Trimethylsilyl ester

Folyphosphoric acid trimethylsilyl ester (PPSE)[1] can be used in sulfolane, CH,Cl2 or nitromethane. It is similar to polyphosphoric acid but the overall conditions arc milder and the work-up more convenient. PPSE has been used in the cydization of ris-arylhydrazones of cyclohexane-l,2-diones to give indolo[2,3-a]carbazole analogues[2],... 

Gas Chromatography Analysis. From a sensitivity standpoint, a comparable technique is a gas chromatographic (gc) technique using flame ioni2ation detection. This method has been used to quantify the trimethylsilyl ester derivative of biotin in agricultural premixes and pharmaceutical injectable preparations at detection limits of approximately 0.3 pg (94,95). 

This ester is formed by standard procedures and is readily cleaved with Pd(Ph3P)4 in CH2CI2 to form trimethylsilyl esters that readily hydrolyze on treatment with water or alcohol or on chromatography on silica gel (73-98% yield). Amines can be protected using.the related carbamate. ... 

Silyl esters are stable to nonaqueous reaction conditions. A trimethylsilyl ester is cleaved by refluxing in alcohol the more substituted and therefore more stable silyl esters are cleaved by mildly acidic or basic hydrolysis. 

Some of the more common reagents for the conversion of carboxylic acids to trimethylsilyl esters are listed below. For additional methods that can be used to silylate acids, the section on alcohol protection should be consulted since many of the methods presented there are also applicable to carboxylic acids. Trimethylsilyl esters are cleaved in aqueous solutions. 

Trimethylsilyl Ester 47. /-Butyldimethylsilyl Ester 50. 5-/-Butyl Ester 59. 2-Alkyl-l,3-oxazolines... 

An interesting synthesis of quinolizidines was achieved using a vinylogous variation of the Bischler-Napieralski reaction. Angelastro and coworkers reported that treatment of amide 26 with PPSE (polyphosphoric acid trimethylsilyl ester) followed by reductive... 

Alkyl esters are efficiently dealkylated to trimethylsilyl esters with high concentrations of iodotrimethylsilane either in chloroform or sulfolane solutions at 25-80° or without solvent at 100-110°.Hydrolysis of the trimethylsilyl esters serves to release the carboxylic acid. Amines may be recovered from O-methyl, O-ethyl, and O-benzyl carbamates after reaction with iodotrimethylsilane in chloroform or sulfolane at 50—60° and subsequent methanolysis. The conversion of dimethyl, diethyl, and ethylene acetals and ketals to the parent aldehydes and ketones under aprotic conditions has been accomplished with this reagent. The reactions of alcohols (or the corresponding trimethylsilyl ethers) and aldehydes with iodotrimethylsilane give alkyl iodides and a-iodosilyl ethers,respectively. lodomethyl methyl ether is obtained from cleavage of dimethoxymethane with iodotrimethylsilane. 

But, as already mentioned, on working at ambient or lower temperatures and normal pressure, and at higher temperatures under pressure, the trimethylsilyl esters 30 react slowly with the liberated ammonia (or trimethylsilylamine 15) to form the primary amides 31 (Scheme 2.5) or their N-monosilylated analogs (cf Section 4.2.1). 

Benzoic acids 194 react at 160 °C with 2 equivalents of anilines 192 in the presence of polyphosphoric acid trimethylsilyl ester (PPSE) 195 (which is prepared by reaction of P2O5 with HMDSO 7) to give the amidines 196 in 69-88% yield [30, 31] (Scheme 4.10)... 

When heated in ethylene chloride at 80 °C for 3h the y9-ketosulfide 1203 reacts with the trimethylsilyl ester of polyphosphoric acid (PPSE) 195 (prepared from P2O5 and HMDSO 7) to give 36% 1204 and 8% 1205, whereas the lactone 1206 affords with PPSE 195 the unsaturated sulfide 1207 in 93% yield [27] (Scheme 8.10). 

Bis(trimethylsilyl)sulfate 559, which is readily available from TCS 14 and H2SO4 in 76% yield [94], reacts on heating with anisole to form the trimethylsilyl ester of p-methoxybenzenesulfonic acid 1331 in 92% yield [95]. The hexamethyldi-siloxane (HMDSO) 7 and H2O formed are removed during the reaction by azeotropic distillation [95]. On heating of acetyl chloride with 559 the probable intermediate 1332 rearranges to give 1333 in 63% yield. The last reaction can be extended to other acid chlorides [95] (Scheme 8.37). 

Sulfur trioxide reacts with MesSiCl 14 to give the trimethylsilyl ester of chloro-sulfonic acid 1334 [96], which on treatment vvdth iodosobenzene forms iodosoben-zenedichloride 1335, HMDSO 7, and regenerated SO3 [97]. Addition of olefins such as cyclohexene leads to the formation of sulfones such as 1336 [97]. With... 

The N-silylated enol acetate 1523 is cyclized by TMSOTf 20 in CHCI3, in 95% yield, giving the oxazole 1524 [57]. The dimeric derivative 1525 affords the 2,2 -bis-oxazole 1526 in 46% yield [57]. 2-Benzoylamino-3-chloropyridine 1527 is cyclized by polyphosphoric acid trimethylsilyl ester (PPSE) 195 on heating for 15 h in boiling 1,2-dichlorobenzene to give 40-60% 2-phenyloxazolo[5,4-f)]pyridine 1528 [58] (Scheme 9.34). 

The trimethylsilyl ester of a-trimethylsilyacetic acid 1613 is converted by LDA and TCS 14 into the C,0,0-tris(trimethylsilyl)ketene acetal 1614 in 91% yield. Reaction of 1614 with benzaldehyde in the presence of ZnBr2 proceeds via 1615 to afford a high yield of trimethylsilyl cinnamate 1616 [18], which gives on work-up free ( )-cinnamic acid in nearly quantitative yield (Scheme 10.7). In contrast, reaction of the lithium salt of 1613 with benzaldehyde then acidic hydrolysis affords a 1 1 mixture of ( )- and (Z)-cinnamic acid in 86% yield [18]. 

Trimethylsilyl esters of tris(thio)phosphonic acids 2070 are readily oxidized by DMSO in toluene at -30 °C to give the dimeric tetra(thia)diaphosphorinanes 2071 and HMDSO 7 [208] (cf. also the oxidation of silylated thiophenol via 2055 to diphenyl disulfide). The polymeric Se02 is depolymerized and activated by reaction with trimethylsilyl polyphosphate 195 to give the corresponding modified polymer... 

In an analogous reaction the catechol titanate 2136 is converted by TMSOTf 20, via 2137, to give the bis-(trimethylsilyl)ester 2138, which eliminates HMDSO 7 to regenerate 2136 [65] (Scheme 13.18). The intermediate compound 2137 apparently serves as catalyst for the reaction of l-0-trimethylsilyl-2,3,5-tri-0-benzyl-D-arabino-furanose with O-silylated alcohols to afford mainly the l-/ -arabinofuranosides. 

TMSI also effects rapid cleavage of esters. The cleavage step involves iodide attack on the O-silylated ester. The first products formed are trimethylsilyl esters, but these are hydrolyzed rapidly on exposure to water.96... 

A similar procedure starting with trimethylsilyl esters generates trimethylsilyl enol... 

Several successful cyclizations of quite complex structures were achieved using polyphosphoric acid trimethylsilyl ester, a viscous material that contains reactive anhydrides of phosphoric acid.58 Presumably the reactive acylating agent is a mixed phosphoric anhydride of the carboxylic acid. 

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