Direct asymmetric amination of α-branched cyclic ketones catalyzed by a chiral phosphoric acid

Here we report the direct asymmetric amination of α-substituted cyclic ketones catalyzed by a chiral phosphoric acid, yielding products with a N-containing quaternary stereocenter in high yields and excellent enantioselectivities. Kinetic resolution of the starting ketone was also found to occur on some of the substrates under milder conditions, providing enantioenriched α-branched ketones, another important building block in organic synthesis. The utility of this methodology was demonstrated in the short synthesis of (S)-ketamine, the more active enantiomer of this versatile pharmaceutical.

Benzothiazoline: versatile hydrogen donor for organocatalytic transfer hydrogenation

CONSPECTUS: The asymmetric reduction of ketimines is an important method for the preparation of amines in optically pure form. Inspired by the biological system using NAD(P)H, Hantzsch ester has been extensively employed as a hydrogen donor in combination with chiral phosphoric acid for the transfer hydrogenation of ketimines to furnish amines with high to excellent enantioselectivities. We focused on 2-substituted benzothiazoline as a hydrogen donor in the phosphoric acid catalyzed transfer hydrogenation reaction of ketimines for the following reasons: (1) benzothiazoline is readily prepared just by mixing 2-aminobenzenethiol and aldehyde, (2) both reactivity (hydrogen donating ability) and enantioselectivity would be controlled by tuning the 2-substituent of benzothiazoline, and (3) benzothiazoline can be stored in a refrigerator under inert atmosphere without conceivable decomposition. Both the 2-position of benzothiazoline and the 3,3'-position of phosphoric acid are tunable in order to achieve excellent enantioselectivity. Benzothiazoline proved to be useful hydrogen donor in combination with chiral phosphoric acid for the transfer hydrogenation reaction of ketimine derivatives to afford the corresponding amines with high to excellent enantioselectivities by tuning the 2-substituent of benzothiazoline. Ketimines derived from acetophenone, propiophenone, α-keto ester, trifluoromethyl ketone, and difluoromethyl ketone derivatives proved to be suitable substrates. Benzothiazoline could be generated in situ starting from 2-aminobenzenethiol and aromatic aldehyde in the presence of ketimine and chiral phosphoric acid and successfully worked in the sequential transfer hydrogenation reaction. The reductive amination of dialkyl ketones also proceeded with high enantioselectivities. Use of 2-deuterated benzothiazoline led to the formation of α-deuterated amines with excellent enantioselectivities. The kinetic isotope effect (kH/kC = 3.8) was observed in the competitive reaction between H- and D-benzothiazoline, which explicitly implies that the cleavage of the C-H (C-D) bond is the rate-determining step in the transfer hydrogenation reaction. Benzothiazoline yielded products with higher enantioselectivity in the transfer hydrogenation reaction of ketimines, particularly ketimines derived from propiophenone derivatives, than Hantzsch ester. DFT study elucidated the mechanism, as well as the difference in selectivity, between benzothiazoline and Hantzsch ester. The chiral phosphoric acid activates ketimines and benzothiazoline by means of the Brønsted acidic site (proton) and the Brønsted basic site (phosphoryl oxygen), respectively, to accelerate the hydride transfer reaction.

Four-component α-bromo-β-phosphoalkoxylation of aromatic α,β-unsaturated carbonyl compounds

Novel α-bromo-β-phosphoalkoxylated carbonyl compounds were produced in moderate to excellent yields via highly selective four-component reaction involving NBS, a cyclic ether, an organic phosphate and an aromatic α,β-unsaturated carbonyl compound.

Organocatalytic asymmetric desymmetrization of 4,4-disubstituted cyclohexadienones via an intermolecular Diels–Alder reaction

Asymmetric desymmetrization of 4,4-disubstituted cyclohexadienones via a chiral phosphoric acid-catalyzed enantioselective intermolecular Diels–Alder reaction is developed.

Four-component reaction leading to highly functionalized sulfoalkoxy carbonyl compounds

Four components together: a new paradigm of four component reaction brings α-bromo-β-sulfoalkoxyl carbonyl compounds with good stereocontrol in moderate to excellent yields.

Catalytic, highly enantioselective, direct amination of enecarbamates

Highly enantioselective electrophilic amination reaction of enecarbamates with dibenzylazodicarboxylate and oxygenated nucleophiles catalyzed by chiral phosphoric acids is reported. Subsequent reduction or oxidation of the resulting precursors of α-hydrazinoimines leads to 1,2-diamines or α-amino acid precursors, respectively, in excellent yields and enantioselectivities.

Enantioselective Addition of Bromonitromethane to Aliphatic N-Boc Aldimines Using a Homogeneous Bifunctional Chiral Organocatalyst

This report details the enantioselective synthesis of β-amino-α-bromo nitroalkanes with β-alkyl substituents, using homogeneous catalysis to prepare either antipode. Use of a bifunctional Brønsted base/acid catalyst allows equal access to either enantiomer of the products, enabling the use of Umpolung Amide Synthesis (UmAS) to prepare the corresponding L- or D-α-amino amide bearing alkyl side chains - overall, in only 4 steps from aldehyde. The approach also addresses an underlying incompatibility between bromonitromethane and solid hydroxide bases.

Enantioselective synthesis of 4,5,6,7-tetrahydroindoles via olefin cross-metathesis/intramolecular Friedel–Crafts alkylation reaction of pyrroles

A sequential catalysis involving olefin cross-metathesis/asymmetric intramolecular Friedel–Crafts alkylation of pyrrole derivatives has been developed.

Synthesis of functionalized tryptamines by Brønsted acid catalysed cascade reactions

A new solvent-free Brønsted acid catalysed cascade reaction has been established, allowing access to highly substituted tryptamines under mild conditions.

Quinine catalysed asymmetric Michael additions in a sustainable solvent

Diethyl carbonate is shown to be a suitable, sustainable solvent in which to carry out quinine catalysed asymmetric Michael additions of malononitriles to enones.

Experimental and computational study of the catalytic asymmetric 4π-electrocyclization of N-heterocycles

The first asymmetric metal-catalyzed Nazarov cyclization of N-heterocycles has been developed. The use of a chiral catalyst allows the enantioselective electrocyclization of N-heterocycles under mild conditions and the corresponding products are obtained in good yields with excellent enantio- and diastereoselectivity. The reaction mechanism and the absolute configuration of the obtained products are explained by means of computational studies.

Catalytic asymmetric Diels-Alder reactions involving aryl vinyl ketones

A catalytic asymmetric Diels-Alder reaction of an aryl vinyl ketone with 1,3-dienylcarbamate has been developed. Cyclohexenes bearing vicinal amino and aroyl groups in a cis-configuration were prepared in excellent ee (>99%) and endo (single diastereomer) selectivity. The absolute configuration of one DA product was unambiguously confirmed using XRD analysis. The transition state structure was proposed on the basis of DFT calculations.

Asymmetric dearomatization of β-naphthols through an amination reaction catalyzed by a chiral phosphoric acid

A highly efficient catalytic asymmetric dearomatization of naphthols by means of an electrophilic amination reaction catalyzed by chiral phosphoric acid is presented. This protocol provides a facile access to functionalized β-naphthalenone compounds with a chiral quaternary carbon center in excellent yields and enantioselectivity (up to 99% yield, up to 96% ee).

Metal-free enantioselective electrophilic activation of allenamides: stereoselective dearomatization of indoles

The effective and unprecedented chiral BINOL phosphoric acid catalyzed (1-10 mol%) dearomatization of indoles through electrophilic activation of allenamides (ee up to 94%), is documented. Besides the synthesis of 3,3-disubstituted indolenine cores, a dearomatization/hydrogen transfer cascade sequence is also presented as a new synthetic shortcut toward highly enantiomerically enriched indolines.

Brønsted acid catalyzed phosphoramidic acid additions to alkenes: diastereo- and enantioselective halogenative cyclizations for the synthesis of C- and P-chiral phosphoramidates

The first highly diastereo- and enantioselective additions of a halogen and phosphoramidic acid to unactivated alkenes have been developed, catalyzed by a chiral Brønsted acid. A unique feature of these additions is the opportunity for stereocontrol at two noncontiguous chiral centers, carbon and phosphorus, leading to cyclic P-chiral phosphoramidates. In addition to their inherent value, the phosphoramidates are precursors to enantioenriched epoxy allylamines.

The ever-expanding role of asymmetric covalent organocatalysis in scalable, natural product synthesis

Following the turn of the millennium, the role of asymmetric covalent organocatalysis has developed into a scalable, synthetic paradigm galvanizing the synthetic community toward utilization of these methods toward more practical, metal-free syntheses of natural products. A myriad of reports on asymmetric organocatalytic modes of substrate activation relying on small, exclusively organic molecules are delineating what has now become the multifaceted field of organocatalysis. In covalent catalysis, the catalyst and substrate combine to first form a covalent, activated intermediate that enters the catalytic cycle. Following asymmetric bond formation, the chiral catalyst is recycled through hydrolysis or displacement by a pendant group on the newly formed product. Amine- and phosphine-based organocatalysts are the most common examples that have led to a vast array of reaction types. This Highlight provides a brief overview of covalent modes of organocatalysis and applications of scalable versions of these methods applied to the total synthesis of natural products including examples from our own laboratory.