Diels bifunctional thiourea catalysts

Scheme 10.17 Diels-Alder reaction catalyzed by bifunctional thiourea catalysts.

In 2008, the first asymmetric Diels-Alder reaction of 3-vinylindoles 91 and maleimides 80 with a bifunctional thiourea catalyst (92) was realized by Bernardi and coworkers (Scheme 38.25) [39]. The resultant highly-enantioenriched fused heterocycles are very useful for the synthesis of biologically important natural or unnatural alkaloids. As shown in the proposed transition state N, catalyst 92 probably activates the diene with the basic moiety and activates the dienophile with the thiourea moiety through hydrogen bonding interactions. 

Wang and coworkers developed an intriguing strategy for the inverse-electron-demand Diels-Alder reaction to afford spiro compounds [51]. The reaction of a cyclopentyl keto-enolate salt 89 with A-tosyl-2-methylenebut-3-enoate 88 was catalyzed by the bifunctional primary amine-thiourea catalyst derived from rosin XXII to afford the spirocycles 90 in excellent yields (85-99%), excellent diastereoselec-tivities (up to >20 1 dr), and excellent enantioselectivities (90-99%, Scheme 10.31). 

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