O -Silylated oximes

The unsaturated oximes 224 (see Table 21) were readily prepared by AT-alkyl-ation of allyl amines with a-bromoketones or O-silyl-a-bromoaldoximes. Heating the oximes 224 in toluene under an argon atm at 110 -180 °C smoothly led to isoxazolidines 225 in good yields with cis ring junction stereochemistry. Even when three stereocenters were generated, as in 225 g-1, a single stereoisomer... 

On treatment of trialkylsilyl nitronates 1043 with MeLi, LiBr, or BuLi in THF the resulting nitrile oxide intermediates 1044 afford, in dilute THF solution (R=Me) the ketoximes 1045 in ca 50-60% yield, whereas in concentrated THF solution the O-silylated hydroxamic acids 1046 are obtained as major products [144] (Scheme 7.35). Analogously, the silyl nitronate 1047 reacts with the 2,3,4,6-tetra-O-acetyl-/ -D-glucopyranosyl thiol/triethylamine mixture to afford, via the thiohydroxi-mate 1048, in high yield, a mixture of oximes 1049 which are intermediates in the synthesis of glucosinolate [145] (Scheme 7.35). 

The chemistry of O-acyl derivatives of trinitromethane was studied in more detail. By analogy with O-silyl ethers of trinitromethane, for intermediate (120) (Scheme 3.103, Eq. 1) it was suggested (222a) that the reaction involves elimination of acetyl nitrate to form the very unstable N-oxide A, which adds acetyl chloride to give derivative (121). Saponification of the latter affords isolable oxime (122). 

O-Substituted oxime derivatives are synthetically useful in a wide variety of transformations. Hoffman and Butani have observed that reaction of a series of aldehydes and ketones with the potassium salt of Af,0-bis(trimethylsilyl)hydroxylamine 4a or 4b (a rapid equilibrium between 4a and its Af,N-bis(silylated) isomer 4b probably exists in solution) gave high yields of the corresponding oximate anion 5, formed via the Peterson-type reaction, together with the silyl ether 6. Anion 5 could be protonated to the oxime 7 or trapped in situ with a variety of electrophiles to give 0-substituted oxime derivatives (Scheme 6). 

The resulting MO-TMS derivatives of numerous important compounds of this class are characterized by standard mass spectra and GC retention indices on standard nonpolar phases for their identification. The use of most active silylating reagents permits us to exclude the stage of O-methyl oxime formation, so far as ketosteroids can form the enol-TMS ethers. For example, androstenedione (II) gives the bis-O-TMS derivative ... 

Indirect replacement of the a-hydrogen atom of carboxylic esters by the nitroso group is remarkable. This procedure uses ketene 0-alkyl O -silyl acetals (1), generated from carboxylic esters, which are treated with nitric oxide or isopentyl nitrite in the presence of titanium(IV) chloride. In the absence of an a-hydrogen a-nitroso esters (2) are obtained. a-Nitroso esters with an a-hydrogen undergo isomerization to oximes of a-keto esters (3 equation 1). Similarly, silyl enol ethers of aldehydes or ketones can be used instead of the carbonyl compound itself for nitrosation. Thus, treatment of enol ether (4) with nitro-syl chloride gives the a-nitroso aldehyde (5 equation 2), which is quite stable at 0 C, but dimerizes at room temperature. 

Other groups that react with chloromethyleneiminium salts include hydroxylamines, hydrazines, oximes, imines, azines, anhydrides, imides and ketene O-alkyl-O -silyl acetal derivatives however, reactions with these compounds have been relatively infrequent. 

Table 24 Condensation Reactions of Silyl Ketene Acetals (219) and O-Benzyl Oxime Ethers (220)...

Treatment of the 1,6-anhydromannose derivative 67 as indicated in Scheme 14 gave the product 68 of radical addition to the oxime ether, and 68 was then convertible to the a-methylene-y-lactone 69, which can be considered as a potential intermediate in a synthesis of tetrodotoxin. Interestingly, the O-silylated form of 67 did not undergo cyclization satisfactorily. A number of related radical annulations were also reported (see Section 4 for one case). ... 

Nevertheless, the direct nucleophilic displacement of support-bound carboxylates to prepare hydroxamates presents some limitations. For example, O-tert-butyldimethylsilyl-protected hydroxylamine displaces common acids from oxime resin however, further treatment with trifluoroacetic acid (TFA) is necessary to remove completely the silyl... 

Another silanization procedure for the derivatization of carbohydrates is the formation of trimethylsilyl oximes. The methyloxime is heated with hexarnethyl-disilazane trifluoroacetic anhydride (9 1 v/v) for 1 h at 100°C. Anthrone O-glucoside is an important ingredient in skin care cosmetics and can be fully silylated by reaction with A/,0-bistrimethylsilylacetamide acetonitrile mixture (1 1 v/v) for 1 h at 90°C, and subsequently separated by GC. 

In a number of instances (e.g., with steroids and sugars), oximes are formed prior to the silylation reaction [201, 202]. Muriatic o-pentafluorobenzylhydroxylamine has been used successfully to protect ketosteroids and increase the sensitivity of detection [203, 204]. It should be noted that oximes (under certain conditions) are converted quantitatively into the corresponding nitriles in a column [205]. In a number of instances it is expedient to use hydrazones to increase the stability of carbonyl compounds, including that of formaldehyde [206,207]... 

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