3-(Methoxycarbonyl)Cyclobutenone as a Reactive Dienophile in Enantioselective Diels-Alder Reactions Catalyzed by Chiral Oxazaborolidinium Ions

γ-Lactam synthesis from cyclobutanone via transoximation and the Beckmann rearrangement

This manuscript describes the synthesis of γ-lactam from the nitrogen insertion reaction of cyclobutanones using an oxime as an aminating reagent with a catalytic amount of Brønsted acid. This method was employed with a more stable oxime reagent, which is a precursor analog of hydroxylamine derivatives with explosive properties. The reaction was tolerated by various substituted cyclobutanones and less strained five- or six-membered ketones. The obtained γ-lactam products could be transformed into γ-aminobutyric acid derivatives via ring-opening hydrolysis. The reaction mechanism is discussed from the perspective of the isotope effect, etc.

Enantioselective Hydrofunctionalization of Cyclobutenones: Total Synthesis of gem-Dimethylcyclobutane Natural Products

Cyclobutanes with a gem-dimethyl group are common motifs in natural products. However, strategies for constructing enantioenriched gem-dimethyl cyclobutanes are still underdeveloped. Herein, we report an enantioselective approach to synthesize a broad group of chiral 2,3-disubstituted cyclobutanones through sequential 1,4-conjugate addition/trapping/cross-coupling of readily available cyclobutenones. The intermediate 2-bromocyclobutanone provides a valuable synthetic handle for further coupling transformations. In addition, this strategy was successfully utilized to synthesize gem-dimethyl cyclobutane-containing natural products, including (+)-β-caryophyllene, (-)-raikovenal, (-)-1β,9αH-5-linoleoyloxy-4,5-secocaryophyllen-4-one, and (-)-rumphellanones A-C.

Asymmetric Transfer Hydrogenation of Cyclobutenediones

Four-membered carbocycles are fundamental substructures in bioactive molecules and approved drugs and serve as irreplaceable building blocks in organic synthesis. However, developing efficient protocols furnishing diversified four-membered ring compounds in a highly regio-, diastereo-, and enantioselective fashion remains challenging but very desirable. Here, we report the unprecedented asymmetric transfer hydrogenation of cyclobutenediones. The reaction can selectively afford three types of four-membered products in high yields with high stereoselectivities, and the highly functionalized products enable a series of further transformations to form more diversified four-membered compounds. Asymmetric synthesis of di-, tri-, and tetrasubstituted bioactive molecules has also been achieved. Systematic mechanistic studies and theoretical calculations have revealed the origin of the regioselectivity, the key hydrogenation transition state models, and the sequence of the double and triple hydrogenation processes. The work provides a new choice for the catalytic asymmetric synthesis of cyclobutanes and related structures and demonstrates the robustness of asymmetric transfer hydrogenation in the accurate selectivity control of highly functionalized substrates.

Enantioselective [3+2]-cycloaddition of 2,3-disubstituted cyclobutenones: vicinal quaternary stereocenters construction and skeletal functionalization

Cycloaddition is a fundamental transformation, featuring the assembly of complex cyclic molecules with multiple stereocenters. We report here a silver-catalyzed [3+2]-cycloaddition of 2,3-disubstituted cyclobutenones with an array of azomethine ylide precursors iminoesters, furnishing azabicycles in good yields and enantioselectivities. Up to three contiguous all-carbon quaternary centers, including two angular stereocenters, could be constructed efficiently, due to high reactivity of strained cyclobutenones. Subsequent skeletal remodeling provided versatile molecules with distinct structural characters.

The Analysis of Two Distinct Strategies toward the Enantioselective Formal Total Synthesis of (+)-Gelsenicine

A full account of a formal enantioselective total synthesis of (+)-gelsenicine is described. Separate strategies based on catalytic cycloisomerization as the central step are considered. One plan involves chirality transfer from enantioenriched substrates, while the other employs asymmetric catalysis. The chirality transfer strategy is less effective, while in the latter, phosphoramidite- and bisphosphine-gold complexes are tested and ultimately provide a key intermediate in high enantiopurity in our Gelsemium alkaloid syntheses.

Asymmetric synthesis of complex tricyclo[3.2.2.0]nonenes from racemic norcaradienes: kinetic resolution via Diels-Alder reaction

Herein, the enantioselective synthesis of complex tricyclo[3.2.2.0]nonenes through the Diels-Alder reaction is reported. Utilizing racemic norcaradienes prepared from the visible-light-mediated dearomative cyclopropanation of m-xylene as dienes and enone derivatives as dienophiles, the overall process represents a kinetic asymmetric transformation in the presence of a chiral cobalt(ii) complex of chiral N,N'-dioxide. High diastereo- and enantioselectivity could be obtained in most cycloaddition processes and part racemization of norcaradiene is observed. The topographic steric maps of the catalysts were collected to rationalize the relationship between reactivity and enantioselectivity with the catalysts.

Cobalt-Catalyzed Diastereo- and Enantioselective Carbon-Carbon Bond Forming Reactions of Cyclobutenes

Catalytic enantioselective functionalization of cyclobutenes constitutes a general and modular strategy for construction of enantioenriched complex cyclobutanes bearing multiple stereogenic centers, as chiral four-membered rings are common motifs in biologically active molecules and versatile intermediates in organic synthesis. However, enantioselective synthesis of cyclobutanes through such a strategy remained significantly limited. Herein, we report a series of unprecedented cobalt-catalyzed carbon-carbon bond forming reactions of cyclobutenes that are initiated through enantioselective carbometalation. The protocols feature diastereo- and enantioselective introduction of allyl, alkynyl, and functionalized alkyl groups. Mechanistic studies indicated an unusual 1,3-cobalt migration and subsequent β-carbon elimination cascade process occurred in the allyl addition. These new discoveries established a new elementary process for cobalt catalysis and an extension of diversity of nucleophiles for enantioselective transformations of cyclobutenes.

Collective Synthesis of Chiral Tetrasubstituted Cyclobutanes Enabled by Enantioconvergent Negishi Cross-Coupling of Cyclobutenones

Cyclobutenones provide a straightforward four-carbon ring platform for further structural elaborations in that every carbon atom of the ring could be potentially functionalized. We report here a nickel catalyzed enantioconvergent Negishi coupling of 4-iodocyclobutenones with an array of aryl or alkenyl zinc reagents to access enantioenriched 4-substituted cyclobutenones, from which a modular approach to the synthesis of 1,2,3,4-tetrasubstituted cyclobutanes was demonstrated.

Stereoselective [2+2]-Cycloadditions of chiral allenic ketones and alkenes: Applications towards the synthesis of benzocyclobutenes and endiandric acids

Cyclobutanes are important motifs that have found utility in many contexts. Prior work has demonstrated an enantioselective isomerization/stereoselective [2 + 2] as a means to access bicyclo [4.2.0] octanes. Herein, the utility of this method is demonstrated towards the synthesis of benzocyclobutenes and a key intermediate towards the endiandric acids.

Stereospecific Nitrogen Insertion Using Amino Diphenylphosphinates: An Aza-Baeyer-Villiger Rearrangement

Amino diphenylphosphinates, which are commercially available or easily prepared from hydroxylamine, undergo ring expansion of cyclobutanones toward γ-lactams under mild conditions. A reaction pathway profoundly different from the common Beckmann reaction is achieved through the ambivalent character of the aminating agent. Thus, rearrangement occurs from a Criegee-like intermediate prior to the formation of the oxime species, which is corroborated by mechanistic experiments. Based on this observation, the migrating aptitude of the adjacent groups is analyzed and found to be in line with the parent Baeyer-Villiger reaction rendering a regioselective (up to >99:1 rr), stereospecific (>99% enantiospecificity), and chemoselective (>99%) insertion process possible. The method thus qualifies for late-stage skeletal editing as showcased by the synthesis of Rolipram and its N-alkylated analogs.

Stereoselective synthesis of novel bis-homoinositols with bicyclo[4.2.0]octane motifs

Starting from cyclooctatetraene, bis-homoconduritols with cis-inositol and allo-inositol (or bicyclo[4.2.0]octane motif) structures were synthesized. Photooxygenation of trans-7,8-dibromo-bicyclo[4.2.0]octa-2,4-diene allowed the preparation of tricyclic endoperoxide. The compound diacetate was obtained by reduction of endoperoxide with thiourea followed by acetylation reaction. Removal of halides with zinc dust in acetic acid yielded the dien-diacetate, a key compound of the designed molecules. OsO₄ oxidation of diendiacetate followed by acetylation gave the corresponding hexaacetates. Finally, the novel desired bis-homoinositols were obtained in high yield by the ammonolysis of acetate groups. The structures of all synthesized compounds were characterized by spectroscopic methods.

Two Steps to Bicyclo[4.2.0]octadienes from Cyclooctatetraene: Total Synthesis of Kingianic Acid A

Synthetic approaches to bicyclo[4.2.0]octadiene natural products frequently employ the synthesis of linear tetraenes to initiate a biosynthetic 8π/6π-electrocyclization cascade. This work forges a functionalized bicyclo[4.2.0]octadiene in two steps from cyclooctatetraene. The versatility of this method is demonstrated through natural product synthesis, including the first total synthesis of kingianic acid A and formal syntheses of kingianins A, D, and F and cryptobeilic acid D ethyl ester. The unexpected formation of an E,E,Z,E-tetraene byproduct is rationalized through density functional theory modeling.

Total Synthesis of (+)-Kingianin A by Enantioselective Cycloaddition of Strained Cyclobutenone

We report here the first asymmetric total synthesis of (+)-kingianin A via dimerization of an enantioenriched bicyclo[4.2.0]octadiene. The synthesis features a chiral oxazaborolidinium ion catalyzed Diels–Alder reaction of strained cyclobutenone and stereoselective functionalization of the cyclobutane ring. A preliminary biological study indicated that (+)-kingianin A exhibits potent anticancer activities.

Gold Catalysis Enabling Furan-Fused Cyclobutenes as a Platform toward Cross Cycloadditions

The inherently strained furan-fused cyclobutenes, in situ generated via cycloisomerizations of allenyl ketones bearing cyclopropyl moiety under gold catalysis, have been utilized as reactive building blocks toward cross cycloadditions. The [4 + 2] and [3 + 2] annulations of these species with benzo[c]isoxazoles and N-iminoquinazolinium ylides furnish various three-dimensional cyclobutane-bridged polyheterocycles in good yields. A wide range of typically electron-deficient 1,3-dienes, heterodienes, and 1,3-dipoles can trap furan-fused cyclobutenes to afford several polycyclic architectures.