Silyl Subject

Selective fluonnation in polar solvents has proved commercially successful in the synthesis of 5 fluorouracil and its pyrimidine relatives, an extensive subject that will be discussed in another section Selective fluonnation of enolates [47], enols [48], and silyl enol ethers [49] resulted in preparation of a/phn-fluoro ketones, fieto-diketones, heta-ketoesters, and aldehydes The reactions of fluorine with these functionalities is most probably an addition to the ene followed by elimination of fluonde ion or hydrogen fluoride rather than a simple substitution In a similar vein, selective fluonnation of pyridmes to give 2-fluoropyridines was shown to proceed through pyridine difluondes [50]... 

Equiv of the imine 2 is dissolved in the appropriate solvent and 1 cquiv of the silyl ketene acetal 1 is added, the mixture is cooled to —70 °C and 0.1 equiv of TMSTf is added. After 15 h the reaction is quenched with H.O. 10% aq NH40H is added to make the piT basic, and the reaction mixture is extracted with F.tOAc. The crude product (obtained after the usual workup) is subjected to silica gel chromatography (pet. cthcr/Et20) to give the pure /J-amino ester 3. 

The first silyl-metal complex was synthesized in 1956 with the compound CpFe(CO)2SiMe3 [93, 94]. Since then, methods for the formation of M-Si bonds have been investigated systematically and developed to an effective set of tools. Numerous articles covering this subject give evidence for the continuous development of this area [64, 95],... 

This area of reactivity has been the subject of excellent reviews (J5). Silyl enol ethers are not sufficiently nucleophilic to react spontaneously with carbonyl compounds they do so under the influence of either Lewis acids or fluoride ion, as detailed above. Few clear trends have emerged from the somewhat limited number of definitive studies reported so far, with ambiguities in diastereoisomeric assignments occasionally complicating the issue even further. 

The Collins reagent in CH2CI2 oxidizes silylated primary alcohols in preference to the more hindered silylated secondary alcohols, as described for oxidation of the prostaglandin intermediate 2963 to the rather labile aldehyde 2964, which is immediately subjected to a Horner-Wittig-reaction to introduce the lower side chain [206] (Scheme 12.61). 

Synthesis of geranyl 6-0-fl-o-xylopyranosyl-(3-D-glucopyranoside (82) Tert-butyldimethylsilylation of 51 gave a silyl ether (84, 63% yield), which was subjected to benzoylation to give a benzoate (85) in 71% yield. Desilylation of 85... 

As with aldol and Mukaiyama addition reactions, the Mannich reaction is subject to enantioselective catalysis.192 A catalyst consisting of Ag+ and the chiral imino aryl phosphine 22 achieves high levels of enantioselectivity with a range of N-(2-methoxyphenyljimines.193 The 2-methoxyphenyl group is evidently involved in an interaction with the catalyst and enhances enantioselectivity relative to other A-aryl substituents. The isopropanol serves as a proton source and as the ultimate acceptor of the trimethyl silyl group. 

The ester 7-1 gives alternative stereoisomers when subjected to Claisen rearrangement as the lithium enolate or as the silyl ketene acetal. Analyze the respective transition structures and develop a rationale to explain these results. 

This approach allows one to functionalize the j3-C atom of AN. For this purpose, AN are initially subjected to double silylation to prepare BENAs, which are then coupled with various stabilized carbocations (512, 513), as well as with sulfenyl and episulfonium cations (514). Molecules containing good leaving groups (e.g., arenesulfenyl chlorides (514)) are used as sources (or precursors) of... 

Both terminal and internal BENAs (434) are readily subjected to this transformation to give the a-azido oximes (465) in very high yields. The improved procedure allows one to prepare compounds (465) virtually without by-products. The use of a large excess of silyl azide and the presence of small additives of triethylamine (5%) are of principal importance. 

Silylation of AN (528b,c,e) with another silylating agent (Me3SiCl/Et3N) gives poorly separable mixtures of unidentified products. However, the reaction of AN (528a) under these conditions produces the silyl derivative of bis-oxime (533), which can be subjected to desilylation to prepare free bis-oxime (534) (491, 497). The stereoselectivity of the reaction with respect to the new C,C double bond is low (E/Z 1.3 1). Silylation of sterically more hindered nitroalkane (528 d)... 

Palladium-catalyzed bis-silylation of methyl vinyl ketone proceeds in a 1,4-fashion, leading to the formation of a silyl enol ether (Equation (47)).121 1,4-Bis-silylation of a wide variety of enones bearing /3-substituents has become possible by the use of unsymmetrical disilanes, such as 1,1-dichloro-l-phenyltrimethyldisilane and 1,1,1-trichloro-trimethyldisilane (Scheme 28).129 The trimethylsilyl enol ethers obtained by the 1,4-bis-silylation are treated with methyllithium, generating lithium enolates, which in turn are reacted with electrophiles. The a-substituted-/3-silyl ketones, thus obtained, are subjected to Tamao oxidation conditions, leading to the formation of /3-hydroxy ketones. This 1,4-bis-silylation reaction has been extended to the asymmetric synthesis of optically active /3-hydroxy ketones (Scheme 29).130 The key to the success of the asymmetric bis-silylation is to use BINAP as the chiral ligand on palladium. Enantiomeric excesses ranging from 74% to 92% have been attained in the 1,4-bis-silylation. 

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