Alkaloids-catalyzed regio- and enantioselective allylic nucleophilic substitution of tert-butyl carbonate of the Morita–Baylis–Hillman products

Direct Enantioselective Allylic Alkylation of α-Amino Esters to Quaternary Glutamates via Strategic Pyridoxal Catalyst Design

The difficulty for N-unprotected α-substituted glycinates as α-C nucleophiles originates from both competing N nucleophilic interference and steric hindrance of the α substituent, which makes direct asymmetric α-C alkylation of N-unprotected α-substituted glycinates with Morita-Baylis-Hillman (MBH) adducts especially challenging. Given the wide utilization of α-quaternary chiral glutamic acid derivatives in therapeutic studies, we took advantage of biomimetic carbonyl catalysis to achieve their efficient synthesis. A bifunctional centrally chiral pyridoxal, featured with an expanded catalytic cavity and reduced steric hindrance around the aldehyde group, was designed and synthesized. In this work, we describe the novel centrally chiral pyridoxal enabled direct asymmetric α-C alkylation of N-unprotected α-substituted glycinates with MBH acetates. A broad range of α-quaternary chiral glutamic acid derivatives with multiple modifications were produced with high reactivity and excellent stereocontrol.

Stereospecific 3-Aza-Cope Rearrangement Interrupted Asymmetric Allylic Substitution-Isomerization

Transition-metal catalyzed asymmetric allylic substitution with alkyl and heteroaryl carbon nucleophiles has been well-established. However, the asymmetric allylic arylation of acyclic internal alkenes with aryl nucleophiles remains challenging and underdeveloped. Herein we report a stereospecific 3-aza-Cope rearrangement interrupted asymmetric allylic substitution-isomerization (Int-AASI) that enables asymmetric allylic arylation. By means of this stepwise strategy, both enantioenriched allylic arylation products and axially chiral alkenes could be readily obtained in high enantioselectivities. Experimental studies support a mechanism involving a cascade of asymmetric allylic amination, stereospecific 3-aza-Cope rearrangement and alkene isomerization. Density functional theory studies detailed the reasons of achieving the high chemoselectivity, regioselectivity, stereoselectivity and stereospecificity, respectively.

Axially Chiral Phenanthroline Ligand-Enabled Pd-Catalyzed Asymmetric Amination and Alkylation of Aryl-Substituted Morita-Baylis-Hillman Adducts

Highly enantioselective allylic amination and alkylation of racemic sterically hindered aryl-substituted Morita-Baylis-Hillman (MBH) adducts have been achieved by using an in situ formed Pd-catalyst from an axially chiral phenanthroline ligand. This dynamic kinetic asymmetric transformation (DYKAT) is compatible with cyclic and acyclic secondary amines, dialkyl malonates, β-keto esters, acetylacetone, and malononitrile, affording the corresponding chiral products, such as β-amino acid esters, in up to 95% yield and with up to a 99:1 enantiomeric ratio.

Pyridine-Catalyzed Chemoselective Four-Component Cascade Reaction of Aromatic Aldehydes, Malononitrile/Cyanoacetates, MBH Carbonates, and Alcohols

An efficient pyridine-catalyzed chemoselective four-component cascade reaction of aromatic aldehydes, malononitrile/cyanoacetates, Morita-Baylis-Hillman (MBH) carbonates, and alcohols has been established. This one-pot reaction progressed in an unusual reaction with solvent participation via a Knoevenagel condensation/oxa-Michael addition/SN2' substitution sequence. This method allowed for facile access to an array of functionalized chain alkylbenzenes and dihydroquinolinones bearing one all-carbon quaternary center in moderate to excellent yields. It is worth noting that the configuration of the all-carbon quaternary center could be modulated by changing only the electron-withdrawing groups via a tandem reduction/cyclization reaction.

Organocatalytic allylic alkylation of alkyne-substituted MBH carbonates: access to quaternary carbon-containing 1,4-enynes

A DABCO-catalyzed allylic alkylation of tertiary propargylic alcohol-derived MBH carbonates with nitromethane was developed. A series of substituted 1,4-enynes with an all-carbon quaternary stereocenter were efficiently obtained in moderate to high yields. The synthetic utility of the product was demonstrated by facile synthesis of 1,4-enyne-embedded 2-pyrrolidinones.

DABCO-Catalyzed Mono-/Diallylation of N-Unsubstituted Isatin N,N′-Cyclic Azomethine Imine 1,3-Dipoles with Morita-Baylis-Hillman Carbonates

Allylation of N-unsubstituted isatin N,N′-cyclic azomethine imines with Morita-Baylis-Hillman carbonates in the presence of 1–10 mol% DABCO in DCM at room temperature, rapidly gave N-allylated and N, β-diallylated isatin N,N′-cyclic azomethine imine 1,3-dipoles in moderate to high yields. The reaction features mild reaction conditions, easily practical operation, and short reaction times in most cases. Furthermore, the alkylated products were transformed into novel bicyclic spiropyrrolidine oxoindole derivatives through the [3+2] or [3+3]-cycloaddition with maleimides or Knoevenagel adducts.

Asymmetric α-Allylation of Glycinate with Switched Chemoselectivity Enabled by Customized Bifunctional Pyridoxal Catalysts

Owing to the strong nucleophilicity of the NH₂ group, free-NH₂ glycinates react with MBH acetates to usually deliver N-allylated products even in the absence of catalysts. Without protection of the NH₂ group, chiral pyridoxal catalysts bearing an amide side chain at the C3 position of the naphthyl ring switched the chemoselectivity of the glycinates from intrinsic N-allylation to α-C allylation. The reaction formed chiral multisubstituted glutamic acid esters as S_N 2'-S_N 2' products in good yields with excellent stereoselectivity (up to 86 % yield, >20 : 1 dr, 97 % ee). As compared to pyridoxal catalysts bearing an amide side arm at the C2 position, the pyridoxals in this study have a bigger catalytic cavity to enable effective activation of larger electrophiles, such as MBH acetates and related intermediates. The reaction is proposed to proceed via a cooperative bifunctional catalysis pathway, which accounts for the high level of diastereo- and enantiocontrol of the pyridoxal catalysts.

Organocatalytic Regio- and Enantioselective [3 + 2]-Annulations of Ninhydrin-Derived Morita-Baylis-Hillman Carbonates with 3-Methyleneoxindoles

A type of Morita-Baylis-Hillman (MBH) carbonates has been developed from ninhydrin. These MBH carbonates have been successfully employed as 3C-synthons in the organocatalytic asymmetric [3 + 2]-annulations of the isatin-derived electron-deficient olefins, affording structurally diverse spirooxindoles in high yield with excellent stereoselectivity. In particular, the regioselectivity of MBH carbonates was controlled by the reaction partner, 3-methyleneoxindoles with carbonyl groups (R = ArCO), affording β-selective products and 3-methyleneoxindoles with ester groups (R = CO₂Me) furnishing γ-selective products. The representative scale-up reactions and transformation of product were examined. The reaction mechanism was expounded by control experiments.

Light-Triggered Catalytic Asymmetric Allylic Benzylation with Photogenerated C-Nucleophiles

Herein is reported the asymmetric allylic benzylation of Morita–Baylis–Hillman (MBH) carbonates with 2-methylbenzophenone (MBP) derivatives as nonstabilized photogenerated C-nucleophiles. The dual activation of both reaction partners, chiral Lewis-base activation of the electrophile and light activation of the nucleophile, enables the stereoselective installation of benzyl groups at the allylic position to forge tertiary and quaternary carbon centers.

Asymmetric synthesis of multifunctional aryl allyl ethers by nucleophilic catalysis

Asymmetric allylic substitution of Morita–Baylis–Hillman (MBH) carbonates with less-nucleophilic phenols mediated by nucleophilic amine catalysis was successfully developed. A variety of substituted aryl allyl ethers were afforded with moderate to high yields with excellent enantioselectivities. The chiral MBH alcohol could be easily accessed from the corresponding aryl allyl ether.

Asymmetric allylic substitution of Morita–Baylis–Hillman (MBH) carbonates with less-nucleophilic phenols mediated by nucleophilic amine catalysis was successfully developed.

Nickel-Mediated Asymmetric Allylic Alkylation between Nitroallylic Acetates and Acyl Imidazoles

A nickel-mediated asymmetric allylic alkylation reaction between imidazole-modified ketones and nitroallylic acetates is presented. This reaction is catalyzed by a simple chiral diamine-nickel catalyst under mild conditions and leads to a series of novel enantioenriched α-allylic adducts in moderate to good yields with excellent enantioselectivities. Furthermore, transformation of the allylic adducts could smoothly lead to chiral γ-nitro-esters containing three continuous stereocenters in good yields.

Catalyst-controlled switch of regioselectivity in the asymmetric allylic alkylation of oxazolones with MBHCs

A catalyst-controlled switch of regioselectivity in asymmetric allylic alkylation of oxazolones with MBHCs was described.

A facile and mild synthesis of trisubstituted allylic sulfones from Morita-Baylis-Hillman carbonates

An efficient and catalyst-free synthesis of trisubstituted allylic sulfones through an allylic sulfonylation reaction of Morita-Baylis-Hillman (MBH) carbonates with sodium sulfinates has been developed. Under the optimized reaction conditions, a series of trisubstituted allylic sulfones were rapidly prepared in good to excellent yields (71%–99%) with good to high selectivity (Z/E from 79:21 to >99:1). Compared with known synthetic methods, the current protocol features mild reaction temperature, high efficiency and easily available reagents.

An asymmetric allylic alkylation reaction of 3-alkylidene oxindoles

An efficient asymmetric allylic alkylation reaction with respect to 3-alkylidene oxindoles and racemic MBH carbonate has been achieved.

Regio- and diastereoselective construction of 1′,2′-(dihydrospiro[indoline-3,3′-pyrrol]-2′-yl)acrylates through phosphine-catalyzed [4 + 1] annulation of Morita–Baylis–Hillman carbonates with oxindole-derived α,β-unsaturated imines

Phosphine-catalyzed [4 + 1] annulation of Morita–Baylis–Hillman carbonates with oxindole-derived imines has been developed.

α-Isocupreine, an enantiocomplementary catalyst of β-isocupreidine

Complementary chemistry! α-Isocupreine (-ICPN) was synthesized for the first time in one step from quinine by treatment with CF3SO3H (see scheme). This compound serves as an enantiocomplementary catalyst to β-isocupreidine (β-ICD) in the Morita–Baylis–Hillman reaction.

Asymmetric allylic alkylation of isatin-derived Morita-Baylis-Hillman carbonates with nitroalkanes

A stereoselective allylic alkylation of isatin-derived Morita-Baylis-Hillman (MBH) carbonates with nitroalkanes has been developed. In the presence of 10 mol % β-isocupreidine (β-ICD), 3,3'-disubstituted oxindoles were prepared with moderate diastereoselectivities and excellent enantioselectivities.