Trimethylsilyl fluoride, synthesis

A similar elimination in which the tin is attacked by fluoride anions (cf. the reaction of silanes with F ) has been used179 to synthesize terminal methylene compounds as in equation (75). An analogous reaction sequence using a trimethylsilyl group in place of the trialkyltin group has been published by Hsiao and Shechter180 as part of a synthesis of substituted 1,3-butadienes. 

A simple procedure for the synthesis of 4,5-disubstituted 1,2,3-triazoles 1247 involves stirring a mixture of nitroethene 1245 with trimethylsilyl azide and tetrabutylammonium fluoride at 30 °C for 3h. No solvent is needed. Triazoline 1246, which forms in the first step of the reaction, eliminates nitrous acid, and the trimethylsilyl group is cleaved off by the fluoride anion to afford triazole 1247. Various aryl and heteroaryl substituents R are used providing triazoles 1247 in 70-90% yield (Scheme 207) <2005JOC6526>. 

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. 

The efficiency of the [2 + 2]-cycloadditions of 417 was utilized in a strategy for the synthesis of cephalosporin derivatives that carry an acetone or acetic acid ester group in the 3-position (Scheme 6.88) [175]. Liberated in the presence of 2-(trimethylsilyl-oxy)propene, 417 underwent cycloaddition leading to 435, treatment of which with tetrabutylammonium or hydrogen fluoride furnished the A3-cephalosporin 436 admixed with the A2-isomer. This mixture was converted to pure 436 by an oxidation-reduction sequence. In addition to the trimethylsilylenol ether of acetone, the... 

Regiospecific mono-C-alkylation (60-90%) of trimethylsilyl enol ethers is promoted by benzyltriethylammonium fluoride [34, 35]. A similar alkylation of tin(IV) enolates is aided by stoichiometric amount of tetra-n-butylammonium bromide and has been utilized in the synthesis of y-iminoketones [36]. Carbanions from weakly acidic carbon acids can be generated by the reaction of their trimethylsilyl derivatives with tetra-n-butylammonium triphenyldifluorosilicate [37] (see also Section 6.3). Such carbanions react readily with haloalkanes. Tautomeric ketones in which the enol form has a high degree of stabilization are O-alkylated to form the enol ether, e.g. methylation of anthrone produces 9-methoxyanthracene [26],... 

Dodd and co-workers (5) reported the first known synthesis of 11//-indolizino[8,7-h]indoles by the cycloaddition reaction of a nonstabilized ylide 21 and diethylacetylene dicarboxylate (DEAD). The azomethine ylide, formed by the alkylation of the 3,4-dihydro-p-carboline (22) with trimethylsilyl methyl triflate to the triflate salt, followed by in situ desilyation with cesium fluoride, underwent cycloaddition with DEAD at low temperature. The expected major cycloadduct 23 was isolated, along with quantities of a minor product 24, presumed to have been formed by initial reaction of the ylide with 1 equiv of DEAD and the intermediate undergoing reaction with a further equivalent of DEAD before cyclization. Dodd offers no explanation for the unexpected position of the double bond in the newly generated five-membered ring, although it is most likely due to post-reaction isomerization to the thermodynamically more stable p-amino acrylate system (Scheme 3.5). 

Scheme 12.10. Synthesis of a-onocerin using butyldimethylsilyl, THF = tetrahydrofuran, a three-component nucleophilic addition/Brook Tf = trifluoromethanesulfonyl, DME = 1,2-rearrangement/alkylation on acylsilanes, by dimethoxyethane, TMS = trimethylsilyl, Corey and co-workers [33]. TBS = t- TBAF = n-tetrabutylammonium fluoride.

SCHEME 14.2 Synthesis of lipid IV by Wong, Cheng, and coworkers. ClAc, chloroacetyl NIS, /V-iodosucci n i mide Phth, phthaloyl PyBOP, benzotriazol-l-yl-oxytripyrrolidinophos-phonium hexafluorophosphate RRV, relative reactivity value TBAF, tetrabutylammonium fluoride TBS, fert-butyldimethylsilyl Tf, trillyl TMS, trimethylsilyl Tol, 4-tolyl. 

Michael additions. Gerlach and Kiinzler report that the lithium enolate of S-t-butyl thioacetate undergoes 1,4-addition to cyclopentenone. They have extended this Michael reaction to a synthesis of methyl jasmonate (5), based on the similar conjugate addition of the trimethylsilyl enolate 1 promoted by tetra-n-butylam-monium fluoride. The adduct 2 was alkylated by l-bromo-2-pentyne in the presence of tetra-n-butylammonium fluoride to give 3 in rather low yield. Remaining steps to 5 were methanolysis and partial hydrogenation of the triple bond. 

Adaptation of similar functionality to the synthesis of imidazole IV-oxides is illustrated by the fluoride ion-promoted cyclization of the trimethylsilyl ether of the oxime (30) derived from l-(dichloroacetylphenyl)aminopropanone (Scheme 15) (80AHC(27)24l). 

Equation 54 describes the preparation of dienes by a similar cyclopropane cleavage followed by proton transfer and elimination of sulfinate. The weak donor ability of a (trimethylsilyl)methyl group also assists Lewis acid-promoted ring cleavage according to equation 55, which allows synthesis of y, -unsaturated ketones Interestingly, the related ester opens smoothly only if fluoride reagents are employed (equation 56) ... 

Oxidation of Schiff bases of the monoamide 8 derived from diamino-maleonitrile gives 4-cyanoimidazole-5-carboxamides (Eq. 4). A new synthesis of 2-formyl-4-methyl-l-phenylimidazole 3-oxide (10) involves the fluoride-ion-promoted cyclization of the trimethylsilyl ether of JV-dichloro-acetyl-A-phenylaminopropanone oxime (9). ... 

Saegusa s group (81JA5250) found that [o-[(trimethylsilyl)alkylamino]-benzyljtrimethylammonium halide underwent the fluoride-anion induced 1,4-elimination under mild conditions to generate an o-quinone methide N-alkylamine intermediate. They performed the formal synthesis of gephyro-toxin 434 on the basis of an intramolecular cycloaddition of the o-quinone methide A-alkylamine 426 (83TL2881). Treatment of azadiene precursor 425... 

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