Ketenes, trimethylsilyl-, synthesis

A similar reaction was used for preparation of the remote stereocenter in 85, an intermediate in Paterson s synthesis of aurisides A and B. The asymmetric catalysis of the aldol reaction with fluoral has also been investigated by Mikami for highly enantioselective syntheses of fluorine-containing aldols. The reaction of the ketene trimethylsilyl acetal 86 with fluoral (87) in the presence of the BfNOL-catalyst 67 yielded the aldol adduct 88 in moderate yield (56%) and high enantioselectivity (90% ee). 

Considerable efforts have been devoted to the stereoselective introduction of a /(-methyl function in intermediates for the synthesis of 1 jS-methylcarbapenems. While the trimethylsilyl trifluoromethanesulfonate catalyzed reaction of a 4-acetoxyazetidinone derivative with ketene acetals shows no selectivity, ketene thioacetals lead to stereoselective formation of the a-methyl isomer108. The zirconium enolate, however, shows high /(-methyl selectivity. 

Tocopheryl)propionic acid (50) is one of the rare examples that the o-QM 3 is involved in a direct synthesis rather than as a nonintentionally used intermediate or byproduct. ZnCl2-catalyzed, inverse hetero-Diels-Alder reaction between ortho-qui-none methide 3 and an excess of <2-methyl-C,<9-bis-(trimethylsilyl)ketene acetal provided the acid in fair yields (Fig. 6.37).67 The o-QM 3 was prepared in situ by thermal degradation of 5a-bromo-a-tocopherol (46). The primary cyclization product, an ortho-ester derivative, was not isolated, but immediately hydrolyzed to methyl 3-(5-tocopheryl)-2-trimethylsilyl-propionate, subsequently desilylated, and finally hydrolyzed into 50. 

More stable alternatives to ketene that have been used in 2-oxetanone synthesis are (trimethylsilyl)ketenes 100 <1996CC1053>. Using methylaluminoimidazolines as catalysts, with aldehydes and 100, 3-(trimethylsilyl)oxetan-2-ones (101a and 101b) were generated with up to 83% ee (Equation 36). 

A diastereoselective Mukaiyama aldol lactonization between thiopyridylsilylketene acetals and aldehydes was used to form the /3-lactone ring in the total synthesis of (-)-panclicin D <1997T16471>. Noyori asymmetric hydrogenation was a key step in a total synthesis of panclicins A-E and was used to establish the stereocenter in aldehyde 140, which in turn directed the stereochemistry of subsequent reactions <1998J(P1)1373>. The /3-lactone ring was then formed by a [2+2] cycloaddition reaction of 140 with alkyl(trimethylsilyl)ketenes and a Lewis acid catalyst. 

The use of substituted pyridines in organic synthesis has broad application. The activation of the pyridine ring toward nucleophilic attack is well known in the literature. The products of such reactions are often dihydropyridines which can serve as intermediates in more complex synthetic strategies. Rudler and co-workers have reported on the nucleophilic addition of bis(trimethylsilyl)ketene acetals to pyridine (26). The 1,4-addition product 27 was then cyclized with iodine to afford bicycle 28 in 90% overall yield <02CC940>. Yamada has elegantly shown that facial selectivity can be achieved and chiral 1,4-dihydropyridines accessed in high yield and de (29—>30) <02JA8184>. 

Roush WR, Hall SE (1981) Studies on the total synthesis of chlorothricol-ide stereochemical aspects of the intramolecular Diels-Alder reactions of methyl undeca-2,8,10-trienoates. J Am Chem Soc 103 5200-5211 Rudler H, Denise B, Xu Y, Parlier A, Vaissermann J (2005) Bis(trimethylsilyl)-ketene acetals as C,0-dinucleophiles one-pot formation of polycyclic y-and 8-lactones from pyridines and pyrazines. Eur J Org Chem 3724-2744 Sekino E, Kumamoto T, Tanaka T, Ikeda T, Ishikawa T (2004) Concise synthesis of anti-HIV-1 Active (+)-inophyllum B and (+)-calanolide A by application of (-)-quinine-catalyzed intramolecular oxo-michael addition. J Org Chem 69 2760-2767... 

Three successive [2+4] cycloadditions were used in the synthesis of the pentacyclic methyl ether of G-2N by Kraus and Zhao [92] and later, by a slightly modified procedure, also of the natural product G-2N (118) [93] (Scheme 31). Thermal reaction of the cyclobutanol 112 with acrylic ester gave the dihydronaphthalene 113 which was demethylated by treatment with boron tribromide and converted into the exocyclic ketene acetal 114. This unstable diene was reacted in a second cycloaddition with 2,6-dichlorobenzoquinone (115) to afford the tetracyclic chloroquinone 116. In a last Diels-Alder reaction, ring E was anella-ted by treatment of 116 with l-methoxy-l,3-bis[(trimethylsilyl)oxy]-l,3-buta-diene (117) to yield the pentacyclic natural product G-2N (118) [93]. 

In studies directed toward intermediates for the synthesis of quadrone, Livinghouse demonstrated the utility of lithiated methoxy(phenylthio)(trimethylsilyl)methane (327) for the conversion of aldehydes and ketones to ketene 0,5-acetals (328) in good to excellent yields (Scheme 46). These Peterson alkena-tions gave predominantly the ( )-double bond isomer. As the example depicted in the scheme demonstrates, this procedure may be used to homologate a carbonyl to the phenyl thioester (329) in excellent yields. 

The [2+2]photocycloaddition of 5-arylfuran-2,3-diones (496) to trimethylsilyl-oxyethylenes (495) occurs with high regio- and stereoselectivity to efficiently yield polyfunctionalised cyclobutanes (497). ° A convenient four step stereoselective synthesis of ( + )-norasteriscanolide (500) has used the [2+2]photoadduct (499) from 2-cyclopentenone and the trimethylsilyl enol ether (498) to assemble the 5/8 ring system present in many terpenoids. 2-Naphthaldehyde undergoes regio-and stereoselective [2+2]photoadditon to the cyclic ketene silyl acetals (501) to... 

Okamoto, Y, and Sakurai, H., Preparation of (dialkoxyphosphinyl)-methyl-substituted ketene alkyl trimethylsilyl acetal derivatives. Synthesis, 497, 1982. 

Rubottom oxidation reactions have been conducted on enolsilanes derived from a number of different carbonyl derivatives including carboxylic acids and esters.15 For example, the Rubottom oxidation of bis(trimethylsilyl)ketene acetal 30 provided a-hydroxy carboxylic acid 31 in 81% yield. Use of alkyl trimethylsilyl ketene acetal substrates generates a-hydroxy esters, as seen in the conversion of 32 to 33.16 The synthesis of 3-hydroxy-a-ketoesters (e.g., 36) has been accomplished via Rubottom oxidation of enolsilanes such as 35 that are prepared via Homer-Wadsworth-Emmons reactions of aldehydes and ketones with 2-silyloxy phosphonoacetate reagent 34.17 The a-hydroxylation of enolsilanes derived from P-dicarbonyl compounds has also been described, although in some cases direct oxidation of the P-dicarbonyl compound is feasible without enolsilane formation.18... 

Aldol-type reactions. The catalytic effect of ZnBr2 is demonstrated in the condensation of ketene bis(trimethylsilyl)acetals with aldimines in a synthesis of /3-amino acids, and in the formation of ( )-enals from A-(f-butyl) bis(trimethylsilyl)-acetaldimine and aldehydes. ... 

Metallation of 2,3,4,5-tetrahydrooxepin with Bu"Li or Bu Li leads to 7-lithio-2,3,4,5-tetra-hydrooxepin, i.e. vinylic deprotonation occurs <82JOC3094>. It is of interest that 2,3-dihydrooxepin, like 2,5-dihydrofuran, undergoes allylic deprotonation. The readily available trimethylsilyl ketene acetal of e-caprolactone is a convenient intermediate for the synthesis of 3-RO-substituted e-caprolactones (R is acyl or tosyl) using, as other reagents, lead(IV) carboxylates <83JOC4940> or [hydroxy(tosyloxy)iodo]benzene <89JOC1101>, respectively. 

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