Stereocontrol in Synthesis of Homoallylic Amines. Syn Selective Direct Allylation of Hydrazones with Allylboronic Acids

Efficient access to general α-tertiary amines via water-accelerated organocatalytic multicomponent allylation

A tetrasubstituted carbon atom connected by three sp³ or sp²-carbons with single nitrogen, i.e., the α-tertiary amine (ATA) functional group, is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. The synthetic approach toward ATA structures is intricate, therefore, a straightforward catalytic method has remained a substantial challenge. Here we show an efficient water-accelerated organocatalytic method to directly access ATA incorporating homoallylic amine structures by exploiting readily accessible general ketones as useful starting material. The synergistic action of a hydrophobic Brønsted acid in combination with a squaramide hydrogen-bonding donor under aqueous condition enabled the facile formation of the desired moiety. The developed exceptionally mild but powerful system facilitated a broad substrate scope, and enabled efficient multi-gram scalability.

The α-tertiary amine functional group is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. Here the authors show an efficient water-accelerated organocatalytic method to access α-tertiary amines incorporating homoallylic amine structures by exploiting ketones as useful starting material.

An expeditious route to sterically encumbered nonproteinogenic α-amino acid precursors using allylboronic acids

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yields and diastereoselectivities. Gram-scale synthesis, broad tolerance of functional groups, excellent stereodivergence, post-synthetic modifications, and easy removal of the chiral auxiliary are some of the key highlights. The protocol is applicable to various amino acids and short peptides, resulting in the incorporation of these precursors at the N-terminal position.

A diastereoselective allylation of N-tert-butane sulfinyl α-iminoesters using allylboronic acids is developed to obtain optically active non-proteinogenic α-amino acid precursors in good yield and diastereoselectivities.

Asymmetric Petasis reaction for the synthesis of chiral α- and β-butadienyl amines

The Petasis reaction using (1S,2R)-1-amino-2-indanol as the substrate and an activator to construct α- and β-butadienyl amines in optically pure forms was realized, which are otherwise difficult to prepare. The reactions feature a metal-free nature, broad substrate scope, complete regioselectivities (γ-selectivity of pinacol homoallenyl- and isoprenylboronates), and high to excellent chirality induction (up to >20 : 1 dr). The favored nucleophilic addition across the Si-face of the imine intermediate was explained using DFT calculations of the six-membered chair-like transition state.

Brønsted Acid-Catalyzed General Petasis Allylation and Isoprenylation of Unactivated Ketones

Brønsted acid-catalyzed general Petasis allylation and isoprenylation of unactivated ketones were developed by using o-hydroxyaniline and the corresponding pinacolyl boronic esters. This robust methodology provided access to a broad variety of quaternary homoallylic amines and dienyl amines in high yields, proved to be applicable to a gram-scale synthesis, and allowed the synthesis of a potentially bioactive quaternary homoallylic aminodiol.

Neighboring Hydroxyl Group-Assisted Allylboration and Lewis Acid-Mediated Carbonyl-Ene Reaction for Access to a Hapalindole Cyclohexane Core with Multiple Contiguous Stereogenic Centers

A neighboring hydroxyl group-assisted allylboration of 3-indolyl ketones with γ,γ-disubstituted allylboronic acids is reported, affording various 3-indolyl-substituted homoallylic alcohols in good yields with excellent diastereoselectivies (up to >20:1 dr). The hydroxyl group not only played a vital role in the challenging allylboration but was elaborated for the subsequent construction of a hapalindole cyclohexane core by a highly diastereoselective Lewis acid-catalyzed carbonyl-ene reaction. In the overall process, four contiguous stereogenic centers including two quaternary stereogenic centers were installed.

1,2-Amino alcohol-dependent Petasis allylboration for racemic and chiral homoallylamines

Asymmetric Petasis allylboration among an aldehyde, 1,2-amino alcohol and pinacolallylboronate has been established at room temperature without any catalysts.

Copper-catalyzed synthesis of allenylboronic acids. Access to sterically encumbered homopropargylic alcohols and amines by propargylboration

Tri- and tetrasubstituted allenylboronic acids were prepared via a new versatile copper-catalyzed methodology. The densely functionalized allenylboronic acids readily undergo propargylboration reactions with ketones and imines without any additives. Catalytic asymmetric propargylborylation of ketones is demonstrated with high stereoselectivity allowing for the synthesis of highly enantioenriched tertiary homopropargyl alcohols. The reaction is suitable for kinetic resolution of racemic allenylboronic acids affording alkynes with adjacent quaternary stereocenters.

Catalytic asymmetric propargyl- and allylboration of hydrazonoesters: a metal-free approach to sterically encumbered chiral α-amino acid derivatives

Metal-free asymmetric catalysis for the synthesis of sterically encumbered amino-acids was developed using allyl- and allenylboronic acid reagents.

Iron-catalyzed boration of allylic esters: an efficient approach to allylic boronates

The first general iron-catalyzed boration of allylic esters has been developed.

Umpolung Addition of Aldehydes to Aryl Imines

One of the classical ways to synthesize amines involves the coupling of carbonyl compounds and imines, either through enolate chemistry or acyl-based carbanion equivalents. We herein report an alternative strategy that is based on the use of aldehydes as alkyl carbanion equivalents in a reductive coupling with aryl imines. A wide array of secondary amines can be synthesized in moderate to high yields. This reaction is mediated by hydrazine and catalyzed by ruthenium(II) complexes, and it tolerates various functional groups, such as esters, amides, and nitriles.

Catalytic Asymmetric Allylboration of Indoles and Dihydroisoquinolines with Allylboronic Acids: Stereodivergent Synthesis of up to Three Contiguous Stereocenters

The catalytic asymmetric allylboration of cyclic imines with γ,γ‐disubstituted allylboronic acids provides products with adjacent stereocenters in high yield and stereoselectivity. Various electrophiles, including 3,4‐dihydroisoquinolines and indoles, were prenylated in a fully stereodivergent fashion by switching the E/Z geometry of the allylboronate and/or the enantiomer of the BINOL catalyst. 3‐Methylindole provided products with three adjacent stereocenters with high stereoselectivity in one synthetic operation.

Accessing Both Retention and Inversion Pathways in Stereospecific, Nickel-Catalyzed Miyaura Borylations of Allylic Pivalates

We have developed a stereospecific, nickel-catalyzed Miyaura borylation of allylic pivalates, which delivers highly enantioenriched α-stereogenic γ-aryl allylboronates with good yields and regioselectivities. Our complementary sets of conditions enable access to either enantiomer of allylboronate product from a single enantiomer of readily prepared allylic pivalate substrate. Excellent functional group tolerance, yields, regioselectivities, and stereochemical fidelities are observed. The stereochemical switch from stereoretention to stereoinversion largely depends upon solvent and can be explained by competitive pathways for the oxidative addition step. Our mechanistic investigations support a stereoretentive pathway stemming from a directed oxidative addition and a stereoinvertive pathway that is dominant when MeCN blocks coordination of the directing group by binding the nickel catalyst.