Synthesis of Oxatricyclooctanes via Photoinduced Intramolecular Oxa-[4+2] Cycloaddition of Substituted o-Divinylbenzenes

Recent advances in the transformation of maleimides via annulation

Over the past five years, maleimide scaffolds have gained considerable attention in organic synthesis for their role in forming cyclized molecules through annulation and C-H activation. As versatile and reactive coupling agents, maleimides have enabled the efficient synthesis of various cyclized products, including annulation, benzannulation, cycloaddition, and spirocyclization, with applications in medicinal chemistry, drug discovery, and materials science. Despite the extensive study of maleimide chemistry, certain reactions-such as cycloaddition-based annulation, photoannulation, and electrochemical transformations-remain underexplored despite their promising potential in the pharmaceutical and chemical industries. Recent advancements, such as photocatalysis and electrochemical methods, have expanded the utility of maleimides, providing more sustainable and selective approaches for synthesizing complex molecules. This review compiles research published between 2019 and 2024, highlighting the substrate scope, reaction diversity, and industrial relevance of maleimide-based annulation strategies. Additionally, we discuss emerging trends and future directions in maleimide chemistry, exploring opportunities for novel reaction pathways and broader applications in synthetic biology and materials science.

Organocatalytic enantio- and diastereoselective assembly of cyclopropane-incorporated polycyclic molecules via isobenzopyrylium ions

A highly enantio- and diastereoselective organocatalytic formation of cyclopropanes embedded in a complex bridged polycyclic architecture is disclosed. In the presence of a chiral phosphoric acid catalyst, this reaction generates four new stereogenic centers and three new C-C bonds efficiently from isochromene acetals and vinylboronic acids under mild conditions. Different from conventional asymmetric cyclopropanation strategies, this process does not involve carbenes or carbenoids. The complex products can serve as precursors to useful homoenolate equivalents. Mechanistically, DFT studies provided insights into the key transition states of the enantiodetermining [4 + 2] cycloaddition, in which the enantioselectivity is induced by the chiral phosphate counter anion of the isobenzopyrylium intermediate.

Mild Intermolecular Synthesis of a Cyclopropane-Containing Tricyclic Skeleton: Unusual Reactivity of Isobenzopyryliums

Cyclopropanes embedded in a polycyclic bridged architecture are a versatile structural motif, but such complex frameworks often impose substantial synthetic challenges. Herein we introduce a new approach for the expedient access to such spring-loaded strained systems via an exceptionally mild intermolecular convergent process between the readily available isobenzopyryliums and vinyl boronic acids. Different from the typical conventional approaches, our protocol does not involve the highly active carbenoid intermediates or strong conditions in order to overcome the disfavored kinetic and thermodynamic problems. Instead, the key cyclopropane ring was formed between the well-positioned nucleophile and electrophile in the adduct from the regioselective [4+2] cycloaddition. Thus, this unusual process also represents a new reactivity of the versatile isobenzopyryliums. The choice of a Brønsted acid catalyst with proper acidity is crucial to the high efficiency and selectivity for this multiple bond-forming process. The strained products are precursors to other useful synthetic building blocks.

Spiro 1,3-indandiones: intramolecular photochemical reactions of carbonyl groups with carbon–carbon double bonds

Spiro-1,3-indandiones involving a double CC bond undergo photochemical intramolecular reactions affording a variety of polycyclic products.

Organophotoredox-Catalyzed Intermolecular Oxa-[4+2] Cycloaddition Reactions

An intermolecular oxa-[4+2] cycloaddition reaction promoted by a thioxanthylium photoredox catalyst under irradiation with green light has been developed. The reaction of ortho-quinone methides with styrenes smoothly affords the desired cycloadducts. Especially styrenes bearing electron-donating groups are efficiently transformed in this reaction. This method represents a sustainable way to carry out oxa-[4+2] cycloaddition reactions using only a catalytic amount of a photocatalyst and visible light.