Reverse electron-demand type Diels-Alder reaction

MO calculations of the reaction of 3,4-bis(methoxycarbonyl)-l,2-dithiete (150) with alkenes to produce 2,3-dihydro-1,4-dithiins (152) confirmed that the reaction is a reverse-electron-demand hetero-Diels-Alder type between the valence isomer 1,2-bis(methoxycarbonyl)ethane-l,2-dithione (151) and the dienophiles (Scheme 58). Sulfur dioxide has been shown to promote its hetero-Diels-Alder and chelotropic additions to 1,2-dimethyUdenecyclohexane. The competition between hetero-Diels-Alder and chelotropic addition of sulfur dioxide with substituted 1,3-dienes has been investigated by studying the theoretical and experimental effects on the relative stability of sultines and sulfolenes. ... 

The present reaction proceeds through a reverse electron-demand type of Diels-Alder reaction, 14, with the enol form 47, which, followed by dehydration, would generate the product 12 (Scheme 15.20). The reaction needed trace amounts of water. 

In another aspect of the mechanism, the effects of electron-donating and electron-withdrawing substituents (p. 1065) indicate that the diene is behaving as a nucleophile and the dienophile as an electrophile. However, this can be reversed. Perchlorocyclopentadiene reacts better with cyclopentene than with maleic anhydride and not at all with tetracyanoethylene, though the latter is normally the most reactive dienophile known. It is apparent, then, that this diene is the electrophile in its Diels-Alder reactions. Reactions of this type are said to proceed with inverse electron demand ... 

Diels-Alder reactions also may occur when the electronic situation of the substrates is completely reversed, that is, when electron-rich dienophiles react with electron-poor dienes. [4+2]-Cycloadditions of this type are called Diels-Alder reactions with inverse electron demand. 1,3-Dienes that contain heteroatoms such as O and N in the diene backbone are the... 

A rarer type is the reverse electron demand Diels-Alder reaction in which the dienophile has electron-donating groups and the diene has a conjugated electron-withdrawing group. 

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