluoro-d-glucitol, 2,6-di-O-benzyl-5-deoxy-5-fluoro-l-glucitol and 1,5-di-O-benzyl-2-deoxy-2-fluoro-d-mannitol. A new catalytic asymmetric route to 1-benzyloxy-4-trimethylsilyl-but-3-yn-2-ol, a common precursor to two advanced allylsilanes, is also described featuring a Noyori asymmetric transfer hydrogenation reaction.     

The first example of a crystallization-induced asymmetric transformation (CIAT) in the Mannich reaction

The novel synthesis of highly enantioenriched N-substituted α-amino-γ-alkyl(aryl)-γ-oxobutanoic acids is described. The process involves the combination of a crystallization-induced asymmetric transformation (CIAT) and the Mannich reaction. The role of retro-Mannich and retro-Michael reactions in the mechanism of the highly stereoselective transformation is also discussed.     

On the regioselectivity of the Friedländer reaction leading to huprines: stereospecific acid-promoted isomerization of syn-huprines to their anti-regioisomers

Racemic 12-amino-6,7,8,11-tetrahydro-7,11-methanocycloocta[b]quinoline derivatives (syn-huprines) substituted at the 9-position with a methyl or ethyl group, and both enantioenriched forms of the 9-ethyl derivative, obtained by chiral MPLC resolution of the racemic mixture, were readily converted to the corresponding anti-isomers (huprines) by stereospecific acid-promoted (AlCl₃ or triflic acid) isomerization of the endocyclic CC double bond from the 9(10)- to the 8(9)-position. These results support the hypothesis that the hitherto unseen syn-huprines are also formed under the usual acidic Friedländer reaction conditions used to prepare the known huprines, but rearrange in situ to the more stable anti-regioisomers (B3LYP/6-31G*).     

Synthesis of Δ-15-deoxy-PG-J1 methyl ester and epi-Δ-15-deoxy-PG-J1

The synthesis of racemic Δ^12,14-15-deoxy-PG-J₁ is readily achieved in six steps employing as the key transformation a one-pot conjugate addition-Peterson olefination sequence using exo-2-trimethylsilyl-3a,4,7,7a-tetrahydro-4,7-methanoinden-1-one. Additionally a Noyori-type three-component coupling approach is employed for the synthesis of enantioenriched epi-Δ¹²-15-deoxy-PG-J₁ from 4(S)-tert-butyldimethylsilyloxycyclopent-2-enone.     

Achiral Inorganic Gypsum Acts as an Origin of Chirality through Its Enantiotopic Surface in Conjunction with Asymmetric Autocatalysis

Achiral inorganic gypsum (CaSO₄⋅2 H₂O) triggers the asymmetric autocatalysis of pyrimidyl alkanol on its two‐dimensional enantiotopic faces to give highly enantioenriched alkanol products with absolute configurations corresponding to the respective enantiotopic surfaces. This is the first example of highly enantioselective synthesis on the enantiotopic surface of an achiral mineral.     

Efficient asymmetric synthesis of aryl difluoromethyl sulfoxides and their use to access enantiopure α-difluoromethyl alcohols

The -CHF₂ moiety has shown a growing interest in pharmaceutical and agrochemical applications over the last few years. Its introduction is therefore a current research topic for organic chemists. Several groups have reported the synthesis of difluoromethylated compounds. However, the more challenging enantioselective introduction of the difluoromethyl group has been scarcely described yet. We recently developed a new strategy, based on the use of an enantiopure difluoromethyl sulfoxide used as chiral and traceless auxiliary, for the synthesis of highly enantioenriched α-difluoromethyl alcohols. The first method developed in our laboratory aims to access highly stereoenriched α,α-difluoro-β-hydroxysulfoxides through the condensation of the enantiopure difluoromethyl sulfoxide on carbonyl derivatives. It is noteworthy that highly diastereo- and enantioenriched α,α-difluoro-β-hydroxysulfoxides can also be accessed after the diastereoselective reduction of highly enantioenriched α,α-difluoro-β-ketosulfoxides. Finally, the expected difluoromethyl-substituted alcohols can be obtained after removal of the chiral auxiliary with complete retention of stereoenrichment at carbon.     

Synthetic studies on the icetexones: enantioselective formal syntheses of icetexone and epi-icetexone

Two strategies for the synthesis of the icetexane diterpenoids icetexone and epi-icetexone that rely on Ga(III)-catalyzed cycloisomerization of alkynyl indene substrates to yield fused [6-7-6] tricycles have been explored. In the first approach, access to a tricycle bearing a gem-dimethyl group paved the way for explorations of C–H functionalization of one of the methyl groups in close proximity to a hydroxyl-directing group. This approach was ultimately unsuccessful and led only to ring cleaved products. In the second approach, an alkynyl indene substrate bearing a cyano substituent was utilized, which was effective in providing a functional handle to access the icetexone subclass of diterpenoids. A key epoxide opening/diazene rearrangement sequence was utilized to complete a formal synthesis of icetexone and epi-icetexone, which is discussed in detail. Furthermore, the cyano-containing substrate has been prepared in enantioenriched form using a Rh-catalyzed conjugate addition reaction, which now provides a route to the enantioselective synthesis of these natural products.     

C2-Symmetric nitroxides and their potential as enantioselective oxidants

The synthesis and evaluation of four C₂-symmetric nitroxides are presented. The nitroxides were evaluated for their ability to mediate the oxidation of several alcohols and found to have good catalytic activity. One enantioenriched nitroxide was found to kinetically resolve selected secondary alcohols with very modest selectivities.     

Catalytic Enantioselective Protonation of Monofluorinated Silyl Enol Ethers towards Chiral α‐Fluoroketones

Described herein is an organocatalytic enantioselective protonation of monofluorinated silyl enol ethers, affording an array of optically active α‐secondary α‐fluoroketones in good to high yields and enantioselectivities, under the catalysis of bifunctional cinchonidine derived squaramide C4 . It represents a rare example of facile synthesis of enantioenriched α‐secondary α‐fluoroketones. With D₂O as the deuterium source and MeOD as the solvent, the first highly enantioselective preparation of chiral α‐deuterated α‐fluoroketones in >92% deuteration is developed.     

Rh2(S-PTAD)4-catalyzed asymmetric cyclopropenation of aryl alkynes

Rh₂(S-PTAD)₄ is an effective catalyst for the asymmetric cyclopropenation of aryl alkynes using a siloxyvinyldiazoacetate as the carbenoid precursor. Upon deprotection of the silyl protecting group, highly enantioenriched cyclopropenes bearing geminal acceptor groups can be accessed. These cyclopropenes undergo regioselective rhodium(II)-catalyzed ring expansion to furans.     

Asymmetric dehydration of β-hydroxy esters and application to the syntheses of flavane derivatives

Catalytic asymmetric dehydration of β-aryl or alkyl substituted β-hydroxy esters via kinetic resolution has been investigated. A brief survey of 10 different chiral ligands is conducted to examine the effects of chiral ligand structure on selectivity of the dehydration. The kinetic resolution of a variety of rac-β-hydroxy tert-butyl esters in the presence of prolinol chiral ligand 2 and BrZnCH₂CO₂-t-Bu can provide highly enantioenriched β-hydroxy esters 14-21 with selectivity factors ranging from 11 to 66. Also, application of this asymmetric synthetic methodology to the preparation of enantioenriched flavane derivatives 25-29 is demonstrated.     

Asymmetric synthesis of 3-hetero-substituted 2,3-dihydro-1H-isoindol-1-ones

An efficient asymmetric synthesis of highly enantioenriched 3-hetero-substituted 2,3-dihydro-1H-isoindolinones is reported. The key step is a diastereoselective nucleophilic addition on N-acylhydrazonium intermediates generated by acidic treatment of hemiaminal precursors bearing an (S)-2-alkoxymethyl-pyrrolidin-1-yl type auxiliary. The auxiliary is removed by an oxidative N,N-bond cleavage with magnesium monoperoxyphthalate.     

A general and enantiodivergent method for the asymmetric synthesis of piperidine alkaloids: concise synthesis of (R)-pipecoline, (S)-coniine and other 2-alkylpiperidines

A simple and very efficient protocol for the preparation of highly enantioenriched 2-alkylpiperidines has been set up, which allows the preparation of the final heterocycles with any wanted configuration at the stereogenic center starting from the same starting material. The key step of the synthesis relies on a diastereodivergent aza-Michael reaction protocol using the readily available and cheap reagent (+)-(S,S)-pseudoephedrine as chiral auxiliary.     

Three-membered ring formation reaction—III : Mechanism of the reaction of α-halogenoacrylic ester with organozinc compounds

The stereochemistry of ring formation in the reaction of α-halogenoacrylic ester with organozinc compounds was studied using β-deuterated α-halogenoacrylic ester. A quantitative study was made on the steric course of every step of the reactions involved in the synthesis of methyl β-deuterio-α-bromoacrylate-cis-d₁ starting from methyl propiolate. The mode of the CC double bond opening of methyl β-deuterio-α-bromoacrylate-cis-_d1 to form dimethyl 1 -bromo-2-propyl-cis-1,2- cyclopropanedicarboxylate-d₂ was confirmed to be cis and trans in a 50 to 50 ratio. Asymmetric syntheses for the cyclopropanedicarboxylic ester were possible, especially under the influence of chiral organozinc alkoxide system. A stepwise mechanism was postulated for the ring formation reaction.     

Highly Enantioselective Tandem Michael Addition of Tryptamine‐Derived Oxindoles to Alkynones: Concise Synthesis of Strychnos Alkaloids

A highly enantioselective tandem Michael addition of tryptamine‐derived oxindoles to alkynones was developed by taking advantage of a chiral N,N′‐dioxide Sc(OTf)₃ catalyst. The reaction enables the facile preparation of enantioenriched spiro[pyrrolidine‐3,3′‐oxindole] compounds, which provides a novel strategy for the synthesis of monoterpenoid indole alkaloids. As a demonstration, the asymmetric synthesis of strychnos alkaloids [(?)‐tubifoline, (?)‐tubifolidine, (?)‐dehydrotubifoline] was achieved in 10–11 steps.     

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.