Nitronate structures silyl nitronates

The UV spectra of nitronates, which are not functionalized at the a-C atom, have an intense absorption at 230 to 240 nm, which is very similar in characteristics to UV absorption of salts of nitro compounds and solutions of aci-nitro compounds in protic solvents. Since standard alkyl- or silyl nitronates cannot have ionic structures, the presence of the above mentioned absorption in the UV spectra of nitronates, unambiguously confirms, that these compounds have the structures of O-esters. 

The reactions of salts of nitro compounds (113) (Scheme 3.95) with silylated thiols (308), hexamethyldisilathiane (308, 309), and hexamethyldisilane (310) afford oximes (114), thiohydroxamates (115), or thiohydroxamic acids (116) as final products depending on the structures of the starting nitronates and the reagents used. 

As mentioned above (66, 393), (see Scheme 3.150) silylation followed by intramolecular enantioselective cycloaddition with five-membered cyclic nitronates, containing the hydroxyl group at C-4, can produce chiral polycyclic structures (293), which are direct precursors of chiral hydroxyamino acids (294) and aminopolyols (295) (Scheme 3.179). 

This chapter is divided into four major sections. The first (Section 2.1) will deal with the structure of both alkoxy and silyl nitronates. Specifically, this section will include physical, structural, and spectroscopic properties of nitronates. The next section (Section 2.2) describes the mechanistic aspects of the dipolar cycloaddition including both experimental and theoretical investigations. Also discussed in this section are the regio- and stereochemical features of the process. Finally, the remaining sections will cover the preparation, reaction, and subsequent functionalization of silyl nitronates (Section 2.3) and alkyl nitronates (Section 2.4), respectively. This will include discussion of facial selectivity in the case of chiral nitronates and the application of this process to combinatorial and natural product synthesis. 

TABLE 2.2. SELECTED X-RAY STRUCTURAL DATA FOR SILYL NITRONATE (9)... 

The primary cycloadduct from combination of a dipolarophile with a silyl nitronate is an isoxazolidine. The and NMR spectra are highly informative for the structural determination of these products. Tables 2.7 and 2.8 (21,25,34,35). Both the and NMR data show that HC(5) are shifted downfield relative to HC(3). An expected downheld shift is also observed with electron-withdrawing or conjugating groups. In the absence of functionalization at C(3), there is a significant upfield shift of the corresponding resonance. The IR data is less reliable. The O—N—O stretch is reported to be 1055 cm (Fig. 2.8), however, this stretching... 

The wide variety of methods for the preparation of alkyl nitronates, gives rise to a broader diversity of structures compared to silyl nitronates. Alkyl nitronates can be grouped into two subclasses, acyclic and cyclic. Both subclasses participate in dipolar cycloadditions with similar reactivity, however, minor differences are manifest in their stability and stereoselectivity. Additionally, the ability to prepare cyclic nitronates allows access to a wide variety of novel, multicyclic ring stractures. 

To a solution of 5.4 mmol lithium diisopropylamide in 15 mL THF stirred at -78 °C under argon were added 0.84 g (5 mmol) p-methoxyphenylnitro-methane, and after 30 min, 0.90 g (6.0 mmol) (/-butyl)dimethylsilyl chloride dissolved in 2 mL THF. The temperature was allowed to rise to 20 °C overnight. The solvent removed in vacuo, and the residue triturated with 25 ml pentane, filtered through celite and concentrated. The 0.61 g (43.6%) of crude silyl nitronate thus obtained was recrystallized from pentane to furnish crystals suitable for X-ray structure analysis. 

A three-atom silyldioxy cleavable tether also has been used for the syntheses of related alkaloids [130c]. As was seen in the aU carbon case, the three-atom linker directs the [3 - - 2] cycloaddition toward the ejco-fold (tether) transition structure. This arrangement creates a cir-relationship at C(3)/ C(3a) (Figure 16.11). For example, the imstable nitroalkene 342 (prepared by silylation of 340 with 341) is used for the [4-1-2] cycloaddition with the chiral dienophile (—)-247 (Scheme 16.67). In the presence of MAPh, nitronate 343... 

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