Anionic oxy-Cope Research Articles

Approach to the Core Structure of Signermycin B.

Among the natural tetramic acids with a decalinoyl part, signermycin B is unique because it contains a cis-decalin. In this paper, we demonstrate that the cis-decalin section of signermycin B can be accessed by an anionic oxy-Cope rearrangement. The substrate, a tricyclic dienol was prepared by an intramolecular Diels-Alder reaction of a masked ortho-benzoquinone, generated by oxidation of an α-methoxyphenol in presence of cis-2-hexenol. After a superfluous bromine on the cycloadduct was removed, reaction of the tricyclic ketone with isopropenylmagnesium bromide led to the tricyclic trienol that underwent the oxy-Cope rearrangement to a cis-decalinone. While we could show, that introduction of the 4-ethyl substituent (signermycin B numbering) is possible by enolate alkylation, the 4-epi-isomer was formed.

Synthetic study toward tridachiapyrone B.

A convergent approach to the skeleton of tridachiapyrone B is described taking advantage of the desymmetrization of α,α'-dimethoxy-γ-pyrone leading to α-crotyl-α'-methoxy-γ-pyrone in one step. To construct the quaternary carbon of the 2,5-cyclohexadienone of the target, a strategy based on the Robinson-type annulation of an aldehyde derived from α-crotyl-α'-methoxy-γ-pyrone was applied. The grafting of the simplified target's side chain was demonstrated through an oxidative anionic oxy-Cope rearrangement of the tertiary alcohol arising from the 1,2-addition of a 1,3-dimethylallyl reagent to 2,5-cyclohexadienone connected to the α'-methoxy-γ-pyrone motif.

Bi(OAc)3/Chiral Phosphoric Acid-Catalyzed Enantioselective 1,2- and Formal 1,4-Allylation Reaction of β,γ-Unsaturated α-Ketoesters.

A highly efficient asymmetric 1,2-allylation reaction of β,γ-unsaturated α-ketoesters was realized by using a Bi(OAc)3/chiral phosphoric acid catalyst system under mild conditions. Meanwhile, using this combined strategy of enantioselective 1,2-allylation and subsequent anionic oxy-Cope rearrangement, the asymmetric formal 1,4-allylation reaction was achieved by a one-pot process. These reactions offer rapid access to an array of homoallylic tertiary alcohols and γ-allyl-α-ketoesters with good yields and excellent enantioselectivities. Density functional theory calculations were conducted to interpret the high enantioselectivity.

Enantioselective Catalysis of an Anionic Oxy-Cope Rearrangement Enabled by Synergistic Ion Binding.

Charge-accelerated rearrangements present interesting challenges to enantioselective catalysis, due in large part to the competing requirements for maximizing reactivity (ion-pair separation) and stereochemical communication. Herein, we describe application of a synergistic ion-binding strategy to catalyze the anionic oxy-Cope rearrangement of a symmetric bis-styrenyl allyl alcohol in up to 75:25 e.r. Structure-reactivity-selectivity relationship studies, including linear free-energy-relationship analyses, with bifunctional urea catalysts indicate that H-bonding and cation-binding interactions act cooperatively to promote the chemo- and enantioselective [3,3]-rearrangement. Implications for catalyst designs applicable to other transformations involving oxyanionic intermediates are discussed.

Tandem Anionic oxy-Cope Rearrangement/Oxygenation Reactions as a Versatile Method for Approaching Diverse Scaffolds.

Tandem anionic oxy-Cope rearrangement/radical oxygenation reactions provide δ,ϵ-unsaturated α-(aminoxy) carbonyl compounds, which serve as convenient precursors to diverse compound classes. Functionalized carbocycles are accessible by very rare all-carbon 5-endo-trig cyclizations, but also common 5-exo-trig radical cyclizations, based on the persistent radical effect. The tandem reactions can be further extended by highly diastereoselective allylation or reduction steps to give complex scaffolds.

Asymmetric Total Synthesis of (-)-Erogorgiaene and Its C-11 Epimer and Investigation of Their Antimycobacterial Activity.

A short, nine-step, highly enantioselective synthesis of (-)-erogorgiaene and its C-11 epimer is reported. The key stereochemistry controlling steps involve catalytic asymmetric crotylation, anionic oxy-Cope rearrangement and cationic cyclisation. (-)-Erogorgiaene exhibited promising antitubercular activity against multidrug-resistant strains of Mycobacterium tuberculosis.

A Novel Allyl Transfer Coupled with a Grob Fragmentation.

A novel acid-promoted rearrangement is disclosed. In the previously unknown transformation, an allyl group migrated to an in situ formed carbocation stabilized by an electron-rich aryl or heteroaryl group, resulting in a stereoselective intramolecular Grob fragmentation. The outcome of the rearrangement observed with an array of substrates can be satisfactorily rationalized using a working hypothesis with the aid of a six-membered transition state similar to those proposed for the anionic oxy-Cope or oxonia-Cope rearrangements, but involving only one instead of two double bonds.

Catalytic asymmetric crotylation of aldehydes: application in total synthesis of (-)-elisabethadione.

A new, highly efficient Lewis base catalyst for a practical enantio- and diastereoselective crotylation of unsaturated aldehydes with E- and Z-crotyltrichlorosilanes has been developed. The method was employed as a key step in a novel asymmetric synthesis of bioactive serrulatane diterpene (-)-elisabethadione. Other strategic reactions for setting up the stereogenic centers included anionic oxy-Cope rearrangement and cationic cyclization. The synthetic route relies on simple, high yielding reactions and avoids use of protecting groups or chiral auxiliaries.

Synthesis of mulinane diterpenoids.

The mulinane class of diterpenoids is a set of tricyclic (5-6-7), biologically active natural products whose members exhibit a variety of oxidation states. Herein, we report the inaugural synthesis of four mulinanes employing a divergent approach that relies on a diastereoselective anionic oxy-Cope rearrangement to set the relative configuration of the C8 stereocenter and an unprecedented vinylogous Saegusa dehydrogenation reaction to address C-ring functionality.

Divergent pathways to furosesquiterpenes: first total syntheses of (+)-zedoarol and (Rac)-gweicurculactone.

A non-natural hydroxy-elemane was found amenable to divergent transformations, producing either polyunsaturated guaianes under basic, oxygen-free conditions, or oxidized furogermacranes when anionic oxy-Cope reaction quenched by an oxidant is employed. Based on these findings, the first total syntheses of zedoarol and gweicurculactone are reported.

ChemInform Abstract: Total Synthesis of the Congested Propellane Alkaloid (‐)‐Acutumine

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Total synthesis of the congested propellane alkaloid (-)-acutumine.

The enantioselective total synthesis of (-)-acutumine is described. The synthetic strategy was inspired by the premise that the cyclohexenone ring could be derived from an aromatic precursor. After successful construction of a propellane model system, an initial attempt to prepare the spirocyclic subunit was thwarted by incorrect regioselectivity in a radical cyclization. A second-generation approach involving a radical-polar crossover reaction was successful, and the chemistry developed in the aforementioned model system was then applied to synthesize the natural product. Key reactions included a phenolic oxidation, a diastereoselective ketone allylation utilizing Nakamura's chiral allylzinc reagent, an anionic oxy-Cope rearrangement, an acid-promoted cyclization of a secondary amine onto an α,β-unsaturated ketal, and a regioselective methyl enol etherification of a 1,3-diketone.

Enantioselective Conjugate Allylation of Cyclic Enones

Enantioselective organocatalytic 1,2-allylation of a cyclic enone followed by anionic oxy-Cope rearrangement delivered the ketone as a mixture of diastereomers. This appears to be a general method for the net enantioselective conjugate allylation of cyclic enones.

Enantioselective Total Synthesis and Confirmation of the Absolute and Relative Stereochemistry of Streptorubin B

The enantioselective total synthesis of the pyrrolophane natural product streptorubin B is described. Key steps in the concise route include the application of a one-pot enantioselective aldol cyclization/Wittig reaction and an anionic oxy-Cope rearrangement to forge the crucial 10-membered ring. Comparisons between CD spectra of synthetic and natural samples of streptorubin B coupled with X-ray crystallography allowed for the determination of the absolute stereochemistry of this natural product for the first time. These studies also provided unambiguous proof of the relative configuration between the butyl side chain and the bispyrrole subunit. Additional studies revealed a novel atropstereoselective Paal-Knorr pyrrole condensation and provided fundamental experimental insight into the barrier for atropisomerization of the natural product.

Fast Oxy-Cope Rearrangements of Bis-alkynes: Competition with Central C−C Bond Fragmentation and Incorporation in Tunable Cascades Diverging from a Common Bis-allenic Intermediate

Fast anionic oxy-Cope rearrangements of 1,5-hexadiyn-3,4-olates can be incorporated into cascade transformations which rapidly assemble densely functionalized cyclobutenes or cyclopentenones via a common bis-allenic intermediate. The competition between fragmentation, 4π-electrocyclic closure, and aldol condensation can be efficiently controlled by the nature of the acetylenic substituents. The rearrangement of bis-alkynes with two hydroxyl substituents opens a conceptually interesting entry in the chemistry of ε-dicarbonyl compounds and suggests a new approach to analogues of rocaglamide/aglafolin.

On the Origin of Altered Diastereomeric Ratios for Anionic versus Neutral Reaction Conditions in the Oxy‐Cope/Ene Reaction: An Interplay of Experiment and Computational Modeling

We report herein a detailed investigation into the reaction mechanism for a sequential oxy-Cope/ene reaction under anionic conditions. With DFT calculations and ab initio molecular dynamics simulations, the observed diastereoselectivity is shown to be the result of an isomerization of the enolate olefin, which would evidently not occur under neutral conditions. The potential energy surface was thoroughly mapped out for the reaction pathways and the proposed mechanism confirmed the different product distributions observed under neutral and anionic oxy-Cope conditions. In addition, other possible pathways are shown to be higher in energy and experimental evidence is given that supports the olefin-isomerization pathway.

Tandem Vinylogous 1,2-Addition/Anionic Oxy-Cope Reaction Leading from Butadiene Sulfone to an Orthogonally Functionalized Bicycle

Here we present a transition metal-free synthesis of a rigid, orthogonally functionalized bicyclic sulfone, starting from readily available reagents. The transformation proceeds via a tandem vinylogous 1,2-addition/anionic oxy-Cope sequence, followed by a second vinylogous ketone addition. Stereochemical assignments suggest that the anionic oxy-Cope reaction proceeds exclusively through a boat-shaped transition state. The product of the two-step sequence can be further functionalized through subsequent chemo- and diastereoselective transformations, suggesting possible applications in medicinal chemistry or materials chemistry.

ChemInform Abstract: Sequential Vinylcyclopropylcarbene and Anionic Oxy-Cope Rearrangements: An Expedient Synthesis of Nine-Membered Rings.

Abstract 5-Alkenyl-2-methoxy-2-cyclopentenones, readily available from thermolysis of 2-alkenylcyclopropylcarbene-chromium complexes, can be converted to nine-membered rings by treatment with vinylcerium reagents and subsequent anionic oxy-Cope rearrangement.

ChemInform Abstract: Thermal Reactions of a 2-Aryl-1-vinylcyclobutanol.

Abstract Thermolysis reactions of the exo (distal) and endo (proximal) isomers of 7-vinyl-6b,8,8a-tetrahydrocyclobut[a]acenaphthylen-7-ol (1a and 1b) have been studied, both in refluxing xylene and in a packed pyrolysis column. These epimers were prepared from the corresponding cyclobutanone (3). An oxy-Cope rearrangement of 1b was not observed under any conditions. Both epimers gave the same thermolysis products, a fused cyclohexanone derivative 15 (a formal [1,3] shift product) and vinyl ketone 16 (a retro-ene product). At temperatures above 250 °C some acenaphthylene was also obtained. The anionic oxy-Cope variant gave only 15. A common diradical intermediate is proposed for the thermal reactions.

ChemInform Abstract: Intramolecularly Competitive Anionic Oxy-Cope Rearrangements.

Abstract Two examples of the title reaction are described. While the kinetically preferred pathway for 2 utilizes the vinyl substituent, that for 16 skirts this option completely. To a great extent, these biases are a function of the structural features inherent to these alochols.