Rhodium-Catalyzed Desymmetric Arylation of γ,γ-Disubsituted Cyclohexadienones: Asymmetric Synthesis of Chiral All-Carbon Quaternary Centers

Enantioselective Divergent Total Syntheses of Cycloaurenones and Dysiherbols

Cycloaurenones and dysiherbols are naturally occurring sesquiterpene quinones/quinols that share a 6/6/5/6 tetracyclic carbon skeleton with either a cis- or trans-decalin system containing four contiguous stereocenters, including three contiguous all-carbon quaternary stereocenters. Total syntheses of cycloaurenones have not been reported. Herein, we present the first enantiodivergent syntheses of cycloaurenones and dysiherbols based on manipulation of a common cyclohexadienone intermediate: namely, a local desymmetric Giese-Baran-type cyclization for cycloaurenones and a copper-catalyzed enantioselective conjugate addition for dysiherbols. Moreover, the key cyclohexadienone intermediate was readily accessible by a bidirectional approach from a chiral bis-Weinreb amide. The 1,4-nonadjacent stereocenters were installed by an unprecedented enantioselective hydrogenation of the corresponding bis-α,β-unsaturated Weinreb amide (>99:1 chiral/meso ratio, >99% enantiomeric excess).

HTE-Enabled Development of an Ene-Reductase-Catalyzed Desymmetrization: Remote Control of All-Carbon Quaternary γ-Centers

We report the remote stereocontrol of all-carbon quaternary γ-centers via an ene-reductase (ERED)-catalyzed desymmetrization of prochiral cyclohexadienones. By leveraging high-throughput experimentation (HTE) protocols, we were able to rapidly identify EREDs capable of desymmetrizing both spirocyclic cyclohexadienones and non-spirocyclic 2,6-disubstituted cyclohexadienone substrates in up to 85% yield with excellent levels of stereoselectivity (up to >99% ee and >20:1 dr) under mild reaction conditions.

Carbon-silicon-switch effect in enantioselective construction of silicon-stereogenic center from silacyclohexadienones

Carbon-silicon-switch strategy, replacing one specific carbon atom in organic molecules with a silicon, has garnered significant interest for developing new functional molecules. However, the influence of a reaction regarding its selectivity and reactivity by carbon-silicon-switch strategy has far less been investigated. Here we discover an unusual carbon-silicon-switch effect in the enantioselective construction of silicon-stereogenic center. It is found that there has been a significant change in the desymmetrization reaction of silacyclohexadienones using asymmetric conjugate addition or oxidative Heck reaction with aryl/alkyl nucleophiles when compared with their carbon analogues cyclohexadienones. Specifically, the carbon-silicon-switch leads to a reversal in enantioselectivity with arylzinc as the nucleophile by the same chiral catalyst, and results in totally different reactivity with arylboronic acid as the nucleophile. Control experiments and density functional theory (DFT) calculations have shown that the unusual carbon-silicon-switch effect comes from the unique stereoelectronic feature of silicon.

Rhodium(I)/Chiral Diene Complexes Catalyzed Asymmetric Desymmetrization of Alkynyl-Tethered 2,5-Cyclohexadienones Through an Arylative Cyclization Cascade

Cis-hydrobenzofurans, cis-hydroindoles, and cis-hydrindanes, privileged structural motifs found in numerous biologically active natural and synthetic compounds, are efficiently prepared by a Rh(I)-catalyzed cascade syn-arylation/1,4-addition protocol. This approach starts with the regioselective syn-arylation of the alkyne tethered to 2,5-hexadienone moieties, using a chiral Rh(I) catalyst generated in situ from a chiral bicyclo[2.2.1]hepatadiene ligand L4f. By forging two new carbon-carbon bonds and introducing two chiral centers, the resulting alkenylrhodium species undergoes desymmetrization via an intramolecular 1,4-addition reaction, delivering annulated products with high yields and enantioselectivities.

sSPhos: A General Ligand for Enantioselective Arylative Phenol Dearomatization via Electrostatically-Directed Palladium Catalysis

Arylative phenol dearomatization affords complex, cyclohexanone-based scaffolds from simple starting materials, and asymmetric versions allow access to valuable enantioenriched structures. However, bespoke chiral ligands must typically be identified for each new scaffold variation. We have addressed this limitation by applying the concept of electrostatically-directed palladium catalysis whereby the chiral sulfonated ligand sSPhos engages in electrostatic interactions with a phenolate substrate via its associated alkali metal cation. This approach allows access to highly enantioenriched spirocyclohexadienones, a process originally reported by Buchwald and co-workers in a predominantly racemic manner. In addition, sSPhos is proficient at forming two other distinct scaffolds, which had previously required fundamentally different chiral ligands, as well as a novel oxygen-linked scaffold. We envisage that the broad generality displayed by sSPhos will facilitate the expansion of this important reaction type and highlight the potential of this unusual design principle, which harnesses attractive electrostatic interactions.

Organocatalytic Enantioselective Intramolecular Michael Addition by In Situ Generated Aminoisobenzofulvenes: Construction of Spiro Quaternary Carbon Stereocenters

An unprecedented enantioselective organocatalytic spirocyclization strategy is presented by in situ generation of aminoisobezofulvenes. The reaction sequence involves a reductive Michael/aldol-condensation/Michael addition cascade by iminium-enamine catalysis. The key success of this spirocyclization was the formation of intermediatory nucleophilic aminoisobenzofuvenes accountable for intramolecular Michael addition. Benzospirononanes featuring an all carbon qauternary spirocenter were obtained using proline-derived amino-organocatalyst in moderate to good yields and excellent diastereo- and enantioselectivities (up to >20 : 1 dr, and 99 % ee). Post-methodological manipulation of benzospirononanes was also demonstrated.

Enantioselective Construction of Carbocyclic and Heterocyclic Tertiary Boronic Esters by Conjunctive Cross-Coupling Reaction

Synthesis of stereodefined carbocyclic and heterocyclic tertiary boronic esters is accomplished by performing a conjunctive cross-coupling reaction on preformed cyclic boron ate complexes. Boronates bearing spirocyclic and aryl bicyclic skeletons can be synthesized enantioselectively using a chiral PHOX-ligated Pd catalyst with achiral starting material, while substrates bearing continuous stereogenic centers can be generated diastereoselectively. A variety of aryl and alkenyl electrophiles are incorporated.