Hetero-Diels-Alder reaction cinchona alkaloid derivatives

Benzoate salt of the vicinal diamine (249) has been found to catalyse a domino reaction of enones R CH=CHCOR with R CH=CHCOR that affords cyclic products containing three to four contiguous stereocentres (with 92-99% ee and >30 1 dr) " The self-assembly of proline and cinchona alkaloid-derived thioureas has been reported to dramatically improve the enantioselectivity of the inverse-electron-demand hetero-Diels-Alder reactions between aldehydes R R CHCH=0 and electron-deficient enones R CH=CHCOR , affording (250) after subsequent oxidation with PCC. °... 

In 2006, Lectka and coworkers reported an asymmetric inverse electron demand hetero-Diels-Alder reaction (HDAR) of acyl chlorides (Bekele et al. 2006 Wolfer et al. 2006) and o-benzoquinone diimides to deliver chiral quinoxalinones (Abraham et al. 2006). In spite of perfect ee values observed by the catalysis of Lewis bases derived from cinchona alkaloids, the reaction conditions were somewhat harsh and metal triflates had to be used as co-catalysts to activate the electrophilic o-benzoquinone diimides (Abraham et al. 2006 Pauli et al. 2(X)8). 

In 2003, Rawal reported the use of TADDOLs 177 as chiral H-bonding catalysts to facilitate highly enantioselec-tive hetero-Diels-Alder reactions between dienes 181 and different aldehydes 86 (Scheme 6.29A) [82], and also BINOL-based catalysts 178 were found to facilitate this reaction with excellent selectivities [83]. TADDOLs were also successfully used as organocatalysts for other asymmetric transformations like Mukaiyama aldol reactions, nitroso aldol reactions, or Strecker reactions to mention a few examples only [84]. In addition, also BINOL derivatives have been employed as efficient chiral H-bonding activators as exemplified in the Morita-Baylis-Hilhnan reaction of enone 184 with different carbaldehydes 86 [85]. The use of chiral squaramides for asymmetric reactions dates back to 2005 when Xie et al. first used camphor-derived squaric amino alcohols as ligands in borane reductions [86]. The first truly organocatalytic application was described by Rawal et al. in 2008 who found that minute amounts of the bifunctional cinchona alkaloid-based squaramide 180 are... 

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