Formal [4+2] cycloaddition of 3-phenylcyclobutanones with nitriles

KOtBu-promoted Michael/aldol/ring-opening cascade reaction of cyclobutanones with chalcones

An efficient Michael/aldol/ring-opening cascade reaction of cyclobutanones with chalcones has been developed. This protocol employs inexpensive potassium tert-butoxide (KOtBu) as a promoter and enables an efficient synthesis of densely substituted cyclohex-3-ene-carboxylic acids in high yields with high diastereoselectivities, which are difficult to prepare through conventional approaches. The significant advantages of this methodology include transition-metal-free conditions, readily available starting materials, wide scope and high atom economy.

Diastereoselective Synthesis of Tetrahydrobenzo[b]azocines by Lu(OTf)3 Catalyzed [4 + 4] Cycloaddition of Donor–Acceptor Cyclobutanes with Anthranils

The construction of N-heterocyclic eight-membered rings remains a challenging due to unfavorable transannular strain. Herein, we report a Lu(OTf)3 catalyzed formal [4 +4] cycloaddition reaction of cyclobutane 1,1-diesters with anthranils...

Lewis acid-catalyzed [4 + 2] cycloaddition of donor–acceptor cyclobutanes with iminooxindoles: access to spiro[piperidine-3,2′-oxindoles]

A Lewis acid-catalyzed [4 + 2] cycloaddition reaction from D–A cyclobutanes and iminooxindoles, providing the corresponding spiro[piperidine-3,2′-oxindoles] under mild conditions.

Synthesis of 2,3-Dihydro-4-pyridones and 4-Pyridones by the Cyclization Reaction of Ester-Tethered Enaminones

2,3-Dihydro-4-pyridone skeleton is an important building block in organic synthesis because it features several reaction sites with nucleophilic or electrophilic properties. Herein, we disclose a method for its formation by intramolecular cyclization of ester-tethered enaminones, which can easily be synthesized from readily available materials, such as amines, activated alkynes, and activated alkenes. 2,3-Dihydro-4-pyridones have been isolated in 41-90% yields. We also demonstrate the transformation of these heterocycles into another important class of compounds, 4-pyridones, by utilizing 2,3,5,6-tetrachloro-p-benzoquinone (chloranil) as an oxidizing agent. The latter products were isolated in 65-94% yields.

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.

Catalytic C-C Cleavage/Alkyne-Carbonyl Metathesis Sequence of Cyclobutanones

A ring-opening/alkyne-carbonyl metathesis sequence of alkyne-tethered cyclobutanones catalyzed by AgSbF₆ is realized for the first time to furnish multisubstituted naphthyl ketones under mild conditions. A range of substrates decorated with various substituents at different positions were all well accommodated. Preliminary mechanistic studies show that silver salt acted as a Lewis acid to facilitate both C-C cleavage of the cyclobutanone moiety and the subsequent metathesis between C═O and C≡C bonds.

Synthesis of Functionalized Tetrahydropyridines by SnCl4 -Mediated [4+2] Cycloaddition between Donor-Acceptor Cyclobutanes and Nitriles

Cycloadditions of strained carbocycles promoted by Lewis acids are powerful methods to construct heterocyclic frameworks. In fact, the formal [3+2] cycloadditions of donor-acceptor (DA) cyclopropanes with nitriles has seen particular success in synthesis. In this work, we report on the first [4+2] cycloaddition of nitriles with DA cyclobutanes by Lewis acid activation. Tetrahydropyridine derivatives were obtained in up to 91 % yield from various aryl-activated cyclobutane diesters and aliphatic or aromatic nitriles.