Asymmetric synthesis of fluorine-containing amines, amino alcohols, α- and β-amino acids mediated by chiral sulfinyl group

This review article provides a critical overview of several different synthetic approaches developed for asymmetric preparation of fluorine-containing amines, amino alcohols, α- and β-amino acids. The common feature of these methods is the application of sulfinyl group as a chiral auxiliary to control the stereochemical outcome of the reactions under study. In particular, the following general methods are critically discussed: diastereoselective methylene transfer from diazomethane to the carbonyl of β-keto-γ-fluoroalkyl sulfoxides as a general approach for preparation of various α-fluoroalkyl α-sulfinylalkyl oxiranes. The resulting compounds were used as true chiral synthons for their further elaboration via oxidative or reductive desulfurization, to numerous fluorine-containing and biologically relevant amino- and hydroxy-containing derivatives. Another general approaches discussed here are asymmetric additions to CN double bond. One of them is addition of chiral sulfoxide stabilized carbon nucleophiles to fluorine-containing imines, leading to convenient preparation of alpha-fluoroalkyl derivatives of alpha amino acids and amines. Another approach is asymmetric Reformatsky reaction between N-sulfinyl imines and ethyl bromodifluoroacetate allowing operationally convenient preparation of α,α-difluoro-β-amino acids in enantiomerically pure form. Finally, structurally similar but mechanistically different addition reactions of diethyl difluoromethylphosphonate to N-sulfinyl imines, as a general approach to asymmetric synthesis of α,α-difluoro-β-aminophosphonates and phosphonic acids, are discussed. Effect of fluorine on the mechanism and stereochemical outcome of these reactions is discussed in detail and compared, where it is possible, with that of the analogous reactions of fluorine-free substrates.     

syn-π-Face- and endo-selective, inverse electron-demand Diels-Alder reactions of 3,4-di-tert-butylthiophene 1-oxide with electron-rich dienophiles

Diels-Alder reactions of 3,4-di-tert-butylthiophene 1-oxide with oxygen (or sulfur)-substituted dienophiles and with simple alkenic dienophiles, which are classified as an inverse electron-demand Diels-Alder reaction on the basis of DFT calculations, took place exclusively at the syn-π-face of the diene with respect to the SO bond to provide the corresponding adducts in high yields.     

Synthesis of spiro[bicyclo[2.2.1]heptane-2,2′-furan]-3-amines via stereoselective cycloadditions of trimethylenemethane to (1S,3EZ,4R)-3-arylimino-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones

Stereoselective [3+2] cycloadditions of trimethylenemethane (TMM) to the exocyclic CO and CN double bonds of (1S,3EZ,4R)-3-arylimino-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones gave the corresponding spiro[bicyclo[2.2.1]heptane-2,2′-furan] and spiro[bicyclo[2.2.1]heptane-3,2′-pyrrolidine] derivatives. Further stereoselective reductions of the CN or CO bond in these cycloadducts furnished new chiral amines, diamines, and a new aminoalcohol. All cycloadditions and reductions of the CN double bonds took place from the less hindered endo-face of the (1S,3EZ,4R)-3-arylimino-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ones, exclusively, thus giving the corresponding products in 100% de. The structures were determined by NMR, NOESY spectroscopy, and by X-ray diffraction.     

Synthesis of optically active β-keto-γ-butyrolactones having a trifluoromethyl group

A number of chiral β-keto-γ-butyrolactones and tetronic acids bearing a trifluoromethyl group on an asymmetric carbon were prepared by ultrasound-promoted Reformatsky-type reactions between an optically active O-trimethylsilylated cyanohydrin of trifluoroacetaldehyde and ethyl α-substituted bromoacetates.     

Carbon-carbon bond formation by radical addition of α-trifluoromethylacrylate with cyclic ethers

The radical addition reactivity of tert-butyl α-trifluoromethylacrylate (CH₂C(CF₃)COOC(CH₃)₃) (BFMA) with cyclic ethers was investigated in order to compare to that of perfluoroisopropenyl ester. One to one addition compound of BFMA with tetrahydrofuran (THF) was produced in fairy high yields in the presence of 2,2′-azobisisobutyronitrile, benzoyl peroxide or di-tert-butyl peroxide to give 2-substituted THF derivative. Time-conversion investigation showed much higher reactivity of BFMA compared to that of 2-benzoxypentafluoropropene [CF₂C(CF₃)OCOC₆H₅]. Radical additions of BFMA with 1,4-dioxane, 1,3-dioxolane and tetrahydropyran were also examined to afford corresponding 1:1 addition products in fairly high yields by achieving carbon-carbon bond formation. It is then concluded that no interconversion of fluoroalkylcarbon radical and hydrocarbon radical may take place in the reaction system of BFMA which possesses two less fluorines in the vinyl group compared to 2-benzoxypentafluoropropene. The method may be a facile way to prepare trifluoromethyl-substituted organic compounds accompanied by the formation of carbon-carbon bonds by the aid of fluorine atoms.     

Asymmetric catalytic aziridination of dihydronaphthalenes for the preparation of substituted 2-aminotetralins

An enantioselective synthesis of substituted 2-aminotetralins from dihydronaphthalenes in four steps is described. The key step is the Jacobsen’s (diimine)copper-catalyzed asymmetric aziridination of dihydronaphthalenes to the respective aziridines in 33-82% yields and 60-87% enantiomeric excess. The enantioselectivity and the yield were dependent on the properties of the nitrene precursor. pTsNIPh appeared in general to give better results than pNsNIPh. Aziridines were ring-opened in the benzylic position by catalytic hydrogenolysis in quantitative yields, and deprotected in two steps to the respective 2-aminotetralins in 66-85% yields. The synthesis of (S)-2-aminotetralin (>98% ee) and (S)-2-amino-7-methoxytetralin (56% ee) were accomplished in 30 and 52% overall yields, respectively.     

Polyfluoroaromatic compounds with a NCClR group—a reactive type of polyfluoroarene

This review deals with syntheses and reactivity of polyfluoroarenes with a NCClR group (including those with RCl). The most promising and convenient methods of synthesis of these type compounds are based on co-pyrolysis reactions of low-basic polyfluoroaromatic amines with RCCl₃ type derivatives and the interaction of such amines and polyfluoroaromatic hydrazines with RCX₃ (XCl,F) compounds in the presence of AlCl₃ at moderate temperature; they have no analogues in the nonfluorinated series. Reactions with different nucleophilic reagents, the formation of electrophilic intermediates of the nitrilium cation type and reactions of the latter with aromatic and fluoroaromatic hydrocarbons, amines, and compounds with CN, C≡N and CO multiple bonds are described. It is shown that polyfluoroaromatic compounds with a NCClR group are promising precursors for syntheses of a wide range of compounds of various classes, including heterocyclic derivatives.     

Synthesis of a chiral phosphonium salt for the preparation of α-substituted alaninol derivatives

We herein report the synthesis of a chiral phosphonium salt, {[(4S)-4-methyl-2-oxo-1,3-oxazolidin-4-yl]methyl}(triphenyl)phosphonium iodide 13 to provide a new Wittig reagent for the general method of synthesizing α-substituted alaninol derivatives. Our method described here is widely applicable to reactions with various types of aldehyde to afford olefin products with high E-selectivity, enabling us to provide a new approach to the synthesis of chiral S1P₁ agonists including our key intermediates, and of the trace amine-associated receptor 1 (TAAR1) agonist.     

Nickel-Catalyzed Kinetic Resolution of Racemic Unactivated Alkenes via Enantio-, Diastereo-, and Regioselective Hydroamination

Kinetic resolution is a powerful strategy for the isolation of enantioenriched compounds from racemic mixtures, and the development of selective catalytic processes is an active area of research. Here, we present a nickel-catalyzed kinetic resolution of racemic α-substituted unconjugated carbonyl alkenes via the enantio-, diastereo-, and regioselective hydroamination. This protocol affords both chiral α-substituted butenamides and syn-β^2,3-amino acid derivatives with high enantiomeric purity (up to 99 % ee) and selectivity factor up to >684. The key to the excellent kinetic resolution efficiency is the distinctive architecture of the chiral nickel complex, which enables successful resolution and enantioselective C−N bond construction. Mechanistic investigations reveal that the unique structure of the chiral ligand facilitates a rapid migratory insertion step with one enantiomer. This strategy provides a practical and versatile approach to prepare a wide range of chiral compounds.     

A facile direct conversion of aldehydes to esters and amides using acetone cyanohydrin

Aromatic aldehydes with electron-withdrawing groups undergo rapid reactions with a variety of alcohols and secondary amines to afford the corresponding esters and amides, respectively, in high yields, when treated with NaCN or acetone cyanohydrin and base under ambient reaction conditions. In case of α,β-unsaturated aldehydes, simultaneous reduction of the CC bond along with esterification occurred to produce the saturated esters in high yields.     

Facile and clean synthesis of α-alkenoyl ketene-(S,S)-acetals via the aldol condensation reactions in water

The aldol condensation reactions of α,α-diacetyl ketene-(S,S)-acetals, 1a and 1b, with aromatic aldehydes 2 in the presence of NaOH in water have been investigated. At room temperature, the base-mediated condensations proceed smoothly to afford the corresponding mono-condensed products, α-alkenoyl ketene-(S,S)-acetals 3, in high yields. At reflux temperature, the reactions produce high yields of symmetric double condensed α,α-dialkenoyl ketene-(S,S)-acetals 4 when 2 equiv of aldehyde are employed. Moreover, compounds 3 can condense with a different aldehyde to give unsymmetric double condensed α,α-dialkenoyl ketene-(S,S)-acetals 5.     

Novel synthetic route to 1,4-dihydropyridines from β-amino acrylates by using titanium(IV) chloride under facile conditions

The reactions of Boc-β-amido- and β-amino acrylates, in which the CC possesses both nucleophilic and electrophilic sites, were investigated under acidic conditions. The trifluoroacetic-acid induced cyclization of the β-amido acrylates to the corresponding oxazolidin-2-ones involves a rarely seen nucleophilic attack of the carbamate carbonyl group. The cyclotrimerization of β-amino acrylates to N-substituted 1,4-dihydropyridines was observed in the presence of a Lewis acid. High yields of 1,4-dihydropyridines (70–83%) were readily obtained by using substoichiometric amount TiCl₄ under mild condition. The cyclotrimerization is presumably occurring via a Hantzsch related mechanism involving three addition/elimination reactions of the amphiphilically reactive CC.     

Enantioselective Reaction of 2H‐Azirines

2H‐Azirines are useful precursors for the synthesis of a variety of chiral aziridine and amine derivatives with a range of biological activities. Owing to the ring strain and the presence of a C=N double bond, 2H‐azirines are more reactive than other types of ketimine, and undergo a range of enantioselective reactions, including reduction and Diels–Alder reactions, as well as nucleophilic addition to the C=N double bond. Therefore, the enantioselective reactions of 2H‐azirines has become a hot topic, in particular within the last few years. In this Minireview, we focus on the enantioselective reactions of 2H‐azirines by using catalytic or stoichiometric amounts of chiral additives, the reaction mechanisms, and the applications of these reactions of 2H‐azirines and related compounds in organic synthesis.     

{(COD)Ru[RNC(H)-C(H)C(Ph)]2} (RMe, Et): The first structurally characterized mononuclear ruthenium complexes with enyl-imino ligands and their relevance in ruthenium catalyzed C-H activation reactions

The reaction of α,β-unsaturated imines with [(1,5-cyclooctadiene)-bis(2-methylallyl)-ruthenium(II)] leads to the formation of mononuclear ruthenium complexes of the general formula {(COD)Ru[RNC(H)-C(H)C(Ph)]₂}. In these complexes the imine ligands are deprotonated in β-position with respect to the imine double bond and coordinate as an enyl-imino ligand. In the case of R = Me, Et the corresponding compounds have been characterized by X-ray crystallography. The relevance of these complexes with respect to ruthenium catalyzed C-C coupling reactions of the same α,β-unsaturated imines is demonstrated by the structural analysis of another mononuclear ruthenium complex in which two imine ligands are reductively coupled (R = Cy). [(1,5-Cyclooctadiene)-bis(2-methylallyl)-ruthenium(II)] also turns out to be a highly effective precatalyst in the reaction of the respective imines with carbon monoxide and ethylene to produce heterocyclic compounds.     

Catalyst-Controlled Regiodivergent and Enantioselective Formal Hydroamination of N,N-Disubstituted Acrylamides to α-Tertiary-α-Aminolactam and β-Aminoamide Derivatives

Enantioenriched α-tertiary-α-aminoacid and α-chiral-β-aminoacid derivatives play an important role in biological science and pharmaceutical chemistry. Thus, the development of methods for their synthesis is highly valuable and yet remains challenging. Herein, an unprecedented catalyst-controlled regiodivergent and enantioselective formal hydroamination of N,N-disubstituted acrylamides with aminating agents has been developed, accessing enantioenriched α-tertiary-α-aminolactam and α-chiral-β-aminoamide derivatives. Sterically-disfavored and electronically-disfavored enantioselective hydroamination of electron-deficient alkenes have been successfully tuned using different transition metals and chiral ligands. Notably, extremely hindered aliphatic α-tertiary-α-aminolactam derivatives were synthesized by Cu−H catalyzed asymmetric C−N bond forming with tertiary alkyl species. Enantioenriched α-chiral-β-aminoamide derivatives have been accessed by Ni−H catalyzed anti-Markovnikov-selective formal hydroaminations of alkenes. This set of reactions tolerates a wide range of functional groups to deliver diverse α-tertiary-α-aminolactam and α-chiral-β-aminoamide derivatives in good yields with high levels of enantioselectivity.     

Synthesis of substituted pyrrolidines and piperidines from endocyclic enamine derivatives. Synthesis of (±)-laburnamine

Functionalization of the α- and β-positions of readily available endocyclic enamine derivatives provides a convenient method for the formation of substituted pyrrolidines and piperidines. α-Alkoxy-β-iodopyrrolidines are formed by the electrophilic addition of iodine to the endocyclic enamine double bond of an N-substituted 2-pyrroline, and nucleophillic attack by an alcohol on the intermediate iodonium ion. The resultant α-alkoxy-β-iodopyrrolidines can be used in radical cyclization reactions to give bicyclic hemiaminal compounds, which can be further elaborated using N-acyliminium chemistry to form α,β-cis-dialkylsubstituted pyrrolidines. A strategy for the incorporation of amino functionality at the β-position was also established by using iodoamination of the enamine double bond, followed by migration of the amine functionality through an aziridination/methanolysis protocol. An alternative method uses an azidomethoxylation protocol using ceric ammonium nitrate (CAN) in the presence of NaN₃ and methanol. Formation and trapping of the N-acyliminium ions derived from these substrates, afforded the 3-carbamate and 3-azido-2-substituted products with good diastereoselectivity, with the preferential formation of the trans and cis stereoisomers, respectively. Using the sequential iodoamination, aziridination in methanol and N-acyliminium transformation, trans-3-NHCO₂Me-2-allyl-pyrrolidine was prepared, which was used as the key precursor in a synthesis of the natural 1-amidopyrrolizidine alkaloid, (±)-laburnamine.     

Rh(II)-Cp-TsDPEN catalyzed aqueous asymmetric transfer hydrogenation of chromenones into saturated alcohol: CC and CO reduction in one step

As an efficient catalyst for asymmetric transfer hydrogenation reaction (ATH reaction) of α,β-unsaturated ketones, Rh-Cp^∗-TsDPEN (Cp^∗ = 1,2,3,4,5-pentamethylcyclopenta-1,3-diene, TsDPEN = N-(p-toluenesulfonyl)-1,2-diphenyl- ethylenediamine) shows high chemoselectivity on CO and CC reduction. In our method, both CO and CC bonds in a variety of chromenone derivatives were reduced efficiently in aqueous media, resulting in at least 98% ee and up to 99% yields in a convenient way without further purification. The product was a useful intermediate for deriving chiral chroman-4-amine, which was reported as an effective agent against hypotension and inflammatory pain by inhibiting human bradykinin B1 receptor.     

4,5-Dihydroisoxazole and 4,5-dihydro-1,2,4-oxadiazole derivatives from cycloaddition reactions of nitrile oxides to alkyl N-(diphenylmethylene)-α,β-dehydroamino acids

The alkyl N-(diphenylmethylene)-α,β-dehydroamino acids 1 have been submitted to 1,3-dipolar cycloadditions with nitrile oxides. The reactivity of these compounds depends on the type and on the stereochemistry of the β-substituents. With the unsubstituted terms 1a,b the reaction occurs on the C,C double bond, providing a good method for the synthesis of the 4,5-dihydroisoxazole derivatives 3a,b,c and for the gem-functionalized 4,5-dihydroisoxazoles amino carboxylic ester 5. The β-substituted compounds 1c,d,e , inert to 1,1-dimethylethylnitrile oxide, undergo the reaction to the N,C double bond, thus giving with 2a,b the 4,5-dihydro-1,2,4-oxadiazole derivatives 4. All the reactions occur with high site- and regioselectivity.     

Alkyl 2-(2-benzothiazolylsulfinyl)acetates as useful synthetic reagents for alkyl 4-hydroxyalk-2-enoates by sulfinyl-Knoevenagel reaction

Isopropyl, ethyl, and methyl 2-(2-benzothiazolylsulfinyl)acetates have been found to be useful synthetic reagents for sulfinyl-Knoevenagel reaction with various aldehydes to give directly the corresponding 4-hydroxyalk-2-enoates [R′CH(OH)CHCHCO₂R], which are ubiquitous structures in biologically active natural products and useful building blocks for organic synthesis of chiral compounds. From the optically pure (R)-2-(2-benzothiazolylsulfinyl)acetates (>99% ee) prepared by the enzymatic kinetic resolution of (±)-2-(2-benzothiazolylsulfinyl)acetates, optically active 4-hydroxyalk-2-enoates (up to 91% ee) have been obtained in good yields.