Butyldimethylsilyl ethers

SEM ethers are stable to the acidic conditions (AcOH, H2O, THE, 45°, 7 h) that are used to cleave tetrahydropyranyl and t-butyldimethylsilyl ethers. 

Rexyn 101 (polystyrene sulfonic acid), 80-91% yield.This method does not cleave the r-butyldimethylsilyl ether. 

The triethylsilyl ether is approximately 10-100 times more stable than the TMS ether and thus shows a greater stability to many reagents. Although TMS ethers can be cleaved in the presence of TES ethers, steric factors will play an important role in determining selectivity. The TES ether can be cleaved in the presence of a /-butyldimethylsilyl ether using 2% HE in acetonitrile. In general, methods used to cleave the TBDMS ether are effective for cleavage of the TES ether. 

NaH, HMPA, 0°, 5 min H2O, 83-84% yield. These conditions selectively cleave a TBDPS ether in the presence of a t-butyldimethylsilyl ether. 

DMSO, H2O, dioxane, reflux, 12 h, 65-99% yield.These conditions cleave a dimethyl ketal in the presence of a r-butyldimethylsilyl ether. 

Bu4N F, THF, 0°, 1 h, 52-95% yield.A primary alcohol protected as the r-butyldimethylsilyl ether is cleaved under these conditions, but a similarly protected secondary alcohol is stable. 

Aryl and alkyl trimethylsilyl ethers can often be cleaved by refluxing in aqueous methanol, an advantage for acid- or base-sensitive substrates. The ethers are stable to Grignard and Wittig reactions and to reduction with lithium aluminum hydride at —15°. Aryl -butyldimethylsilyl ethers and other sterically more demanding silyl ethers require acid- or fluoride ion-catalyzed hydrolysis for removal. Increased steric bulk also improves their stability to a much harsher set of conditions. An excellent review of the selective deprotection of alkyl silyl ethers and aryl silyl ethers has been published. ... 

From intermediate 12, the path to key intermediate 7 is straightforward. Reductive removal of the benzyloxymethyl protecting group in 12 with lithium metal in liquid ammonia provides diol 27 in an overall yield of 70% from 14. Simultaneous protection of the vicinal hydroxyl groups in 27 in the form of a cyclopentanone ketal is accompanied by cleavage of the tert-butyldimethylsilyl ether. Treatment of the resultant primary alcohol with /V-bromosuccini-mide (NBS) arid triphenylphopshine accomplishes the formation of bromide 7, the central fragment of monensin, in 71 % yield from 27. 

The successful implementation of this strategy is shown in Scheme 4. In the central double cyclization step, the combined action of palladium(n) acetate (10 mol %), triphenylphosphine (20 mol %), and silver carbonate (2 equiv.) on trienyl iodide 16 in refluxing THF results in the formation of tricycle 20 (ca. 83 % yield). Compound 20 is the only product formed in this spectacular transformation. It is noteworthy that the stereochemical course of the initial insertion (see 17—>18) is guided by an equatorially disposed /-butyldimethylsilyl ether at C-6 in a transition state having a preferred eclipsed orientation of the C-Pd a bond and the exocyclic double bond (see 17). Insertion of the trisubstituted cycloheptene double bond into the C-Pd bond in 18 then gives a new organopal-... 

Although some methods for reductive etherifications of carbonyl compounds have been reported [152-162], the iron-catalyzed version possesses several advantages (1) fairly short reaction times are needed, (2) not only trimethylsilyl ether but also triethylsilyl and butyldimethylsilyl ethers and alcohols are adaptable, and (3) a broad substrate scope. 

The same coordination is used to account for the observed anti preference in the allylation of (t-hydroxybutanal with allyl bromide/indium in water (Scheme 8.16). The intermediate leads to the anti product. In support of the intramolecular chelation model, it is found that if the hydroxy group is converted to the corresponding benzyl or t-butyldimethylsilyl ether, the reaction is not stereoselective at all and gives nearly equal amounts of syn and anti products. 

Aliphatic and aromatic carboxamides, with the exception of p-nitrobenzamide, are dehydrated in this way in high yield. Acid-labile protective groups such as tetra-hydropyranyl and tert-butyldimethylsilyl ether and base-sensitive compounds are not attacked. A,A -Sulfinyldi-1,2,4-triazole, easily prepared from thionylchloride and triazole [THF, (C2H5)3N, 0 °C, 1 h] in 85-95% yield, was used without further purification. 

A similar strategy can also be used in a seven-step procedure of preparation of 3/3,25-dihydroxy-cholesta-5,7-diene from ergosterol giving a total yield of 30%. The 3-hydroxy function of ergosterol is protected as /-butyldimethylsilyl ether before the 5,7-diene system is treated with l,4-dihydrophthalazine-l,4-dione. In this case, the key step is a very mild method for the cleavage of the hetero Diels-Alder adduct using lithium naphthalenide <1999S1331>. 

Silica gel-based catalytic systems have been described as efficient promoters for a number of organic reactions.28 Illustrative examples include the oxidative cleavage of double bonds catalyzed by silica-supported KM11O4,29 reaction of epoxides with lithium halides to give /i-halohydrins performed on silica gel,30 selective deprotection of terf-butyldimethylsilyl ethers catalyzed by silica gel-supported phosphomolybdic acid (PMA),31 and synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts.32... 

G. D. Kishore Kumar and S. Baskaran, A facile, catalytic, and environmentally benign method for selective deprotection of ferf-butyldimethylsilyl ether mediated by phosphomolybdic acid supported on silica gel, /. Org. Chem., 70 (2005) 4520-4523. 

A nickel-chromium catalyst prepared from chromous chloride and (p-diphenylphos-phinopolystyrene)nickel dichloride mediates the ring-closure of the ene-allene 236 (R = H) to a mixture of 3.4 parts of 237 and 1 part of 238 (equation 120)121. An analogous reaction of the t-butyldimethylsilyl ether of 236 yields solely the (E)-isomer 237 (R = t-BuMeaSi). Cyclization of the ene-allene 239 affords the perhydroindane 240 in 72%... 

A tandem enolate-arylation-allylic cyclisation, in which an essential z-butyldimethylsilyl ether protecting group delays the cyclisation step until the Pd-catalysed arylation is complete, enables 1-vinyl-l//-[2]benzopyrans 54 to be prepared from 2-bromobenzaldehyde (Scheme 32) <00CC1675>. 4-Substituted isochromans 55 are formed from aldehydes by a Pd-catalysed termolecular queuing cascade. The sequence involves cyclisation of an aryl iodide onto a proximate alkyne followed by an allene insertion. Transmetallation with indium then allows addition to the aldehyde (Scheme 33) . 

Addition of allylic zinc bromides to nitrones, generated in situ from allylbro-mides and zinc powder in THF (670), allyltributylstannane (671) and lithiated allyl ferf-butyldimethylsilyl ether (672), proceeds regioselectively in good yields and is used to synthesize homoallyl hydroxylamines (Scheme 2.189). The latter were subjected to an iodo cyclization reaction (see Scheme 2.186). 

Trost et al 1 have observed product distribution to be dependent in part on the steric and electronic properties of the substrate. For example, linear enyne 48 (Equation (30)) cyclized exclusively to the Alder-ene product 49, whereas branching at the allylic position led to the formation of 1,3-diene 50 (Equation (31)) under similar conditions. Allylic ethers also give 1,3-dienes this effect was determined not to be the result of chelation, as methyl ethers and tert-butyldimethylsilyl ethers both gave dialkylidene cyclopentanes despite the large difference in coordinating ability. 

The potentiality of the present methodology is demonstrated by the synthesis of y-undecalactone, as shown in Scheme 18 [37,47], The treatment of the THP-protected cu-hydroxyalkyl iodide with the anion of methoxybis(trimethylsilyl) methane gave the corresponding alkylation product. Acidic deprotection of the hydroxyl group followed by Swern oxidation produced the aldehyde. The aldehyde was allowed to react with heptylmagnesium bromide, and the resulting alcohol was protected as tm-butyldimethylsilyl ether. The electrochemical oxidation in methanol followed by the treatment with fluoride ion afforded the y-undeealactone. 

Substituted cyclobutenediones. These useful precursors to quinones (13,209-210) can be prepared from commercially available dialkoxycyclobutenediones (1, dialkyl squarates). Thus a wide variety of organolithium reagents add to 1 at - 78°, and the adducts (a) are hydrolyzed under mild conditions to the cyclobutenediones 2.2 - Protection of a as the /-butyldimethylsilyl ether (b) permits a second addition... 

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