Thiourea catalysis Diels-Alder reactions

The self-assembly of cinchona alkaloid-derived thioureas and proline, which enables the inverse-electron-demand hetero-Diels-Alder reactions between aldehydes R R CHCH=0 and electron-deficient enones R CH=CHCOR", affording (260), has been discussed in the paragraph covering enamine catalysis. ... 

Mine [7] and Kelly [8] were the first to prepare synthetic equivalents of this bidentate motif, using biphenylene diols as catalysts. Shortly after Etter had studied the hydrogen bonding patterns in supramolecular assemblies of various carbonyl compounds [9, 10], Curran introduced diarylurea derivatives as further bidentate organocatalysts [11, 12]. This interplay between supramolecular chemistry and non-covalent catalysis has turned out to be very fruitful ever since [13-15]. Schreiner subsequently showed that thioureas are also potent organocatalysts, which offer several advantages compared to urea derivatives, e.g., better solubility [13, 16]. In a proof-of-principle study, thiourea derivative 1 (Fig. 1, right) was used to catalyze the Diels-Alder reaction shown in Scheme 1 [16]. 

Hydrogen-bonding catalysis of the Diels-Alder reaction of 2-vinylindoles 39 and methylene-indolinones 69 has been accomplished by the Barbas group using a C2-symmetric bis-thiourea organocatalyst [31]. This efficient transformation provided a powerful approach by which to construct carbazole-spirooxindole derivatives 71 extremely rapidly (less than 10 min) with excellent enantiopurity and structure diversity (Scheme 38.20). 

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