FeCl(3)-Catalyzed Stereoselective Construction of Spirooxindole Tetrahydroquinolines via Tandem 1,5-Hydride Transfer/Ring Closure

An efficient FeCl(3)-catalyzed stereoselective intramolecular tandem 1,5-hydride transfer/ring closure reaction was developed. The method allows for the formation of structurally diverse spirooxindole tetrahydroquinolines in high yields (up to 98%) with good to excellent levels of diastereoselectivity (up to 99:1 dr). The catalytic enantioselective variant of this process was also investigated preliminarily with a chiral BINOL-derived phosphoric acid.     

Chirality transfer in Au-catalyzed cyclization reactions of monoallylic diols: selective access to specific enantiomers based on olefin geometry

The gold(I)-catalyzed cyclization of monoallylic diols to form tetrahydropyrans is shown to be highly stereoselective when chiral allylic alcohols are employed. Substrates that differ only in olefin geometry provide enantiomeric products from formal S(N)2' reactions in high yields with excellent chirality transfer. The allylic alcohol stereochemistry also efficiently controls the facial selectivity when the substrates include additional stereocenters.     

A stereoselective method for the synthesis of enantiopure 3-substituted 4-hydroxyproline derivatives via 1,3-dipolar cycloadditions

The stereoselective 1,3-dipolar cycloaddition between (Z)-1,4-dichloro-2-butene and a menthone-derived nitrone led to the corresponding isoxazolidine. Regioselective azidation of a single chlorine atom followed by another Cu(I)-catalyzed azide–alkyne 1,3-dipolar cycloaddition (CuAAC) induced enantiopure 1,2,3-triazolyl-functionalized isoxazolidines. The subsequent acidic cleavage of the menthone chiral auxiliary and reductive cleavage of the isoxazolidine N–O bond triggered an intramolecular cyclization through the displacement of the second chlorine. This rapid and stereoselective synthetic strategy provided reliable access to a series of enantiopure 3-substituted 4-hydroxyproline derivatives.     

Development of Stereocontrolled Palladium(II)-Catalyzed Domino Heck/Suzuki β,α-Diarylation Reactions with Chelating Vinyl Ethers and Arylboronic Acids

A stereoselective and 1,4-benzoquinone-mediated palladium(II)-catalyzed Heck/Suzuki domino reaction involving metal coordinating cyclic methylamino vinyl ethers and a number of electronically diverse arylboronic acids has been developed and studied. Diastereomeric ratios up to 39:1 and 78 % isolated yields were obtained. The stereoselectivity of the reaction was found to be highly dependent on the nature of the arylboronic acid and the amount of water present in the reaction mixture. Thus, a domino β,α-diarylation–reduction of chelating vinyl ethers can now be accomplished and stereochemically controlled, given that optimized conditions and an appropriate chiral auxiliary are used. To the best of our knowledge, this represents the first example of a stereoselective, oxidative Heck/Suzuki domino reaction in the literature.     

Synthetic studies towards pentacyclic quassinoids: total synthesis of unnatural (-)-14-epi-samaderine E and natural (-)-samaderine Y from (S)-(+)-carvone

First total syntheses of unnatural (-)-14-epi-samaderine E (5) and natural (-)-samaderine Y (2) were accomplished from (S)-(+)-carvone (6) in 18 and 21 steps, respectively. The syntheses are short, efficient (with an average yield of 80 % plus for each transformation), enantiospecific, and produce nine new chiral centers. The crucial points of the syntheses included a regioselective allylic oxidation on ring C, regio- and stereoselective reduction of ketone, a stereocontrolled epoxidation, an epoxymethano-bridge formation, a chemoselective Grignard reaction, an intramolecular Diels-Alder reaction, an intramolecular aldol addition, and a newly developed manganese(III)-catalyzed allylic oxidation on ring A.     

RuCl3/CeCl3/NaIO4: a new bimetallic oxidation system for the mild and efficient dihydroxylation of unreactive olefins

[reaction: see text] The catalytic dihydroxylation of olefins represents a unique synthetic tool for the generation of two C,O-bonds with defined relative configuration. Whereas OsO(4) has been established as a very general dihydroxylation catalyst within the past 30 years, the less expensive and toxic isoelectronic RuO(4) has found only limited use for this type of oxygen-transfer reaction. High catalyst loading and undesired side reactions were severe drawbacks in RuO(4)-catalyzed oxidations of C,C-double bonds. Recently, we were able to improve the RuO(4)-catalyzed dihydroxylation by addition of Bronsted acids to the reaction mixture. This protocol proved to be of general applicability, however, certain limitations were observed. To address these problematic functional groups a new Lewis acid accelerated oxidation was developed. The use of only 10 mol % of CeCl(3) allowed a further decrease in the catalyst concentration down to 0.25 mol % while broadening the scope of the reaction. Silyl ethers and nitrogen containing functional groups are now tolerated in this optimized protocol. Furthermore, competing scission reactions are supressed in the presence of Lewis acid allowing longer reaction times and the successful oxidation of electron-deficient tetrasubstituted double bonds that cannot be oxidized using known dihydroxylation protocols.     

Formal asymmetric synthesis of (+)-tofacitinib

Tofacitinib is an efficient and selective Janus kinase 3 (JAK3) inhibitor, and is used as an immunosuppressant drug for the treatment of rheumatoid arthritis and transplant patients. Herein we report a concise formal asymmetric synthesis of tofacitinib from homochiral 1,3-dioxolanone 10b, which was elaborated through a highly stereoselective Michael addition followed by solvent-free removal of the chiral auxiliary and ring cyclization to furnish chiral imide 8. The preparation of tofacitinib’s precursor 16 could be obtained after reduction of 8 followed by sequential oxidation, reductive amination and S_NAr reactions.     

Cyclopropanation of enantiopure metal alkenyl carbenes with 2-methoxyfuran: a practical route to carboxycyclopropylglycine precursors

We have examined the reactivity of enantiopure alkenyl Fischer carbene complexes 1, readily available from the chiral pool, with 2-methoxyfuran 4. In this reaction, polyfunctionalised cyclopropylcarbenes 5 are obtained under very mild conditions and with high selectivity, the major stereoisomer being isolated in an enantiopure form. The reaction involves the conjugate nucleophilic addition of 2-methoxyfuran 4 to the carbene complexes 1 followed by ring closure of the resulting zwitterionic intermediate species. The oxidation of the carbene 5 a results in the formation of the enantiopure cyclopropane diester 6. Further elaboration of the cyclopropane 6 allows for an efficient enantioselective access to alcohols or diols 7-9 as well as to cyclopropanecarbaldehydes 10-12. The protocol described herein provides a very simple entry to interesting enantiopure precursors of carboxycyclopropylglycine derivatives from readily available starting materials. In order to test this potential as carboxycyclopropylglycine precursors, the aminocyanation of the cyclopropanecarbaldehyde 10 was undertaken and the alpha-aminocyano derivative 13 was isolated as a single diastereosiomer.     

A novel highly selective chiral auxiliary for the asymmetric synthesis of L- and D-alpha-amino acid derivatives via a multicomponent Ugi reaction

This paper describes the synthesis of a bicyclic beta-amino acid scaffold in both pure enantiomeric forms and its application as chiral auxiliary in an intramolecular version of the Ugi multicomponent reaction (U-5C-4CR) to prepare alpha-amino acid derivatives of both D- and L-series in a straightforward and very stereoselective manner. The mild conditions required for the Ugi condensation and for the removal of the chiral auxiliary make this method very attractive to prepare a wide range of differently structured N-alkylated and unalkylated amino acid derivatives.     

New perspective for natural products synthesis: concise synthesis of (+)-sch 642305 by chiral auxiliary multiuse methodology

The synthesis of (+)-Sch 642305 is an example of chiral auxiliary multiuse methodology, which shows a new perspective for the synthesis of compounds with multiple asymmetric centers. Thus, (+)-Sch 642305 was concisely synthesized from the known compound. Every reaction is stereoselective, and the chiral nonracemic hydrobenzoin worked as chiral auxiliary for desymmetrization of diene, as a template for attaining regio- and stereoselective reactions, as an oxygen source at the C4-position, and as a protecting group of hydroxyl functions. Namely, the chiral auxiliary played a role in every step throughout the synthesis. Furthermore, the synthesis contains a new protocol for obtaining alpha'-alkylated enone compounds.     

Total synthesis of (-)-salinosporamide A

A concise and stereoselective total synthesis of (-)-salinosporamide A (1), a potent inhibitor of the 20S proteasome that is in clinical development as an anticancer drug candidate, has been accomplished in 14 steps with 19% overall yield from 4-pentenoic acid. Our synthesis features a stereoselective alkylation utilizing a chiral auxiliary, formation of a pyrrolidine unit, and oxidation of the pyrrolidine to a γ-lactam. To demonstrate the scalability of our synthesis, (-)-salinosporamide A has been synthesized on a gram scale.     

Chiral Auxiliary Approach for Gold(I)-Catalyzed Cyclizations

Two different classes of stereoselective cyclizations have been developed using a chiral auxiliary approach with commercially available [JohnPhosAu(MeCN)SbF₆] as catalyst. First, a stereoselective cascade cyclization of 1,5-enynes was achieved using the Oppolzer camphorsultam as chiral auxiliary. In this case, a one-pot cyclization-hydrolysis sequence was developed to directly afford enantioenriched spirocyclic ketones. Then, the stereoselective alkoxycyclization of 1,6-enynes was mediated by an Evans-type oxazolidinone. A reduction-hydrolysis sequence was selected to remove the auxiliary to give enantioenriched β-tetralones. DFT studies confirmed that the steric clash between the chiral auxiliary and alkene accounts for the experimentally observed diastereoselective cyclization through the Si face.     

Enantiopure Sulfinyl Aniline as a Removable and Recyclable Chiral Auxiliary for Asymmetric C(sp3)−H Bond Activation

An original and recyclable chiral bidentate aniline‐sulfoxide‐based directing group has been developed. This auxiliary allows challenging stereoselective Pd‐catalyzed direct functionalization of small cycloalkanes through C–aryl and C–alkyl bond formation. Although moderate diastereoselectivities are observed, both optically pure enantiomers of the highly functionalized products can be obtained separately by simple silica gel chromatography and cleavage of the chiral auxiliary. This strategy was further applied to the preparation of enantiomerically pure 1,2,3‐trisubstituted cyclopropane carboxylic acid derivatives, with three stereogenic centers and bearing both alkyl and aromatic substituents. These molecular scaffolds are not yet reported in the literature. The synthetic utility of this approach is validated by the chiral auxiliary being readily cleaved and recovered posteriori to the C?H activation step, without deterioration of its optical purity. Finally, an unprecedented palladacycle intermediate generated through C?H activation of the cyclopropane moiety has been isolated and fully characterized. Initial DFT calculations shed additional light on the reactivity of this original intermediate.     

Lewis‐Base‐Mediated Diastereoselective Silylations of Alcohols: Synthesis of Silicon‐Stereogenic Dialkoxysilanes Controlled by Chiral Aryl BINMOLs

In the past years, stereoselective functionalizations of hydroxyl groups of alcohol substrates with chlorosilanes leading to silyl ether formation have evolved from a functional‐group protection to an enantioselective synthetic strategy. This work comprises a controlled desymmetrization of dichlorosilanes by using a family of structurally specific chiral diols, chiral 1,1′‐binaphthalene‐2‐α‐arylmethanol‐2′‐ol (Ar‐BINMOL). This process led to the facile construction of silicon‐stereogenic organosilicon compounds with high yields and good diastereoselectivities. In addition, the diasteroselective silylation of chiral diols might not only be of interest for the development of highly stereoselective nucleophilic silylation, but also shed light on the construction of novel chiral phosphine ligands bearing a silicon‐stereogenic center.     

Regiodivergent reactions through catalytic enantioselective silylation of chiral diols. Synthesis of sapinofuranone A

Through the use of an amino acid based imidazole catalyst, a regiodivergent silylation of chiral diols in cases where there is not a significant steric and electronic difference between the regioisotopic hydroxyl groups has been developed. This transformation allows for the conversion of racemic diols into regioisomeric, enantiomerically enriched, monosilylated products. The utility of this process is highlighted in the efficient enantioselective preparation of a useful synthetic intermediate and the natural product, sapinofuranone A.     

Chiral tricyclic iminolactone derived from (1R)-(+)-camphor as a glycine equivalent for the asymmetric synthesis of alpha-amino acids

The development of a highly efficient and stereoselective methodology for the preparation of alpha-amino acids is described. The chiral template, tricyclic iminolactone 7, was synthesized from (1R)-(+)-camphor in five steps in 50% overall yield. Alkylation of iminolactone 7 afforded the alpha-monosubstituted products in good yields (74-96%) and excellent diastereoselectivities (>98%). Hydrolysis of the alkylated iminolactones furnished the desired alpha-amino acids in good yields and enantioselectivities with nearly quantitative recovery of the chiral auxiliary 4.     

Recent advances in the application of the Oppolzer camphorsultam as a chiral auxiliary

Oppolzer’s camphorsultam has attracted much attention as an efficient chiral auxiliary, and is one of the most powerful synthetic tools in asymmetric synthesis. The sultam chiral auxiliary can be applied in a variety of different reactions such as alkylations, allylations, 1,3-dipolar cycloadditions, cyclopropanation, reductions, Diels–Alder, aldol and ene reactions. These applications have been highly successful in the stereoselective construction of a number of important natural products via total synthesis. The present review is focused on the utility and versatility of the sultam in various asymmetric reactions.     

Efficient asymmetric synthesis of novel gastrin receptor antagonist AG-041R via highly stereoselective alkylation of oxindole enolates

An efficient method for asymmetric synthesis of the potent Gastrin/CCK-B receptor antagonist AG-041R was developed. Core oxindole stereochemistry was established by asymmetric alkylation of oxindole enolates with bromoacetic acid esters, using l-menthol as a chiral auxiliary. The key alkylation reaction of the oxindole enolates generated tetrasubstituted chiral intermediates with high diastereoselectivity. The stereoselective alkylation reactions are described in detail.     

Synthesis of PDE IVb inhibitors. 1. Asymmetric synthesis and stereochemical assignment of (+)- and (-)-7-[3-(cyclopentyloxy)-4-methoxyphenyl]hexahydro-3H-pyrrolizin-3-one

Asymmetric synthesis of GlaxoSmithKline's highly potent phosphodiesterase inhibitor 1 has been accomplished in nine steps and 16% overall yield. The original strategy suggested involves as a key step the silylation of enantiopure six-membered cyclic nitronates 4 obtained by a highly stereoselective [4 + 2]-cycloaddition of an appropriate nitroalkene 5 to trans-1-phenyl-2-(vinyloxy)cyclohexane. Functionalization of the resulting 5,6-dihydro-4H-1,2-oxazine and subsequent stereoselective reduction of 1,2-oxazine ring in intermediate 2 furnished the pyrrolizidinone framework with the recovery of chiral auxiliary alcohol.     

Stereoselective Preparation of β-Aryl-β-Boronyl Enoates and Their Copper-Catalyzed Enantioselective Conjugate Reduction

A new methodology has been developed for the stereoselective preparation of β-aryl-β-boronyl α,β-unsaturated esters via Heck coupling, and their subsequent copper(I)-catalyzed enantioselective conjugate reduction. Various chiral secondary boronate derivatives can be accessed in excellent yields and good to high levels of enantioselectivity through the efficient copper-catalyzed process using polymethylhydrosiloxane (PMHS) as the hydride source.