Precursors silylation

Probably the most widdy applicable conditions developed for palladium catalysts utilize silyl enol ethers.In one instance,an excellent yield of enone was ob ned using 0.5 equiv. each of palla-dium(II) acetate and p-benzoquinone in acetonitrile. The method has the advantage that the position of the double bond is determine by the geometry of the precursor silyl enol ether (Scheme 26). Palla-... 

An alternative route to condensation with HC1 elimination is the use of precursor silyl compounds (here eliminating Me3SiCl). Hence reaction of Me3SiN(H)COMe with ClPPh2 in toluene at 60 °C gave Ph2PN(H)COMe (221) (Scheme 13) in 84% yield.467... 

MuFaiyarfia-AJdoJ- Silyl Enol Ethers as an enolate precursors,... 

On the other hand, the corresponding tin precursor (63) undergoes smooth cycloaddition with a wide variety of aldehydes to produce the desired methylene-tetrahydrofnran in good yields [32, 33]. Thus prenylaldehyde reacts with (63) to give cleanly the cycloadduct (64), whereas the reaction with the silyl precursor (1) yields only decomposition products (Scheme 2.20) [31]. This smooth cycloaddition is attributed to the improved reactivity of the stannyl ether (65) towards the 7t-allyl ligand. Although the reactions of (63) with aldehydes are quite robust, the use of a tin reagent as precursor for TMM presents drawbacks such as cost, stability, toxicity, and difficult purification of products. 

The synthetic problem is now reduced to cyclopentanone 16. This substance possesses two stereocenters, one of which is quaternary, and its constitution permits a productive retrosynthetic maneuver. Retrosynthetic disassembly of 16 by cleavage of the indicated bond furnishes compounds 17 and 18 as potential precursors. In the synthetic direction, a diastereoselective alkylation of the thermodynamic (more substituted) enolate derived from 18 with alkyl iodide 17 could afford intermediate 16. While trimethylsilyl enol ether 18 could arise through silylation of the enolate oxygen produced by a Michael addition of a divinyl cuprate reagent to 2-methylcyclopentenone (19), iodide 17 can be traced to the simple and readily available building blocks 7 and 20. The application of this basic plan to a synthesis of racemic estrone [( >1] is described below. 

This synthetic approach is known from the synthesis of L M(alkene)H compounds from LnM(CO)alkane precursors and can easily be applied to the analogous silyl complexes. The Si—H bond even shows an increased activity for oxidative addition reactions [42, 43]. 

Blechert s synthesis of the piperidine alkaloid (-)-halosaline (387) by Ru-catalyzed RRM is outlined in Scheme 76 [160]. In the presence of 5 mol% of catalyst A, the ring rearrangement of metathesis precursor 385 proceeded cleanly with formation of both heterocyclic rings in 386. In situ deprotection of the cyclic silyl ether in 386, followed by selective reduction and removal of the to-syl group led to 387. 

Recent progress of basic and application studies in chitin chemistry was reviewed by Kurita (2001) with emphasis on the controlled modification reactions for the preparation of chitin derivatives. The reactions discussed include hydrolysis of main chain, deacetylation, acylation, M-phthaloylation, tosylation, alkylation, Schiff base formation, reductive alkylation, 0-carboxymethylation, N-carboxyalkylation, silylation, and graft copolymerization. For conducting modification reactions in a facile and controlled manner, some soluble chitin derivatives are convenient. Among soluble precursors, N-phthaloyl chitosan is particularly useful and made possible a series of regioselective and quantitative substitutions that was otherwise difficult. One of the important achievements based on this organosoluble precursor is the synthesis of nonnatural branched polysaccharides that have sugar branches at a specific site of the linear chitin or chitosan backbone [89]. 

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

All of the ethynylated cyclobutadienes are completely stable and can be easily manipulated under ambient conditions, as long as the alkyne arms carry substituents other than H. For the deprotected alkynylated cyclobutadiene complexes, obtainable by treatment of the silylated precursors with potassium carbonate in methanol or tetrabutylammonium fluoride in THF, the stability is strongly dependent upon the number of alkyne substitutents on the cyclobutadiene core and the nature of the stabilizing fragment. In the tricarbonyUron series, 27b, 27c, 29 b, and 28b are isolable at ambient temperature and can be purified by sublimation or distillation under reduced pressure. The corresponding tetraethynylated complex 63 e, however, is not stable under ambient conditions as a pure substance but can be stored as a dilute solution in dichloro-methane. It can be isolated at 0°C and kept for short periods of time with only... 

HMDSO 7 [39]. With 20/DIPEA sulfoxide 1233 affords 76% of 1235 [40]. Analogous silylation of the S-oxide function in 1237 with the O-silylketene acetal 1214 and subsequent cyclization with ZnCl2 or Znl2 affords 1238, a precursor of thie-namycin [41-43] (Scheme 8.16). 

We also reported that CpFe(CO)2Me acts as a precursor for the Si-O-Si bond formation reaction from hydrosUane and DMF (Scheme 51)[ 166,167]. In this reaction, tertiary silanes and bis(silyl) compounds are converted into the corresponding disUox-anes and the polymers with (-R-Si-O- i)n backbone, respectively. 

Scheme 2.12 shows some representative Mannich reactions. Entries 1 and 2 show the preparation of typical Mannich bases from a ketone, formaldehyde, and a dialkylamine following the classical procedure. Alternatively, formaldehyde equivalents may be used, such as l>is-(di methyl ami no)methane in Entry 3. On treatment with trifluoroacetic acid, this aminal generates the iminium trifluoroacetate as a reactive electrophile. lV,A-(Dimethyl)methylene ammonium iodide is commercially available and is known as Eschenmoser s salt.192 This compound is sufficiently electrophilic to react directly with silyl enol ethers in neutral solution.183 The reagent can be added to a solution of an enolate or enolate precursor, which permits the reaction to be carried out under nonacidic conditions. Entries 4 and 5 illustrate the preparation of Mannich bases using Eschenmoser s salt in reactions with preformed enolates. 

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