Diels-Alder reactions radical cation catalysed

A number of electrocyclic reactions under PET conditions have been reported. In this way, A-benzyl-2.3-diphcnylaziridinc (40) underwent a 3 + 2-cycloaddition with alkene and alkyne dipolarophiles to afford substituted pyrrole cycloadducts (41) via the radical cation intermediate (42) see Scheme 7.80 Elsewhere, novel arylallenes have been used as dienophiles in a radical cation-catalysed Diels-Alder cycloaddition reaction with 1,2,3,4,5-pentafluromethylcyclopentadiene, which often occurred with peri-, chemo-, facial- and stereo-selectivity.81... 

Nitrosonium terafluoroborate initiates the cation radical-mediated imino-Diels-Alder reaction of IV-arylimines with IV-vinylpyrrolidinones to give cw-4-(2-oxopyr- (g) rolidin-l-yl)tetrahydroquinolines.175 Also, 2,4,6-triphenylpyrylium tetrafluoroborate catalyses the Diels-Alder addition of IV-arylimines with IV-vinylpyrrolidinone and IV-vinylcarbazole to yield the corresponding 2-oxopyrrolidin-l-yl- and carbazole-9-yl-tetrahydroquinolines.176 The tricyclic core (151) of the batzelladine alkaloids has been prepared by a diastereoselective 4 + 2-annelation of the vinylcarbodiimide (150)... 

Figure 4 13C KIEs for the radical-cation catalysed Diels-Alder reaction of indole with cyclohexadiene.

The cation-radical Diels-Alder reactions of cis- and fran5 -l,2-(diaryloxy)ethenes with butadienes are stereospecific, in agreement with a concerted cycloaddition mechanism. " Tris(4-bromophenyl)aminium hexachloroantimonate catalyses the two-step, non-stereospecific cation-radical Diels-Alder reaction of cis- and traui-prop-l-enyl aryl ethers with cyclopenta-1,3-diene in CH2CI2 solution. 

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