Silyl esters

Catalytic reduction of secondary functionaUties in sulfonates, in which the sulfonate moiety is unchanged, is accompHshed using standard hydrogenation techniques (21). Sulfonic acids may be converted to the corresponding silyl esters in very high yields (22). 

A newer method for the preparation of nitronic esters, namely utilizing the (9-trimethyl-silyl ester, has been reported and these are prepared by the reaction of alkylnitro compounds and (V,(V-bis(trimethylsilyl)acetamide. These nitronic esters also undergo cycloaddition with alkenes to produce isoxazolidines (equation 54) (74MIP41601, 74DOK109, 78ACS(B)ll8). 

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. 

Schemes 28 and 29 illustrate Curran s synthesis of ( )-hirsutene [( )-1]. Luche reduction58 of 2-methylcyclopentenone (137), followed by acetylation of the resulting allylic alcohol, furnishes allylic acetate 138. Although only one allylic acetate stereoisomer is illustrated in Scheme 28, compound 138 is, of course, produced in racemic form. By way of the powerful Ireland ester enolate Clai-sen rearrangement,59 compound 138 can be transformed to y,S-unsaturated tm-butyldimethylsilyl ester 140 via the silyl ketene acetal intermediate 139. In 140, the silyl ester function and the methyl-substituted ring double bond occupy neighboring regions of space, a circumstance that favors a phenylselenolactonization reac-...

On the other hand, syn-carboxylic acids are obtained from a deprotonation of the /5-silyl ester, giving the (E)-enolate, followed by reaction with different aldehydes and subsequent hydrogenolysis. No diastereomers of the aldol product are detected720. 

The addition of the lithium enolates of methyl acetate and methyl (trimelhylsilyl)acetate to ( + )-(S)-2-(4-methylphenylsulfinyl)-2-cycloalkenones gives, after desulfurization, (/ -substituted cycloalkenones. A higher level of selectivity is observed with the a-silyl ester enolate and in the cyclohexenone series13. The stereochemical outcome is rationalized by assuming attack on a ground-state conformation analogous to that in Section 1.5.3.2.1. 

The use of HMDS (ca. 1.5 mmol) and saccharin (0.01 mmol) per mmol of substrate in refluxing dichloromethane or chloroform has been recommended (5) for easy silylation of carboxylic acids, including azetidin-2-one-4-carboxyIic acids. Clear solutions result, i.e., no ammonium salts are present at completion of the reaction, and consequently the silyl esters can be obtained by direct distillation, or merely by evaporation of solvent. 

A solution of LDA (11 mmol) in THF (30 ml) was cooled to -78°C, and HMPA (CAUTION—CANCER SUSPECT AGENT) (3 ml) then added. To this solution was added dropwise 3-acetoxyoct-l-ene (10 mmol), and then TBDMSC1 (11 mmol) in THF (2 ml) over 5 min. The pale yellow solution was stirred at -78°C for an additional 2 min, and the reaction mixture was allowed to warm to 25DC over 30min. It was stirred at this temperature for a further 2 h, and then quenched with water and pentane. The combined pentane extracts were concentrated, the crude oily silyl ester was dissolved in THF (25 ml) and dilute aqueous HC1 (5 ml, 3 m) and the solution was then stirred for 45 min at 25 °C to complete hydrolysis. The mixture was then poured into aqueous sodium hydroxide (30 ml, I m) and... 

The use of silylated monomers is an interesting alternative method of aromatic polyester synthesis since the silylated gaseous by-products cannot participate in the reverse reaction, shifting polyesterification toward polymer formation. Reactions between silyl esters and acetates (Scheme 2.23) and reactions between silyl ethers and acid chlorides (Scheme 2.24) have been applied to the synthesis of linear265-267 and hyperbranched wholly aromatic polyesters202,268 269 (see Section 2.4.5.2.2). 

Okazoe, T. Takai, K. Oshima, K. Utimoto, K. J. Org. Chem., 1987, 52, 4410. This procedure is also successful for silyl esters, to give silyl enol ethers Takai, K. Kataoka, Y. Okazoe, T Utimoto, K. Tetrahedron Lett, 1988, 29, 1065. 

Thermolysis of the dicarbamic acid silyl ester (CH2 N[SiMe3]C02SiMe3)2) gives the 1,3-diazetane derivative 26 <96JOM93>. Alkylation of the 1,3-dithietane tetraoxide 27 with a,co-dihaloalkanes yields the dispiro compounds 28 (n = 1-4) <95ZOR589>. The first 1,2-dithiete S-oxide 29 is reported <95TL8583>. 

Hydrates of acids such as Ts0H-H20 can probably also be dehydrated by treatment with silyl esters such as TsOSiMe3 (Scheme 13.1). Likewise, Ts0H-H20 is dehydrated in situ during aminations of hydroxy-N-heterocycles such as purines 242 (Scheme 4.24) or (lH,3H)-quinazoline-2,4-dione 250 (Scheme 4.27) by HMDS 2, in the presence of higher-boiling primary or secondary amines, to give the ami-... 

Carboxylic acids can be protected as oxazolines [96, 105-107, 186, 191] or as ester functions. Alkynic esters such as silyl esters [153, 211], tert-butyl esters [216], and even benzyl esters [153, 211] have been successfully hydrozirconated when the reactive site was a terminal alkyne or vinyl group (Scheme 8-27). 

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... 

Application of amino acid silyl esters or A-silyl amino acid silyl esters as amino components is very convenient in peptide synthesis with CDI, because the resulting peptide silyl esters are easily hydrolyzed to dipeptides during the usual work up. They need not be saponified in a separate step, as would be the case with the corresponding alkyl esters. Furthermore, no racemization occurs with this method.tl8],tl9]... 

Figure 11.11 Pyrogram of a paint sample collected from a decorative frame of the Universal Judgement by Bonamico Buffalmacco (fourteenth century, Monumental Cemetery of Pisa, Italy). Pyrolysis was performed with a microfurnace pyrolyser, at 600°C, in the presence of HMDS. 1, Benzene 2, ethyl acrylate 3, methyl methacrylate 4, acetic acid, trimethyl silyl ester 5, pyrrole 6, toluene 7, 2 methylpyrrole 8, 3 methylpyrrole 9, crotonic acid 10, ben zaldehyde 11, phenol 12, 2 methylphenol 13, 4 methylphenol 14, 2,4 dimethyl phenol 15, benzyl nitrile 16, 3 phenylpropionitrile 17, indole 18, phthalate 19, phthalate 20, ben zyl benzoate HMDS pyrolysis products [27]...

The fe-ester derivatives of trithiophosphinic acids, RP(S)(SR )2, have also been studied and, similar to the metal and ammonium salts, show enhanced thermal stability compared to their parent acids. Trithiophosphonic acid Zj zA (tr im e t lr y 1 s i 1 y 1) esters have been synthesised from organo-/u.v(trim-ethylsilyl)phosphanes with elemental sulfur in toluene (Equation 40).53 These 6z,s(silyl esters) can be readily converted into the parent trithiophosphinic acid by a very slow, controlled reaction with water or methanol.53... 

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