Dienophiles nitro olefins

Electron-deficient olefins which are effective dienophiles in Diels-Alder reactions include 3-phenylthio-nitro-olefins and their... 

The extensive use of nitro-compounds in synthesis stems not only from their ability to stabilise simple a-carbanions but also in their ability to activate both aromatic and olefinic systems towards nucleophilic attack. The additional reactivity of nitro-olefins as dienophiles in Diels-Alder and other [4+2] cyclo-... 

Nitro-olefins are known to be excellent dienophiles, but now the reaction can be coupled with a highly selective de-nitration, using tri-n-butyltin hydride, so providing a novel regioselective entry to cyclohexenes. cw-l-Octalones (133)... 

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. 

The most extensively stodied double intermolecular cycloadditions involve vinyl ether dienophiles (F = -OR, Scheme 16.53) and unsaturated ester dipolarophiles (A = — CO2R)- In this case, the final product of hydrogenolysis is pyrrolizidinone (such as 250, Scheme 16.54), which may contain five of the six original stereogenic centers in the nitroso acetal. The pyrrolizidine is a common motif in many natural products and the double intermolecular cycloaddition of nitro olefins has been extensively employed for its construction in syntheses efforts. 

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