Enantioselective synthesis of alpha-substituted ketones by asymmetric addition of chiral zinc enamides to 1-alkenes

A bifunctional squaramide-catalysed enantioselective vinylogous Michael addition/cyclization cascade reaction of 4-unsaturated isoxazol-5-ones and α,α-dicyanoalkenes

An effective strategy for the stereoselective synthesis of spiro isoxazolone-cyclohexenimines was developed using a bifunctional squaramide-catalysed vinylogous Michael addition/cyclization cascade reaction of 4-unsaturated isoxazol-5-ones and α,α-dicyanoalkenes. The atom-economical cascade process can proceed smoothly under extremely low catalyst loading (1 mol%) and mild conditions, and the corresponding products were obtained in moderate to good yields (45% to 90%) and enantioselectivitites (51% to 96% ee). Meanwhile, the scale-up reaction and transformation of the products were also demonstrated.

Formation of α-chiral centers by asymmetric β-C(sp3)-H arylation, alkenylation, and alkynylation

The enzymatic β-C-H hydroxylation of the feedstock chemical isobutyric acid has enabled the asymmetric synthesis of a wide variety of polyketides. The analogous transition metal-catalyzed enantioselective β-C-H functionalization of isobutyric acid-derived substrates should provide a versatile method for constructing useful building blocks with enantioenriched α-chiral centers from this abundant C-4 skeleton. However, the desymmetrization of ubiquitous isopropyl moieties by organometallic catalysts has remained an unanswered challenge. Herein, we report the design of chiral mono-protected aminomethyl oxazoline ligands that enable desymmetrization of isopropyl groups via palladium insertion into the C(sp³)-H bonds of one of the prochiral methyl groups. We detail the enantioselective β-arylation, -alkenylation, and -alkynylation of isobutyric acid/2-aminoisobutyric acid derivatives, which may serve as a platform for the construction of α-chiral centers.

Construction of Furan Derivatives with a Trifluoromethyl Stereogenic Center: Enantioselective Friedel-Crafts Alkylations via Formal Trienamine Catalysis

An asymmetric Friedel-Crafts alkylation reaction of 2-furfuryl ketones with β-trifluoromethyl enones has been developed via formal trienamine catalysis of a bifunctional primary amine-thiourea substance derived from L-tert-leucine, delivering the furan derivatives incorporating a stereogenic trifluoromethyl (CF₃) group in good to high yields with excellent enantioselectivity.

Advances in tandem reactions with organozinc reagents

A review of recent advances in tandem reactions with organozinc reagents that highlight their utility in organic synthesis.

Carbolithiation of Styrenes with N-tert-butyl aldimines

The addition of aldimine anions to styrene occurs upon the treatment of N-tert-butyl aldimine with LDA followed by the addition of styrene to give the corresponding aldehyde after hydrolysis. Cyclohexenyl amines are generated when the reactions are performed with unsaturated aldimines. This approach affords 4-phenyl substituted butanals and cyclohexenyl amines in a simple and inexpensive way.

Copper-Catalyzed Enantioselective Intramolecular Conjugate Addition/Trapping Reactions: Synthesis of Cyclic Compounds with Multichiral Centers

The Cu-catalyzed enantioselective conjugate addition of dialkylzinc to bis-alpha,beta-unsaturated carbonyl compounds followed by the intramolecular trapping of the zinc enolate in the presence of chiral phosphoramidite ligands was investigated. Cyclic and heterocyclic compounds with multichiral centers were formed as a mixture of two diastereomers with excellent diastereoselectivities (up to 99:1) and enantioselectivities (up to 94 % ee, ee=enantiomeric excess). The stereochemistry was determined to be trans,trans for the major products and trans,cis for the minor products. The absolute configuration of the cyclic compounds was assigned by comparison with the analogous adduct derived from trans-3-nonen-2-one and Et(2)Zn or the adduct obtained through conjugate addition of Me(2)Zn to trans-1-phenyl-non-2-en-1-one. Further transformation of these cyclic products into more complex compounds is under investigation in our laboratory.

Chiral amino alcohol-mediated asymmetric conjugate addition of arylalkynes to nitroolefins

[reaction: see text] The asymmetric reaction of nitroolefins with arylalkynes was mediated by dimethylzinc (or diethylzinc) and (1R,2R)-2-(dimethylamino)-1,2-diphenylethanol in toluene to provide the corresponding conjugate alkynylation products with high enantiomeric excess of up to 99% in good yields. The presence of 0.03 equiv of galvinoxyl improved the reaction yield.

Rhenium-Catalyzed Insertion of Terminal Acetylenes into a C−H Bond of Active Methylene Compounds

[reaction: see text] A rhenium complex, [ReBr(CO)(3)(thf)](2), catalyzed the intermolecular reactions of 1,3-dicarbonyl compounds with terminal acetylenes and gave the corresponding alkenyl derivatives in excellent yields. These reactions could apply to an intramolecular version and gave the corresponding cyclic compounds quantitatively.

Chiral Ligands Derived from Abrine 8. An Experimental and Theoretical Study of Free Ligand Conformational Preferences and the Addition of Diethylzinc to Benzaldehyde

Three structurally similar series of 1,2,3,4-tetrahydro-beta-carboline ligands, 4a-d, 6a-d and 7a-d, and two series of chiral oxazolidines, 8a-d and 9a-g, were synthesized and used as chiral catalysts in the addition of diethylzinc to benzaldehyde. The enantioselectivities of the resulting 1-phenyl-1-propanol were obtained in each case, and these ee values were, in most cases, related to the conformational populations of the free ligand as expressed by the calculated differences in the energies of the ligand conformations formed by inversion at nitrogen. This suggested the possible existence of a linear free energy relationship. The effect on enantioselectivity of the carbon chain length of the R group located (1) on the C-3 substituent of 4a-d, 6a-d, and 7a-d or (2) at C-5 in 8a-d and 9a-g was studied in detail. On the basis of the correlations observed and the ligands' structural characterization, a structure was proposed for the transition state during ethyl group transfer when using ligands 8a-d. Furthermore, the change in enantioselectivity was successfully predicted when diastereomeric ligands 11 and 12 were compared in this chiral addition.

Lithium Diisopropylamide-Mediated Lithiations of Imines: Insights into Highly Structure-Dependent Rates and Selectivities

Lithium diisopropylamide-mediated lithiations of N-alkyl ketimines derived from cyclohexanones reveal that simple substitutions on the N-alkyl side chain and the 2-position of the cyclohexyl moiety afford a 60,000-fold range of rates. Detailed rate studies implicate monosolvated monomers at the rate-limiting transition structures in all instances. Comparisons of experimentally derived regioselectivities and rates, taken in conjunction with density functional theory computational studies, reveal a number of factors that influence reactivities including: (a) axial versus equatorial disposition of the proton on the cyclohexane ring, (b) syn versus anti orientation of the lithiation relative to the N-alkyl group, (c) the presence or absence of a potentially chelating methoxy moiety on the N-alkyl group, (d) the presence of a 2-methyl substituent at the geminal or distal alpha-carbon, and (e) branching in the N-alkyl group. The isolated contributions are not large, yet they display a strong and predictable additivity leading to a kinetic resolution of imines derived from racemic 2-methylcyclohexanone.