Selective formation of non-conjugated olefins by samarium(II)-mediated elimination/isomerization of allylic benzoates

Schaefer, Sara L.; Roberts, Connor L.; Volz, Erasmus O.; Grasso, Monika R.; O’Neil, Gregory W.

doi:10.1016/j.tetlet.2013.08.136

Cited by 7 publications

(3 citation statements)

References 23 publications

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“…10 Our group became interested in the use of SmI 2 for the reduction of acyloxysulfones as part of a masked-alkene metathesis protocol. 11 More recently we reported that allylic benzoates 1a or 1b can be reduced with SmI 2 in the presence of an alcohol additive (R′OH), 12 converging to the corresponding reduced products 2 with high regioselectivity (Scheme 1). The regioselectivity of this reaction can be rationalized by steric considerations of the organosamarium intermediate, and a pericyclic protonation mechanism involving a samarium-bound alcohol molecule.…”

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confidence: 99%

Chelation and Stereodirecting Group Effects on Regio- and Diastereoselective Samarium(II)-Water Allylic Benzoate Reductions

Stockdale

Leitch²,

O’Neil

2020

Synthesis

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SmI2(H2O)n reductions of allylic benzoates adjacent to a trisubstituted alkene occur in high yields with complete regioselectivity and good diastereoselectivity (up to 90:10) for substrates containing properly positioned stereodirecting- and chelating groups. The outcome of these reactions can be rationalized by ring conformation considerations of a proposed chelated organosamarium intermediate, and a mechanism involving intramolecular protonation by a samarium-bound water.

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Chelation and Stereodirecting Group Effects on Regio- and Diastereoselective Samarium(II)-Water Allylic Benzoate Reductions

Stockdale

Leitch²,

O’Neil

2020

Synthesis

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“…Recently, our group reported a general approach to non-conjugated olefins of this type by samarium-mediated benzoyl ester elimination/isomerization. 8 The reaction is proposed to occur via an organosamarium intermediate of type 5, accessable via allylic benzoates 6 or 7 (Scheme 1). Internal delivery of a proton from a coordinated proton donor (e.g.…”

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confidence: 99%

“…Previously, we had reported that commercial aldehydes 10 and 11 could be converted to 8 and 9 via allylic benzoates 12 and 13 by samarium-mediated benzoyl ester reduction (Scheme 2). 8 With water as the proton source, 8 and 9 were obtained in 60% and 76% yield respectively, with only marginal selectivity for the non-conjugated (γ) vs. the conjugated (α) isomer (γ:α = 85:15 for 8 and 65:35 for 9, Scheme 2). After some experimentation it was found that by switching to methanol as the additive, 9 both the yields and selectivities for these reactions greatly improved, affording 8 and 9 in ≥ 90% yield and >10:1 selectivity for the γ-isomer.…”

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Total synthesis of honokiol by selective samarium-mediated allylic benzoate reduction

Wright

O’Neil

2016

Tetrahedron Letters

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The total synthesis of the biologically relevant compound honokiol has been completed featuring a samarium-mediated bis-benzoyl ester reduction to simultaneously install both allyl substituents found in the natural product. This reaction was performed after a Suzuki coupling was used to generate the biphenyl core, thereby avoiding problems associated with the acidity of these allyl groups and their propensity to isomerize. In this way, the synthesis of honokiol could be completed in 4 steps and 42% overall yield. Honokiol (1) is a neolignan contained in the bark of Magnolia occifinalis that continues to elicit significant interest due to its range of promising biological activities including both antitumor 1 and anti-angiogenic 2 properties (Figure 1). Isolation of honokiol from the natural source is hampered by the presence of its isomeric compound magnolol (2). 3 The different bioactivities of magnolol 4 yet very similar physical properties to honokiol makes separation both necessary and challenging. 5 Thus several groups have pursued a total synthesis of honokiol, 6 the first by Takeya et al. 6a in 1986 and more recently Harada et al. in 2014. 6h Many of these syntheses have utilized an aromatic Claisen rearrangement (Figure 2). 6a,c-f This reaction is however limited in terms of regiochemistry, used primarily to generate ortho-allyl substituted phenols. 7 For instance Reddy et al. reported obtaining roughly 1:1 mixtures of honokiol (1) and isohonokiol (3) from the bis-Oallylbiphenyl 4. 6f

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Samarium(II) iodide

Ho¹,

Fieser²,

Fieser³

et al. 2017

Fieser and Fieser's Reagents for Organic Synthesis

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Selective formation of non-conjugated olefins by samarium(II)-mediated elimination/isomerization of allylic benzoates

Cited by 7 publications

References 23 publications

Chelation and Stereodirecting Group Effects on Regio- and Diastereoselective Samarium(II)-Water Allylic Benzoate Reductions

Chelation and Stereodirecting Group Effects on Regio- and Diastereoselective Samarium(II)-Water Allylic Benzoate Reductions

Total synthesis of honokiol by selective samarium-mediated allylic benzoate reduction

Samarium(II) iodide

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