Range and Preparation of Dienophiles

As illustrated above, simple olefins and acetylenes, unactivated by electron withdrawing groups, serve well as dienophiles in the intramolecular Diels-Alder reaction. This is in contrast to the intermolecular case. Heterosub-stituted olefins can also serve as dienophiles. In addition to the two examples cited above [58, 70] the preparation and cyclization of enamides should be noted [82]. 

Olefins activated by electron withdrawing groups are avid dienophiles. For the preparation of heterocyclic systems, esters and amide dienophiles can be prepared from the (often commercially available) acids [45, 50]. For 

Enones have most often been prepared by oxidation of vinyl carbinols [89, 90]. They have also been prepared by addition of vinyl Uthium to a carboxyhc acid, and by aldol condensation [91,92]. In addition, reagents have been developed that allow direct incorporation of the enones [93,94]. The latter example is an interesting case of a tandem Claisen/Diels-Alder sequence. 

A variety of other dienophiles have also been investigated. Unsaturated aldehydes [95], amidines [96], nitriles [97] and o-quinones [98] have been used. By combining electron-withdrawing group activation with heterosubstitution, it is possible to directly assemble highly functionahzed structures [99-101]. 

Acetylenic esters appear to be avid dienophiles [102, 103]. Klemm [69] has extensively investigated the addition of acetylenic esters to styrene systems. Allenes [104] and ketenes [105] appear to also be active dienophiles. 

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