Visible-Light-Mediated Intermolecular [2 + 2]-Cycloaddition Reaction of 3-Alkylideneindolin-2-one with Alkenes via Triplet Energy Transfer for the Synthesis of 3-Spirocyclobutyl Oxindoles

Asymmetric Synthesis of Strained Multichiral Spirocyclobutanes through Cage-Confined Cross [2 + 2] Photocycloaddition

Chiral spirocycles possess the ability to undergo diverse modifications in three-dimensional space, offering advantages in terms of physicochemical property and structural variability over conventional organic scaffolds and holding promising potential for the design of biologically active molecules and drugs. Among them, highly strained spirocyclobutanes with multiple chiral center-containing four-membered rings have attracted significant attention, but their viable and efficient synthesis poses a great challenge. By virtue of cage-confined asymmetric photocatalysis, we successfully construct spirocycle and bispirocycle compounds containing multiple quaternary and tertiary chiral carbon centers in cyclobutane rings through cross [2 + 2] photocycloaddition with visible-light-induced and mild reactions. The mechanistic studies unveil that the chiral open pockets of a cage photoreactor facilitate dynamic bimolecular recognition to render preferential heteromolecular cross-cycloaddition with enhanced reactivity, unconventional enantioselectivity, and good substrate tolerance, providing a promising direction for enzyme-mimetic catalyst design for challenging asymmetric photochemical transformations.

Synthesis of Highly Functionalized Indolizines via NIS-Promoted Spiroannulation/Ring-Opening Aromatization of Alkylidene Oxindoles with 2-(Pyridin-2-yl)acetate Derivatives

A novel NIS-promoted domino reaction of alkylidene oxindoles with 2-(pyridin-2-yl)acetate derivatives has been established, enabling the efficient and straightforward synthesis of a vast variety of highly functionalized indolizines via sequential spiroannulation and ring-opening aromatization processes. The protocol features mild reaction conditions, broad substrate scope, high efficiency, scalability, and applicability for the preparation of CF3-containing indolizines. Furthermore, the functional groups in the indolizine framework provide the feasibility for follow-up derivatization. Based on mechanistic studies, a plausible radical mechanism is proposed to elucidate the formation of indolizines.

Oxidative Difunctionalization of N-Aryl Bicyclobutyl Amides with Aldehydes: Divergent Synthesis of Acylated and Alkylated 3-Spirocyclobutyl Oxindoles

An efficient and operationally simple oxidative radical difunctionalization of N-aryl bicyclobutyl (BCB) amides with aldehydes is described. It was found that acylated 3-spirocyclobutyl oxindoles were generated from the coupling of BCB-amides and aromatic aldehydes, while reactions gave exclusively decarbonylative alkylarylation products using alkyl aldehydes as radical precursors.

InCl3/TfOH-Mediated Convenient Synthesis of 3-Alkylideneoxindoles from 2-Oxindoles with 1,3-Diones, Ketones, or Aldehydes

Two efficient and convenient methods for the synthesis of 3-alkylideneoxindoles are described in this paper. The InCl3/TfOH-mediated tandem Knoevenagel condensation-deacylation sequence of various 2-oxindoles with 1,3-diones or acetoacetate furnished 3-alkylideneoxindoles in satisfactory to excellent yields (up to >99% yield). Employing the reaction system, the condensation of 2-oxindoles with ketones or aldehydes also proceeded smoothly to produce 3-alkylideneoxindoles. This protocol can be amenable to scale up. The effect of acids on this condensation reaction and intermolecular competition experiments were investigated to understand the aspect of the reaction.

Photoredox-Catalyzed Alkylarylation of N-Aryl Bicyclobutyl Amides with α-Carbonyl Alkyl Bromides: Access to 3-Spirocyclobutyl Oxindoles

A visible-light-induced radical alkylarylation of N-aryl bicyclobutyl amides with α-carbonyl alkyl bromides for the synthesis of functionalized 3-spirocyclobutyl oxindoles is described in which β-selective radical addition of the alkyl radical to N-aryl bicyclobutyl amides forms a key radical intermediate followed by interception with intrinsic arene functional group. This approach can be applicable to a wide range of α-carbonyl alkyl bromides, including primary, secondary, and tertiary α-bromoalkyl esters, ketones, nitriles, and nitro compounds.