1-Oxabutadiene electron-withdrawing

In the Diels-Alder reaction with inverse electron demand, the overlap of the LUMO of the 1-oxa-l,3-butadiene with the HOMO of the dienophile is dominant. Since the electron-withdrawing group at the oxabutadiene at the 3-position lowers its LUMO dramatically, the cycloaddition as well as the condensation usually take place at room or slightly elevated temperature. There is actually no restriction for the aldehydes. Thus, aromatic, heteroaromatic, saturated aliphatic and unsaturated aliphatic aldehydes may be used. For example, a-oxocarbocylic esters or 1,2-dike-tones for instance have been employed as ketones. Furthermore, 1,3-dicarbonyl compounds cyclic and acyclic substances such as Meldmm s acid, barbituric acid and derivates, coumarins, any type of cycloalkane-1,3-dione, (1-ketoesters, and 1,3-diones as well as their phosphorus, nitrogen and sulfur analogues, can also be ap-... 

A major breakthrough in the Diels-Alder reaction of oxabutadienes has been accomplished through the introduction of an electron-withdrawing group in the 3-position. In several papers we have demonstrated the usefulness of this concept which found broad acceptance after our discoveries. 

Unsaturated acyl cyanides, an oxabutadiene system possessing a C-2 electron-withdrawing group, have been shown to participate as 4n components in thermal and Lewis acid-catalyzed [4 + 2] cycloadditions with electron-rich and simple olefinic dienophiles [Eq. (27)].113... 

Oxabutadiene electron-donating, 185-187 electron-withdrawing, 177-185 hetero, 201-202... 

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... 

Perhaps the most successful approach for promoting the 47r participation of 1-oxabutadiene systems in intermolecular Diels-Alder reactions employs a,/8-unsaturated carbonyl compounds substituted with an additional C-3 (a) electron-withdrawing group. The addition of the C-3 electron-withdrawing substituent increases the electron-deficient character of the oxabutadiene system, decreases the LUM0oxabutadiene, and as expected, enhances the observed [4 + 2] cycloaddition rate and regioselec-... 

Until recently, the reaction of a,/3-unsaturated esters with electron-rich olefins has been reported to afford cyclobutane [2 + 2] cycloaddition products. Amice and Conia first proposed the intermediacy of [4 + 2] cycloadducts in the reaction of ketene acetals with methyl acrylate,108 and the first documented example of the 4v participation of an a,/3-unsatu-rated ester in a Diels-Alder reaction appears to be the report of Snider and co-workers of the reversible, intramolecular cycloaddition of 1-allylic-2,2-dimethyl ethylenetricarboxylates.142 Subsequent efforts have recognized that substitution of the a,/3-unsaturated ester with a C-3 electron withdrawing substituent permits the 4w participation of such oxabutadiene systems in inverse electron demand Diels-Alder reactions with electron-rich olefins. In the instances studied, the rate of the [4 + 2] cycloaddition showed little dependence on solvent polarity [ aeetomtnie/ cycio-hexane — 3, Eq. (15) j acctomtnic toiuene 10, Eq. (20)], and reactions generally... 

An effective approach to promoting the 4ir participation of oxabutadiene systems in Diels-Alder reactions employs the intramolecular cycloaddition reactions of a,/3-unsaturated carbonyl compounds.27 If this intramolecular variant of the oxabutadiene [4 + 2] cycloaddition is combined with the use or generation of a,j8-unsaturated carbonyl compounds bearing an additional C-3 (a) electron-withdrawing group, exceptionally effective room temperature, regio-, diastereo-, and enantioselective [4 + 2] cycloadditions are observed.134-138 Table 7-II details the intramolecular [4 + 2] cycloadditions of a,j3-unsaturated carbonyl compounds.27,131-145... 

Synthesis of furanofurans has been achieved by the rearrangement of 4-acetylpyrans. Thus the hetero Diels-Alder reaction between 1-oxabutadienes (160)-(162) bearing electron-withdrawing groups with 2,3-dihydrofuran afforded 4//-4a,5,6,6a-tetrahydrofuro[2,3-h]pyrans (163) and (164) which have been successfully rearranged to difuro[2,3-h 3, 4 -d]furans (11) and (165) <92T5883>. 

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