Ethers, a-silyl

Acylsilanes are a class of compounds in which a silyl group is directly bound to the carbonyl carbon, and they have received considerable research interest from the point of view of both physical organic and synthetic organic chemistry [15]. Acylsilanes have a structure quite similar to the structure of a-silyl-substituted ethers a silyl group is attached to the carbon adjacent to the oxygen atom, although the nature of the C-O bond is different. Therefore, one can expect /1-silicon effects in the electron-transfer reactions of acylsilanes. 

TIPS ether) A silyl ether of formula R -O-Sid-Prh commonly used to protect alcohol groups. Formed from an alcohol with TIPSC1 and a tertiary amine. Deprotected using aqueous fluoride salts, (p. 645)... 

A common OH protecting group is a silyl ether. A silyl ether has a new O-Si bond in place of the O-H bond of the alcohol. The most widely used silyl ether protecting group is the tert-butyldimethylsilyl ether. 

Alkenyl allyl ethers. A silyl enol ether nixed acetal of iodoacetaldehyde. The eliminaot BuLi furnishes the alkenyl allyl ether with a defi ractor is that the elimination pattern in DME is... 

Alkenyl allyl ethers. A silyl enol ether, an allylic alcohol, and NIS react to form mixed acetal of iodoacetaldehyde. The elimination of an iodine atom and the siloxy group with BuLi furnishes the alkenyl allyl ether with a defined (Z)- or ( )K onfiguration. An important factor is that the elimination pattern in DME is strictly anti, while in hexane it is syn. 

Another type of protecting group for alcohols is a silyl ether. A silyl ether has the general formula of R —O—SiR3. The reaction of an alcohol with a trialkylsilyl halide leads to a trialkylsilyl ether in near-quantitative yield... 

The (partial) description of the synthesis and coupling of the five fragments starts with the cyclohexyl moiety C —C. The first step involved the enantio- and diastereoselective harpless epoxidation of l,4-pentadien-3-ol described on p. 126f. The epoxide was converted in four steps to a d-vinyl d-lactone which gave a 3-cyclohexenecarboxylate via Ireland-CIaisen rearrangement (cf. p. 87). Uncatalysed hydroboration and oxidation (cf. p. 131) yielded the desired trans-2-methoxycyclohexanol which was protected as a silyl ether. The methyl car-... 

The optically active ethoxylaetam 13 (see Appendix) gives similar results with an allylsilane102. The reaction with a silyl enol ether, however, proceeds with low diastereoselectivity. 

The Lewis acid induced reaction of silyl enol ethers and silyl ketene (thio)acetals with 4-acetoxyazetidinones is often used for introduction of a carbon substituent in the 4-position of the jS-lactam ring. Numerous examples are known, both with and without substituents at nitrogen, some of which are shown. 

The final example concerns cyclization of a silyl enol ether, connected to yet another carbon atom. The (.Ej-enol ether 23 appears to be converted with high stereoselectivity into the aldehyde 24 in 70- 90% yield, while the (Z)-enol ether 23 affords the epimeric aldehyde 25 in similar yield and selectivity164. 

The enol acetates, in turn, can be prepared by treatment of the parent ketone with an appropriate reagent. Such treatment generally gives a mixture of the two enol acetates in which one or the other predominates, depending on the reagent. The mixtures are easily separable. An alternate procedure involves conversion of a silyl enol ether (see 12-22) or a dialkylboron enol ether (an enol borinate, see p. 560) to the corresponding enolate ion. If the less hindered enolate ion is desired (e.g., 126), it can be prepared directly from the ketone by treatment with lithium diisopropylamide in THE or 1,2-dimethoxyethane at —78°C. ... 

Attack by iodine and COgH on the double bond of (30) must be trans, hence I is axial in (29) and elimination to (28) is easy. Epoxidatlon occurs on the less hindered side of the double bond to give (27) which is opened by bromide to trans diaxial (26). This compound was protected as a silyl ether. 

The tert-butyldimethylsilyl (TBDMS) protection of 50 gave a silyl ether... 

Synthesis of geranyl 6-0-fl-o-xylopyranosyl-(3-D-glucopyranoside (82) Tert-butyldimethylsilylation of 51 gave a silyl ether (84, 63% yield), which was subjected to benzoylation to give a benzoate (85) in 71% yield. Desilylation of 85... 

Scheme 2.2 illustrates several examples of the Mukaiyama aldol reaction. Entries 1 to 3 are cases of addition reactions with silyl enol ethers as the nucleophile and TiCl4 as the Lewis acid. Entry 2 demonstrates steric approach control with respect to the silyl enol ether, but in this case the relative configuration of the hydroxyl group was not assigned. Entry 4 shows a fully substituted silyl enol ether. The favored product places the larger C(2) substituent syn to the hydroxy group. Entry 5 uses a silyl ketene thioacetal. This reaction proceeds through an open TS and favors the anti product. 

Scheme 2.9 gives some examples of use of enantioselective catalysts. Entries 1 to 4 are cases of the use of the oxazaborolidinone-type of catalyst with silyl enol ethers and silyl ketene acetals. Entries 5 and 6 are examples of the use of BEMOL-titanium catalysts, and Entry 7 illustrates the use of Sn(OTf)2 in conjunction with a chiral amine ligand. The enantioselectivity in each of these cases is determined entirely by the catalyst because there are no stereocenters adjacent to the reaction sites in the reactants. 

The scope of the conjugate addition reaction can be further expanded by use of Lewis acids in conjunction with enolate equivalents, especially silyl enol ethers and silyl ketene acetals. The adduct is stabilized by a new bond to the Lewis acid and products are formed from the adduct. 

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