Silazanes/catalytic bases: mild, powerful and chemoselective agents for the preparation of enol silyl ethers from ketones and aldehydes
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We have developed an efficient method for the preparation of enol silyl ethers using novel agents, silazanes together with NaH or DBU catalyst, wherein TMS and TBDMS groups were smoothly and chemoselectively introduced into ketones and aldehydes under mild conditions.Cite
6-(Pyridyl)salicylates were regioselectively prepared by formal [3+3] cyclization of 1,3-bis(silyl enol ethers) with 3-(pyridyl)-3-silyloxy-2-en-1-ones. These reactions represent what are, to the best of our knowledge, the first [3+3] cyclizations of 1,3-bis(silyl enol ethers) with heterocyclic substrates.
Formal synthesis
Enol ether
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Enol ether
Silyl ether
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Abstract Stereochemistry in amine-promoted enol trimethylsilylation of meso- and dl-α,α′-dichloro ketones, RCHClCOCHClR (1a; R = Me and 1b; R = i-Pr), and in ketonization of the resultant enol silyl ethers (2a and 2b) has been studied. The Et3N-promoted silylation of 1 in benzene shows a marked diastereoselectivity; the racemic isomer exhibits 84% and 98% (E)-selectivities for 1a and 1b, while the meso isomer, 97% and 98% (Z)-selectivities respectively. Both stereo- and diastereo-selectivities markedly depend upon solvent polarity and base strength. For example, both meso- and dl-1a showed small (Z)-selectivities in DMF. The diastereoselectivity markedly decreased with increasing base strength; both 1a and 1b exclusively gave the (E)-isomer of 2 irrespective of the configuration of the ketone when treated with lithium diisopropylamide in the presence of chlorotrimethylsilane at −78 °C. The ketonization of 2a and 2b was also diastereoselective in the direction opposite that observed in the forward reaction; the (E)-isomer predominantly gave the meso ketone (24% d.e.), while the (Z)-isomer, the racemic ketone (ca. 70% d.e.) upon protonation with concd hydrochloric acid in THF.
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Triethylborane
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The reaction of CCl4 with ketene silyl acetals and silyl enol ethers in the absence of a promoter at ambient temperature or at reflux, or under photo-irradiation, was performed to form products via addition of the trichloromethyl group to those silyl substrates.
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The Lewis acid-catalyzed decarboxylative coupling of cyclic enol carbonates, prepared by the fixation of carbon dioxide onto propargyl alcohols, with silyl enol ethers including ketene silyl acetals, was developed to afford 1,4-dicarbonyl compounds in good-to-high yields. As the plausible reaction mechanism, it was proposed that the decarboxylative formation of an oxyallyl cation intermediate or its equivalent and the sequential nucleophilic addition of silyl enol ethers proceeded to afford 1,4-dicarbonyl products. In addition, the synthetic utility of the obtained-1,4-dicarbonyl compounds was also demonstrated by applying them to the construction of multisubstituted heterocycles.
Propargyl
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Abstract 1,5-Dicarbonyl compounds and their enol silyl ethers were prepared by the reaction of α,β-unsaturated ketones and enol silyl ethers or ketene silyl acetals by using an electrogenerated acid (EG acid) as a catalyst. Similar reaction of 1-trimethylsiloxy-1,3-butadiene with enones produced six-membered adducts as the result of a double-Michael process.
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Enol ether
Silyl enol ether
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The last few years have seen an explosive growth in the use of silyl enol ethers and silyl ketene acetals in organic synthesis. The aim of this review is to provide a reasonably comprehensive account of the newer reactions of these compounds, illustrated by some examples from synthetic pathways. 1. Introduction 2. Preparation 2.1. Silyl Enol Ethers from Ketones and Aldehydes 2.2. Silyl Enol Ethers from Enones 2.3. Silyl Enol Ethers from Acylsilanes 2.4. Silyl Enol Ethers by Other Methods 2.5. 1-(Trimethylsilyl)-silyl Enol Ethers 2.6. Silyl Ketene Acetals and Related Compounds 3. Reactions 3.1. Protonolysis 3.2. Reduction 3.3. Oxidation 3.4. Reactions with Non-Carbon Electrophiles 3.5. Reactions with Carbon Electrophiles 3.6. Reactions with Carbon Nucleophiles 3.7. Concerted Reactions 4. Siloxybutadienes (Silyl Dienol Ethers) 4.1. Preparation 4.2. Reactions with Electrophiles 4.3. [4+2]-Cycloadditions 5. Addendum 6. Conclusions
Trimethylsilyl
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