Silyloxy carbonyl compound

Oxidation of silyl enol ethers. Oxidation of silyl enol ethers to a-hydroxy aldehydes or ketones is usually effected with w-chloroperbenzoic acid (6, 112). This oxidation can also be effected by epoxidation with 2-(phenylsulfonyl)-3-( p-nitrophenyl) oxaziridine in CHC1, at 25-60° followed by rearrangement to a-silyloxy carbonyl compounds, which are hydrolyzed to the a-hydroxy carbonyl compound (BujNF or H,0 + ). Yields are moderate to high. Oxidation with a chiral 2-arene-sulfonyloxaziridine shows only modest enantioselectivity. 

Cyclization off silyloxy carbonyl compounds. Five- to seven-membered ethers can be obtained from y-, 8-, and e-silyloxy aldehydes or ketones by silyl... 

Another example where a bismuth tribromide salt was found efficient is in the synthesis of the diastereomers of (6-methyltetrahydropyran-2-yl) acetic acid. Cycliza-tions of two different 5-silyloxy carbonyl compounds under either BiBrs-initiated reductive etherification (leading to the cis isomer) or tandem cyclization-addition (trans-isomer) etherification protocols from the corresponding P-keto ester or aldehyde respectively, has efficiently provide the target molecules [47]. 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

Addition to carbonyl compounds. In the presence of ZnCl2 or SnCl2, N,Si(CH,), adds to aldehydes or ketones to form gem-di azides. Reactions catalyzed by NaN and 15-crown-5 provide a-silyloxy azides exclusively. The adducts of aldehydes in both reactions are obtained in higher yield than the adducts of ketones. 

Silyloxy esters.l Silyl ketene acetals are known to undergo aldol condensation with carbonyl compounds in the presence of TiCl4 (12,268) to afford (3-silyloxy esters. The same products can be obtained in a one-step reaction of an a,p-unsaturated ester with trimethylsilane and a carbonyl compound in the presence of RhCl3H20. 

Rearrangement of epoxy silyl ethers. When treated with 1 equiv. of TiCl4, a-silyloxy epoxides rearrange to p-hydroxy carbonyl compounds. 

Conditions carbonyl compound (1.0 equiv.), 2-(trimethylsilyloxy)furan or 3-methyl-2-(trimethyl-silyloxy)furan (1.5 equiv.), Bi(0Tf)3-4H20 (x mol%), Et20, —45 °C, 0.5-3 h na not applicable... 

Carbonyl compounds are reduced to symmetrical ethers, probably by way of reduction of some of the starting material to a silyl ether (9), reaction to form the mixed ketal (10) and then reductive replacement of the silyloxy group. Some hydrocarbon may be obtained as a by-product by reduction of (9 Scheme 4). Among the acid partners that have been used are trifluoroacetic acid, trityl perchlorate (with aldehydes) and electrogenerated protons. With Nafion resin symmetrical ethers are obtained from aldehydes, but silyl ethers are obtained from ketones. ... 

The reaction of silyl-enol ethers with carbonyl compounds, also known as the Mukaiyama-z Ao reaction 236,237), represents one of the most useful methods for the (stereoselective) construction of carbon-carbon bonds 238-240). As an example of special interest, the y-butenolide moiety is present in over 13,000 natural products and the coupling of silyloxy furans and aldehydes using chiral Lewis acids is one of the most versatile strategies for butenolide syntheses 241-243). 

Lithio ethyl acetate is prepared in quantitative yield by reaction of LHMDS with ethyl acetate in THF at —78 °C. Reaction with carbonyl compounds leads to condensation products in high yield (eqs 3 and 4). No racemization of the a-silyloxy esters occurs (eq 4). 

Another organocatalyst proposed as altemative to proline was the 4-silyloxy proline derivative 25, which was able to catalyze the enantioselective a-anfinoxylation of ketones very efficiently, yielding the final products after only 1 min and with good chemical yields and excellent enantioselectivities [97]. Ranaikably, in this case neither a-amino ketones nor dihydroxy ketone derivatives could be detected in the crude reaction mixture. Moreover, this catalyst could perform the reaction with substrates, which failed with the initial proline protocol, such as cycloheptanone (50%, >99% ee). Also, the triflamide 21b (R=CF3) has been used as organocatalyst in the reaction of ketones, as well as aldehydes, with nitrosobenzene to give the expected hydroxy-lated carbonyl compounds with high level of enantioselectivity [98]. 

Rubottom oxidation reactions have been conducted on enolsilanes derived from a number of different carbonyl derivatives including carboxylic acids and esters.15 For example, the Rubottom oxidation of bis(trimethylsilyl)ketene acetal 30 provided a-hydroxy carboxylic acid 31 in 81% yield. Use of alkyl trimethylsilyl ketene acetal substrates generates a-hydroxy esters, as seen in the conversion of 32 to 33.16 The synthesis of 3-hydroxy-a-ketoesters (e.g., 36) has been accomplished via Rubottom oxidation of enolsilanes such as 35 that are prepared via Homer-Wadsworth-Emmons reactions of aldehydes and ketones with 2-silyloxy phosphonoacetate reagent 34.17 The a-hydroxylation of enolsilanes derived from P-dicarbonyl compounds has also been described, although in some cases direct oxidation of the P-dicarbonyl compound is feasible without enolsilane formation.18... 

---