Synthesis of enantiopure triazolium salts from pyroglutamic acid and their evaluation in the benzoin condensation

A family of enantiopure 1,2,4-triazolium salts were prepared starting from the inexpensive (S)-pyroglutamic acid. After treatment with base, the corresponding N-heterocyclic carbenes were tested as organocatalysts in the asymmetric benzoin condensation and gave good yields and up to 95% ee.

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(S)-5-[Diphenyl(trimethylsilyloxy)methyl]-2-phenyl-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazol-2-ium tetrafluoroborateC27H30BF4N3OSi[α]D23=-130.2 (c 1.01, CHCl3)Source of chirality: (S)-glutamic acidAbsolute configuration: (S)

(S)-5-[(tert-Butyldimethylsilyloxy)diphenylmethyl]-2-phenyl-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazol-2-ium tetrafluoroborateC30H36BF4N3OSi[α]D23=-137.5 (c 1.01, CHCl3)Source of chirality: (S)-glutamic acidAbsolute configuration: (S)

(S)-2-Phenyl-5-[2-(trimethylsilyloxy)propan-2-yl]-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazol-2-ium tetrafluoroborateC17H26BF4N3OSi[α]D23=-46.3 (c 1.01, CHCl3)Source of chirality: (S)-glutamic acidAbsolute configuration: (S)

(S)-5-Benzhydryl-2-phenyl-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazol-2-ium tetrafluoroborateC24H22BF4N3OSi[α]D23=+14.5 (c 1.01, CHCl3)Source of chirality: (S)-glutamic acidAbsolute configuration: (S)