Trimethylsilylketene

In stepwise additions, ketenes are usually the nucleophilic component, so that such additions can be catalyzed by Lewis acids, such as the additions of trimethylsilylketenes to aldehydes, catalyzed by BF3 (Scheme 14) (79JOC733). However, the roles can be reversed, such as in the addition of chlorocyanoketene to benzaldehyde (79JA5435). 

Trimethylsilylketene and acyl isocyanates generate 4-trimethylsiloxy-l,3-oxazin-6-ones 12 in situ, which smoothly react with the enamines of cycloalkanones to give bicyclic 2-pyridones 13 <96TL(37)4977>. The heterocycles 12 also undergo the Diels-Alder reaction with dimethyl acetylenedicarboxylate or methyl propiolate to furnish substituted 2-pyridones <96TL(37)4973>. 

Azomethine ylide cycloadditions have been utilized to prepare a number of novel fused pyrroles including pyrrolo[2,1 -a isoquinolincs <06CHJC279, 06TL1469> and pyrrolo[l,2-Zdpyridazines <06SL804>. Fused hydroxypyrroles were obtained in cycloaddition reactions with trimethylsilylketenes (TMS ketene) <06TL1469>. 

Trimethylsilylketene reacts smoothly with u./V-diarylnitrones to give oxoin-doles in good yields. On the other hand, the reaction of trimethylsilylketene with N-arylmethylnitrones gives a mixture of N,N-diacylamines and N-acylamines (Scheme 2.315) (836). 

More recently, Pirrung and co-workers established the facility with which the Rh2(OAc)4 catalyzed reaction of 2-diazocyclohexane-l,3-dione (306) and its substituted derivatives (Scheme 8.76) occurs with dihydrofuran and dihydropyran (351-353), vinyl acetates (354) (306 307), terminal alkynes (355) (306 308), methoxyallene (355), trimethylsilylketene (serving as a synthetic equivalent for ketene) (355), and heteroaromatic compounds (353). This reaction is quite useful... 

Using diazomethane as the limiting reagent, silyl- and germyl-substituted ketenes5-7 in certain cases gave cyclopropanones which were isolated as stable compounds.5,6 Transformations of trimethylsilylketene and triethylgermylkelene to the 2- and 3-substituted cyclobutanones was accomplished in 90 and 82% yield, respectively. Mild reaction conditions (— 78 °C) in diethyl ether solutions were employed. 

Kociehski s group assembled the 3-lactone segment utilizing a Lewis acid-catalyzed [2 + 2] cycloaddition strategy. In the presence of a catalytic amount of boron trifluoride etherate, the [2 + 2] cycloaddition between aldehyde 28 and trimethylsilylketene 29 took place rapidly and cleanly to give a mixture of diastereomers of P-lactone 30. After a delicate desilylation and a flash chromatography, the desired diastereomer 31 was obtained in 55% yield. 

Reaction between trimethylsilyldiazomethane and trimethylsilylketene produces the disilylcyclopropanone. This was reacted with trimethylsilylazide to form the corresponding azetidinone in high yields after heating.122... 

Salicylaldehyde reacts with trimethylsilylketene dithioacetal in the presence of a Lewis acid to form the chroman 502, the product of a deoxygenative divinylation (Equation 208) <2001JOC3924>. This reaction can also be applied to salicylaldimines <2003JOC4947>. Treatment of 3,5-dibromosalicylaldehyde with methyl vinyl ketone (MVK) in the presence of DABCO leads to a chroman-4-ol as the major product <2002J(P1)1318>. A stereoselective one-pot synthesis of vy/z-fused chromans from salicylaldehydes, aromatic amines and cyclic enol ethers is carried out in the... 

The reactions of the trimethylsilylketene 313 with silyl ketene acetals 314 provided /J,y-unsaturated esters 315 (equation 195) via a 1,5-silyl migration in the intermediate adduct 316461 462. 

Triplex catalysis.1 1,3-Cyclohexadiene does not undergo cycloadditions under usual conditions although it is known to dimerize on irradiation in the presence of a sensitizer. Actually, 1,3-cyclohexadiene can undergo both [2 + 2]- and [4 + 2]cycloaddition with a silyl enol ether such as 1-trimethylsiloxy-l-phenylethyl-ene (1). Similar results obtain with trimethylsilylketene acetal. These sensitized... 

Since metal ynolates are metallated ketene equivalents, metallation of ketenes is expected to afford metal ynolates. Direct metallation of monoalkylketenes is, however, fairly difficult due to the high lability of these ketenes (e.g. dimerization) and the strong elec-trophilicity of the carbonyl carbon . In contrast, silylketenes are so stable that lithiation of trimethylsilylketene (45) with BuLi at —100 °C provides the lithium ynolate 46 in good yield (equation 18). The f-butyldimethylsilylketene (47) is also lithiated to afford the lithium ynolate 48 (equation 19). ... 

The number of chiral diazaaluminolidine catalysts has been extended by Dymock, Kocienski and Pons, who introduced the more convenient to handle trimethylsilyl-ketene [31]. The catalysts in this study were prepared from slightly different sulfonamides but asymmetric induction was comparable with that obtained with the ketene and similar aldehydes. With trimethylsilylketene, two diastereomers are possible and in all examples studied the cis isomer 126 was the predominate product. TTie reactions in Table 7 were performed with 30 mol % catalyst—with 20 mol % catalyst the reaction is incomplete. A more active catalyst can be prepared from the bis-trifluoro-methylsulfonyl derivative of 128, but asymmetric induction was low. It was reported that ortho substituents on the aryl sulfonamide were necessary for higher induction but data were provided only for the aryl sulfonamide substituents summarized in Table 7. Both symmetrical and unsymmetrical diazaaluminolidines were examined as catalysts in an attempt to optimize asymmetric induction but significant differences were not found. The catalyst prepared from the symmetric bis-sulfonamide 128 with Ar = 2,4,6-tri-/yo-propylphenyl did not give any reaction even at 100 mol %. 

For the introduction of the TMS group under neutral conditions, trimethylsilyl chloride can be activated with lithium sulfide.Also trimethylsilylketene acetals (39), silyl enol ethers (40) (in particular those of pentane-2,4-dione), A -trimethylsilylacetamide (41), A, 0-bis(trimethylsilyl)acetamide (42), A A -bis(trimethylsilyl)urea (43) and A, 0-bis(trimethylsilyl)carbamate (44) are effective silylating agents under neutral conditions (Scheme 39). ... 

Elimination reactions. 1-Trimethylsilylethylbenzotriazoles deliver alkenes on exposure to CsF in DMF. In an oxidative homologation of aldehydes CsF effects the aldol condensation with 2-trimethylsilylketene 0,0-bistrimethylsilylacetal and the elimination step. 

Quite recently, Doyle and coworkers found that dirhodium(II) complexes such as rhodium(II) acetate and Rh2(4S-MEAZ)4 (14) also act as highly active Lewis addcatalysts (1 mol%) for the reaction of trimethylsilylketene and ethyl glyoxalate, affording the P-lactonel5 (Table4.6) [42]. However, theuseofthechiral Rh-complex, Rh2(4S-M EAZ)4, alone afforded almost no asymmetric induction (5% ee for (S)-isomer) (entry 2). The use of quinine (10 mol%) as a cobase catalyst to activate the ketene simultaneously provided exceptional enantiocontrol (99% ee) and enhanced reactivity (entry 5). 

A useful sabinene (767) synthesis has been described by Rousseau and Slougui. The keto ester 768 (R = O) was prepared conventionally from ethyl 4-methyl-3-oxopentanoate and converted to the methylene compound 768 (R = CH2) by a Wittig reaction. The trimethylsilylketene acetal 769 was then treated with bromoform and diethylzinc in pentane, yielding 70% of the ester 765 (R =... 

Dienic carboxylic acids. Prater has described a method for addition of an isoprene unit to an allylic alcohol. The method is iUustrated for the conversion of the ester (1) of geraniol into farnesenic acid (3). The ester is treated with lithium N-isopropylcyclohexylamide (LilCA, 4, 306-309) and then with trimethylchlorosilane. The resulting trimethylsilylketene acetal (a) undergoes... 

Coumarins and Isocoumarins.- Several new or improved syntheses of coumarins have been reported. A new one-pot synthesis under mild conditions promises to be a widely applicable route from salicylaldehydes or 2-hydroxyaryl alkyl (or aryl) ketones (145 R1 = H, alkyl, OH, N02 R2=H, Me or Ph) and trimethylsilylketene... 

Investigations of cyclic ketenes, like the pentafulvene (8.102), generated photo-chemically from 2-diazo-l,2-benzoquinone (Urwyler and Wirz, 1990), and its benzo derivative (8.103) (Barra et al., 1992 Andraos et al., 1993, 1994 Almstead et al., 1994) are in accordance with the mechanisms discussed above. Trimethylsilylketene [(CH3)3SiCH = C = 0] reacts slower by a factor of 4 x 10. This can be ascribed to the stabilization of the acylium ion by silicon ((CH3)3SiCH2(S = 0, Allen and Tidwell, 1991). 

Trimethylsilylketen and bis(trifluoromethyl)keten give the methylene-/3-lactone (76) at -78 °C, but this rearranges to the isomeric compound (77) at room temperature. Solid 4-diazo-l-phenyl-5,6-dioxo-l,4,5,6-tetrahydropyridazine... 

The addition of trimethylsilylketene to an a-ketoester in the presence of a Cj-symmetric bis(oxazoline)-Cu(II) complex affords 4-substituted 2-oxetanone 4-carboxylates in high yield and with 83-91% ee. These p-lactones react with soft nucleophiles to give acyclic P-substituted carboxylic acids <01OL2125>. 

Ester etiolate Claisen rearrangement (4, 307-308). Ireland et al. have reported details of this [3,3]sigmatropic rearrangement and also recent improvements. Rearrangement of the r-butyldimethylsilylketene acetal of the enolate (equation I) is generally superior to rearrangement of the trimethylsilylketene acetal as used previously. A further advantage is that the silyl esters can be converted into the... 

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