Enantioselective synthesis of cyclic enol ethers and all-carbon quaternary stereogenic centers through catalytic asymmetric ring-closing metathesis

The first examples of catalytic asymmetric ring-closing metathesis (ARCM) reactions of enol ethers are reported. To identify the most effective catalysts, various chiral Mo- and Ru-based catalysts were screened. Although chiral Ru catalysts (those that do not bear a phosphine ligand) promote ARCM in some cases, such transformations proceed in <10% ee. In contrast, Mo-based alkylidenes give rise to efficient ARCM and deliver the desired products in the optically enriched form. Thus, Mo-catalyzed enantioselective transformations allow access to various five- and six-membered cyclic enol ethers in up to 94% ee from readily available achiral starting materials. The first examples of catalytic ARCM that lead to the formation of all-carbon quaternary stereogenic centers are also disclosed. Mechanistic models that offer a plausible rationale for the identity of major enantiomers as well as the observed levels of enantioselectivity are provided. Representative examples demonstrate that the enol ether moiety and the unreacted alkene of the ARCM products can be discriminated with excellent site selectivity (>98%).     

Development of a practical mass spectrometry based assay for determining enantiomeric excess. A fast and convenient method for the optimization of PLE-catalyzed hydrolysis of prochiral disubstituted malonates

A practical mass spectrometry-based enantioselectivity assay is presented which makes use of enantiomerically enriched, but not enantiomerically pure, probe molecules readily obtained from esterase hydrolysis of prochiral malonates. The technique presented here allows us to recycle materials obtained from esterase hydrolysis which give substantial, but synthetically insufficient, enantiomeric excess as probe molecules in an enantioselectivity assay. The enantiomerically enriched products are esterified using deuterium-labelled alcohol. The enantiomeric excess is measured using mass spectrometry (LC–MS and LDI) by measuring the D₅/H₅ ratio in the resulting products obtained from an enzymatic hydrolysis. The D₅/H₅ ratio is corrected to account for the enantiomeric purity of the probe. Herein we report the results obtained from Pig Liver Esterase hydrolyses of prochiral malonate esters and outline the strengths and limitations of this approach to enantioselectivity determinations. This assay strategy was able to identify reaction conditions that led to an improvement in ee from 70% ee to >97% ee in the PLE-catalyzed hydrolysis of a prochiral malonate used to prepare unnatural serine analogues.     

Asymmetric synthesis of cyclic hydroxy ketones derived from enol ethers via sharpless asymmetric dihydroxylation. A study in the correlation of the enol ether chain length and enantioselectivity

The Sharpless asymmetric dihydroxylation reaction of enol ethers derived from their corresponding cyclic ketones, gave alpha-hydroxyketones with high enantioselectivity. The enantiomeric excess was found to be proportional to the length of the unbranched enol ether chain with a maximum ee for the pentyl enol ether. An efficient synthesis of alpha-hydroxy chromanone in >90% ee was demonstrated using this method.     

Z- and Enantioselective Ring-Opening/Cross-Metathesis with Enol Ethers Catalyzed by Stereogenic-at-Ru Carbenes: Reactivity, Selectivity, and Curtin-Hammett Kinetics

The first instances of Z- and enantioselective Ru-catalyzed olefin metathesis are presented. Ring-opening/cross-metathesis (ROCM) reactions of oxabicyclic alkenes and enol ethers and a phenyl vinyl sulfide are promoted by 0.5-5.0 mol % of enantiomerically pure stereogenic-at-Ru complexes with an aryloxy chelate tethered to the N-heterocyclic carbene. Products are formed efficiently and with exceptional enantioselectivity (>98:2 enantiomer ratio). Surprisingly, the enantioselective ROCM reactions proceed with high Z selectivity (up to 98% Z). Moreover, reactions proceed with the opposite sense of enantioselectivity versus aryl olefins, which afford E isomers exclusively. Preliminary DFT calculations in support of Curtin-Hammett kinetics as well as initial models that account for the stereoselectivity levels and trends are provided.     

Polymer-supported l-prolinol-based catalysts for the enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinyl)imines

l-Prolinol-based ligands anchored to Merrifield or Wang-type resins have been shown to form efficient catalysts for the enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinyl)imines. The enantioselectivity achieved with the polymeric catalyst (ee up to 88%) is slightly lower than the one obtained with the homogeneous ligand N-benzyl-l-prolinol, but the polymer-supported ligand presents the advantage of its recyclability: it can be recovered and used in up to six consecutive catalytic cycles with only a slight decrease in the enantiomeric excess. The phosphinamides obtained as addition products can be transformed into the corresponding enantiomerically enriched α-branched primary amines under mild acidic conditions.     

An easy access to (S)-pyrrolidinones and -pyrrolidines from chiral benzylic malonates

From chiral benzylic malonic acid esters (R)-(+)-4, available with high enantiomeric excesses by enzymatic hydrolysis (PLE acetonic powder), enantiomerically enriched pyrrolidinones 1 and pyrrolidines 2 were prepared. This rapid and competitive method was developed via enol ether formation, and subsequent one-pot cyclisation, in good overall yield.     

Catalytic and stoichiometric approaches to the desymmetrisation of centrosymmetric piperazines by enantioselective acylation: a total synthesis of dragmacidin A

The enantioselective desymmetrisation of centrosymmetric piperazines was investigated using both catalytic and stoichiometric asymmetric acylation approaches. The catalytic approach involved the desymmetrisation of 2,5-trans-dimethylpiperazine under the control of chiral DMAP analogues. With one equivalent of piperazine, relative to the acylating agent, low yields of products were obtained in up to 70% ee. It was shown that an inevitable 'proof reading' effect was occurring which increased the enantiomeric excess of the desymmetrised product through its kinetic resolution. The desymmetrisation of centrosymmetric piperazines with chiral acylating agents [(1R,2R)-N-formyl-1,2-bis(pentafluoro-benzenesulfonamido)cyclohexane and (1R,2R)-N-acetyl-1,2-bis(trifluoromethanesulfonamido)-cyclohexane] was also studied. The yield and enantioselectivity of the process was highly dependent on the solvent used and the substitution of the piperazine. However, in some cases, good yields of enantiomerically enriched products could be obtained (up to 87% based on the limiting chiral reagent) in good enantiomeric excesses (up to 84% ee). The approach was exploited in the total synthesis of Dragmacidin A.     

An auxiliary induced asymmetric synthesis of functionalized cyclobutanes by means of catalytic (2+2)-cycloaddition reaction

A new entry to optically active hydroxycyclobutanes is described. Treatment of silyl enol ether and (−)-8-phenylmenthyl acrylate in the presence of a catalytic amount of EtAlCl₂ affords enantiomerically enriched multi-substituted cyclobutane compounds in a good yield and diastereofacial selectivity.     

Catalytic Enantioselective Olefin Metathesis in Natural Product Synthesis. Chiral Metal‐Based Complexes that Deliver High Enantioselectivity and More

Chiral olefin metathesis catalysts enable chemists to access enantiomerically enriched small molecules with high efficiency; synthesis schemes involving such complexes can be substantially more concise than those that would involve enantiomerically pure substrates and achiral Mo alkylidenes or Ru‐based carbenes. The scope of research towards design and development of chiral catalysts is not limited to discovery of complexes that are merely the chiral versions of the related achiral variants. A chiral olefin metathesis catalyst, in addition to furnishing products of high enantiomeric purity, can offer levels of efficiency, product selectivity and/or olefin stereoselectivity that are unavailable through the achiral variants. Such positive attributes of chiral catalysts (whether utilized in racemic or enantiomerically enriched form) should be considered as general, applicable to other classes of transformations.     

Chiral Lewis acid catalyzed resolution of racemic enol ester epoxides. Conversion of both enantiomers of an enol ester epoxide to the same enantiomer of acyloxy ketone

This paper describes an efficient kinetic resolution of racemic enol ester epoxides via a chiral Lewis acid catalyzed rearrangement. Both enantiomerically enriched enol ester epoxides and alpha-acyloxy ketones can be obtained through this resolution. A positive nonlinear effect is observed in this process. By taking advantage of the mechanistic duality in acid-catalyzed enol ester epoxide rearrangement, we can completely convert a racemic enol ester epoxide into an enantiomerically enriched alpha-acyloxy ketone by treatment with a catalytic amount of a chiral Lewis acid followed by a catalytic amount of an achiral protic acid.     

Sequential catalytic isomerization and allylic substitution. Conversion of racemic branched allylic carbonates to enantioenriched allylic substitution products

A catalytic protocol for the conversion of readily accessible racemic, branched aromatic allylic esters to branched allylic amines, ethers, and alkyls has been developed. Palladium-catalyzed isomerization of branched allylic esters to terminal allylic esters, followed by sequential iridium-catalyzed allylic substitution, gave the branched allylic products in good yield with high regioisomeric and enantiomeric selectivity. Both electron-rich and electron-poor branched allylic esters gave products in >90% ee. High enantiomeric excesses were also observed for the products from the reactions of 2-thienyl acetates and dienyl carbonates.     

Asymmetric Substitutions of 2‐Lithiated N‐Boc‐piperidine and N‐Boc‐azepine by Dynamic Resolution

Proton abstraction of N‐tert‐butoxycarbonyl‐piperidine (N‐Boc‐piperidine) with sBuLi and TMEDA provides a racemic organolithium that can be resolved using a chiral ligand. The enantiomeric organolithiums can interconvert so that a dynamic resolution occurs. Two mechanisms for promoting enantioselectivity in the products are possible. Slow addition of an electrophile such as trimethylsilyl chloride allows dynamic resolution under kinetic control (DKR). This process occurs with high enantioselectivity and is successful by catalysis with substoichiometric chiral ligand (catalytic dynamic kinetic resolution). Alternatively, the two enantiomers of this organolithium can be resolved under thermodynamic control with good enantioselectivity (dynamic thermodynamic resolution, DTR). The best ligands found are based on chiral diamino‐alkoxides. Using DTR, a variety of electrophiles can be used to provide an asymmetric synthesis of enantiomerically enriched 2‐substituted piperidines, including (after Boc deprotection) the alkaloid (+)‐β‐conhydrine. The chemistry was extended, albeit with lower yields, to the corresponding 2‐substituted seven‐membered azepine ring derivatives.     

Catalytic Asymmetric Propionate Aldol Reactions via Acyl Halide-Aldehyde Cyclocondensations

Catalyzed asymmetric acyl halide-aldehyde cyclocondensation (AAC) reactions afford highly enantiomerically enriched 3,4-disubstituted-2-oxetanones. These reactions constitute catalytic asymmetric propionate aldol additions. An optically active Al(III)-triamine complex (10 mol %) catalyzes the di(isopropyl)ethylamine-mediated cyclocondensation of propionyl bromide and a variety of aldehydes to afford beta-lactone adducts with uniformly high enantioselection (90 --> 98% ee), diastereoselection (74 --> 98% de), and chemical yields (78-90%). Lactone ring opening reveals that the enantiomerically enriched beta-lactones act as surrogates for syn propionate aldol adducts.     

Enantioselective Diels-Alder approach to C-3-oxygenated angucyclinones from (SS)-2-(p-tolysulfinyl)-1,4-naphthoquinone

Chiral racemic vinylcyclohexenes 2, bearing oxygenated substituents and/or a methyl group at the C-5 position of the cyclohexene ring, were submitted to Diels-Alder reactions with enantiomerically pure (SS)-(2-p-tolylsulfinyl)-1,4-naphthoquinone [(+)-1]. The domino cycloaddition/pyrolytic sulfoxide elimination process led to the formation of enantiomerically enriched angularly tetracyclic quinones anti-6 and syn-7, which were obtained from the kinetic resolution of the racemic diene. In all cases, (SS)-(2-p-tolylsulfinyl)-1,4-naphthoquinone reacted from the less hindered face of the more reactive s-cis conformation, to form products in good enantiomeric excesses. Steric effects and torsional interactions in the corresponding approaches account for the observed pi-facial diastereoselectivities at both partners. The usefulness of this methodology is illustrated with the four-step totally asymmetric synthesis of the C-3-oxygenated angucyclinone derivative (-)-8-deoxytetrangomycin 10 in 26% overall yield and with 50% enantiomeric purity.     

Enantioselective synthesis of 2-alkylidenetetrahydrofurans based on a ‘cyclization/enzymatic resolution’ strategy

Enantiomerically pure 2-alkylidenetetrahydrofurans have been prepared by TiCl₄ mediated enantiospecific reactions of 1,3-bis-silyl enol ethers with enantiomerically pure epichlorohydrin. In addition, the enzymatic kinetic resolution of 2-alkylidenetetrahydrofurans, using Candida antarctica lipase B (CAL-B), was studied. Enzymatic kinetic resolution of monocyclic 5-vinyl-2-alkylidenetetrahydrofuran with CAL-B afforded the enantiomerically pure ester with 97% ee. For a bicyclic 2-alkylidenetetrahydrofuran, this proceeded with excellent enantioselectivity (E >100) affording the enantiomerically pure acid with 98% ee. 2-Alkylidenetetrahydrofurans were prepared by [3+2] cyclization reactions of 1,3-dicarbonyl dianions (‘free dianions’) or 1,3-bis-silyl enol ethers (‘masked dianions’).     

Mechanism of the α,α-diarylprolinol trimethylsilyl ether-catalyzed enantioselective C-C, C-N, C-F, C-S, and C-Br bond forming reactions

Theoretical calculations were employed to investigate the enantioselectivity of the α,α-diarylprolinol trimethylsilyl ether-catalyzed α-functionalization of aldehydes with various different electrophiles, via an enol intermediate. The reactions investigated were (i) Michael-aldol condensation, (ii) Michael addition, (iii) Mannich reaction, (iv) α-amination of an aldehyde, (v) α-fluorination of an aldehyde, (vi) α-sulfenylation of an aldehyde, and (vii) α-bromination of an aldehyde. In all seven cases, our proposed enol mechanism is able to account for the experimentally observed enantioselectivity of the products. Our calculations strongly suggest that these catalyzed reactions proceed via an enol intermediate and not via an enamine intermediate.     

Asymmetric epoxidation of (Z)-enol esters catalyzed by titanium(salalen) complex with aqueous hydrogen peroxide

Titanium(salalen) complex 1 was an effective catalyst for asymmetric epoxidation of enol esters. Although (E)-enol esters were reluctant to proceed, (Z)-enol esters underwent asymmetric epoxidation to give the epoxides in high yields with high enantioselectivity ranging from 86 to >99% ee in the presence of aqueous hydrogen peroxide as the stoichiometric oxidant. Complete enantioselectivity was observed in the reaction of (Z)-3,3-dimethylbut-1-en-1-yl 4-methoxybenzoate. The obtained epoxide was readily transformed into the corresponding 1,2-diol by reduction with lithium borohydride without erosion of the high enantiomeric excess.     

Gold-catalyzed multicomponent synthesis of aminoindolizines from aldehydes, amines, and alkynes under solvent-free conditions or in water

A gold(III)-catalyzed multicomponent coupling/cycloisomerization reaction of heteroaryl aldehydes, amines, and alkynes under solvent-free conditions or in water has been developed. This methodology provides rapid access to substituted aminoindolizines with high atom economy and high catalytic efficiency. Especially, the coupling of enantiomerically enriched amino acid derivatives produces the corresponding N-indolizine-incorporated amino acid derivatives without loss of enantiomeric purity.     

Aluminum-catalyzed asymmetric alkylations of pyridyl-substituted alkynyl ketones with dialkylzinc reagents

Alkylations of pyridyl-substituted ynones with Et2Zn and Me2Zn, promoted by amino acid-based chiral ligands in the presence of Al-based alkoxides, afford tertiary propargyl alcohols efficiently in 57% to >98% ee. Two easily accessible chiral ligands are identified as optimal for reactions of the two dialkylzinc reagents. Catalytic alkylations with Et2Zn require a chiral ligand carrying two amino acid moieties (valine and phenylalanine) along with a p-trifluoromethylphenylamide C-terminus. In contrast, reactions with Me2Zn are most effectively promoted in the presence of a chiral ligand containing a single amino acid (benzyl cysteine), capped by an n-butylamide. Enantiomerically enriched tertiary alcohols bearing a pyridyl and an alkyne substituent can be functionalized in a variety of manners to furnish a wide range of difficult-to-access acyclic and heterocyclic structures; two noteworthy examples are Cu-catalyzed protocols for conversion of tertiary propargyl alcohols to enantiomerically enriched tetrasubstituted allenes and bicyclic amides that bear an N-substituted quaternary carbon stereogenic center. Mechanistic models that account for the trends and enantioselectivity levels are provided.     

Stereoselective synthesis of substituted 1,3-oxazolidines via Pd-catalyzed carboamination reactions of O-vinyl-1,2-amino alcohols

The stereoselective synthesis of 2,4- and 2,5-disubstituted 1,3-oxazolidines is accomplished via Pd-catalyzed carboamination of O-vinyl-1,2-amino alcohol derivatives. The transformations generate cis-disubstituted products with good to excellent diastereoselectivity, and enantiomerically enriched substrates are converted without loss of optical purity. In addition to yielding synthetically useful products that are difficult to generate with existing methods, these transformations illustrate that electron-rich enol ethers are viable substrates for alkene carboamination processes.