Silyl enol ethers hydrolysis

Amidoalkylation of silyl enol ethers with /V-acyliiiiiiiium ions containing camphanoyl-derived acyl functions (see Appendix) as the chiral auxiliary leads to optically active 2-substituted piperidine derivatives with moderate to high diastereoselectivity, depending on the chiral auxiliary and the cnol ether82 99. The auxiliary is removed by hydrolysis with base or acid. 

The Simmons-Smith reaction has been used as the basis of a method for the indirect a methylation of a ketone. The ketone (illustrated for cyclohexanone) is first converted to an enol ether, an enamine (16-12) or silyl enol ether (12-22) and cyclopropanation via the Simmons-Smith reaction is followed by hydrolysis to give a methylated ketone. A related procedure using diethylzinc and diiodomethane allows ketones to be chain extended by one carbon. In another variation, phenols can be ortho methylated in one laboratory step, by treatment with Et2Zn and... 

Certain other metal ions also exhibit catalysis in aqueous solution. Two important criteria are rate of ligand exchange and the acidity of the metal hydrate. Metal hydrates that are too acidic lead to hydrolysis of the silyl enol ether, whereas slow exchange limits the ability of catalysis to compete with other processes. Indium(III) chloride is a borderline catalysts by these criteria, but nevertheless is effective. The optimum solvent is 95 5 isopropanol-water. Under these conditions, the reaction is syn selective, suggesting a cyclic TS.63... 

Nitroalkenes are also reactive Michael acceptors under Lewis acid-catalyzed conditions. Titanium tetrachloride or stannic tetrachloride can induce addition of silyl enol ethers. The initial adduct is trapped in a cyclic form by trimethylsilylation.316 Hydrolysis of this intermediate regenerates the carbonyl group and also converts the ad-nitro group to a carbonyl.317... 

The use of oxygen-containing dienophiles such as enol ethers, silyl enol ethers, or ketene acetals has received considerable attention. Yoshikoshi and coworkers have developed the simple addition of silyl enol ethers to nitroalkenes. Many Lewis acids are effective in promoting the reaction, and the products are converted into 1,4-dicarbonyl compounds after hydrolysis of the adducts (see Section 4.1.3 Michael addition).156 The trimethylsilyl enol ether of cyclohexanone reacts with nitrostyrenes in the presence of titanium dichloride diisopropoxide [Ti(Oi-Pr)2Cl2], as shown in Eq. 8.99.157 Endo approach (with respect to the carbocyclic ring) is favored in the presence of Ti(Oi-Pr)2Cl2. Titanium tetrachloride affords the nitronates nonselectively. 

Hydrogenation of silyl enol ethers with the DIOP catalyst followed by hydrolysis [Eq. (52)] has yielded a route to optically active alcohols with low optical purities, 7% ee NMDPP (12) and MePhPR (R = n-Pr, Et, benzyl) systems were less effective (299). 

The use of /i-ketocstcrs and malonic ester enolates has largely been supplanted by the development of the newer procedures based on selective enolate formation that permit direct alkylation of ketone and ester enolates and avoid the hydrolysis and decarboxylation of ketoesters intermediates. Most enolate alkylations are carried out by deprotonating the ketone under conditions that are appropriate for kinetic or thermodynamic control. Enolates can also be prepared from silyl enol ethers and by reduction of enones (see Section 1.3). Alkylation also can be carried out using silyl enol ethers by reaction with fluoride ion.31 Tetraalkylammonium fluoride salts in anhydrous solvents are normally the... 

By allowing Cjq to react with 2-trimethylsilyloxy-1,3-butadiene in toluene at reflux [38, 94], another stable Diels-Alder adduct was obtained (Scheme 4.14). The ketone 93 is formed after hydrolysis of the uncharacterized intermediate silyl enol ether under flash chromatographic conditions. 

The dominance of an alkoxy group over a silyl group is manifested in the generation of amides from reaction of oc-silyl enol ethers with chlorosulfonyl isocyanate, followed by hydrolysis. Simple vinylsilanes and alkynylsilanes undergo carbamidation [34],... 

Cyclobutenes. This derivative of ketene undergoes [2 + 2]cycloaddition with ethyl propiolate in refluxing methylene chloride to produce the cyclobutene 1 in 65% yield. The ester group activates 1 sufficiently for Diels-Alder addition with the silyl enol ether 2 to give the I I adduct 3 under mild conditions. Hydrolysis of 3 can... 

Desilylation of silyl eno ethers. Hydrolysis of silyl enol ethers of aldehydes with oxalic acid in aqueous fill can result in partial degradation to a ketone (equation... 

Mukaiyama aldol condensation (6, 590-591).8 This reaction can be effected in the absence of a Lewis acid catalyst under high pressure (10 kbar). Surprisingly the stereoselectivity is the reverse of that of the TiCl4-catalyzed reaction (equation I). The reaction can also be effected in water with the same stereoselectivity, but the yield is low because of hydrolysis of the silyl enol ether. Yields are improved by use of water-oxolane (1 1) and by sonication.9... 

Cyclopentanones may also be synthesized from a,/ -unsaturated ketones and diiodo-methane. The ketone is converted to the O-silyl enol, and carbene is added to the enol double bond using the Simmons-Smith reaction (see p. 74f.). Thermal rearrangement of the resulting 1-siloxy-l-vinylcydopropane and add-catalyzed hydrolysis of the silyl enol ether leads to cyclopentanones in excellent yields (C. Girard, 1974). Very high temperatures, however, are needed, and this obviously limits the generality of this rearrangement reaction. 

Lithium alkoxides of bis(trimethylsilyl) carbinols react with benzophenone to produce silyl enol ethers of acyl silanes in good yields93. The alcohols were prepared in reasonable yields by hydrolysis of the bis(trimethylsilyl) carbinol silyl ethers94,95, which in turn were produced from the corresponding esters using another silyl acyloin reaction, which itself, ironically, proceeds through an acyl silane intermediate (Scheme 21)94. 

A number of silyl enol ethers of acyl silanes have been produced from alkenes by subjection to 50 atmospheres of carbon monoxide in the presence of 0.1 equivalents of trialkylsilane and 2 mol% of an iridium catalyst (Scheme 26)102. Hydrolysis to the acyl silanes was achieved using hydrochloric acid-acetone. 

In another attempt, we reached the tetracyclic structure 42 via two consecutive [4 + 2] cycloaddition reactions. Reaction of 3-cyano-4-benzopyrone 31 with Danishefsky s diene 38 in toluene at 300 °C for 96 h provided the desired cycloadduct 39 in 80% yield with an endo exo ratio of 1 2 [Scheme 8]. Hydrolysis of the silyl enol ether in 39 using TMSBr in CH3CN at room temperature proved to be feasible but slow, and afforded the enone 40 in 90% yield. Reaction of diene 41 with enone 40 in the presence of 2.5 equiv of BF3-Et20 yielded tetracycle 42 in 25% yield with an endo exo ratio of 1 1 after 120 h at room temperature.49... 

Allyl cyanides can be added across alkynes in the presence of a nickel catalyst prepared from (COD)2Ni and (4-CF3CeH4)3P in situ to give functionalized di- or tri-substituted acrylonitriles in a highly stereoselective manner, presumably via n-allylnickel intermediates. a-Siloxyallyl cyanides also react at the y -position of a cyano group with both internal and terminal alkynes to give silyl enol ethers, which can be converted into the corresponding aldehydes or ketones upon hydrolysis.70... 

Hydrosilane HSiR.3 behaves similar to H2 toward transition metal complexes in some cases. When HSiR.3 is used instead of hydrogen in hydroformylation, two reactions are expected. One is a hydrocarbonylation-type reaction, by which formation of the silyl enol ethers 62 via the acylmetal intermediate 61, and the acylsilanes 64 via the acyl complex 63, are expected in practice both reactions are observed. The other possibility is silylformylation to form 65, which is unknown, even though silylformylation of alkynes is known. When Co2(CO)8 is used, the silyl enol ether of aldehyde 66 is obtained [36], However, the silyl enol ether 67 of acylsilane 68 is obtained when an Ir complex is used, and converted to the acylsilane 68 by hydrolysis [37],... 

Enones are also bis-silylated. Asymmetric 1,4-bis-silylation of the enone 505 catalyzed by Pd-BINAP afforded the silyl enol ether 507, and the keto alcohol 509 with 87% ee was obtained after hydrolysis and oxidation of 508 [195]. In this case, the unsymmetric disilane 506 should be used. 

Addition of monohydrosilane to the a,/ -unsaturated ketone 593 affords the silyl enol ether 594, and its hydrolysis produces the saturated ketone 595. However, the 1,2-adduct 596 is produced with dihydrosilane, and allylic alcohol 597 is obtained after hydrolysis [228]. The ketene silyl acetal 599 is formed by the Rh-catalysed... 

In addition to benzene rings, cycloheptatriene is activated or protected by forming the stable if complex 300. An example of the strong stabilization effected by coordination is shown by isolation of the optically active l,3,5-cycloheptatrien-3-ols 301, 304 and 305 as their enol forms. l,3,5-Cycloheptatrien-3-ol was isolated as complex 301 by hydrolysis of silyl enol ether 300. The triene system is stabilized by coordination,... 

The presumed Z-enoxysilane 28 undergoes rearrangement prior to warming to room temperature furnishing siloxy cycloheptadiene 29. Hydrolysis of silyl enol ether 29 yields (6S, 8/ )-cycloheptenone 7. 

However, the presence of an alkene exo to the chain 40 stops the reaction, presumably because the 120° angle holds the ends too far apart. The solution is conjugate addition of an amine 41 the acyloin then works well 42 and the synthesis of the flavouring compound corylone 43 is completed simply by a silica column.7 Hydrolysis of the silyl enol ethers leads to elimination of Me2NH under the slightly acidic conditions. 

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