Cation Diels-Alder reaction

Extensive studies by Gorman and Gassman have shown that an allyl cation can be a 27r-electron component in a normal electron-demand cationic Diels-Alder reaction and, since a carbocation is a very strong electron-withdrawing group, the allyl cation is a highly reactive dienophile [19a, 21]. 

Saito N., Grieco P. A. Development of Cationic Diels-Alder Reaction in Highly Polar Media and Total Syntheses of Natural Products Yuki Gosei Kagaku Kyo-kaishi 2000 58 39-49... 

Keywords cationic Diels-Alder reaction, asymmetric synthesis, stereoselective Diels-Alder reaction... 

It is important to note that the reactions are fundamentally different from similar radical cation Diels-Alder reactions initiated with the use of a photochemical electron-transfer reaction [35, 36]. In photochemical reactions, a one-electron oxidation of the substrate leads to a cycloaddition that is then terminated by a back electron transfer . No net change is made in the oxidation state of the substrate. However, the reaction outlined in Scheme 13 involves a net two-electron oxidation of the substrate. Hence, the two pathways are complementary. 

Gieseler, A., Steckhan, E., Wiest, O., and Knoch, F. (1991) Photochemically induced radical cation Diels-Alder reaction of indole and electron-rich dienes. Journal of Organic Chemistry, 56, 1405-1411. 

Most dienes studied in radical cation Diels-Alder reactions are derivatives of 1,3-cyclohexadiene 8. As shown in Scheme 4.3, the dimerization of 8a can already lead to four different products 9a-12a that are commonly observed in many crossed radical cation Diels-Alder reactions. Reacting cyclohexadiene 8a with styrenes 13a-c thus... 

The Diels-Alder reaction is one of the most powerful and efficient processes for formation of six-membered rings with the potential of controlling the relative and absolute stereochemistry at four newly created stereogenic centers [1]. Relative stereochemistry is usually well-defined because of the formation of a cyclic transition state arising from suprafacial-suprafacial interaction, with endo approach [2]. The reaction can be accelerated by Lewis acids, high pressure, or radical cations. Diels-Alder reactions catalyzed by Lewis acids are generally more regio- and stereoselective than their thermal counterparts [3]. 

The radical cation Diels-Alder reaction has been the subject of many mechanistic and theoretical investigations and has been shown to have much synthetic potential. With regard to heteroaromatics, the reaction has been exploited by Steckhan in the cycloaddition of indoles and 1,3-dienes. This reaction occurs smoothly upon photosensitization by triarylpyrrilium tetrafluoroborates. The reaction is satisfactory rationalized as involving addition of the indole radical cation to electron-rich dienes (Scheme 35), and the regioselectivity is in accord with theoretical predictions [104]. The reaction with exocyclic dienes has been developed for the synthesis of carbazole derivatives such as 52 and 53 [105]. 

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