Enantioselective Diynylation of Cyclic N-Acyl Ketimines: Access to Chiral Trifluoromethylated Tertiary Carbinamines

Enantioselective Synthesis of C2-Quaternary Indolin-3-ones by Pt-Catalyzed Alkynylation of 2-Aryl-3H-indol-3-one with Alkynylsilanes

An efficient method for the synthesis of five-membered chiral propargylic amines from 2-aryl-3H-indol-3-one and alkynylsilanes has been developed. The reaction proceeded under the catalytic system of PtCl4, oxazoline-based ligand L11, Zn(CF3COO)2, and AcOH in DCE at 95 °C via in situ desilylation of TMS-alkynes. This methodology also highlights a new protocol for the in situ desilylation of alkynylsilanes. The reaction showed a broad substrate scope with good yields and enantioselectivity.

Dearomative Insertion of Fluoroalkyl Carbenes into Azoles Leading to Fluoroalkyl Heterocycles with a Quaternary Center

The skeletal ring expansion of heteroarenes through carbene insertion is gaining popularity in synthetic chemistry. Efficient strategies for heterocyclic ring expansion to access heterocycles containing a fluoroalkyl quaternary carbon center through fluoroalkyl carbene insertion are highly desirable because of their broad applications in medicinal chemistry. Herein, we report a general strategy for the dearomative one-carbon insertion of azoles using fluoroalkyl N-triftosylhydrazones as fluoroalkyl carbene precursors, resulting in ring-expanded heterocycles in excellent yields with good functional-group compatibility. The broad generality of this methodology in the late-stage diversification of pharmaceutically interesting bioactive molecules and versatile transformations of the products has been demonstrated.

Mechanistic Study of Asymmetric Alkynylation of Isatin-Derived Ketimine Mediated by a Copper/Guanidine Catalyst

In this work, we performed a mechanistic study of asymmetric alkynylation of isatin-derived N-Boc ketimine that was first reported by Feng, Liu, and co-workers (Chem. Commun. 2018, 54, 678-681). Guanidine-amide promoted the formation of highly nucleophilic copper acetylene species by abstracting the terminal proton of phenylacetylene with an imine moiety. The guanidinium salt-Cu(I) complex was the most active species in the addition of the C═N bond, in which copper acetylene coordinated to the O atom of the amide moiety, and the isatin-derived ketimine substrate was activated by hydrogen bonding as well as tert-butoxycarbonyl···Cu(I) coordination. Due to weak interaction between Cu(I) and the Ph group in the amide of guanidine, as well as the repulsion between the tert-butyl group in ketimine and the cyclohexyl group in guanidine, the copper acetylene preferred to attack isatin-derived ketimine from the re-face, leading to the S-configuration product with excellent stereoselectivity. The affinity of the counterion for the Cu(I) center in the copper salt affected the deprotonation of phenylacetylene and the formation of guanidinium salt active species. In contrast to CuBr and CuCl, the combination of CuI with aniline-derived guanidine-amide exhibited high catalytic activity and a chiral induction effect, contributing to a high turnover frequency (9.70 × 10^-4 s^-1) in catalysis and ee%.

A bifunctional iminophosphorane squaramide catalyzed enantioselective synthesis of hydroquinazolines via intramolecular aza-Michael reaction to α,β-unsaturated esters

An efficient synthesis of enantioenriched hydroquinazoline cores via a novel bifunctional iminophosphorane squaramide catalyzed intramolecular aza-Michael reaction of urea-linked α,β-unsaturated esters is described. The methodology exhibits a high degree of functional group tolerance around the forming hydroquinazoline aryl core and wide structural variance on the nucleophilic N atom of the urea moiety. Excellent yields (up to 99%) and high enantioselectivities (up to 97 : 3 er) using both aromatic and less acidic aliphatic ureas were realized. The potential industrial applicability of the transformation was demonstrated in a 20 mmol scale-up experiment using an adjusted catalyst loading of 2 mol%. The origin of enantioselectivity and reactivity enhancement provided by the squaramide motif has been uncovered computationally using density functional theory (DFT) calculations, combined with the activation strain model (ASM) and energy decomposition analysis (EDA).

The activation of both aromatic and aliphatic ureas as N-centered nucleophiles in intramolecular Michael addition reactions to α,β-unsaturated esters was achieved under bifunctional iminophosphorane squaramide superbase catalysis.

Asymmetric Mannich reactions of (S)-N-tert-butylsulfinyl-3,3,3-trifluoroacetaldimines with yne nucleophiles

In the present work, arylethynes were studied as new C-nucleophiles in the asymmetric Mannich addition reactions with (S)-N-tert-butylsulfinyl-3,3,3-trifluoroacetaldimine. The reactions were conducted under operationally convenient conditions affording the corresponding Mannich adducts with up to 87% yield and 70:30 diastereoselectivity. The isomeric products can be separated using regular column chromatography to afford diastereomerically pure compounds. The purified Mannich addition products were deprotected to give the target enantiomerically pure trifluoromethylpropargylamines. A mechanistic rationale for the observed stereochemical outcome is discussed.

Catalytic asymmetric de novo construction of dihydroquinazolinone scaffolds via enantioselective decarboxylative [4+2] cycloadditions

The first de novo construction of enantioenriched dihydroquinazolinones via an intermolecular strategy has been established. This approach also represents the first catalytic asymmetric [4+2] cycloaddition of vinyl benzoxazinanones with sulfonyl isocyanates.

Copper/guanidine-catalyzed asymmetric alkynylation of isatin-derived ketimines

Asymmetric alkynylation of isatin-derived ketimines was achieved using an easily available chiral guanidine ligand in combination with CuI under mild reaction conditions.

Highly Enantioselective Synthesis of Propargyl Amides through Rh-Catalyzed Asymmetric Hydroalkynylation of Enamides: Scope, Mechanism, and Origin of Selectivity

Chiral propargyl amides are particularly useful structural units in organic synthesis. The enantioselective synthesis of propargyl amide is highly desirable. Conventional approach involves the use of a stoichiometric amount of metal reagent or chiral auxiliary. In comparison, direct alkynylation with terminal alkyne is attractive because it avoids the use of stoichiometric organometallic reagent. The asymmetric coupling of aldehyde, amine, and alkyne (A³-coupling) provides an efficient method for the synthesis of N-alkyl and N-aryl-substituted propargyl amines, but this strategy is not amenable for the direct enantioselective synthesis of propargyl amide. We have developed a new strategy and report here a Rh-catalyzed asymmetric hydroalkynylation of enamides. Alkynylations occur regioselectively at the α position of an enamide to produce chiral propargyl amides. High yield and enantioselectivity were observed. Previous alkynylation methods to prepare chiral propargyl amine involve the nucleophilic addition to an electron-deficient imine. In contrast, our current approach proceeds through regioselective hydroalkynylation of an electron-rich alkene. Kinetic studies indicated that migratory insertion of the enamide to the rhodium hydride is turnover limiting. Computational studies revealed the origin of regio- and enantioselectivities. This novel strategy provides an efficient method to access chiral propargyl amides directly from terminal alkynes.

Synthesis of 4-substituted 3-(2-hydroxyphenyl)-quinolines through an unexpected iron(iii) chloride promoted reaction of cyclic imine dibenzo[b,f][1,4]oxazepines with alkynes

An unexpected FeCl₃-promoted reaction of cyclic imine dibenzo[b,f][1,4]oxazepines with alkynes was studied to produce novel 4-substituted 3-(2-hydroxyphenyl)-quinolines with 34–88% yield.

Enantioselective alkynylation of N-sulfonyl α-ketiminoesters via a Friedel–Crafts alkylation strategy

Enantioselective Ni(ClO₄)₂-catalyzed alkynylation of N-sulfonyl-α-ketiminoesters is developed via a Friedel–Crafts-type pathway, yielding propargylic amides bearing quaternary stereocenters in excellent enantioselectivities.

Direct access to N-unprotected tetrasubstituted propargylamines via direct catalytic alkynylation of N-unprotected trifluoromethyl ketimines

Direct catalytic alkynylation of N-unprotected trifluoromethyl ketimines allowed an unprecedented direct access to N-unprotected α-tetrasubstituted primary amines without additional deprotection steps.

Cu(i)-catalyzed Michael addition of ketiminoesters to β-trifluoromethyl β,β-disubstituted enones: rapid access to 1-pyrrolines bearing a quaternary all-carbon stereocenter

Chiral pyrrolines with two contiguous stereocenters, including a trifluoromethylated quaternary all-carbon stereocenter, were provided from β-trifluoromethyl β,β-disubstituted enones and glycine ketoimino ester.

Mechanistic Studies and Expansion of the Substrate Scope of Direct Enantioselective Alkynylation of α-Ketiminoesters Catalyzed by Adaptable (Phebox)Rhodium(III) Complexes

Mechanistic studies and expansion of the substrate scope of direct enantioselective alkynylation of α-ketiminoesters catalyzed by adaptable (phebox)rhodium(III) complexes are described. The mechanistic studies revealed that less acidic alkyne rather than more acidic acetic acid acted as a proton source in the catalytic cycle, and the generation of more active (acetato-κ(2)O,O')(alkynyl)(phebox)rhodium(III) complexes from the starting (diacetato)rhodium(III) complexes limited the overall reactivity of the reaction. These findings, as well as facile exchange of the alkynyl ligand on the (alkynyl)rhodium(III) complexes led us to use (acetato-κ(2)O,O')(trimethylsilylethynyl)(phebox)rhodium(III) complexes as a general precatalyst for various (alkynyl)rhodium(III) complexes. Use of the (trimethylsilylethynyl)rhodium(III) complexes as precatalysts enhanced the catalytic performance of the reactions with an α-ketiminoester derived from ethyl trifluoropyruvate at a catalyst loading as low as 0.5 mol % and expanded the substrate scope to unprecedented α-ketiminophosphonate and cyclic N-sulfonyl α-ketiminoesters.

Cascade reaction for the construction of CF3 containing tetrasubstituted furan ring

Activated olefins are well known nucleophiles, but proton abstraction from the substituted α-methyl group in nitroolefin is rare.

Highly enantioselective copper(i)-catalyzed conjugate addition of 1,3-diynes to α,β-unsaturated trifluoromethyl ketones

The conjugate diynylation of α,β-unsaturated trifluoromethyl ketones with terminal diynes is carried out. Pre-metalation of the terminal 1,3-diyne is not required. Diynes bearing a propargylic stereogenic center are obtained with good yields and ee's.

Core-structure-inspired asymmetric addition reactions: enantioselective synthesis of dihydrobenzoxazinone- and dihydroquinazolinone-based anti-HIV agents

An overview of the asymmetric synthesis of dihydrobenzoxazinone- and dihydroquinazolinone-based anti-HIV agents (Efavirenz, DPC 961, DPC 963, DPC 083) and analogues was presented.