Silyl ethers migration

Acetates are the most commonly used of the esters for hydroxyl protection, particularly for peracylation (Figure 2.32) [47] although selective partial acetylation can be achieved, the tendency of acetates to migrate can lead to a loss in observed selectivity. In some cases, as with silyl ethers, migration can be sufficiently reliable to be synthetically useful for example, acetate migration can be used efficiently to lead to simultaneous release of the 4-OH and protection of the 6-OH in glucose derivatives (Figure 2.33) [48]. 

EtMgBr, Et20, it, 1 h, 90-100% yield. These conditions were used to prevent a neighboring silyl ether from migrating. Ethylmagnesium chloride is much more reactive thus, the reaction can be run at —42°, giving a 90% yield of the alcohol. Acetates and pivaloates are also cleaved. 

The use of boron trifluoride-diethyl ether complex as the Lewis acid in these reactions promotes silyl group migration and gives rise to the formation of tetrahydrofurans with excellent stereoselectivity82. 

The combination of a silyl-migration from carbon to oxygen and a prototropic isomerization leads to allenyl silyl ethers [246]. 

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

For the removal of this protecting group, tetrabutylammonium fluoride in oxolane is the most frequently used [388, 389, 409-411]. A much simpler reagent to prepare, potassium fluoride — crown ether, has been introduced for the same purpose [427]. Silyl group at 0-2 of nucleosides is cleaved more rapidly [411] than at 0-5. Acyl migrations occurred under the tetrabutylammonium fluoride-catalyzed desilyla-tion [432, 434, 443], Differencies between the primary and secondary position were also observed for acid- or base-catalyzed solvolysis [391, 409-412], 5 -0-(7ert-butyl-dimethylsilyl)nucleosides are much more labile towards acid than either 2 - or 3 -silyl ethers [391, 410-412], whereas the situation is reversed for base hydrolysis [411], /V-Bromosuccinimide in aqueous DMSO is another alternative for the removal of this type of silyl group [444]. 

When softer counter cations such as sodium and potassium ions were used instead of the lithium ion, a reversal of the migration was found. The reaction of 249 in the presence of a catalytic amount of NaH in DMF followed by hydrolysis gave 250 in excellent yield (equation 157), while the reaction of 249 with 0.2 equiv of MeLi in THF did not result in any detectable amount of silyl ether 250 even after a prolonged reaction time. 

Anionic 1,4-silyl migrations of O-trialkylsilyl methyl ketoximes (268) were found to give o -trialkylsilyl ketoximes 269 after hydrolysis (equation 167). Intramolecularity of this migration was confirmed by crossover experiments. Upon heating 269 at 100 °C, a reverse migration to 268 took place. The reaction appeared to be limited to silyl ethers of methyl ketoximes having the (E)-configuration413. 

An anionic 1,6-silyl migration from C to O (1,6-Brook rearrangement) was observed during the deprotonation of e-silyl alcohol 324, which gave the corresponding silyl ether 325 (equation 199)466. 

Acylated or mesylated aldehyde adducts 301 react with enol silyl ethers 302 to provide products 303, after a phenylsulfanyl migration reaction. Oxidation of compounds 303 with MCPBA followed by heating under mesitylene reflux and acidic hydrolysis afforded 1,3-diketones 304, whereas treatment with potassium fert-butoxide followed by acidic hydrolysis provided 1,4-diketones 305 (Scheme 79)478. 

Anionic migration of an alkylsilyl group from carbon to oxygen to afford silyl ether a-oxycarbanion is known as a Brook rearrangement, and the reverse reaction is called... 

Allyl silyl ethers 29 derived from the corresponding allylic alcohols 28 are selectively isomerized to silyl enol ethers 30 via carbon-carbon double bond migration catalyzed by a ruthenium hydride complex, RuH2(PPh3)4 (Eq. 12.11) [17], The generality of the reaction was demonstrated for the silyl ethers of methallyl alcohol, ciima-myl alcohol, 2,4-pentadienyl alcohol, and so on. 

Anomeric silyl ethers have been prepared from 1-OH sugars and the corresponding silyl chloride in the presence of a base. When the hydroxyl group at C-2 is unprotected silyl group migrations away from the anomeric center have been observed [458]. 

Acylsilanes are versatile intermediates for carbon-carbon bond formation reactions, and may serve as precursors for the synthesis of silyl enol ethers, aldehydes, or carboxylic acids. In the presence of a base or certain nucleophiles, they undergo the Brook rearrangement, where the silyl moiety migrates from carbon to oxygen (see below in this section). 

Isomerisation ofallyl silyl ethers. Double bond migration promoted by cationic Ir complexes provides silyl enol ethers (10 examples, 71-97%). 

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