Olefins, activated nitroalkenes

Keywords Organozinc, Organolithium, Grignard reagent, Copper, Conjugate addition, Nucleophilic addition, Alkylation, Activated olefin, Enone, Nitroalkene, p-Substituted carbonyl... 

The most reactive Michael acceptors, such as alkylidene malonates, gem-dicyanoalkenes and nitroalkenes, react with a-halozinc esters in a conjugate fashion. Beautiful examples were offered by two stereocontrolled conjugate additions to piperidinone 102 and pyrro-lidinone 104 leading to optically active bicyclic lactams 103147 (equation 60) and 105 (equation 61)148. With these electron-poor alkenes a Grignard two-step protocol is to be adopted in order to avoid the single electron transfer reactions from the metal to the Michael acceptor, which should afford olefin dimers. The best solvent is found to be a... 

Michael/Henry/dehydration/aromatisation reaction of 2-(2-oxoethyl) benzal-dehydes and nitroalkenes mediated by pyrrolidine to obtain polysubstituted naphthalene derivatives. DBU catalysed the conjugate additions of alcohols to ot,p-unsaturated nitriles, esters and ketones. Perhaps more important are the aza-Michael addition reactions of amines to a,p-unsaturated ketones, nitriles and esters. Recently, Costa, Vilarrasa and coworkers described the addition of lactams, imides, 2-pyridone, pyrimidine-2,4-diones and inosines to methyl propiolate and other similar compounds, DABCO and DMAP being the best catalysts. As mentioned before, tertiary amines give zwitterionic species with activated allynes. It was as early as 1932 when Diels and Alder used the reaction of pyridine with dimethyl acetylenedicarbojylate (DMAD) for the synthesis of heterocycles. The interception of the corresponding intermediate with Al-tosylimines and activated olefins provided access to l-azadienes ° and highly substituted butadienes (Scheme 2.6). When the quenching species of the zwitterionic intermediate is a 1,2-diketone, dibenzoyl maleates or cyclopentenedione derivatives could be obtained (Scheme 2.6). The interception of the zwitterionic species of N-methyl imidazole (NMI) and DMAD with ketenes to obtain unsaturated esters has also been shown. ... 

S)-Pyrrolidine trifluoromethanesulfonamide 3a is a highly active catalyst for the asymmetric conjugate addition of aldehydes and ketones to nitroalkenes. The use of catalyst 3a (20 mol%) in i-PrOH at 0 °C, afforded a,a-dialkyl aldehydes with good to high yields and high to excellent levels of diastereo- and enantioselectivity (Scheme 9.20). In addition, the reactions of various cyclic and acyclic ketones with aromatic nitro-olefins indicated the usefulness of the catalyst. " Computational results corresponded with the observed stereoselectivities of 3a-catalysed Michael reactions. 

Despite of the fact that Baylis and Hillman reported the synthesis of a-hydr-oxyethylated nitroethylene through the reaction between nitroethylene and acetaldehyde in the presence of DABCO, nitroalkenes employed as activated olefins in MBH reaction have not received much attention until recently. Prompted by the fact that nitroalkenes have shown superior Michael acceptor abilities, and that the first step in the MBH reaction is the Michael-type addition of the catalyst to substrate, Namboothiri et al. have published a series of papers on nitroalkenes involved the MBH reaction. The MBH reactions between nitroalkenes and various electrophiles such as formaldehyde, activated carbonyl compounds, imines, alkenes and azodicarboxylates " in the presence... 

In the reactions using NO as a reagent, there is always a problem that the true active species is unidentified. This is due to the possibility that a trace amount of oxygen or a metal ion can function as a catalyst for the reaction of NO. Pure NO does not react with alkenes, for example, isobutene or similar alkyl-substituted olefins [22]. The main isolated products were found to be nitroalkenes, and it was postulated that the small amounts of NO formed from the oxidation of NO ... 

Cycloaddition reactions represent another important area of nitro olefin chemistry. Typically, nitroalkenes react in normal-electron-demand [4 + 2] cycloadditions as highly activated dienophiles (2ji-components) and produce nitro-substimted cyclohexenes [53]. However, under certain conditions nitroalkenes can switch the mode of reactivity in cycloadditions (periselectivity) and behave as electron-poor heterodienes (47i-components). Nitroalkenes react as 471-partners with electron-rich dienophiles under Lewis-acid activation [19], high pressure [54], and even thermal activation in some cases [55-57]. The products of such cycloadditions are six-membered cyclic nitronates (see Scheme 16.2, for example), whose properties will be discussed later in this text. 

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