Ring-Opening Coupling of Cyclopropanol with 1,2,3-Triazole for the Synthesis of Fused Triazoles

A radical ring-opening arylation of cyclopropanol with 1,2,3-triazole has been achieved. This synthetic protocol provides straightforward access to a wide range of structurally diverse and chiral 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyridines with high efficiency from readily available chiral cyclopropanols.

Synthesis of Vicinal anti-Amino Alcohols from N-tert-Butanesulfinyl Aldimines and Cyclopropanols

The stereoselective synthesis of vicinal amino alcohols derivatives from 1-substituted cyclopropanols and chiral N-tert-butanesulfinyl imines is described. Cyclopropanols are easily prepared from carboxylic esters upon reaction with ethylmagnesium bromide in the presence of titanium tetraisopropoxide and undergo carbon-carbon bond cleavage by means of diethylzinc to produce, upon base deprotonation, enolized zinc homoenolates, which react with chiral sulfinyl imines in a highly regio- and stereoselective manner.

Et2 Zn-Mediated Gem-Dicarboxylation of Cyclopropanols with CO2

An unprecedented Et2 Zn-mediated gem-dicarboxylation of C─C/C─H single bond of cyclopropanols with CO2 is disclosed, which provides a straightforward and efficient methodology for the synthesis of a variety of structurally diverse and useful malonic acids in moderate to excellent yields. The protocol features mild reaction conditions, excellent functional group compatibility, broad substrate scope, and facile derivatization of the products. DFT calculations confirm that the transition-metal-free transformation proceeds through a novel ring-opening/α-functionalization/ring-closing/ring-opening/β-functionalization (ROFCOF) process, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) plays dual important roles in the transformation.

Uracil-Cu(i) catalyst: allylation of cyclopropanols with Morita-Baylis-Hillman alcohols under water-tolerant conditions

Inspired by the high affinity of copper with DNA and RNA, a uracil-copper catalytic system was developed to promote ring-opening allylation of cyclopropanols with allylic alcohols under water-tolerant conditions. A new C-OH bond-breaking model can well resolve the trade-off between the need for acidic activators for C(allyl)-OH bond cleavage and the demand for strong basic conditions for generating homoenolates. Therefore, Morita-Baylis-Hillman alcohols, rather than their pre-activated versions, could be incorporated directly into dehydrative cross-coupling with cyclopropanols delivering water as the only by-product. A variety of functionalized δ,ε-unsaturated ketones were obtained in good-to-high yield with high E-selectivity.

Site-selective ring opening of bicyclo[n.1.0]alkanols: an Fe(II)-catalyzed 1,6-conjugate addition to p-quinone methides

Herein, we report an efficient synthetic strategy for an Fe(ii)-catalyzed site-selective ring opening of bicyclo[n.1.0]alkanols and their concomitant 1,6-conjugate addition to p-quinone methides. Access to tertiary carbon centers with appendaged carbocycles of distinct sizes and functional groups are achieved, under a substrate-controlled bond scission of the fused cyclopropanols. Synthetic derivatizations further enhance the utility of the protocol.

Promoting Catalytic C-Selective Sulfonylation of Cyclopropanols against Conventional O-Sulfonylation Using Readily Available Sulfonyl Chlorides

Against the backdrop of the well-known O-sulfonylation of cyclopropyl alcohols with sulfonyl chlorides, we examined the feasibility of conducting regioselective C-sulfonylation. By emulating an umpolung strategy-guided design, we report for the first time the Cu(II)-catalyzed β-sulfonylation of cyclopropanols by a mechanism that potentially involves an oxidative addition of a sulfonyl radical to a metal homoenolate. Unlike reported methods, this protocol allows a practical synthetic route to γ-keto sulfone building blocks from cyclopropanols by leveraging commercially available aryl- and alkyl-sulfonyl chlorides, common reagents in organic chemistry laboratories. Using operationally simple open-flask conditions, the preparative scope of starting materials was demonstrated using an array of aryl- and alkyl-substituted sulfonyl chlorides and cyclopropanols (43 examples, up to 96% yield).

A Homo-Mannich Reaction Strategy Enables Collective Access to Ibophyllidine, Aspidosperma, Kopsia, and Melodinus Alkaloids

We report here a homo-Mannich reaction of cyclopropanol with an iminium ion, generated by an asymmetric allylic dearomatization of indole, to construct a tricyclic hydrocarbazole core, which is shared by a variety of monoterpenoid indole alkaloids across families. Through this approach, an all-carbon quaternary stereogenic center as well as an allyl and a ketone group were installed. Using this functionalized hydrocarbazole as the structural platform, D ring and E rings of different sizes (i.e., five-, six-, and seven-membered) were successively or simultaneously assembled, leading to a collective asymmetric synthesis of seven alkaloids belonging to the ibophyllidine, Aspidosperma, Kopsia, and Melodinus alkaloid families.

A Bischler-Napieralski and homo-Mannich sequence enables diversified syntheses of sarpagine alkaloids and analogues

Sarpagine alkaloids offer signicant opportunities in drug discovery, yet the efficient total syntheses and diverse structural modifications of these natural products remain highly challenging due to the architectural complexity. Here we show a homo-Mannich reaction of cyclopropanol with imines generated via a Bischler-Napieralski reaction enables a protecting-group-free, redox economic, four-step access to the tetracyclic sarpagine core from L-tryptophan esters. Based on this advancement, diversified syntheses of sarpagine alkaloids and analogues are achieved in a short synthetic route. The systematic anticancer evaluation indicates that natural products vellosimine and Na-methyl vellosimine possess modest anticancer activity. Intensive structural optimization of these lead molecules and exploration of the structure-activity relationship lead to the identification of analogue 15ai with an allene unit showing a tenfold improvement in anticancer activities. Further mechanism studies indicate compound 15ai exertes antiproliferation effects by inducing ferroptosis, which is an appealing non-apoptotic cell death form that may provide new solutions in future cancer therapies.

Palladium-catalyzed coupling of amides and cyclopropanols for the synthesis of γ-diketones

A palladium catalytic method has been developed for the coupling of amides and cyclopropanols to γ-diketones, through simultaneous C-N and C-C activation. Heteroatom ligand exchange and heteroatom-to-carbon ligation mode switching enable the achievement of molecular cross-coupling in an amide N-atom structural context-dependent manner, avoiding any stoichiometric organometallic reagent or base.

Arylation of Cyclopropanol with Pyrrole: Asymmetric Synthesis of Indolizidine 167B, Indolizidine 209D, and Monomorine I

A Fe(NO₃)₃-mediated ring-opening arylation of cyclopropanol with the electron-rich pyrrole has been developed, which might proceed through oxidative radical ring opening of cyclopropanol followed by cyclization to the pyrrole motif and then aromatization. This method enables direct arylation of cyclopropanol without prefunctionalization and thus allows rapid access to a diverse array of chiral 5,6,7,8-tetrahydroindolizines from easily available chiral amino acid esters. The synthetic utility has been demonstrated by the asymmetric synthesis of alklaoids (-)-indolizidine 167B, (+)-indolizidine 209D, (+)-monomorine I, and a natural product analogue.

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.

Diastereoselective Transfer of Tri(di)fluoroacetylsilanes-Derived Carbenes to Alkenes

Stereoselective cyclopropanation reaction of alkenes is usually achieved by metal complexes via singlet-metal-carbene intermediates. However, previous transition-metal-catalyzed cyclopropanation of alkenes with acylsilanes afforded low diastereoselectivity. Herein, we report the first visible-light-induced transition-metal-free cyclopropanation reaction of terminal alkenes with trifluoroacetylsilanes and difluoroacetylsilanes. Both aromatic and aliphatic alkenes as well as electron-deficient alkenes are suitable substrates for the highly cis-selective [2+1] cyclization reaction. A combination of experimental and computational studies identified triplet carbenes as being key intermediates in this transformation. The gram scale reaction and late-stage functionalization demonstrated the synthetic potential of this strategy.

Palladium-Catalyzed Stagewise Strain-Release-Driven C-C Activation of Bicyclo[1.1.1]pentanyl Alcohols

A palladium-catalyzed chemoselective coupling of readily available bicyclo[1.1.1]pentanyl alcohols (BCP-OH) with various halides is reported, which offers expedient approaches to a wide range of cyclobutanone and β,γ-enone skeletons via single or double C-C activation. The chemistry shows a broad substrate scope in terms of both the range of BCP-OH and halides including a series of natural product derivatives. Moreover, dependency of reaction chemodivergence on base additive has been investigated through experimental and density functional theory (DFT) studies, which is expected to significantly enrich the reaction modes and increase the synthetic potential of BCP-OH in preparing more complex molecules.

Nickel-Catalyzed Carbonylation of Cyclopropanol with Benzyl Bromide for Multisubstituted Cyclopentenone Synthesis

Herein, we reported a Ni-catalyzed carbonylation of cyclopropanol with benzyl bromide to afford multisubstituted cyclopentenone under 1 atm of CO. The reaction proceeds through cascade carbonylation of benzyl bromides, followed by generation of nickel homoenolate from cyclopropanols via β-C elimination to afford 1,4-diketones, which undergoes intramolecular Aldol condensation to furnish highly substituted cyclopentenone derivatives in moderate to good yields. The reaction exhibits high functional group tolerance with broad substrate scope.

Zinc-Mediated Hydroxyallylation of Aldehydes with Cyclopropanols: Direct Access to Vicinal anti-sec,tert-Diols via Enolized Homoenolates

Direct and diastereoselective synthesis of vicinal anti-sec,tert-diols has been achieved by zinc-mediated α-hydroxyallylation of aldehydes with cyclopropanols. The reaction features the action of the zinc-enolized homoenolate as a γ-oxyallyl nucleophile toward the carbonyl electrophile. The diastereoselectivity of the present reaction is ascribed to the strong preference for a chelated (Z)-configuration of the enolized homoenolate as well as the bicyclic chairlike transition state it forms with the aldehyde, where the aldehyde substituent prefers to occupy the pseudoaxial position.

Asymmetric Total Syntheses of Schizozygane Alkaloids

The concise, collective, and asymmetric total syntheses of four schizozygane alkaloids, which feature a "Pan lid"-like hexacyclic core scaffold bearing up to six continuous stereocenters, including two quaternary ones, are described. A new method of dearomative cyclization of cyclopropanol onto the indole ring at C2 was developed to build the ABCF ring system of the schizozygane core with a ketone group. Another key skeleton-building reaction, the Heck/carbonylative lactamization cascade, ensured the rapid assembly of the hexacyclic schizozygane core and concurrent installation of an alkene group. By strategic use of these two reactions and through late-stage diversifications of the functionalized schizozygane core, the first and asymmetric total syntheses of (+)-schizozygine, (+)-3-oxo-14α,15α-epoxyschizozygine, and (+)-α-schizozygol and the total synthesis of (+)-strempeliopine have been accomplished in 11-12 steps from tryptamines.

Zinc-Catalyzed β-Functionalization of Cyclopropanols via Enolized Homoenolate

We report herein a zinc-catalyzed β-allylation of cyclopropanols with Morita-Baylis-Hillman (MBH) carbonates with retention of the cyclopropane ring. The reaction is promoted by a zinc aminoalkoxide catalyst, affording cyclopropyl-fused α-alkylidene-δ-valerolactone derivatives in moderate to good yields. Mechanistic experiments suggest that the present reaction does not proceed via direct β-C-H cleavage of the cyclopropanol, but involves zinc homoenolate and its enolization to generate a key bis-nucleophilic species. α-Allylation of this "enolized homoenolate" with MBH carbonate would be followed by regeneration of the cyclopropane ring and irreversible lactonization. The enolized homoenolate mechanism has also been proven to allow for β-functionalization with alkylidenemalononitrile as the reaction partner. A sequence of the present reaction and known cyclopropanol transformation provides an opportunity to transform a simple cyclopropanol into α,β- or β,β-difunctionalized ketones.

Nickel-Catalyzed Ring-Opening Allylation of Cyclopropanols via Homoenolate

We report herein a nickel-catalyzed ring-opening allylation of cyclopropanols with allylic carbonates that occurs under mild and neutral conditions. The reaction displays linear selectivity for both linear and branched acyclic allylic carbonates and is also applicable to cyclic allylic carbonates, affording a variety of δ,ε-unsaturated ketones in moderate to good yields. Mechanistic experiments are in accord with a catalytic cycle involving decarboxylative oxidative addition of allylic carbonate to Ni(0), alkoxide exchange with cyclopropanol, cyclopropoxide-to-homoenolate conversion on Ni(II), and C-C reductive elimination.

Enantioselective Conjugate Addition of Catalytically Generated Zinc Homoenolate

We report herein an enantioselective conjugate addition reaction of a zinc homoenolate, catalytically generated via ring opening of a cyclopropanol, to an α,β-unsaturated ketone. The reaction is promoted by a zinc aminoalkoxide catalyst generated from Et₂Zn and a chiral β-amino alcohol to afford 1,6-diketones, which undergo, upon heating, intramolecular aldol condensation to furnish highly substituted cyclopentene derivatives with good to high enantioselectivities. The reaction has proved applicable to various 1-substituted cyclopropanols as well as chalcones and related enones. The chiral amino alcohol has proved to enable ligand-accelerated catalysis of the homoenolate generation and its conjugate addition. Positive nonlinear effects and lower reactivity of a racemic catalyst have been observed, which can be attributed to a stable and inactive heterochiral zinc aminoalkoxide dimer.