<i>B</i>-Alkylcatecholborane-Mediated Tandem Radical Conjugated Addition−Aldol Cyclization

Beauseigneur, Alice; Ericsson, Cecilia; Renaud, Philippe; Schenk, Kurt

doi:10.1021/ol9014943

Cited by 24 publications

(13 citation statements)

References 14 publications

(24 reference statements)

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“…Herein, we report the enantioselective coppercatalyzed domino conjugate boration/aldol cyclization of enone diones to give a range of highly functionalized bicyclic products. [12][13][14][15][16][17] This desymmetrization process results in the formation of one boron-carbon bond, one carbon-carbon bond, and four contiguous stereocenters, two of which are quaternary, with high levels of diastereo-and enantioselection.Our studies commenced with evaluation of various common chiral bisphosphine ligands (L1-L4; 5.5 mol %) in the domino conjugate boration/aldol cyclization of the enone dione 1 a with B 2 (pin) 2 (1.1 equiv) in the presence of CuCl (5 mol %) and NaOtBu (7.5 mol %) in THF (0.1m) at room temperature (Table 1, entries 1-4). Although the desired bicyclic product 3 a was formed in greater than 95:5 d.r.[(major isomer):(AE other isomers)] in several cases, appreciable quantities of the product 2 a resulting from conjugate boration without cyclization were often observed (Table 1, entries 1, 2, and 4).…”

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Enantioselective Copper(I)‐Catalyzed Borylative Aldol Cyclizations of Enone Diones

2012

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The in situ trapping of metal enolates generated by the catalytic hydrometalation of a,b-unsaturated carbonyl compounds has proven to be a mild and versatile method for carbon-carbon bond construction. [1] Not only does this strategy allow precise control of the site of enolization of substrates containing several acidic sites, the use of chiral metal/ligand complexes can also enable products to be formed with high levels of diastereo-and enantiocontrol. In this context, we and others have developed various copper(I)catalyzed or copper(I)-mediated reductive aldol, [2][3][4][5] Mannich, [6] and Michael [7] reactions to furnish products with high diastereo-and enantioselectivities.While these processes are effective, the development of related transformations in which metal enolate generation is initiated not by the formation of a carbon-hydrogen bond, but by a carbon-heteroatom bond which can then be exploited in subsequent functionalizations, should also be of high value.Given the recent developments in enantioselective copper(I)catalyzed conjugate boration reactions, [8,9] we envisaged a domino conjugate boration/aldol cyclization sequence in which a copper enolate generated from the initial conjugate boration is trapped by a pendant ketone to deliver cyclic products containing multiple stereocenters (Scheme 1).Relevant precedent for such a process is relatively limited. [10] Hoveyda and co-workers have described racemic N-heterocyclic-carbene-catalyzed conjugate boration of a cyclic enone with subsequent in situ trapping of the resulting boron enolate with benzaldehyde, [10a] whereas Kanai, Shibasaki, and co-workers reported a similar enantioselective process catalyzed by a chiral copper/bisphosphine complex. [10b] Recently, Riant and co-workers developed a racemic copper-catalyzed conjugate boration/intermolecular aldol sequence of acyclic or cyclic a,b-unsaturated carbonyl compounds. [10c] However, analogous processes in which the aldol reaction occurs in an intramolecular fashion have not, to our knowledge, been reported, despite the potential for the generation of useful functionalized cyclic building blocks.In view of the precedent set by Krische and co-workers, who described highly diastereo-and enantioselective rhodium-catalyzed conjugate arylation/aldol cyclizations of enone diones, [11] we wondered whether a related process involving copper-catalyzed conjugate boration could be developed. Herein, we report the enantioselective coppercatalyzed domino conjugate boration/aldol cyclization of enone diones to give a range of highly functionalized bicyclic products. [12][13][14][15][16][17] This desymmetrization process results in the formation of one boron-carbon bond, one carbon-carbon bond, and four contiguous stereocenters, two of which are quaternary, with high levels of diastereo-and enantioselection.Our studies commenced with evaluation of various common chiral bisphosphine ligands (L1-L4; 5.5 mol %) in the domino conjugate boration/aldol cyclization of the enone dione 1 a with B 2 (pin) 2 (1.1 equ...

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Enantioselective Copper(I)‐Catalyzed Borylative Aldol Cyclizations of Enone Diones

2012

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“…The reaction is highly diastereoselective leading to diquinanes and hydrindanes bearing a hydroxyl group at the ring junction (Scheme 10). 15 Scheme 10 Radical conjugate addition of B-alkylcatecholborane to enones…”

Section: Borane-promoted Radical Cyclizationsmentioning

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Diastereo- and Enantioselective Synthesis of Polyfunctionalized Diquinanes, Hydrindanes, and Decalins Bearing a Hydroxyl Group at the Ring Junction

et al. 2016

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Bicyclo[3.3.0]octan-1-ol (diquinane ring system), bicyclo[4.3.0]nonan-1-ol (hydrindane ring system), and bicyclo[4.4.0]decan-1-ol (decalin ring system) belong to the medium-sized rings family which represent important scaffolds of numerous natural products. Thus, the development of new routes leading to such scaffolds is of importance because such carbocycles are ubiquitous in numerous biologically active compounds. This short review will focus on the main approaches reported since 2000, leading to these bicyclic scaffolds with high diastereo-and/or enantioselectivities. 1 Introduction 2 Diastereoselective Approaches 3 Diastereo-and Enantioselective Catalytic Approaches 4 Conclusion

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“…In an attempt to develop a highly efficient and stereoselective carbon-carbon bond-forming reaction involving a novel hybrid radical-ionic reaction process, [50][51][52][53][54][55][56][57][58][59] we investigated the domino radical addition aldol-type reaction of conjugated oxime ethers.…”

Section: Sequential Radical Addition and Aldol-type Reactionmentioning

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Radical Addition-Initiated Domino Reactions of Conjugated Oxime Ethers

Ueda

2014

CHEMICAL & PHARMACEUTICAL BULLETIN

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The application of conjugated oxime ethers to the synthesis of complex chemical scaffolds using domino radical reactions has been described in detail. The triethylborane-mediated hydroxysulfenylation reaction allows for the regioselective construction of a carbon-sulfur bond and a carbon-oxygen bond in a single operation for the formation of β-hydroxy sulfides. This reaction proceeds via a radical pathway involving regioselective thiyl addition and the subsequent trapping of the resulting α-imino radical with O₂, where the imino group enhances the stability of the intermediate radical. Hydroxyalkylation reactions that occur via a carbon radical addition reaction followed by the hydroxylation of the resulting N-borylenamine with O₂ have also been developed. We investigated sequential radical addition aldol-type reactions in detail to explore the novel domino reactions that occur via the generation of N-borylenamine. The radical reaction of a conjugated oxime ether with triethylborane in the presence of an aldehyde affords γ-butyrolactone via sequential processes including ethyl radical addition, the generation of N-borylenamine, an aldol-type reaction with an aldehyde, and a lactonization reaction. A novel domino reaction has also been developed involving the [3,3]-sigmatropic rearrangement of N-boryl-N-phenoxyenamine. The triethylborane-mediated domino reactions of O-phenyl-conjugated oxime ethers afforded the corresponding benzofuro[2,3-b]pyrrol-2-ones via a radical addition/[3,3]-sigmatropic rearrangement/cyclization/lactamization cascade.

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B-Alkylcatecholborane-Mediated Tandem Radical Conjugated Addition−Aldol Cyclization

Abstract: A one-pot procedure involving radical conjugate addition of B-alkylcatecholboranes to enones followed by intramolecular aldol reaction is reported. Application to the stereoselective synthesis of monocyclic and bicyclic products with up to four contiguous stereogenic centers is presented.

Cited by 24 publications

References 14 publications

Enantioselective Copper(I)‐Catalyzed Borylative Aldol Cyclizations of Enone Diones

Enantioselective Copper(I)‐Catalyzed Borylative Aldol Cyclizations of Enone Diones

Diastereo- and Enantioselective Synthesis of Polyfunctionalized Diquinanes, Hydrindanes, and Decalins Bearing a Hydroxyl Group at the Ring Junction

Radical Addition-Initiated Domino Reactions of Conjugated Oxime Ethers

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