An exploratory study of type II [3 + 4] cycloadditions between vinylcarbenoids and dienes

Dearomative [4 + 3] cycloaddition of furans with vinyl-N-triftosylhydrazones by silver catalysis: stereoselective access to oxa-bridged seven-membered bicycles

A practical dearomative [4 + 3] cycloaddition of furans with vinylcarbenes to access oxa-bridged seven-membered carbocycles, with complete and predictable stereoselectivity, is achieved by merging silver catalysis and vinyl-N-triftosylhydrazones.

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.

Synthetic applications of type II intramolecular cycloadditions

Type II intramolecular cycloadditions ([4+2], [4+3], [4+4] and [5+2]) have emerged recently as an efficient and powerful strategy for the construction of bridged ring systems. In general, type II cycloadditions provide access to a wide range of bridged bicyclo[m.n.1] ring systems with high regio- and diastereoselectivity in an easy and straightforward manner. In each section of this review, an overview of the corresponding type II cycloadditions is presented, which is followed by highlights of method development and synthetic applications in natural product synthesis. The goal of this review is to provide a survey of recent advances in the field covering literature up to 2020. The review will serve as a useful reference for organic chemists engaged in the total synthesis of natural products containing bridged bicyclo[m.n.1] ring systems and provide strong stimulus for invention and further advances in this exciting research field.

Enantioselective Total Syntheses of Pentacyclic Homoproaporphine Alkaloids

Herein we report the first enantioselective total syntheses of pentacyclic homoproaporphine alkaloids by means of a route, which includes a tandem retro-oxa-Michael addition and nucleophilic substitution to generate the oxa-benzobicyclco[3.3.1]nonane core structure, a Pictet-Spengler cyclization to construct the fused B and C rings, and sequential Baeyer-Villiger oxidation and pinacol-type cyclization to install the hydroxyl-lactol moiety of D ring. With this unified route, six pentacyclic homoproaporphine alkaloids have been synthesized enantioselectively.

Pharmacophore-Directed Retrosynthesis Applied to Rameswaralide: Synthesis and Bioactivity of Sinularia Natural Product Tricyclic Cores

A pharmacophore-directed retrosynthesis strategy applied to rameswaralide provided simplified precursors bearing the common 5,5,6 (red) and 5,5,7 (blue) skeleton present in several cembranoid and norcembranoids from Sinularia soft corals. Key steps include a Diels-Alder lactonization organocascade delivering the common 5,5,6 core and a subsequent ring expansion affording a 5,5,7 core serviceable for the synthesis of rameswaralide. Initial structure-activity relationships of intermediates en route to the natural product have revealed interesting differential and selective cytotoxicity.

Synthesis of 1,2-divinylcyclopropanes by metal-catalyzed cyclopropanation of 1,3-dienes with cyclopropenes as vinyl carbene precursors

The synthesis of 1,2-divinylcyclopropanes by the reaction of cyclopropenes with 1,3-dienes is reported. The process relies on the ability of ZnCl₂ or [Rh₂(OAc)₄] to generate metal-vinyl carbene intermediates from cyclopropenes, which effect cyclopropanation of 1,3-dienes. Most of the reactions proceeded in reasonable yields while the diastereoselectivity strongly depends on the structure of the diene. An example of an intramolecular process as well as the use of furan and 1,4-cyclohexadiene as dienes are also reported.

Model Studies To Access the [6,7,5,5]-Core of Ineleganolide Using Tandem Translactonization-Cope or Cyclopropanation-Cope Rearrangements as Key Steps

Recently, we reported a convergent cyclopropanation-Cope approach to the core of ineleganolide, which was the first disclosed synthesis of the core of the norditerpene natural product ineleganolide. In this complementary work, a model system for the core of ineleganolide has been prepared through a series of tandem cyclopropanation-Cope and translactonization-Cope rearrangements. Work with this model system has enriched our understanding of the cyclopropanation-Cope rearrangement sequence. Additionally, research into this model system has driven the development of tandem translactonization-Cope rearrangements.

Cycloaddition reactions of enoldiazo compounds

Enoldiazo esters and amides have proven to be versatile reagents for cycloaddition reactions that allow highly efficient construction of various carbocycles and heterocycles. Their versatility is exemplified by (1) [2+n]-cycloadditions (n = 3, 4) by the enol silyl ether units of enoldiazo compounds with retention of the diazo functionality to furnish α-cyclic-α-diazo compounds that are themselves subject to further transformations of the diazo functional group; (2) [3+n]-cycloadditions (n = 1-5) by metallo-enolcarbenes formed by catalytic dinitrogen extrusion from enoldiazo compounds; (3) [2+n]-cycloadditions (n = 3, 4) by donor-acceptor cyclopropenes generated in situ from enoldiazo compounds that produce cyclopropane-fused ring systems. The role of dirhodium(ii) and the emergence of copper(i) catalysts are described, as are the different outcomes of reactions initiated with these catalysts. This comprehensive review on cycloaddition reactions of enoldiazo compounds, with emphasis on methodology development, mechanistic insight, and catalyst-controlled chemodivergence, aims to provide inspiration for future discoveries in the field and to catalyze the application of enoldiazo reagents by the wider synthetic community.

Enantioselective, Convergent Synthesis of the Ineleganolide Core by a Tandem Annulation Cascade

An enantioselective and diastereoselective approach toward the synthesis of the polycyclic norditerpenoid ineleganolide is disclosed. A palladium-catalyzed enantioselective allylic alkylation is employed to stereoselectively construct the requisite chiral tertiary ether and facilitate the synthesis of a 1,3-cis-cyclopentenediol building block. Careful substrate design enabled the convergent assembly of the ineleganolide [6,7,5,5]-tetracyclic scaffold by a diastereoselective cyclopropanation-Cope rearrangement cascade under unusually mild conditions. Computational evaluation of ground state energies of late-stage synthetic intermediates was used to guide synthetic development and aid in the investigation of the conformational rigidity of these highly constrained and compact polycyclic structures. This work represents the first successful synthesis of the core structure of any member of the polycyclic norcembranoid diterpene family of natural products. Advanced synthetic manipulations generated a series of natural product-like compounds that were shown to possess selective secretory antagonism of either interleukin-5 or interleukin-17. This bioactivity stands in contrast to the known antileukemic activity of ineleganolide and suggests the norcembranoid natural product core may serve as a useful scaffold for the development of diverse therapeutics.

Enantioselective iron-catalyzed intramolecular cyclopropanation reactions

An iron-catalyzed asymmetric intramolecular cyclopropanation was realized in high yields and excellent enantioselectivity (up to 97% ee) by using the iron complexes of chiral spiro-bisoxazoline ligands as catalysts. The superiority of iron catalysts exhibited in this reaction demonstrated the potential abilities of this sustainable metal in asymmetric carbenoid transformation reactions.

Gold-catalyzed cycloisomerization of 1,6,8-dienyne carbonates and esters to cis-cyclohepta-4,8-diene-fused pyrrolidines

A synthetic approach that provides access to cis-cyclohepta-4,8-diene-fused pyrrolidines efficiently through Au(I) -catalyzed cycloisomerization of 1,6,8-dienyne carbonates and esters at a low catalyst loading of 2 mol % is reported. Starting carbonates and esters with a pendant alkyl group on the terminal alkenyl carbon center were found to favor tandem 1,2-acyloxy migration/cyclopropanation followed by Cope rearrangement of the resulting cis-3-azabicyclo[3.1.0]hexane intermediate. On the other hand, substrates containing a terminal diene or starting materials in which the distal alkene moiety bears a phenyl substituent were observed to undergo competitive but reversible 1,3-acyloxy migration prior to the nitrogen-containing bicyclic ring formation. The delineated reaction mechanism also provides experimental evidence for the reversible interconversion between the oft-proposed organogold intermediates obtained in this step of the tandem process.

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.

Total synthesis of monosporascone and dihydromonosporascone

The first synthesis of monosporascone has been achieved in five steps and 57% overall yield.

Synthetic studies and mechanistic insight in nickel-catalyzed [4+2+1] cycloadditions

A new nickel-catalyzed procedure for the [4+2+1] cycloaddition of (trimethylsilyl)diazomethane with alkynes tethered to dienes has been developed. A broad range of unsaturated substrates participate in the sequence, and stereoselectivities are generally excellent. Stereochemical studies provided evidence for a mechanism that involves the [3,3] sigmatropic rearrangement of divinylcyclopropanes.

Sequential Cycloaddition Approach to the Tricyclic Core of Vibsanin E. Total Synthesis of (±)-5-epi-10-epi-Vibsanin E

[chemical reaction: see text]. A direct access to (+/-)-5-epi-10-epi-vibsanin E is described, based on three key cycloaddition steps, a rhodium-catalyzed [4 + 3] cycloaddition, a heteronuclear [4 + 2] cycloaddition, and a photochemically induced [4 + 2] cycloaddition.

An Efficient [4 + 2 + 1] Entry to Seven-Membered Rings

A new nickel-catalyzed procedure for the [4 + 2 + 1] cycloaddition of trimethylsilyl diazomethane with alkynes tethered to dienes has been developed. A broad range of unsaturated substrates participate in the sequence, and stereoselectivities are generally excellent. Three possible mechanisms are proposed, and each involves the generation of a transient nickel carbene species.

Controllable Diastereoselective Cyclopropanation. Enantioselective Synthesis of Vinylcyclopropanes via Chiral Telluronium Ylides

Novel chiral telluronium salts 1 are designed for asymmetric synthesis of 1,3-disubstituted 2-vinylcyclopropanes. The allylides, generated in situ from the corresponding telluronium salt in the presence of different base, reacted with alpha,beta-unsaturated esters, ketones, and amides to afford cis-2-silylvinyl-trans-3-substituted or trans-2-silylvinyl-trans-3-substituted cyclopropane derivatives with high diastereoselectivity and excellent enantioselectivity in good to high yields. Thus, either one of the two diastereomers could be enantioselectively synthesized at will just by the choice of LiTMP/HMPA or LDA/LiBr. The first examples of catalytic ylide reaction for enantioselective synthesis of 1,3-disubstituted 2-vinylcyclopropanes with high distereoselectivity is also achieved.