Nitroalkenes determination

The addition of (Z)-3-(alkylamino)-2-butenoic acid esters to nitroalkenes derived from open-chain sugars gave a 50 50 mixture of diastereomeric products 32. The reaction of chiral 2-sub-stituted l-(2-nitroethenyl)pyrrolidines with zinc enolates of 3-substituted tetrahydro-2//-pyran-2-oncs in 1,2-dimethoxyethane at — 78 °C afforded the corresponding 3,3-disubstituted products in 82-96% ee via an addition-elimination process33. The stereochemical course of the reaction was determined by chemical correlation of (S)-( )-3-ethyltetrahydro-3-(2-ni-troethenyl)-2//-pyran-2-one with ( + )-quebrachamine. 

Nitroalkenes with Chiral Auxiliaries The use of carbohydrates as chiral auxiliary in Diels-Alder reactions for the stereoselective preparation of carbocyclic and heterocyclic chiral rings is well documented.48 For example, D-manno-nitroalkene reacts with 2,3-dimethyl-1,3-butadiene to give a 65 35 mixture of adducts, as shown in Eq. 8.29. The configurations at C-4 and C-5 have been determined to be (4R,5R) and (45,55), respectively. Hydrolysis of the product followed by degradative oxidation of the sugar side chains leads to enantiomerically... 

Initially, a complex of nitroalkene (42) with LA (A) is reversibly formed. The efficient concentration of the latter is determined by the reaction conditions and the nature of heterodiene (42) and LA. This complex acts as a Michael substrate and adds alkene (43) to give bipolar adduct B, which undergoes cycliza-tion to give cationic intermediate C. The latter eliminates LA to yield target nitronate (35). In the case of nonconcerted cycloaddition, ionic intermediate B can undergo different isomerization reactions, some of which are considered below. The stereoselectivity of the process depends on the reactive conformation... 

The PE spectra and electronic structures of some nitroalkenes like nitroethene149 and the isomeric nitropropenes150, including their thermolysis151, have been studied recently. A correlation diagram for the experimentally determined ionization potentials for the nitropropenes is reproduced in Figure 20. 

A review appeared on the determination of nitroalkanes, polynitroalkanes, nitroalkenes, aromatic nitro and polynitro compounds, heterocyclic nitro derivatives and inactive compounds after nitration, by polarography, voltammetry and HPLC with electrochemical detection441. 

Scheme 6.138 Product range of the 121-catalyzed asymmetric (3-hydroxylating Michael addition of oximes to aliphatic nitroalkenes. The product configurations were not determined.

When Michael additions of chiral enolates to nitroalkenes were studied, it was found that lithium enolates (132) of l,3-dioxolan-4-ones (131), derived from the corresponding a-hydroxy acids, afford the adducts (133) with high diastereoselectivity (Scheme 50).144 Recrystallization leads, in general, to diastereomerically pure products, which in turn can efficiently be converted to homochiral compounds like (134), (135) or (136). A number of other chiral enolates (137M140) were also shown to undergo highly selective additions to nitroalkenes however, product configurations were not determined in these cases. 

The use of DCP-derived propenyl ethers in nitroalkene [4 + 2] cycloaddition allows for the installation of an additional stere-ogenic center in the tandem cycloadducts. The methyl substituent also provides a stereochemical marker to allow for the determination of endo/exo selectivity in the [4 + 2] cycloaddition. DCP-derived E-propenylvinylether (E-4) has been employed in the... 

Evidently, although the system tolerates both aromatic and aliphatic aldehydes, the introduction of an electron-withdrawing substituent on the aromatic substrate results in a decreased yield. To gain information about the mechanism of the overall tandem reaction, kinetic studies were carried out to identify the rate-determining step. Changing the catalyst concentration in the reaction between 3-methylbutyraldehyde, nitromethane and dimethyl malonate revealed that the reaction is first order in nickel catalyst, indicating that the Michael addition of dimethyl malonate to the nitroalkene is the ratedetermining step. 

The Cu(II) complex of 117 outperforms the congener possessing a BINOL moiety in conjugate addition of organometallics to nitroalkenes, the o,o -dioxydiphenylmethane unit is subject to conformational changes as determined by the bis-(a-phenethyl)amino chirality/ ... 

The molecular mechanism of the domino reaction of nitroalkenes with silyl enol ethers to give nitroso-acetal adducts has been investigated using computational procedures at the PM3 semiempirical level. The reaction has been found to comprise three consecutive steps the first and rate-determining step is the nucleophilic attack of the... 

Alternative to the Nef pathway, some OYEs can bioreduce p,p-disubstituted nitroalkenes to oxazetes. This reaction proceeds through reduction of the nitro group to the nitroso alkene, which spontaneously forms 1,2-oxazete derivatives. At elevated temperatures by a retro-[2 + 2]-cycloaddition, HCN and the ketone are formed as final products. The bioreduction pathway was determined by the OYE used. Thus, using xenobiotic reductase A oxazete formation was predominant, while the PETN reductase-catalyzed process favored the Nef pathway (Scheme 2.15). 

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