Application of oxygen/nitrogen substituted donor-acceptor cyclopropanes in the total synthesis of natural products

Stereoselective Photoredox Catalyzed (3+3) Dipolar Cycloaddition of Nitrone with Aryl Cyclopropane

By resorting to the principle of remote activation, we herein demonstrate the first photoredox catalyzed (3+3) dipolar cycloaddition of nitrones with aryl cyclopropanes. Key to the fidelity of the reaction resides in a facile manner of substrate activation by single-electron transfer (SET) oxidation with photoredox catalysis, and the reaction takes place through a stepwise cascade encompassing a three-electron-type nucleophilic substitution triggered cyclopropane ring-opening and a diastereoselective 6-endo-trig radical cyclization manifold. The reaction proceeds under mild conditions with excellent regio- and stereoselectivity, nicely complementing the well-developed Lewis acid catalyzed cycloaddition of donor-acceptor cyclopropanes. Other merits of the protocol include wide scope of aryl cyclopropanes with diversified substitution patterns and good functional-group compatibility. A mechanism involving an aryl radical cation promoted remote activation mode was also proposed and supported by mechanistic experiments.

Asymmetric cascades of the π-allyl complex: a journey from transition-metal catalysis to metallaphotocatalysis

The enantioselective catalytic cascade involving Tsuji-Trost allylation has provided a viable strategy for the construction of multiple asymmetric C-C and C-X centres and numerous methods have been developed around it for the synthesis of various vital scaffolds. The synthetic utility of this strategy was enhanced by replacing the customary allyl acetates with ethylene diacetates/dicarbonates, vinyl epoxides, vinyl oxetanes, vinyl ethylene carbonates, vinyl cyclopropanes, enynes, and dienes using transition-metal catalysis. One more milestone was achieved when metallaphotocatalysis provided the necessary platform for these cascades by using a cheaper metal. This review will provide a summary of these enantioselective catalytic cascades from 2015.

Donor-Acceptor Cyclopropanes: Activation Enabled by a Single, Vinylogous Acceptor

A novel class of highly activated donor-acceptor cyclopropanes bearing only a single, vinylogous acceptor is presented. These strained moieties readily undergo cycloadditions with aldehydes, ketones, thioketones, nitriles, naphth-2-ols and various other substrates to yield the corresponding carbo- and heterocycles. Diastereocontrol can be achieved through the choice of catalyst (Brønsted or Lewis acid). The formation of tetrahydrofurans was shown to be highly enantiospecific when chiral cyclopropanes are employed. A series of mechanistic and kinetic experiments was conducted to elucidate a plausible catalytic cycle and to rationalize the stereochemical outcome.

New Report: Genome Mining Untaps the Antibiotics Biosynthetic Gene Cluster of Pseudoalteromonas xiamenensis STKMTI.2 from a Mangrove Soil Sediment

The marine bacterium Pseudoalteromonas xiamenensis STKMTI.2 was isolated from a mangrove soil sediment on Setokok Island, Batam, Indonesia. The genome of this bacterium consisted of 4,563,326 bp (GC content: 43.2%) with 1 chromosome, 2 circular plasmids, 2 linear plasmids, 4,824 protein-coding sequences, 25 rRNAs, 104 tRNAs, 4 ncRNAs, and 1 clustered, regularly interspaced, short palindromic repeated (CRISPR). This strain possessed cluster genes which are responsible for the production of brominated marine pyrroles/phenols (bmp), namely, bmp8 and bmp9. Other gene clusters responsible for the synthesis of secondary metabolites were identified using antiSMASH and BAGEL4, which yielded five results, namely, non-ribosomal peptides, polyketide-like butyrolactone, Lant class I, and RiPP-like, detected in chromosome 1, while prodigiosin was detected in the unnamed plasmid 5. This suggests that these whole genome data will be of remarkable importance for the improved understanding of the biosynthesis of industrially important bioactive and antibacterial compounds produced by P. xiamenensis STKMTI.2.

Cycloadditions of Donor-Acceptor Cyclopropanes and -butanes using S=N-Containing Reagents: Access to Cyclic Sulfinamides, Sulfonamides, and Sulfinamidines

We present (3+2)‐ and (4+2)‐cycloadditions of donor–acceptor (D–A) cyclopropanes and cyclobutanes with N‐sulfinylamines and a sulfur diimide, along with a one‐pot, two‐step strategy for the formal insertion of HNSO₂ into D–A cyclopropanes. These are rare examples of cycloadditions with D–A cyclopropanes and cyclobutanes whereby the 2π component consists of two different heteroatoms, thus leading to five‐ and six‐membered rings containing adjacent heteroatoms.

Synthesis of substituted 1,2-dihydroisoquinolines via Ni(ii) and Cu(i)/Ag(i) catalyzed double nucleophilic addition of arylamines to ortho-alkynyl donor-acceptor cyclopropanes (o-ADACs)

A concise approach for the synthesis of substituted 1,2-dihydroisoquinolines via double nucleophilic addition of primary arylamines to ortho-alkynyl donor-acceptor cyclopropanes (o-ADACs) in the presence of a catalytic Ni(ClO₄)₂·6H₂O and CuI/AgOTf system has been developed. Further applying this protocol, some of the derived malonates were converted into the corresponding monoesters under Krapcho decarboxylation reaction conditions. Thereafter, these esters were transformed into the respective acids and alcohols. In addition, multifunctionalized 4-(2,2,2-trifluoroacetyl) 1,2-dihydroisoquinolines were also obtained with excess TFAA.

Tin(iv) chloride mediated (3 + 2) annulation of trans-2-aroyl-3-styrylcyclopropane-1,1-dicarboxylates with nitriles: diastereoselective access to 5-vinyl-1-pyrroline derivatives

A tin(iv) chloride promoted (3 + 2) annulation of trans-2-aroyl-3-styrylcyclopropane-1,1-dicarboxylates with nitriles is reported. The transformation involves the Lewis acid assisted formation of 1,5-dipolar intermediates from the cyclopropane dicarboxylates and nitriles followed by cyclization. The reactions proceed in a highly diastereoselective manner and afford 5-vinyl-1-pyrroline derivatives in 60-88% yields.

Recent advances in ring-opening of donor acceptor cyclopropanes using C-nucleophiles

Ring-opening transformations of donor-acceptor cyclopropanes (DAC) with carbon-centered nucleophiles is a simple, straight-forward approach to 1,3-bifunctional compounds that has witnessed remarkable progress over the past several years. To date, different reactivity patterns of DACs have been successfully exploited in racemic/stereoselective syntheses of various acyclic compounds or carbocycles with an impressive structural diversity. The thriving strategies have been successfully utilized in multistep synthesis of complex target molecules. Herein, the recent advances (2015-present) in the ring-opening of DAC involving electron rich arenes and indoles, active methylene compounds, various dipolarophiles, organoborates/boronates, vinyl ethers etc. following Friedel-Crafts alkylation, annulation/formal cycloaddition reaction, organocatalytic reaction, Nazarov cyclisation etc. are presented.

(3+2)-Cycloaddition of Donor–Acceptor Cyclopropanes with Thiocyanate: A Facile and Efficient Synthesis of 2-Amino-4,5-dihydrothiophenes

An easy and efficient route to obtain 2-amino-4,5-dihydrothiophenes is presented. A formal (3+2)-cycloaddition of donor–acceptor cyclopropanes and ammonium thiocyanate catalyzed by Yb(OTf)3 delivers the desired products in good to excellent yields. A broad range of functional groups is tolerated during this process.

Cascade intramolecular rearrangement/cycloaddition of nitrocyclopropane carboxylates with alkynes/alkenes: access to uncommon bi(hetero)cyclic systems

A straightforward approach for the one-pot synthesis of fused bi(hetero)cyclic systems via cascade intramolecular rearrangement/cycloaddition reaction of nitrocyclopropane carboxylates with substituted alkynes/alkenes has been demonstrated.

Applications of Sesquiterpene Lactones: A Review of Some Potential Success Cases

Sesquiterpene lactones, a vast range of terpenoids isolated from Asteraceae species, exhibit a broad spectrum of biological effects and several of them are already commercially available, such as artemisinin. Here the most recent and impactful results of in vivo, preclinical and clinical studies involving a selection of ten sesquiterpene lactones (alantolactone, arglabin, costunolide, cynaropicrin, helenalin, inuviscolide, lactucin, parthenolide, thapsigargin and tomentosin) are presented and discussed, along with some of their derivatives. In the authors’ opinion, these compounds have been neglected compared to others, although they could be of great use in developing important new pharmaceutical products. The selected sesquiterpenes show promising anticancer and anti-inflammatory effects, acting on various targets. Moreover, they exhibit antifungal, anxiolytic, analgesic, and antitrypanosomal activities. Several studies discussed here clearly show the potential that some of them have in combination therapy, as sensitizing agents to facilitate and enhance the action of drugs in clinical use. The derivatives show greater pharmacological value since they have better pharmacokinetics, stability, potency, and/or selectivity. All these natural terpenoids and their derivatives exhibit properties that invite further research by the scientific community.

Ring-opening 1,3-arylboration of arylcyclopropanes mediated by BCl3

Herein, we report a ring-opening 1,3-arylboration of aryl cyclopropanes using BCl₃ in the presence of arene nucleophiles.

Beyond the Corey-Chaykovsky Reaction: Synthesis of Unusual Cyclopropanoids via Desymmetrization and Thereof

Desymmetrization-based protocols for the synthesis of highly functionalized indeno-spirocyclopropanes and cyclopropa-fused indanes have been established through unexpected reactions triggered by the Corey-Chaykovsky reagent. These structures were further elaborated in one step to privileged scaffolds such as fluorenones, indenones, and naphthaphenones. For instance, an acid-catalyzed transformation of indeno-spirocyclopropanes provided fluorenones via a homo-Nazarov-type cyclization, and naphthaphenones were obtained via an acid-catalyzed cyclopropane ring-opening/retro-Michael sequence.

Brønsted acid mediated intramolecular cyclopropane ring expansion/[4 + 2]-cycloaddition

A cascade reaction of 3-hydroxy-2-phenylisoindolin-1-one and cyclopropyl ketone has been developed via a BrØnsted acid-promoted ring-opening/intramolecular cross-cycloaddition/[4 + 2]-cycloaddition process. The developed methodology provides straightforward access to pentacyclic isoindolin-1-one derivatives under simple reaction conditions.

Chameleon-Like Activating Nature of the Spirooxindole Group in Donor-Acceptor Cyclopropanes

The concept of a chameleon activating group is considered in the context of donor-acceptor cyclopropane chemistry. When spiro-conjugated with cyclopropane, oxindole can act as an acceptor or a donor depending on the electronic nature of vicinal substituents. This dichotomy is reflected in the alteration of chemoselectivity of spiro[oxindole-1,3'-cyclopropane] ring opening with nucleophiles.

Donor-Acceptor Cyclopropanes in the Synthesis of Carbocycles

Donor-acceptor cyclopropanes not only participate in a broad range of ring openings with nucleophiles, electrophiles, radical and red-ox agents, but also are excellent substrates for various (3+n)-cycloaddition and (3+n)-annulation processes. Moreover, under treatment with Lewis acid donor-acceptor cyclopropanes can produce new ring systems via isomerization or cyclodimerization. Authors' contribution to the synthesis of diverse carbocycles from donor-acceptor cyclopropanes is summarized in this account.

DBU-Mediated Cyclization of Acylcyclopropanecarboxylates with Amidines: Access to Polysubstituted Pyrimidines

DBU-mediated cyclization of 2-acyl-1-cyanocyclopropanecarboxylates with amidines for the synthesis of multisubstituted pyrimidine derivatives is described. This reaction gives a practical method for producing a diverse set of pyrimidines, having simple experimentation, readily available starting materials, a wide substrate scope, and very good yields.

Stereocontrolled [3+2] Cycloaddition of Donor-Acceptor Cyclopropanes to Iminooxindoles: Access to Spiro[oxindole-3,2'-pyrrolidines]

A novel stereocontrolled assembly of spiro[oxindole-3,2'-pyrrolidines] via [3+2]-cycloaddition of donor-acceptor cyclopropanes to electron-poor ketimines, iminooxindoles, was developed. The method allows for efficient employment of common readily available donor-acceptor cyclopropanes, functionalized with ester, keto, nitro, cyano etc. groups, and N-unprotected iminooxindoles. The stereospecificity of the initial S_N2-like imine attack on a cyclopropane molecule together with a high diastereoselectivity of further C-C bond formation facilitate a rapid access to spiro[oxindole-3,2'-pyrrolidines] in their optically active forms. Preliminary in vitro testing of the synthesized compounds against LNCaP (p53+) and PC-3 (p53-) cells revealed good antiproliferative activities and p53-selectivity indices for several compounds that are intriguing in terms of their further investigation as inhibitors of MDM2-p53 interaction.

Synthesis of Cyclopropanoids via Substrate-Based Cyclization Pathways

A series of unexpected reactions triggered by the dimethyloxosulfonium methylide led to the discovery of unconventional approaches for the synthesis of cyclopropa-fused tetralones and indeno-spirocyclopropanes. These highly functionalized structures were further elaborated in one step to privileged scaffolds such as tetralones, indenones, and fluorenones. As a whole, the results presented herein establish new diversity-oriented folding pathways.

Highly regio- and diastereoselective Cu-catalyzed hydroborylation and hydrosilylation of difluorocyclopropenes with B2pin2 and PhMe2Si–Bpin

An efficient method for the synthesis of trans-difluorocyclopropyl boronates and silanes through a Cu-catalyzed hydroborylation and hydrosilylation of difluorocyclopropenes was reported.

Expanding the Reactivity of Donor-Acceptor Cyclopropanes: Synthesis of Benzannulated Five-Membered Heterocycles via Intramolecular Attack of a Pendant Nucleophilic Group

Lewis-acid-induced domino transformations of donor-acceptor cyclopropanes, possessing a nucleophilic center embedded in a donor group, into functionalized 2,3-dihydrobenzo[ b]furans and 2,3-dihydrobenzo[ b]thiophenes are reported herein. An unusual switch of the electrophilic center in the three-membered ring, from the atom bearing a donor substituent to an unsubstituted carbon atom, was achieved by a judicious choice of Lewis acid, which induces the isomerization of a cyclopropane to an electrophilic alkene, and the length of linker, connecting a nucleophilic moiety and the small ring.

Anti-addition of Dimethylsulfoxonium Methylide to Acyclic α,β-Unsaturated Ketones and Its Application in Formal Synthesis of an Eicosanoid

Cyclopropanation using dimethylsulfoxonium methylide (Corey-Chaykovsky reaction) was examined with a series of linear α,β-unsaturated ketones, and the results showed that the major trajectory for the addition of the sulfur ylide to the enones is anti, related to the γ-substituent. The stereochemical assignment for the generated cyclopropanes was achieved by X-ray crystallography or comparing with the reported spectroscopic data. We found that the diastereoselectivity was influenced by several factors, including the protecting groups, solvents, and temperatures, and good anti/syn ratios (>10:1) were often obtained using the tert-butyldimethylsilyl and tert-butyldiphenylsilyl-protected substrates. The method was applied to a formal synthesis of a natural eicosanoid with good efficiency.