C-silyl ester

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

Transesterification has been carried out with phase-transfer catalysts, without an added solvent. Nonionic superbases (see p. 365) of the type P(RNCH2CH2)3N catalyze the transesterification of carboxylic acid esters at 25°C. ° Silyl esters (R C02SiR3) have been converted to alkyl esters (R C02R) via reaction with alkyl halides and tetrabutylammonium fluoride. Thioesters are converted to phenolic esters by treatment with triphosgene-pyridine and then phenol. 

Strong evidence for intramolecular fluorine exchange in PhaPFg has been obtained from and P n.m.r. spectra in the temperature range 32— 170 °C. Silyl esters of bis(trifluoromet%l)- and difluoro-phosphinic acids and their thio-analogues result from reactions of the type ° ... 

Hydroxy-3-phenylpropyl)-diphenylphosphine oxide and sulfide were prepared and acylated. The two P-chalcogenides resulted in dilferent products on deprotonation by LDA and subsequent treatment by TMSCl. The phosphine oxide was deprotonated on the a carbon atom, followed by intramolecular cyclization and trimethylsilylation to result in eventually a diphenylphosphinoyl tetrahydrofuran derivative. However, under the same conditions, the phosphine sulfide was transformed to the C-silylated ester (Scheme 41). ... 

In their total synthesis of (-i-)-ophiobolin in 1989, Kishi et al. found that treatment of a cyclopentenyl ester under the typical Ireland conditions gave principally C-silylated ester [63]. Heating of a C-silyl ester (prepared by acylation using a C-silyl acyl chloride) at 230 °C resulted in a 1,3-Brook rearrangement followed by an Ireland-Claisen rearrangement to give the desired product as a 6 1 ratio of isomers at C2 of the pentenoic acid (Scheme 4.63). The major product could have arisen through either a chair transition state of the Z-sUyl ketene acetal or a boat transition state of the E-silyl ketene acetal. 

Ph3Si ethylene oxide catalytically adds PhSH to give a- and -adducts in 6 1 ratio, and serves as a vinyl dication equivalent with RCu to give alkenes stereoselectively, while epoxides rearrange in the presence of silylated nucleophiles Diphenylmethylsilylation of cyclopropane carboxylates gives the C-silylated ester which then form C-silylated cyclopropyl ketones, P-silyl cyclopropyl carbinols, and alkylidene cyclopropancs, which ring expand to the... 

Full details have appeared of application of the C-silylated ester Me3 iCH2C02Et to silylations of alcohols mediated by fluoride ion (cf. 1,170)." In an extension of earlier work with the TMS enol ethers of esters (4,154), ketene methyl TBDMS acetal (50) has been advocated as a stable readily available reagent for conversion of alcohols to TBDMS ethers under mild conditions." In a related approach the O-silylated derivatives (51) and (52), readily formed from acetylacetone (R = Me) or methyl acetoacetate (R = OMe), have been found to be effective reagents for the transformation of alcohols to their TBDMSand TMS ethers," respectively reagents (51) require acid catalysis whereas (52) react without catalyst. Some further details have appeared... 

The f-butyl derivatives ( )-6 and (Z)-6 were employed since the methyl analogs were contaminated by C-silylated esters. The reactions of (E)-6 with representative aldehydes proceeded smoothly at room temperature to afford, predominantly, the syn aldol products. The fact that an 89/11 E/Z mixture of 6 afforded a >95/5 syn/anti mixture of aldol products most likely arises from... 

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... 

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]...

Two procedures were developed for C,C-coupling reactions of silyl esters of primary AN. One approach involves two steps and the synthesis of intermediate SENAs according to standard procedures. Another procedure is based on the one-pot reaction of AN with the DBU/TBSOTf system in a ratio of 1 1.1 followed by the addition of silyl ketene acetal and a catalytic amount of TBSOTf. 

C-Glucosidation. Reaction of the protected a-D-glucopyranosyl chloride 1 with silyl enol ethers activated with AgOTf results in a-C-glucosyl esters or ketones. 

A solution of ethyl 3-phenyl-3-(trimethylsiloxy)propanoate (2.5 g, 10 mmol) in benzene (50 mL) was mixed with PhPF4 (1.8 g, 10 mmol) in benzene (10 mL) and stirred at 20 C until the reaction was complete (the disappearance of the GC peak due to the silylated ester). Then the solvent was evaporated and the residue was chromatographed (hexane/EtOAc, 9 1) yield 1.86 g (95%). 

The RCS XH salts (X = piperidine) also react with chlorotrimethyl-germane to give the complexes (RDta)Ge(Me)3 (R = tolyl, Ph, p-Cl—Ph). These complexes show two intense bands in the IR spectra at 1240 and 1050cm 1 (silyl esters) and 1220 and 1050 cm 1 (germyl esters) that are assigned to the C=S vibrations of the monodentate RCS2 ligands (378). The (Ph)3Ge(EtXant) derivatives also show the C=S vibration at 1040 cm 1 (565). Undoubtedly these... 

The C-Si bonds of aryl-, heteroaryl-, vinyl-, and allylsilanes are stable towards alco-holates or weak reducing agents, but can be cleaved under mild conditions by treatment with acids or fluoride to yield a hydrocarbon and a silyl ester or silyl fluoride. Several linkers of this type have been tested and have proven useful for the preparation of unfunctionalized arenes and alkenes by cleavage from insoluble supports. Typical loading procedures for these linkers are sketched in Figure 3.42. 

Dimethylalkenes.1 The readily available a-silyl esters, obtained by C-silylation of lithium ester enolates with 1, are useful precursors to trisubstituted alkenes, including 1,1-dimethylalkenes. 

The hydroxyl group of a primary, secondary, or tertiary acyclic or alicyclic alcohol may be protected by conversion into (a) an ether, (b) a silyl ether, or (c) an ester. The most important method for the protection of a 1,2- or 1,3-diol is conversion into (d) a cyclic acetal. 

The mechanism of selenocyclization of yS,y-unsaturated acids and their derivatives has been studied. The reactions of ( )-4-phenylbut-3-enoic acid and its silyl and alkyl esters (15 R = H, SiMe3, alkyl) with benzeneselenenyl halide PhSeX (X = Cl, Br) have been examined by VT-NMR and in situ IR spectroscopic methods. Whereas the reactions of the acid in the presence of a base were irreproducible and complicated, reactions of the silyl esters were clean and spontaneously and quantitatively afforded the corresponding chloroselenylation adduct at -70 °C as a single (Markovnikov) isomer. This adduct underwent three processes as the temperature was raised (1) reversal to the starting materials, (2) isomerization to the anti-Markovnikov product, and (3) cyclization to the selenolactone (16). All of these processes are believed to proceed via a seleniranium ion, the intermediacy of which was established by independent synthesis and spectroscopic identification. The reversible formation of chloroselenide adducts was unambiguously established by crossover experiments. The reaction of (15) with PhSeBr was found to be rapid but thermodynamically unfavourable at room temperature.29... 

The palladium [Pd(Ph3)4]-catalysed 3 + 3-cycloaddition of trimethylenemethane with azomethineimines produced hexahydropyridazine derivatives under mild conditions (40 °C).171 The Lewis acid-catalysed formal oxa-[3 + 3]-cycloaddition of a,f+ unsaturated aldehydes with 6-methyl-4-hydroxy-2-pyrone, 1,3-diketones, and viny-logous silyl esters yielded a variety of pyrones at room temperature.172 Croton-aldehyde has been converted to 6-hydroxy-4-methylcyclohex-l-enecarboxaldehyde by an enantioselective 3 + 3-cycloaddition catalysed by proline. This methodology was used in the synthesis of (—)-isopulegol hydrate, (—)-cubebaol, and (—)-6-hydroxy-4-methylcyclohex-l-ene-1-methanol acetate, an intermediate in the total synthesis of the alkaloid magellanine.173... 

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