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A variety of asymmetric intramolecular reactions was developed featuring stabilized carbocations as intermediates. In 2010, the Antilla group demonstrated that chiral phosphoric acids could catalyze the enantioselective semi‐pinacol rearrangement of an indolyl diol via intermediacy of the indolyl cation/chiral phosphate ion‐pair (Scheme 4.37) [122]. A vinylogous variant of this reaction was reported in 2016 by the Zhu group, forming β,γ‐unsaturated ketones in high yields and enantioselectivities [123]. Other intramolecular reactivity was shown in 2018 by the Ariafard and Chan groups, with an enantioselective Nazarov‐type electrocyclization of allylic alcohols, forming chiral indenes and tricyclic compounds in high yields and enantioselectivities [124].

Scheme 4.34. Asymmetric counteranion‐directed catalysis involving stabilized oxocarbenium intermediates.

Source: Based on [115].

Scheme 4.35. Asymmetric counteranion‐directed catalysis with unstabilized oxocarbenium intermediates.

Source: Based on [118].

Scheme 4.36. Trityl cation/ chiral phosphate salt activation of α‐ketoesters.

Source: Based on [121].

Scheme 4.37. Intramolecular asymmetric counteranion‐directed catalysis with carbocation intermediates.

Source: Based on [122].

Intermolecular transformations utilizing carbocations in conjunction with chiral‐anions have been of recent synthetic interest. In 2017, the Sun group demonstrated an asymmetric intermolecular reaction involving ion‐pairing with carbocations (Scheme 4.38) [125]. In this example, a propargyl alcohol is ionized by protonation/dehydration with N‐triflylphosphoramide catalysts. This intermediate was then shown to undergo nucleophilic attack by 1,3‐diketones and thioacetic acid in high yields and enantioselectivities. Following this, the Peng and Yang groups [126], as well as the Kartika group [127], demonstrated enantioselective intermolecular arylation reactions of allyl alcohols with indoles.

Scheme 4.38. Intermolecular asymmetric counteranion‐directed catalysis with carbocation intermediates.

Source: Based on [125].