Asymmetric [4 + 3] Cycloadditions between Benzofuranyldiazoacetates and Dienes: Formal Synthesis of (+)-Frondosin B

Toward Efficient and Stereoselective Aromatic and Dearomative Cope Rearrangements: Experimental and Theoretical Investigations of α-Allyl-α'-Aromatic γ-Lactone Derivatives

The aromatic Cope rearrangement is an elusive transformation that has been the subject of a limited number of investigations compared to those seemingly close analogues, namely the Cope and aromatic Claisen rearrangement. Herein we report our investigations inspired by moderate success observed in the course of pioneering works. By careful experimental and theoretical investigations, we demonstrate that key substitutions on 1,5-hexadiene scaffold allow fruitful transformations. Especially, efficient functionalisation of the heteroaromatic rings results from the aromatic Cope rearrangement, while highly stereoselective interrupted aromatic Cope rearrangements highlight the formation of chiral compounds through a dearomative process.

Skipped dienes in natural product synthesis

Covering: 2000-2020The 1,4-diene motif, also known as a skipped diene, is widespread across various classes of natural products including alkaloids, fatty acids, terpenoids, and polyketides as part of either the finalized structure or a biosynthetic intermediate. The prevalence of this nonconjugated diene system in nature has resulted in numerous encounters in the total synthesis literature. However, skipped dienes have not been extensively reviewed, which could be attributed to overshadowing by the more recognized 1,3-diene system. In this review, we aim to highlight the relevance of skipped dienes in natural products through the lens of total synthesis. Subjects that will be covered include nomenclature, structural properties, prevalence in natural products, synthetic strategies and the future direction of the field.

Palladium-Catalyzed Aminomethylation and Cyclization of Enynol to O-Heterocycle Confined 1,3-Dienes

The rational tuning of the electrophilicity of the allylpalladium intermediates enables the regioselectively intramolecular 1,2-addition of enynol in the presence of aminal. This aminomethylation and cyclization reaction via C-N bond activation and intramolecular nucleophilic addition provides a rare example for the synthesis of O-containing heterocycle-confined 1,3-dienes, which is of synthetic potential for further derivatization. The method possesses broad substrate generality as well as functional group compatibility and efficiently affords a wide range of desired products with 5-, 6-, and 8-membered O-containing heterocycles with various functional groups.

A General Strategy for the Construction of Calyciphylline A-Type Alkaloids: Divergent Total Syntheses of (-)-Daphenylline and (-)-Himalensine A

An efficient general strategy for the synthesis of the Daphniphyllum alkaloids via the rapid construction of a common core intermediate has been established, based on which a divergent total synthesis of (-)-daphenylline and (-)-himalensine A has been accomplished in 16 and 19 steps, respectively. The present work features an enantioselective Mg(ClO₄ )₂ -catalyzed intramolecular amidocyclization to construct the aza-bridged core structure; a Cu-catalyzed intramolecular cyclopropanation and subsequent phosphine-catalyzed Cope-type rearrangement to furnish the himalensine A scaffold; and a one-pot Diels-Alder/aromatization method to assemble the aromatic skeleton of daphenylline.

Aromatic Cope rearrangements

Reviewed herein is the aromatic Cope rearrangement, a Cope rearrangement where one (or both) of the alkenes of the 1,5-diene are part of a greater aromatic system. While the Cope rearrangement of 1,5-dienes has seen wide utility, variation, and application in chemical synthesis, the aromatic Cope rearrangement, comparatively, has not. This review summarizes the ∼40 papers dating back to 1956 on this topic and is divided into the following sections: (1) introduction, including kinetic and thermodynamic challenges of the aromatic Cope rearrangement, and (2) key substrate features, of which there are four general types: (i) α-allyl-α-aryl malonates (and related substrates), (ii) 1-aryl-2-vinylcyclopropanes, and (iii) anion-accelerated aromatic oxy-Cope substrates, and (iv) the concept of synchronized aromaticity. Ultimately, we hope this review will draw attention to a potentially valuable transformation for arene functionalization that warrants further studies and development.

Zinc-Catalyzed Asymmetric Formal [4+3] Annulation of Isoxazoles with Enynol Ethers by 6π Electrocyclization: Stereoselective Access to 2H-Azepines

6π electrocyclization has attracted interest in organic synthesis because of its high stereospecificity and atom economy in the construction of versatile 5-7-membered cycles. However, examples of asymmetric 6π electrocyclization are quite scarce, and have to rely on the use of chiral organocatalysts, and been limited to pentadienyl-anion- and triene-type 6π electrocyclizations. Described herein is a zinc-catalyzed formal [4+3] annulation of isoxazoles with 3-en-1-ynol ethers via 6π electrocyclization, leading to the site-selective synthesis of functionalized 2H-azepines and 4H-azepines in good to excellent yields with broad substrate scope. Moreover, this strategy has also been used to produce chiral 2H-azepines with high enantioselectivities (up to 97:3 e.r.). This protocol not only is the first asymmetric heptatrienyl-cation-type 6π electrocyclization, but also is the first asymmetric reaction of isoxazoles with alkynes and the first asymmetric catalysis based on ynol ethers.

Finding Opportunities from Surprises and Failures. Development of Rhodium-Stabilized Donor/Acceptor Carbenes and Their Application to Catalyst-Controlled C-H Functionalization

Catalyst-controlled C-H functionalization by means of the C-H insertion chemistry of rhodium carbenes has become a powerful synthetic method. The key requirements for the development of this chemistry are donor/acceptor carbenes and the chiral dirhodium tetracarboxylate catalysts. This perspective will describe the stages involved in developing this chemistry and illustrate the scope of the donor/acceptor carbene C-H functionalization.

A Tandem Iridium-Catalyzed "Chain-Walking"/Cope Rearrangement Sequence

An iridium-catalyzed tandem olefin migration/Cope rearrangement of alkenyl ω-ene cyclopropanes is reported. By this means, a variety of complex annulenes are obtained as single diastereomers starting from cyclopropyl ester derived from simple 1,ω-dienes and alkenyldiazo compounds. Long-range olefin migration over up to 10 positions could be realized and coupled with an efficient Cope rearrangement to yield valuable scaffolds. Various functional groups are well-tolerated, giving rise to densely functionalized products. Furthermore, the present methodology could be successfully extended to yield bicyclic cycloheptenones starting from readily available alkenyl cyclopropanols via a Kulinkovich reaction.

Application of (4+3) cycloaddition strategies in the synthesis of natural products

This review summarizes the applications of (4+3) cycloadditions, both classical and formal, in the syntheses of natural products in the last two decades.

Ni-Catalyzed Dimerization and Arylation of Diarylacetylenes with Arylboronic Acids

A new, facile, and efficient protocol for the synthesis of polysubstituted conjugated 1,3-dienes through Ni-catalyzed tandem dimerization/cross-coupling reaction of diarylacetylenes and arylboronic acids in the presence of a catalytic amount of B₂pin₂ has been developed. A series of arynes and arylboronic acids with different substituents participated well in this catalytic system, affording a variety of useful conjugated 1,3-dienes.

Platinum-Catalyzed α,β-Unsaturated Carbene Formation in the Formal Syntheses of Frondosin B and Liphagal

Formal syntheses of tetracyclic terpenoids frondosin B and liphagal are described. Both synthetic routes rely on the use of platinum-catalyzed α,β-unsaturated carbene formation for the key C-C bond forming transformations. The successful route toward frondosin B utilizes a formal (4 + 3) cycloaddition, while the liphagal synthesis features the vinylogous addition of an enol nucleophile as a key step. Both synthetic routes are discussed, revealing insights into structural requirements in the catalytic α,β-unsaturated carbene reaction manifold.

A computational triage approach to the synthesis of novel difluorocyclopentenes and fluorinated cycloheptadienes using thermal rearrangements

Electronic structure calculations have been used for the effective triage of substituent effects on difluorinated vinylcyclopropane precursors and their ability to undergo vinyl cyclopropane rearrangements (VCPR). Groups which effectively stabilised radicals, specifically heteroarenes, were found to result in the lowest energy barriers. Ten novel precursors were synthesised to test the accuracy of computational predictions; the most reactive species which contained heteroarenes underwent thermal rearrangements at room temperature to afford novel difluorocyclopentenes and fluorinated benzocycloheptadienes through competing VCPR and [3,3]-rearrangement pathways, respectively. More controlled rearrangement of ethyl 3-(1'(2'2'-difluoro-3'-benzo[d][1,3]dioxol-5-yl)cyclopropyl)propenoate (22) allowed these competing pathways to be monitored at the same time and activation energies for both reactions were determined; E_a(VCPR) = (23.4 ± 0.2) kcal mol^-1 and E_a([3,3]) = (24.9 ± 0.3) kcal mol^-1. Comparing our calculated activation energies with these parameters showed that no single method stood out as the most accurate for predicting barrier heights; (U)M05-2X/6-31+G* methodology remained the best for VCPR but M06-2X/6-31G* was better for the [3,3]-rearrangement. The consistency observed with (U)B3LYP/6-31G* calculations meant that it came closest to a universal method for dealing with these systems. The developed computational design model correctly predicted the observed selectivity of rearrangement pathways for both our system and literature compounds.

Preparation of cycloheptane ring by nucleophilic cyclopropanation of 1,2-diketones with bis(iodozincio)methane

The nucleophlic cyclopropanation of hexa-1,5-diene-3,4-diones with bis(iodozincio)methane afforded the Zn alkoxides of cyclohepta-2,7-diene-1,2-diols stereospecifically via Zn alkoxides of cis-dialkenylcyclopropane-1,2-diols.

Reversal of the regiochemistry in the rhodium-catalyzed [4+3] cycloaddition between vinyldiazoacetates and dienes

A regio-, diastereo-, and enantioselective [4+3] cycloaddition between vinylcarbenes and dienes has been achieved using the dirhodium tetracarboxylate catalyst [Rh2(S-BTPCP)4]. This methodology provides facile access to 1,4-cycloheptadienes that are regioisomers of those formed from the tandem cyclopropanation/Cope rearrangement reaction of vinylcarbenes with dienes.

Cyclopropene cycloadditions with annulated furans: total synthesis of (+)- and (-)-frondosin B and (+)-frondosin A

The asymmetric total syntheses of the natural products (+)- and (-)-frondosin B and (+)-frondosin A are reported based on a diastereoselective cycloaddition between tetrabromocyclopropene and an annulated furan to provide a highly functionalized common building block. The bridged bicyclic intermediate could be stereo- and chemoselectively manipulated to produce the two structurally distinct members of the frondosins. Both syntheses feature regioselective palladium-coupling reactions and an unprecedented phosphine-mediated ether bridge cleavage. Surprisingly, the planned enantioselective synthesis of frondosin B led to the opposite epimer of the natural product, suggesting an unusual late stage stereoinversion at C8. Frondosin A, but not frondosin B, was shown to have selective antiproliferative activity against several B-cell lines.

Recent applications of the divinylcyclopropane-cycloheptadiene rearrangement in organic synthesis

This review summarizes the application of the divinylcyclopropane–cycloheptadiene rearrangement in synthetic organic chemistry. A brief overview of the new mechanistic insights concerning the title reaction is provided as well as a condensed account on the biological relevance of the topic. Heteroatom variants of this rearrangement are covered briefly.

Rhodium-catalyzed tandem cyclopropanation/Cope rearrangement of 4-alkenyl-1-sulfonyl-1,2,3-triazoles with dienes

Take your pick… A practical method for the synthesis of structurally diverse rhodium vinylcarbenes from stable 1-sulfonyl-1,2,3-triazole precursors has been developed. The reaction is general for a broad range of 4-alkenyl triazoles and dienes, enabling the stereoselective synthesis of a variety of polycyclic imines, which are readily converted into amines or aldehydes in a one-pot process.

A novel, facile approach to frondosin B and 5-epi-liphagal via a new [4 + 3]-cycloaddition

A new [4 + 3]-cycloaddition between benzofuran allylic alcohols and dienes, promoted by camphorsulfonic acid, has been identified. A novel strategy which used this cycloaddition as a key step has been developed for the synthesis of 6,7,5-tricyclic skeleta, and syntheses toward frondosin B (1) and 5-epi-liphagal (2) have been achieved via short routes in good yields.

Sequential rhodium-, silver-, and gold-catalyzed synthesis of fused dihydrofurans

A triple cascade process was developed for the rapid synthesis of polycyclic benzo-fused dihydrofurans. The first step is a rhodium-catalyzed cyclopropanation of α-aryldiazoketones with alkenes. This is followed by a silver-catalyzed ring expansion to dihydrofurans, which then undergo a gold-catalyzed cyclization to form benzo-fused dihydrofurans.

Enantioselective synthesis of 2,2,5-tri- and 2,2,5,5-tetrasubstituted tetrahydrofurans via [4 + 2] cycloaddition and ring-opening cross-metathesis

A chiral vinyl sulfoxide has been developed that undergoes highly diastereoselective Diels-Alder cycloadditions with various substituted furans in excellent yield. The cycloadducts can be stereoselectively transformed into 2,2,5-tri- and 2,2,5,5-tetrasubstituted tetrahydrofurans, which are structural subunits of many natural products, via regioselective ring-opening metathesis/cross-metathesis or oxidative cleavage/refunctionalization.