Trialkylsilyl oxides

This reaction was initially reported by Flood in 1933. It is a synthesis of trialkylsilyl halide involving the treatment of the mixture of di-trialkylsilyl oxide and concentrated sulfuric acid with sodium halide or ammonium halide. The resulting trialkylsilyl halide can be extracted by petroleum ether and then purified via distillation. It was found that when di-trimethylsilyl oxide is mixed with concentrated sulfuric acid, trimethylsilyl sulfate can be isolated as a white crystalline (m.p., Sb-SS C), which forms trimethylsilyl halide when it reacts with ammonium halide. The reaction has been improved by continuous extraction of the reaction mixture with pentane to yield more and purer trialkylsilyl halide and by the addition of ammonium bisulfate to the reaction mixture. ... 

Nitro compounds have been converted into various cyclic compounds via cycloaddidon reactions. In particular, nitroalkenes have proved to be nsefid in Diels-Alder reactions. Under thermal conditions, they behave as electron-deficient alkenes ind react v/ith dienes to yield 3-nitrocy-clohexenes. Nitroalkenes c in also act as heterodienes ind react v/ith olefins in the presence of Lewis acids to yield cyclic alkyl nkronates, which undergo [3- 2 cycloaddidon. Nitro compounds are precursors for nitnie oxides, alkyl nitronates, and trialkylsilyl nitronates, which undergo [3- 2 cycloaddldon reacdons. Thus, nitro compounds play important roles in the chemistry of cycloaddidon reacdons. In this chapter, recent developments of cycloaddinon chemistry of nitro compotmds and their derivadves are summarized. 

The per(silylethynyl)ated octadehydro[12]annulenes 58a, b and the dodecade-hydro[18]annulenes 59 a, b were prepared by oxidative Hay macrocyclization of the corresponding czs-bis(trialkylsilyl)-protected TEEs [84, 85]. X-ray crystal... 

On treatment of trialkylsilyl nitronates 1043 with MeLi, LiBr, or BuLi in THF the resulting nitrile oxide intermediates 1044 afford, in dilute THF solution (R=Me) the ketoximes 1045 in ca 50-60% yield, whereas in concentrated THF solution the O-silylated hydroxamic acids 1046 are obtained as major products [144] (Scheme 7.35). Analogously, the silyl nitronate 1047 reacts with the 2,3,4,6-tetra-O-acetyl-/ -D-glucopyranosyl thiol/triethylamine mixture to afford, via the thiohydroxi-mate 1048, in high yield, a mixture of oximes 1049 which are intermediates in the synthesis of glucosinolate [145] (Scheme 7.35). 

Nef reaction.4 Nitro compounds, primary or secondary, are converted to tri-alkylsilyl nitronates in greater than 90% yield by reaction with a trialkylsilyl chloride and DBU in CH2C12. The silyl nitronates derived from secondary nitro compounds are oxidized by C1QH4C03H at 25° to ketones in high yield. This sequence is not useful for conversion of primary nitro compounds to aldehydes. 

Scheme 8.13 and Eqs. 8.6-8.10 reveal that lithiated methoxyallene 42 is sufficiently reactive towards a variety of electrophiles such as alkyl halides [44, 45], ethylene oxide [12c], tosylated aziridine 45 [46], dimethyl disulfide [12b], trialkylstannyl and trialkylsilyl chlorides [47, 48] and iodine [49]. These substitution reactions proceed with excellent regioselectivity and the corresponding a-functionalized products are obtained in good to high yields. An exceptional case was found by treatment of 42 with a guanidinium salt, which led to a 60 40 mixture of a- and y-adducts 50 and 51 (Eq. 8.11) [50],... 

Since enol silyl ethers are readily accessible by a number of methods in a regioselective manner and since the trialkylsilyl moiety as a potential cationic leaving group facilitates the termination of a cyclization sequence, unsaturated 1-trialkylsilyloxy-1-alkenes represent very promising substrates for radical-cation cyclization reactions. Several methods have been reported on the synthesis of 1,4-diketones by intermolecular oxidative coupling of enol silyl ethers with Cu(II) [76, 77], Ce(IV) [78], Pb(IV) [79], Ag(I) [80] V(V) [81] or iodosoben-zene/BFa-etherate [82] as oxidants without further oxidation of the products. 

Alternatively, lactone 392 was condensed with 2-furaldehyde, and the aldol adduct was dehydrated to give 394. Treatment of 394 with methanolic sodium methoxide afforded the methyl ester 395, which, after (trialkylsilyl)ation, was transformed by ozonolysis into the unstable keto ester 396. Compound 396 was converted into showdomycin, as well as into some 6-azapseudouridines.261 A number of a,a -dibro-moketones react262 with furan, to give substituted analogs of the bicy-elic ketone 390. Appropriately substituted substrates have been converted,263 by way of Baeyer-Villiger oxidation and treatment of the resulting lactone with tcrt-butoxybis(dimethylamino)methane, into pseudouridines 397 modified at C-5. 

Acyl silanes by anodic oxidation of2-alkyl-2-trialkylsilyl-l,3-dithianes... 

The protection of alcohols as silyl ethers has been reviewed62, as have the relative stabilities of the different trialkylsilyl groups63. Their stability under alcohol oxidation conditions and their oxidative deprotection have been discussed64. Methods for selective deprotection of the various silyl ethers have been the subject of an excellent review65. 

The mechanism of Scheme 42 is an alternative pathway to the hydride abstraction pathway of oxidation of trialkylsilyl enol ethers185. The silyl derivatives of 4-aza-3-ketosteroids... 

The trialkylsilyl group can be introduced to aryl or alkenyl groups using hexaalkyldisilanes. The oxidative addition of alkenyl iodide and transmetallation,... 

Diorgano chalcogen oxides replaced tellurium in bis[trialkylsilyl] telluriums by oxygen4... 

This deprotection-oxidation can also be applied to the preparation of quinones from trialkylsilyl-pro-tected hydroquinones. This method has wider applicability than that reported employing PCC, since substrates with electron-releasing and those with electron-withdrawing groups are oxidized. Use of the reagent prepared in situ appears to be preferable to the preformed reagoit. 

Carbon-centered nucleophiles can also be used to advantage in the reaction with epoxides. For example, the lithium enolate of cyclohexanone 96 engages in nucleophilic attack of cyclohexene oxide 90 in the presence of boron trifluoride etherate to give the ketol 97 in 76% yield with predominant syn stereochemistry about the newly formed carbon-carbon bond <03JOC3049>. In addition, a novel trimethylaluminum / trialkylsilyl triflate system has been reported for the one-pot alkylation and silylation of epoxides, as exemplified by the conversion of alkenyl epoxide 98 to the homologous silyl ether 99. The methyl group is delivered via backside attack on the less substituted terminus of the epoxide <03OL3265>. 

Secondly it has been found that a number of trialkylsilyl groups can be added stereoselectively to the /j-position of the C —C double bond in dioxanone 10 (derived from enantiomerically pure 3-hydroxybutanoic acid)23. In this case the presumed silylcuprate is generated from a chlorosilane and a higher-order organocuprate. In the example shown, with addition of phenyldimethylsilyl chloride, a diastereomeric ratio of 91 9 was obtained. Improved ratios were obtained with other groups (e g., 98 2 for trimethylsily 1), but unfortunately none of the latter were susceptible to oxidative cleavage. The first two steps of the procedure produce a mixture of 11 and its silyl enol ether, which is then desilylated to 11 by treatment with tetrabutylammonium fluoride. 

Trialkylsilyl enolates can be oxidized to the corresponding a, S-nnsatnrated ketones with trityl tetrafluoborate or trityl methyl ether in the presence of BFsOEta. It has been observed that the use of trityl cations often leaves snbstantial amonnts of saturated ketones. The amonnt of these species can be rednced by the nse of ddq as in the case of... 

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