Silylative under

Alkylation of pyrazinones and quinoxalinones may be carried out under a variety of conditions and it is usually observed that while O-alkylation may occur under conditions of kinetic control, to yield the corresponding alkoxypyrazines or alkoxyquinoxalines, under thermodynamic control the A-alkylated products are formed. Alkylation using trialkyl-oxonium fluoroborate results in exclusive O-alkylation, and silylation under a variety of conditions (75MI21400) yields specifically the O-silylated products. Alkylation with methyl iodide or dimethyl sulfate invariably leads to A-methylation. 

TBDMSCl, imidazole, DMF, 25°, 10 h, high yields. This is the most common method for the introduction of the TBDMS group on alcohols with low steric demand. The method works best when the reactions are mn in very concentrated solutions. This combination of reagents also silylates phenols, hydroperoxides, and hydroxyl amines. Thiols, amines, and carboxylic acids are not effectively silylated under these conditions. ... 

The reductive silylation under these conditions renders possible an access to useful heterocyclic intermediates which undergo various electrophilic displacements (Scheme 21)105 of the TMS moiety. In the first step phenazine (149) is converted to the corresponding 5,10-bis(TMS)-5,10-dihydrophenazine (150) that can be acylated in excellent yields to the desired 5,10-diacetyl-5,10-dihydro-phenazine (151). 

The rate of derivatization of different compounds varies when trimethylsilyl derivatives are prepared (Lundquist and Kirk 1971). Another complication arises from the fact that enolization and subsequent formation of enol derivatives occur during the derivatization treatment (Lundquist and Kirk 1971). Ketol la (Fig. 6.1.1), for example, has been shown to give rise to three different derivatives (Fig. 6.1.3) (Lapierre et al. 1983a). One approach for overcoming these difficulties consists of derivatization under mild conditions in combination with a gas chromatographic study of the rate of derivatization (Lundquist and Kirk 1971, Pometto and Crawford 1985). A second solution to this problem consists of performing the silylation under conditions that secure complete derivatization and analysis of all types of derivatives formed (Lapierre et al. 1983a). The latter principle has been adopted in the preferred acidolysis procedure. 

This first product is a very electrophilic 1,2-dione and it accepts electrons from sodium atoms Details in B M Trost and group even more readily that the original esters. The product is an ene diolate that is also silylated under j. Org. Chem., 1978, 43, 4559. die reaction conditions. 

Typical fragmentation of both types of base involves the loss of small neutral molecules such as HCN, CO, and CH3CN, as well as some extensive rearrangements in aromatic compounds. The most common derivatization, silylation under normal conditions (TMSC, HMDS/TMSC, BSA, or BSTFA/TMSC, V, 20 min - 2 hr), with or without pyridine as a solvent and base catalyst, usually leads to the enolic forms of the bases via the corresponding silyl ether. The persilylated derivatives are volatile enough to be used in gas chromatography-mass spectrometry (GC-MS). This enhancement of volatility compared to that of the free bases is the key factor making GC-MS a widely used analytical method in this area. 

Summarizing all these results, it can be said that three different kinds of products can be obtained due to reaction conditions The alternatives are (a) pure C-silylation under reduction (b) dimerizing reduction ( duplication reductrice ) (c) pure reduction ( simple reduction ). 

With ketones we come to the problem of regioselectivity, and the situation from chapter 3 is that methyl ketones 98 and ketones with one primary and one secondary alkyl group, particularly cyclic ketones such as 103 give the less substituted lithium enolate 97 or 102 by kinetically controlled deprotonation with LDA, and the more substituted silyl enol ether 99 or 104 on silylation under equilibrium conditions. Either derivative (lithium enolate or silyl enol ether) may be used to make the other, e.g. 96 and 100. 

SUylatioH of carbonyl compounds, Ketones, diketones, esters, and amides are silylated under mild conditions by this reagent in the presence of triethylamine. Examples ... 

Secondary and tertiary alcohols can be silylated under mild conditions using Et3SiOTf and 2,6-lutidine or a trialkylamine as a proton scavenger (eqs 1 and... 

Silylation of Alcohols, pyridiniump-toluenesulfonate can be used as the catalyst in the silylation of primary and secondary alcohols and phenols (eq 3). Even alcohols sensitive to acid-catalyzed rearrangements can be successfully silylated under these conditions. Continuous removal of the water generated during the reaction is necessary and is achieved via the use of a Soxhlet extractor filled with 4A molecular sieves. 

Benzylation of the 5-amino-5-deoxy-pentoside 3 via its 2,3-O-dibutylstannylene derivative afforded a 1 1 mixture of the 2-0- and 3-0-benzyl ethers, whereas tritylation and silylation under the same conditions generated predominantly the 2-O-protected derivative. Similarly, benzylation of the D-erythronolactone 4 via its 0-dibutylstannylene derivative gave predominantly the 2-O-benzyl ether 5 while reductive opening (TiCU, HSiEts) of the corresponding 2,3-0-benzylidene compound afforded the 3-0-benzyl ether 6. Partial benzylation (0.9 eq NaH, DMF, BnBr) of sucrose has afforded 42% of 2-0-benzyl-sucrose, isolated as its 3,4,6,r,3, 4, 6 -heptaacetate. The molecular electrostatic potential profile of sucrose apparently predicts that the 2-OH group is the most electropositive of the eight hydroxy-groups. ... 

Pyridinium salts of some sugar sulfates can be desulfated and silylated under certain silylating conditions. ... 

Trimethylsilylacetate esters may he converted to the enolate by treatment with lithium dialkylamide bases (LDA in Eq. 7.28) in THF at -78°C. These will add to ketones or aldehydes quickly at -78°C, followed by elimination of MOjSiOLi and formation of a,p-unsaturated esters in high yields, uncontaminated by p,y-unsaturated isomers [47]. This is known as the Peterson reaction [48, 49]. The requisite ethyl trimethylsilylacetate is made by the reaction of cldorotrimethylsilane, ethyl bromoacetate, and zinc [50]. Esters of longer-chain acids give mostly 0-silylation under these conditions, but diphenylmethylchlorosilane gives C-silylation selectively. These diphenyl-methylsilylated esters also give the Peterson reaction (Eq. 7.29) [51]. 

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