Regioselective [3 + 2] Annulation of β,γ-Alkynyl-α-ketimino Esters with 1,3-Dicarbonyls: The Synthesis of Highly Functionalized Dihydrofurans

A regioselective [3 + 2] annulation of β,γ-alkynyl-α-ketimino esters with 1,3-dicarbonyls is disclosed. A series of Z-selective dihydrofurans bearing an exocyclic double bond and a quaternary carbon center are accessed without the usage of base. Control and deuterium-labeling experiments have been investigated to probe into the reaction mechanism. The catalyst and base-free nucleophilic addition highlights the transformation.

Copper-catalyzed tandem cyclization reaction of ethynylbenzoxazinones and thiols: facile construction of 2-thiomethylene indoles

The first successful copper-catalyzed decarboxylative cyclization reaction of ethynylbenzoxazinones and thiols has been developed. A rarely studied α-addition process to a copper-allenylidene intermediate promoted this reaction. Using this protocol, a range of 2-thiomethylene indole compounds have been obtained. This methodology offers significant advantages including mild reaction conditions, cheap catalysts, good yields and broad substrate compatibility.

Doubly Stereoconvergent Propargylic Alkylation of α-Cyanocarbonyls: Enantioselective Construction of Vicinal Stereocenters

An asymmetric propargylic alkylation of α-cyanocarbonyls is developed for the first time under cooperative Cu(I) and organocatalysis. With ethynyl benzoxazinanones as the propargylic electrophile, this decarboxylative doubly stereoconvergent reaction evades alkyne hydroamination to furnish acyclic α-propargylic cyanocarbonyls, bearing vicinal tertiary and quaternary stereocenters, with high diastereo- and enantioselectivity (up to >20:1 dr and 99.5:0.5 er).

Decarboxylative Cyclization of Ethynyl Benzoxazinanones with Imidazolidines to Access 2,3-Indole-Fused 1,4-Diazocines

2,3-Indole-fused 1,4-diazocines represent a new family of indole alkaloid compounds and are difficult to access by the reported protocols. Herein, we report a copper-catalyzed decarboxylative cyclization of cyclic propargylic carbamates with imidazolidines via sequential C-N/C-N/C-C bond formation to deliver a series of 2,3-indole-fused 1,4-diazocines, with a broad substrate scope and mild conditions.

Ligand-Controlled Diastereodivergency in Propargylic Alkylation of Vinylogous Aza-Enamines: Construction of 1,3-Stereocenters

The first diastereodivergent propargylic alkylation reaction is developed. This Cu(I)-catalyzed formal decarboxylative [4+2] cycloaddition between ethynyl benzoxazinanone and vinylogous aza-enamine delivers each diastereomer of tetrahydroquinoline derivatives, bearing 1,3-stereocenters, using either i-Pr-Pybox or BINAP as the ligand under otherwise identical reaction conditions. This is the first application of vinylogous aza-enamines in a transition metal-catalyzed transformation and the first example of the creation of 1,3-stereocenters in a propargylic substitution reaction.

Rh(III)-Catalyzed C-H Activation of Benzamides and Chemodivergent Annulation with Benzoxazinanones: Substrate Controlled Selectivity

Decarboxylative annulation of propargyl carbamates with benzamides has been realized via rhodium-catalyzed C-H bond activation under mild conditions, delivering two distinct classes of heterocycles in high efficiency and selectivity under substrate control. This protocol provides a direct synthetic method for the preparation of functionalized 1,8-naphthyridines and isoindolinones.

Mild Base-Promoted Tandem Nucleophilic Substitution/Decarboxylation/Hydroamination: Access to 3-Sulfonylindoles and 2-Methyleneindophenols

We have developed an efficient construction of 3-sulfonylindoles and 2-methyleneindophenols via decarboxylative propargylation/hydroamination of ethynyl benzoxazinanones using sodium sulfonates and phenols as the nucleophiles. The reaction featured mild conditions (K₂CO₃), simple operation, and high chemoselectivity and was transition-metal-free. Our protocol could also be extended to other nucleophiles, including malonates, alcohols, and indoles. The broad substrate scope and scalability made this protocol practical for the synthesis of indole derivatives.

Construction of poly-N-heterocyclic scaffolds via the controlled reactivity of Cu-allenylidene intermediates

Controlling the sequence of the three consecutive reactive carbon centres of Cu-allenylidene remains a challenge. One of the impressive achievements in this area is the Cu-catalyzed annulation of 4-ethynyl benzoxazinanones, which are transformed into zwitterionic Cu-stabilized allenylidenes that are trapped by interceptors to provide the annulation products. In principle, the reaction proceeds via a preferential γ-attack, while annulation reactions via an α- or β-attack are infrequent. Herein, we describe a method for controlling the annulation mode, by the manipulation of a CF₃ or CH₃ substituent, to make it proceed via either a γ-attack or an α- or β-attack. The annulation of CF₃-substituted substrates with sulfamate-imines furnished densely functionalized N-heterocycles with excellent enantioselectivity via a cascade of an internal β-attack and an external α-attack. CH₃-variants were transformed into different heterocycles that possess a spiral skeleton, via a cascade of an internal β-attack and a hydride α-migration followed by a Diels-Alder reaction.

Synthesis of Heterobiaryl 4-Aryl Furans through a Base-Promoted Decarboxylative Propargylation/Cycloisomerization Annulation

A transition-metal-free synthesis of heterobiaryl 4-aryl furans through a base-promoted decarboxylative propargylation/cycloisomerization annulation of ethynyl benzoxazinanones and readily accessible β-keto esters or 1,3-diketones has been developed. A series of novel heterobiaryl 4-aryl furans were accessed efficiently in the presence of base under mild reaction conditions. This protocol is significant for probing the reaction mechanism of ethynyl benzoxazinanones and even other propargylic compounds.

Asymmetric Total Syntheses of Kopsane Alkaloids via a PtCl2 -Catalyzed Intramolecular [3+2] Cycloaddition

A concise and asymmetric total synthesis of five kopsane alkaloids that share a unique heptacyclic caged ring system was accomplished. The key transformation in the sequence involved a remarkable PtCl₂ -catalyzed intramolecular [3+2] cycloaddition, which allowed for the rapid assembly of pentacyclic carbon skeletons bearing 2,3-quaternary functionalized indoline. Expeditious construction of diverse indoline scaffolds with excellent control of diastereoselectivity demonstrated the broad scope and versatility of this key transformation.

Two Catalytic Annulation Modes via Cu-Allenylidenes with Sulfur Ylides that Are Dominated by the Presence or Absence of Trifluoromethyl Substituents

We disclose the Cu-catalyzed enantioselective synthesis of 3-methyl-3-propargyl-indolines, which contain a quaternary stereogenic carbon center, via the decarboxylative [4 + 1] annulation of 4-methyl-4-propargyl-benzoxazinanones with variety of sulfur ylides. The reaction proceeds predominantly through a γ-attack at the Cu-allenylidene intermediates by sulfur ylides to provide the corresponding indolines in good yield and high enantioselectivity (up to 91% ee). In contrast, the reaction of 4-trifluoromethyl-4-propargyl-benzoxazinanones with sulfur ylides delivers 3-trifluoromethyl-2-functionalized indoles in good to high yield via an unexpected α-attack at the Cu-allenylidene intermediates. Control over the α/γ-attack at the Cu-allenylidene intermediates by the same interceptors was achieved for the first time by the use of trifluoromethyl substituents.

Synthesis of cyclobutane-fused oxygen-containing tricyclic framework via thermally promoted intramolecular cycloaddition of cyclohexadienone-tethered allenes

We have developed a unique approach for the thermally promoted cycloaddition of cyclohexadienone-tethered allenes to form a versatile cyclobutane-fused oxygen-containing tricyclic framework in an environmentally friendly and atomic economic fashion with high regioselectivity. The reaction encompasses a broad substrate scope and functional group tolerance of cyclohexadienone moieties. Moreover, the cycloaddition was also applicable to the late-stage functionalization of pharmaceutically relevant compounds.

A concise construction of 4-alkynylquinazolines via [4 + 2] annulation of 4-alkynylbenzoxazinanones with acylhydroxamates under transition-metal-free conditions

A concise and highly efficient method for the construction of valuable 4-alkynylquinazolines under transition-metal-free conditions was developed via [4 + 2] annulation of 4-alkynylbenzoxazinanones with acylhydroxamates in good to excellent yields.