A regiocontrolled and stereocontrolled synthesis of allylsilanes from β-silyl enolates

Fleming, Ian; Gil, Salvador; Sarkar, Achintya Kumar; Schmidlin, Tibur

doi:10.1039/p19920003351

Cited by 34 publications

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“…15 We developed two syntheses of enantiomerically enriched allylsilanes, the first of which was fairly good, but not good enough for the purpose in hand. 16 The second, adapted from a general method for the synthesis of unsymmetrical allylsilanes, 17 allowed us to make a pair of allylsilanes E-and Z-16 that were, within experimental error, enantiomerically pure, and this in turn allowed us to measure the degree of stereospecificity in the S E 2Ј reaction with greater accuracy than anyone had before. We knew from Eschenmoser's and Kumada's work that it would be high, hence the need for high levels of enantiomeric purity in order to measure it accurately, but we did not know just how high it would be.…”

Section: Introductionmentioning

confidence: 99%

Accurate determinations of the extent to which the SE2? reactions of allyl-, allenyl- and propargylsilanes are stereospecifically anti

et al. 2004

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The allylsilanes, (R)-E- and (R)-Z-4-trimethylsilylpent-2-ene 16, were prepared in essentially an enantiomerically and geometrically pure state (er >99.95 : 0.05, E : Z and Z : E >99.95 : 0.05) by, successively, conjugate addition of lithium dimethylcuprate to N-[(E)-3'-trimethylsilylpropenoyl]-(7S)-10,10-dimethyl-4-aza-5-thiatricyclo[5.2.1.0(3,7)]decane 5,5-dioxide 13, to give N-[(E)-(3'R)-3'-trimethylsilylbutanoyl]-(7S)-10,10-dimethyl-4-aza-5-thiatricyclo[5.2.1.0(3,7)]decane 5,5-dioxide, removal of the chiral auxiliary with bromomagnesium benzyloxide, aldol reaction with acetaldehyde, and decarboxylative elimination, to give either the Z- or E-isomer. Both the E- and Z-allylsilanes 16 reacted with the adamantyl cation to give mixtures of E- and Z-4-adamantylpent-2-enes 17. The E-allylsilane gave the E- and Z-products in a ratio of 40 : 60, and the Z-allylsilane gave the E- and Z-products in a ratio of 99.8:0.02. The enantiomer ratio was >99:1 for the reaction of the E-allylsilane giving the Z-product, 90:10 for the E-allylsilane giving the E-product, and 95 : 5 for the Z-allylsilane giving the E-product, showing that the reactions were stereospecific to a high degree, but not always quite completely so. The allenylsilane, 2-trimethylsilylpenta-2,3-diene 29, was prepared enantiomerically highly enriched (er 99:1) by copper-catalysed reaction of methylmagnesium chloride with (S)-4-trimethylsilylbut-3-yn-2-yl camphor-10-sulfonate 28. The allenylsilane 29 reacted with the adamantyl cation to give (S)-4-adamantylpent-2-yne (S)-30 with the same level of enantiomeric purity, showing that the reaction was, as accurately as can be measured, completely stereospecific. The allenylsilane 29 also reacted with isobutanal in the presence of titanium tetrachloride to give 2,4-dimethylhept-5-yn-3-ol as a mixture of diastereoisomers, syn 31 and anti 32, in a ratio of 95:5, with the major diastereoisomer present as a mixture of enantiomers (4R,5R):(4S,5S) in a ratio of 99:1, showing that the reaction was, as accurately as can be measured, completely stereospecific in the anti sense. The corresponding propargylsilane, 4-trimethylsilylpent-2-yne 37, reacted with the adamantyl cation to give dienes assigned the structures 2,3-diadamantyl-1,3-pentadiene 42 and 2,4-diadamantyl-1,3-pentadiene 43, and reacted with isobutanal in the presence of titanium tetrachloride to give 2-(1-hydroxy-2-methylpropyl)-3-trimethylsilylpenta-1,3-dienes 45 and 2,4-dimethyl-5-trimethylsilylhept-5-en-3-one 46. The enantiomerically enriched propargylsilane (R)-1,3-bis(trimethylsilyl)but-1-yne (er >99.7:0.3) was prepared from the sultam 13, by removal of the chiral auxiliary with lithium ethoxide, reduction of the ethyl ester to give (R)-3-trimethylsilylbutanal 60, enol triflate formation, beta-elimination and C-silylation. The propargylsilane reacted with 2,4-dinitrobenzaldehyde in the presence of titanium tetrachloride to give the allenes, 1-(2,4-dinitrophenyl)-2-trimethylsilylpenta-2,3-dienols 63-66, as two diastereoisomers in a ratio of 2 : 1, each of whi...

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Section: Introductionmentioning

confidence: 99%

Accurate determinations of the extent to which the SE2? reactions of allyl-, allenyl- and propargylsilanes are stereospecifically anti

et al. 2004

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“…Similar diastereoselectivities (94:6 to 92:8) were observed for a series of alkyl halides (RX, where R ¼ Me, Et, n-Bu, i-Pr, PhCH 2 , CH 2 bCHCH 2 , and MeO 2 CCH 2 ; X ¼ I, Br). Stereospecific syn [49a] or anti [49b] decarboxylative elimination of the b-hydroxy acids selectively affords either the E or the Z allylsilane (Scheme 3.7) [50]. This strategy has been employed in a synthesis of the Prelog-Dejerassi lactone [47].…”

Section: Conjugate Addition Reactionsmentioning

confidence: 99%

Heteroatomcuprates and α‐Heteroatomalkylcuprates in Organic Synthesis

Dieter

2002

Modern Organocopper Chemistry

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“…For methyl fluoride, attack on the naked acetaldehyde enolate occurs at an angle of 106°relative to the plane of the enolate, [12] though recent calculations using a higher level of theory revise the attack angle upward to 116° (Figure 1). [13] The 116°trajectory correlates with the Bürgi-Dunitz angle [14] for attack in enolate alkylations, [15] aldol reactions, [16] and conjugate additions. [17] Particularly pertinent for secondary electrophiles is the inversion at the electrophilic carbon and movement of the methylene protons out of the enolate plane (8) which effectively causes one proton (H*) to straddle the olefinic methylene protons (8′, Figure 1).…”

Section: Introductionmentioning

confidence: 99%

Diastereoselective Electrophile‐Directed Alkylations

2019

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Electrophile‐directed alkylations in which a chiral, sp3‐hybridized electrophile, dictates the facial attack on a prochiral nucleophile, provides a powerful method of asymmetric induction. The strategy is inherently efficient because the asymmetry of an sp3‐hybridized electrophile is propagated to install two contiguous chiral centers, the first through an SN2 displacement and the second through preferential facial recognition. The two primary modes of topological matching are through steric approach control and chelation between the electrophile and nucleophile. Electrophiles capable of chelation generally induce higher diastereoselectivity for secondary and primary electrophiles. Key parameters that influence the stereoselectivity are those expected for enolate/anion alkylations: solvent, cation, complexing agents. A generalized enolate‐electrophile alkylation model is used to provide an evaluative framework for collecting related alkylations with the aim of providing insight into the origin of the electrophile‐directed diastereoselectivity as a powerful synthetic strategy.

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A regiocontrolled and stereocontrolled synthesis of allylsilanes from β-silyl enolates

Cited by 34 publications

References 39 publications

Accurate determinations of the extent to which the SE2? reactions of allyl-, allenyl- and propargylsilanes are stereospecifically anti

Accurate determinations of the extent to which the SE2? reactions of allyl-, allenyl- and propargylsilanes are stereospecifically anti

Heteroatomcuprates and α‐Heteroatomalkylcuprates in Organic Synthesis

Diastereoselective Electrophile‐Directed Alkylations

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