Stereoselective S<sub>E</sub>2′ Protonation of α‐Hydroxyallylsilanes Mediated by a Brook Rearrangement

Sasaki, Michiko; Shirakawa, Yuri; Kawahata, Masatoshi; Yamaguchi, Kentaro; Takeda, Kazuya

doi:10.1002/chem.200802499

Cited by 16 publications

(2 citation statements)

References 26 publications

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“…A similar result was obtained with 4b to give (Z)-7. 9 It should be noted that both (E)-and (Z)-7 are products arising from a syn-attack ring-opening of the epoxide as shown in 4a′.…”

Section: Chirality Transfer From Epoxide To Carbanionmentioning

confidence: 98%

Synthetic Potential of Configurationally Unstable Chiral Carbanions

Sasaki¹,

Takeda²

2012

Synlett

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This account summarizes our recent efforts in exploring the synthetic potential of configurationally labile chiral carbanions next to electron-withdrawing groups that have previously been considered to be impossible even to generate. We place particular emphasis on the generation and trapping of chiral α-nitrile carbanions as well as evaluation of the configurational stability of those carbanions and effects of solvent on the stability.

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“…A similar result was obtained with 4b to give (Z)-7. 9 It should be noted that both (E)-and (Z)-7 are products arising from a syn-attack ring-opening of the epoxide as shown in 4a′.…”

Section: Chirality Transfer From Epoxide To Carbanionmentioning

confidence: 98%

Synthetic Potential of Configurationally Unstable Chiral Carbanions

Sasaki¹,

Takeda²

2012

Synlett

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“…We previously reported that the Brook rearrangement mediated S E 2 protonation of allylsilanes having an oxygen substituent on the stereogenic center proceeds in an anti fashion. [7] The synthesis of racemic siloxyallenes by a Brook rearrangement was originally reported independently by the Kuwajima [8] and Reich [9] groups. They generated a-hydroxypropargylsilane, a precursor for the Brook rearrangement, by reactions of acylsilanes with lithium acetylides.…”

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confidence: 99%

Enantioselective Synthesis of Siloxyallenes from Alkynoylsilanes by Reduction and a Brook Rearrangement and Their Subsequent Trapping in a [4+2] Cycloaddition

et al. 2011

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Enantioselective synthesis through the use of chiral allenes has attracted much attention because of their capability to transfer their axial chirality to one or more new stereogenic centers.[1] A well-established approach to enantiomerically enriched allenes relies on the S N 2' substitution of homochiral propargylic derivatives with organocuprates. [2] We became interested in developing new methodologies for the synthesis of chiral, nonracemic allenes based on the enantioselective Meerwein-Ponndorf-Verley-type reduction of acylsilanes by chiral lithium amide that was developed by us.[3] We envisaged that if alkynoylsilane 1 could be enantioselectively reduced by 2, [4] the resulting a-silyl alkoxide 3 would provide optically active siloxyallene derivatives through a Brook rearrangement; [5] subsequent S E 2' electrophilic substitutions [6] of the silicate intermediate 4, would result in the enantioselective preparation of 1-unsubstituted siloxyallenes 5 or ent-5 depending on the mode of the S E 2 process (Scheme 1). We previously reported that the Brook rearrangement mediated S E 2 protonation of allylsilanes having an oxygen substituent on the stereogenic center proceeds in an anti fashion. [7] The synthesis of racemic siloxyallenes by a Brook rearrangement was originally reported independently by the Kuwajima [8] and Reich [9] groups. They generated a-hydroxypropargylsilane, a precursor for the Brook rearrangement, by reactions of acylsilanes with lithium acetylides. Scheidt et al. recently reported the synthesis of enantiomerically enriched siloxyallenes by the treatment of a-hydroxypropargylsilane, which was obtained by a catalytic asymmetric addition of acetylide to acylsilane, with a catalytic amount of nBuLi.[10] Consequently, their methods are limited to the synthesis of 1-alkyl-substituted siloxyallene derivatives.We report here some preliminary results for the enantioselective synthesis of 1-unsubstituted 1-siloxyallenes and their trapping by [4+2] cycloaddition.When 1 a was treated with 2 at À80 8C in toluene for 30 min followed by addition of tBuOH (1.2 equiv) in THF and then warming to À20 8C, siloxyallene (+)-6 a [11] was obtained in 52 % yield and with e.r. 95:5 together with 7 a (32 %; Table 1, entry 1). [12,13] The selectivity was markedly improved by a change in the substituent X to a 3-phenylpropyl group and the allene derivative (+)-6 b was obtained in 86 % yield (Table 1, entry 2). Our initial choice of 1 a as a substrate was based on the assumed stabilization of the allene structure owing to the a-anion-stabilizing nature of the silyl group. The results showing that the less bulky alkyl derivative 1 b Scheme 1. Tandem process for the enantioselective formation of siloxyallenes. El = electrophile.

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The Brook Rearrangement

Yang

et al. 2020

Organic Reactions

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The Brook rearrangement describes the intramolecular silyl group migration from a carbon to an oxygen atom in a “through‐space” fashion. This feature has been extensively applied to develop numerous useful transformations of diverse organosilanes. Some representative examples include formal [3 + 2] and [4 + 3] annulations to construct diverse ring systems, syntheses of configurationally defined silyl enol ethers, multi‐component reactions by Brook rearrangement‐based ARC (anion relay chemistry), and various umpolung processes such as silyl benzoin condensations and sila‐Stetter reactions. This chapter presents a thorough overview of the Brook rearrangement. The scope and limitations are categorized according to the organosilane type. The mechanism and stereochemical course of this process are discussed, as is its application in the synthesis of complex natural products. The literature is covered up to the end of 2019.

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Stereoselective S_E2′ Protonation of α‐Hydroxyallylsilanes Mediated by a Brook Rearrangement

Cited by 16 publications

References 26 publications

Synthetic Potential of Configurationally Unstable Chiral Carbanions

Synthetic Potential of Configurationally Unstable Chiral Carbanions

Enantioselective Synthesis of Siloxyallenes from Alkynoylsilanes by Reduction and a Brook Rearrangement and Their Subsequent Trapping in a [4+2] Cycloaddition

The Brook Rearrangement

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Stereoselective SE2′ Protonation of α‐Hydroxyallylsilanes Mediated by a Brook Rearrangement

Cited by 16 publications

References 26 publications

Synthetic Potential of Configurationally Unstable Chiral Carbanions

Synthetic Potential of Configurationally Unstable Chiral Carbanions

Enantioselective Synthesis of Siloxyallenes from Alkynoylsilanes by Reduction and a Brook Rearrangement and Their Subsequent Trapping in a [4+2] Cycloaddition

The Brook Rearrangement

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Stereoselective S_E2′ Protonation of α‐Hydroxyallylsilanes Mediated by a Brook Rearrangement