Trimethylsilyl nitronates

The liberated trimethylsilanol 4, however, reacts with the trimethylsilyl nitronate... 

If an electron-attracting group is present in the position yS to the nitro moiety, formation of trimethylsilyl nitronates from, for example 1050, leads, via 1051, to the oximes 1052 in 73-78% yield [106]. Subsequent transsilylations of 1052 with methanol give the free oximes in high yield [106] (Scheme 7.36). 

Because of the relative instabihty of many trimethylsilyl nitronates 1036, 1037, they should be reacted in situ with olefins 1053 [103-105] or acetylenes [127] to generate the isooxazolidines 1054 [103-105, 107-117, 119-133] or isoxazoles [127] (Scheme 7.37) The isoxazolidines 1054 with R2=H readily ehminate trimethylsilanol 4 in the presence of acids such as TsOH to form the isoxazolines 1055 in high yields [104, 105] (Scheme 7.37 cf. also the cycloadditions with acrylonitrile in Scheme 7.42). 

The unsaturated Oppolzer amide 1072 adds O-trimethylsilyl nitronates 1073 to give a 9 1 mixture of isoxazolines 1074 and 1075 in 96% yield [123, 124] (Scheme 7.41). 

Methyl 4-nitrobutyrate 1099, on treatment with Me3SiBr 16/NEt3, affords the unsaturated O-trimethylsilylated oxime 1101, in 71% yield, via the O-trimethylsilyl nitronate 1100 and elimination of trimethylsilanol 7 [107, 131] whereas the nitro compound 1102 is converted by TCS 14/NEt3, via 1103, into the / -chlorooxime 1104, in 39% yield [107] (Scheme 7.45). 

O-Trimethylsilyl nitronates 1036 have been used in fluoride-catalyzed aldol-type condensations with aldehydes and ketones to give a-trimethylsilyloxy-nitro com-... 

The synthesis of O-trimethylsilyl nitronates [143 a], the many synthetic apphca-tions of O-trimethylsilyl nitronates ]143b, 158, 159], and the chemistry of conjugated nitrosoalkenes have been reviewed [109]. 

Bicyclic trimethylsilyl nitronates undergo stereoselective Henry reactions with benzalde-hyde in the presence of fluoride ion to give cyclic hemiacetals in good yield with high diastereo-selectivity (95% ds) (Eq. 3.68).110... 

Initial investigations showed that the treatment of trimethylsilyl nitronate 23a (R1 = Me) with benzaldehyde (R2 = Ph) in the presence of (S,S)-6b (X = HF2, 2 mol %) in TH F at —98 °C for 1 h and at —78 °C for 1 h and subsequent hydrolysis with 1M HC1 at 0 °C, resulted in clean formation of the corresponding nitroalkanol 24 (R1 = Me, R2 = Ph) in 83% yield (anti/syn = 74 26) with 33% ee (anti isomer) (entry 1 in Table 9.5). Notably, the poor diastereo- and enantioselectivities were dramatically improved by switching the catalyst to (S,S)-6c (X = HF2) possessing a radially extended 3,3-aromatic substituent (Ar), and 24 (R1 = Me, R2 = Ph) was obtained in 92% yield (anti/syn = 92 8 with 95% ee (anti isomer) (Table 9.5, entry 2). This asymmetric nitroaldol protocol tolerates various aromatic aldehydes to afford anti-nitroaldols selectively, being complementary to Shibasaki s method... 

The nitroaldol reaction of silyl nitronates with aldehydes promoted by ammonium fluorides, which was originally introduced by Seebach and Colvin in 1978 [13], is a useful method for the preparation of 1,2-functionalized nitroallcanols. We have developed an asymmetric version of high efficiency and stereoselectivity by using a designer chiral quaternary ammonium bifluoride 15 as catalyst this was readily prepared from the corresponding bromide using the modified Shioiri method (Scheme 4.8) [14]. For example, the treatment of trimethylsilyl nitronate 16a with benzaldehyde in the presence of (S,S)-15a (2 mol%) in THF at —98 °C... 

Michael addition of diethyl cyanomethylphosphonate to a-nitroalkenes in the presence of LDA (1 eq) followed by treatment with Mc,SiOI provides a one-pot synthesis of 2-isoxazoline derivatives. The reaction proceeds through a 1,3-dipolar cycloaddition of trimethylsilyl nitronate with an a-cyanovinylphosphonate. Similarly, the 1,4-addition of the sodium diethyl cyanomethylphosphonate to the azo-ene system of conjugated alkenes results in the formation of l,2-diamino-3-(dietlioxyphosphinyl)pyrroles in moderate to good yields via a hydrazonic intermediate. - In a route to phosphonylated pyrones, the sodium diethyl cyanomethylphosphonate reacts with 3-anilinometliylene derivatives of 4-hydroxycouinaiins in DMF with displacement of aniline. ... 

A stereoselective Henry reaction has also been observed between bicyclic trimethylsilyl nitronates and benzaldehyde with fluoride ion as the catalyst. The reaction results in the formation in high yield of a cyclic hemiacetal (equation 20) and is highly diastereoselective (95% ds)P-... 

There is one more way for conversion of ort/to-nitroarylacetonitriles into indoles. Alkylation of such nitriles with allyl or benzyl halides followed by treatment of the compounds obtained with basic agents results in a multistep transformation, which is likely to proceed via intermediate nitrosoarenes, to produce 1-hydroxyindoles. For instance, alkylation of ort/io-nitroarylacetonitriles with 3-phenylallyl bromide gives the compounds that in the presence of chlorotri-methylsilane and triethylamine undergo cyclization into 3-cyano-l-hydroxy-2-vinylindoles (Scheme 70) [188]. Presumably, this reaction proceeds via 0-silylation of the nitronate anion and 1,5-elimination of trimethylsilanol from the intermediate trimethylsilyl nitronate, followed by cyclization and a hydrogen shift. 

Anderson and coworkers expanded the copper-catalyzed addition of trimethylsilyl nitronates to a broader range of imine substrates [49] Cu(OTf)2/t-Bu-BOX (13) catalyzed the addition of trimethylsilyl nitropropanate (163) to PMP-protected imines (167) derived from aryl, aliphatic, and heteroaryl aldehydes (Scheme 17.35). 

Handling, Storage, and Precautions although there are indications that some trimethylsilyl nitronates are thermally unstable, there have been no reports of violent decompositions. SUyl nitronates are, of course, extremely sensitive to moisture, and they are more resistant to base than to acid. All silyl nitronates should be kept under an inert atmosphere and stored in a freezer. 

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