Catalytic Kinetic Resolution by Enantioselective Aromatization: Conversion of Racemic Intermediates of the Barton–Zard Reaction into Enantioenriched 3‐Arylpyrroles
2019; Wiley; Volume: 58; Issue: 27 Linguagem: Inglês
10.1002/anie.201903589
ISSN1521-3773
AutoresSheng‐Cai Zheng, Qian Wang, Jieping Zhu,
Tópico(s)Porphyrin and Phthalocyanine Chemistry
ResumoAbstract Racemic 3,4‐dihydro‐2 H ‐pyrroles, hypothetical intermediates of the Barton–Zard reaction, were synthesized in a highly diastereoselective manner and fully characterized for the first time. Kinetic resolution of the dihydropyrroles with a quinine‐derived thiourea afforded the (+)‐3‐arylpyrrole products and recovered (+)‐3,4‐dihydro‐2 H ‐pyrroles with high efficiency (s‐factor up to 153). The resolved (+)‐3,4‐dihydro‐2 H ‐pyrroles underwent subsequent aromatization with a quinidine‐derived thiourea catalyst to afford (−)‐3‐arylpyrroles with excellent central‐to‐axial chirality transfer. In contrast to the well‐accepted Barton–Zard mechanism, the aromatization of the 3,4‐dihydro‐2 H ‐pyrroles in the presence of a bifunctional catalyst is believed to proceed by an unprecedented sequence involving syn elimination of HNO 2 and aromatization.
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