Generation and trapping of electron-deficient 1,2-cyclohexadienes. Unexpected hetero-Diels-Alder reactivity

Total Synthesis of Lissodendoric Acid A

We describe a full account of our synthetic strategy leading to the first total synthesis of the manzamine alkaloid lissodendoric acid A . These efforts demonstrate that strained cyclic allenes are valuable synthetic building blocks and can be employed efficiently in total synthesis.

Diels-Alder Cycloadditions of Oxacyclic Allenes and α-Pyrones

Reactions of α-pyrones with oxacyclic allenes in Diels-Alder trappings are described. We investigate regioselectivity trends and perform competition experiments to assess the influence of structural and electronic features on relative reaction rates. We also demonstrate the stereospecific trapping of an oxacyclic allene, which proceeds in high optical yield. This study provides insight into strained cyclic allene reactivity, as well as new synthetic tools for the rapid construction of complex, heterocyclic scaffolds.

Access to Complex Scaffolds Through [2 + 2] Cycloadditions of Strained Cyclic Allenes

We report the strain-induced [2 + 2] cycloadditions of cyclic allenes for the assembly of highly substituted cyclobutanes. By judicious choice of trapping agent, complex scaffolds bearing heteroatoms, fused rings, contiguous stereocenters, spirocycles, and quaternary centers are ultimately accessible. Moreover, we show that the resulting cycloadducts can undergo thermal isomerization. This study provides an alternative strategy to photochemical [2 + 2] cycloadditions for accessing highly functionalized cyclobutanes, while validating the use of underexplored strained intermediates for the assembly of complex architectures.

A Cascade of Strain-Driven Events Converting Benzynes to Alkynylbenzocyclobutenes to 1,3-Dien-5-ynes to Cyclic Allenes to Benzocyclohexadienones

Here, we report a strain-promoted cascade reaction that proceeds via multiple strained intermediates, ultimately driven by the high potential energy inherent in alkyne triple bonds (C≡C). More specifically, four alkynes (three from an HDDA benzyne precursor and the fourth from a conjugated enyne reaction partner) are transformed into eight of the skeletal carbons in the benzocyclohexadienone products. The reaction pathway proceeds sequentially via strained benzyne, benzocyclobutene, and cyclic allene intermediates. DFT computations suggest that the slowest step following benzyne generation is the 4π-electrocyclic ring-opening of the alkynylbenzocyclobutene to a 1,3-dien-5-yne (an alkynylxylylene) intermediate. The activation energy for the subsequent 6π-electrocyclic ring-closure is lower than that for related acyclic dienynes because of the aromaticity that is being regained in the transition structure. Finally, the isolation of the benzocyclohexadienone products rather than their phenolic tautomers is notable.

Stereoselective Intramolecular [2+2] Trapping of 1,2-Cyclohexadienes: a Route to Rigid, Angularly Fused Tricyclic Scaffolds

1,2-Cyclohexadienes generated under mild fluoride-mediated desilylative conditions undergo efficient intramolecular [2+2] trapping, providing tricyclic alkylidene cyclobutanes with complete diastereoselectivity for the cis-fused products. Pendent styrenes or electron-deficient olefins can trap simple 1,2-cyclohexadienes or their oxygenated counterparts, with 14 substrates being disclosed. Reactions proceed at ambient temperature using just cesium fluoride in up to 91 % yield, and the necessary precursors are easily accessed from substituted 2-bromocyclohexenones. Multiple synthetic routes have been developed to install the appropriate functional groups required for [2+2] trapping.

Cyclic Allene Approach to the Manzamine Alkaloid Keramaphidin B

We report an approach to the core of the manzamine alkaloid keramaphidin B that relies on the strain-promoted cycloaddition of an azacyclic allene with a pyrone trapping partner. The cycloaddition is tolerant of nitrile and primary amide functional groups and can be complemented with a subsequent retro-Diels-Alder step. These efforts demonstrate that strained cyclic allenes can be used to build significant structural complexity and should encourage further studies of these fleeting intermediates.

Catalyst-Controlled Annulations of Strained Cyclic Allenes with π-Allylpalladium Complexes

Strained cyclic allenes are a class of in situ-generated fleeting intermediates that, despite being discovered more than 50 years ago, has received significantly less attention from the synthetic community compared to related strained intermediates. Examples of trapping strained cyclic allenes that involve transition metal catalysis are especially rare. We report the first annulations of highly reactive cyclic allenes with in situ-generated π-allylpalladium species. By varying the ligand employed, either of two isomeric polycyclic scaffolds can be obtained with high selectivity. The products are heterocyclic and sp3-rich and bear two or three new stereocenters. This study should encourage the further development of fragment couplings that rely on transition metal catalysis and strained cyclic allenes for the rapid assembly of complex scaffolds.

A Distinct Mode of Strain-Driven Cyclic Allene Reactivity: Group Migration to the Central Allene Carbon Atom

Strained cyclic allenes are reactive species that can be trapped in a variety of complementary fashions that capitalize on their inherent high potential energy. 1,2,4-Cyclohexatrienes represent a subclass of allenes that, notably, can be conveniently generated by a net [4 + 2] cycloaddition within a 1,3-enyne bearing a tethered alkyne via a tetradehydro-Diels-Alder reaction. A limitation to the use of this type of thermally generated cyclic allene as a construct for the introduction of molecular complexity is their propensity to isomerize to benzenoids via a simple net 1,5-hydrogen atom migration. We have discovered that when the enyne component of the substrate is modified as an enol silyl ether (or an enol ester), migration of the silyl (or acyl) group can become the predominant event. Specifically, an appropriately electrophilic group can migrate from the O atom to the central allene carbon adjacent to the 1-siloxy(acyloxy) substituent. This process leads to highly substituted phenolic products (e.g., o-silyl phenols) following tautomerization of the intermediate cyclohexa-2,4-dienone. Experimental studies show that this novel mode of reactivity is general; DFT studies reveal the unimolecular nature of the group migration.

2 + 2 Trapping of Acyloxy-1,2-cyclohexadienes with Styrenes and Electron-Deficient Olefins

Oxygenated-1,2-cyclohexadienes and their unsubstituted counterpart can be generated under mild conditions by fluoride-induced desilylation and undergo intermolecular [2 + 2]-cycloaddition reactions with a variety of alkene traps to afford bicyclo[4.2.0]octenes. Both styrenes and electron-deficient olefins react in good conversion and with complete regioselectivity in favor of cyclobutane formation at the unsubstituted C2/C3 carbons of the C1-substituted cyclic allenes. Diastereoselectivities are modest (1.1-5.7:1) with a preference for the exo-isomer.

Merging Metals and Strained Intermediates

Strained intermediates such as cyclic alkynes and allenes are most commonly utilized in nucleophilic additions and cycloadditions, but have seen increased use in a third area of reactivity: metal-mediated transformations. The merger of strained intermediates and metal catalysis has enabled rapid access to complex, polycyclic systems. Following a discussion of relevant landmark studies involving metals and strained intermediates, this article highlights recent advances in transition metal-mediated transformations from our laboratory. Specifically, this includes the use of arynes in the synthesis of decorated organometallic complexes, and the utilization of cyclic allenes to access enantioenriched heterocycles. Moreover, the broad applicability of such transformations, and exciting future areas of research are discussed.

Ethyl 10-cyano-7-hy-droxy-6-oxo-3-phenyl-8,9,10,10a-tetra-hydro-6H-benzo[c]chromene-10-carboxyl-ate

In the title compound, C23H19NO5, the cyano group adopts an axial orientation and the ester group an equatorial orientation. The dihedral angle between the pendant phenyl group and the benzene ring of the fused-ring system is 25.97 (8)°. Intra-molecular O-H⋯O and C-H⋯O hydrogen bonds are observed and the packing is consolidated by C-H⋯O and C-H⋯π inter-actions.

Interception of 1,2-cyclohexadiene with TEMPO radical

Transient strained cyclic intermediates, such as strained cyclic allenes, are useful building blocks for the synthesis of structurally and stereochemically complex scaffolds. Trappings of strained cyclic allenes are thought to occur primarily through either two or one electron processes. Regarding the latter, diradical intermediates have been invoked in (2 + 2) cycloadditions and (3 + 2) nitrone cycloadditions. The present study questions if a monoradical pathway could exist for strained cyclic allene reactivity, as examined in the reaction of 1,2-cyclohexadiene and TEMPO radical. Our findings suggest the viability of this monoradical pathway.

Leveraging Fleeting Strained Intermediates to Access Complex Scaffolds

Arynes, strained cyclic alkynes, and strained cyclic allenes were validated as plausible intermediates in the 1950s and 1960s. Despite initially being considered mere scientific curiosities, these transient and highly reactive species have now become valuable synthetic building blocks. This Perspective highlights recent advances in the field that have allowed access to structural and stereochemical complexity, including recent breakthroughs in asymmetric catalysis.

Palladium-Catalyzed Annulations of Strained Cyclic Allenes

We report Pd-catalyzed annulations of in situ generated strained cyclic allenes. This methodology employs aryl halides and cyclic allene precursors as the reaction partners in order to generate fused heterocyclic products. The annulation proceeds via the formation of two new bonds and an sp3 center. Moreover, both diastereo- and enantioselective variants of this methodology are validated, with the latter ultimately enabling the rapid enantioselective synthesis of a complex hexacyclic product. Studies leveraging transition metal catalysis to intercept cyclic allenes represent a departure from the more common, historical modes of cyclic allene trapping that rely on nucleophiles or cycloaddition partners. As such, this study is expected to fuel the development of reactions that strategically merge transition metal catalysis and transient strained intermediate chemistry for the synthesis of complex scaffolds.

Origins of Endo Selectivity in Diels-Alder Reactions of Cyclic Allene Dienophiles

Strained cyclic allenes, first discovered in 1966 by Wittig and co-workers, have recently emerged as valuable synthetic building blocks. Previous experimental investigations, and computations reported here, demonstrate that the Diels-Alder reactions of furans and pyrroles with 1,2-cyclohexadiene and oxa- and azaheterocyclic analogs proceed with endo selectivity. This endo selectivity gives the adduct with the allylic saturated carbon of the cyclic allene endo to the diene carbons. The selectivity is very general and useful in synthetic applications. Our computational study establishes the origins of this endo selectivity. We analyze the helical frontier molecular orbitals of strained cyclic allenes and show how secondary orbital and electrostatic effects influence stereoselectivity. The LUMO of carbon-3 of the allene (C-3 is not involved in primary orbital interactions) interacts in a stabilizing fashion with the HOMO of the diene in such a way that the carbon of the cyclic allene attached to C-1 favors the endo position in the transition state. The furan LUMO, allene HOMO interaction reinforces this preference. These mechanistic studies are expected to prompt the further use of long-avoided strained cyclic allenes in chemical synthesis.