Trialkylsilyl nitronates

Nitro compounds have been converted into various cyclic compounds via cycloaddidon reactions. In particular, nitroalkenes have proved to be nsefid in Diels-Alder reactions. Under thermal conditions, they behave as electron-deficient alkenes ind react v/ith dienes to yield 3-nitrocy-clohexenes. Nitroalkenes c in also act as heterodienes ind react v/ith olefins in the presence of Lewis acids to yield cyclic alkyl nkronates, which undergo [3- 2 cycloaddidon. Nitro compounds are precursors for nitnie oxides, alkyl nitronates, and trialkylsilyl nitronates, which undergo [3- 2 cycloaddldon reacdons. Thus, nitro compounds play important roles in the chemistry of cycloaddidon reacdons. In this chapter, recent developments of cycloaddinon chemistry of nitro compotmds and their derivadves are summarized. 

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). 

A very short and elegant synthesis of the 16-rtiembered dilactone ( )-pyrenophorin (515) has been accomplished by the dipolar cycloaddition reaction of a trialkylsilyl nitronate (81TL735). Nitromethane was added to 3-buten-2-one and the carbonyl group of the product reduced with sodium borohydride. The nitro alcohol (511) was converted to the acrylate (512) which was then subjected to a dimerization-cyclization reaction by treatment with chlorotrimethylsilane and triethylamine in dry benzene. Hydrogenation of the mixture of isoxazoline products (513) over palladium on charcoal followed by double dehydration of the intermediate bis-/3-hydroxyketone (514) led to ( )- and meso-pyrenophorin (Scheme... 

Very mild conditions, which do not affect enol ethers, involve treatment of trialkylsilyl nitronates with MCPBA (equation 36) . Nitro derivatives themselves do not react with MCPBA. 

Method G Highruiri-selecdvity is also observed in the fluoride-catalyzed reacdonof silyl nitronates v/ith aldehydes. Trialkyl silyl nitronates are prepared in good yield from primary nitroalkanes by consecndve treatment v/ith iithiiim dusopropylamide and trialkylsilyl chloride at -78 C in THF. 

Finally, boryl nitronates can be synthesized by a method based on the replacement of the trialkylsilyl fragment in SENAs (217) (Scheme 3.71). 

In studies of alkyl- or silyl nitronates, the problem of comparison of covalent and ionic stmctures is not urgent because it is impossible to imagine a stable product containing alkyl- or trialkylsilyl cations. However, this is not so evident for other elementorganic nitronates, and special studies were required to solve these questions (see, e.g., Ref. 232). 

Nef reaction.4 Nitro compounds, primary or secondary, are converted to tri-alkylsilyl nitronates in greater than 90% yield by reaction with a trialkylsilyl chloride and DBU in CH2C12. The silyl nitronates derived from secondary nitro compounds are oxidized by C1QH4C03H at 25° to ketones in high yield. This sequence is not useful for conversion of primary nitro compounds to aldehydes. 

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