1-Oxabutadiene electron-donating

Oxabutadiene electron-donating, 185-187 electron-withdrawing, 177-185 hetero, 201-202... [Pg.188]

Oxabutadiene Systems Bearing C-2, C-3, or C-4 Electron-Withdrawing Substituents 1-Oxabutadiene Systems Beanng C-3 or C-4 Electron-Donating Substituents... [Pg.271]

Oxabutadiene Systems Bearing C-3 or C-4 Electron-Donating Substituents... [Pg.279]

The addition of electron-donating substituents to C-3 or C-4 of a 1-oxabutadiene system would be expected to decrease the facility with which the system participates as the 4tt component of Diels-Alder reactions. As a consequence of this expected behavior only a select set of such systems have been investigated and shown to successfully participate as 47t components of Diels-Alder reactions. [Pg.279]

However, the C-4 substitution of the 1-oxabutadiene system with strong electron-donating substituents (—OR, —NR2) does serve to increase the nucleophilic character of the a,/3-unsaturated carbonyl compound to the extent that stepwise, addition-cyclization [4 + 2] cycloadditions, often in competition with [2 + 2] cycloadditions, with reactive electrophilic olefins, e.g., ketenes121-127 or sulfenes,127-130 have been observed and extensively investigated (Scheme 7-II). For early investigations, prior reviews should be consulted.5... [Pg.280]

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