Four-Electron Electrocyclic Ring-Opening/Intermolecular [4+2] Cycloadditions of α-Hydroxycyclobutenones: Stereoselective Synthesis of Multiple Substituted δ-Lactams

Brønsted acid catalyzed Ficini [2 + 2] cycloaddition of ynamides with enones

Herein, we describe a novel metal-free Brønsted acid-catalyzed Ficini [2 + 2] cycloaddition of ynamides with enones under mild reaction conditions, leading to the formation of various cyclobutenamides in generally good to excellent yields within short reaction times. This work represents the first example of ynamides involved in a nonmetal-catalyzed [2 + 2] cycloaddition with enones.

[2 + 2] Cycloaddition of Ynamides to Construct 3-Aminocyclobutenones

An efficient Tf₂NH-catalyzed [2 + 2] cycloaddition of ynamides under mild conditions has been developed. Within short reaction times, various ynamides are transformed into the corresponding 3-aminocyclobutenones in good to excellent yields. This is the first example for the metal-free intermolecular [2 + 2] self-cycloaddition of ynamides. Meanwhile, the desired cycloaddition products can be easily transformed into aminonaphthol derivatives.

[4+3]-Cycloaddition Reaction of Sulfilimines with Cyclobutenones: Access to Benzazepinones

A catalyst-free [4+3]-cycloaddition reaction of N-aryl sulfilimines with cyclobutenones is described, which provides a straightforward protocol for synthesizing 1,5-dihydro-2H-benzo[b]azepin-2-ones under mild reaction conditions. This reaction features a broad substrate scope and good functional group tolerance and does not require catalysts or additives. Moreover, using N-pyridinyl sulfilimine as the substrate, a series of pyridoazepinones have also been prepared.

Cleavage of Carbon-Carbon σ-Bonds of Four-Membered Rings

This article reviews synthetic transformations involving cleavage of a carbon-carbon bond of a four-membered ring, with a particular focus on the examples reported during the period from 2011 to the end of 2019. Most significant is the progress of catalytic reactions involving oxidative addition of carbon-carbon bonds onto transition metals or β-carbon elimination of transition metal alkoxides. When they are looked at from synthetic perspectives, they offer unique and efficient methods to build complex natural products and structures that are difficult to construct by conventional methods. On the other hand, β-scission of radical intermediates has also attracted increasing attention as an alternative elementary step to cleave carbon-carbon bonds. Its site-selectivity is often complementary to that of transition metal-catalyzed reactions. In addition, Lewis acid-mediated and thermally induced ring-opening of cyclobutanone derivatives has garnered renewed attention. On the whole, these examples demonstrate unique synthetic potentials of structurally strained four-membered ring compounds for the construction of organic skeletons.

Nickel-catalyzed ring-opening of α-hydroxycyclobutenones with a remarkable ligand effect

A Ni-catalyzed ring-opening of α-hydroxycyclobutenones is reported herein.