We
report the enantioselective fluorination of α-substituted
β-diketones using β,β-diaryl serines as a primary
amine organocatalyst. The reaction affords the corresponding fluorinated
products in yields of 74 to 99% with excellent enantioselectivity
(75–95% ee). Moreover, for synthetic applications,
the diol, aldols, and the allylic fluoride were synthesized from 2a, maintaining excellent enantioselectivity (94% ee). The control experiment reveals that the CO2H group of the β,β-diaryl serines plays an important
role in inducing the high enantioselectivity.
A general synthetic procedure was developed for enantiomerically pure β,β‐diaryl β‐hydroxy‐α‐amino acids 5, starting from readily available oxazolidine ester 1 and a wide variety of Grignard reagents. The problematic oxidative decomposition of β‐aminodiols 3, due to their instability in the presence of metallic oxidants, was successfully suppressed using Parikh‐Doering oxidation and subsequent Pinnick oxidation. Additionally, the organocatalytic ability of β,β‐diaryl β‐hydroxy‐α‐amino acids 5 was evaluated with respect to asymmetric fluorination of 2‐phenylpropionaldehyde 6, and moderate enantioselectivity, 61%, was achieved using compound 5 f.
Recently, organocatalysis with amino acids has been recognized as one of the most environmentally benign catalytic reactions. However, the structural diversity of amino acids is still limited; therefore, developments of syntheses for the new enantiomerically‐pure amino acid are highly demanded. In their Full Paper, Satoru Arimitsu and co‐workers on page 238 in Issue 2, 2020 (DOI: 10.1002/ajoc.201900694) report a conventional synthesis for β,β‐diaryl β‐hydroxy‐α‐amino acids and an investigation of their ability for organocatalysis. Cover design: Ms. Barbara Sebastian, a.k.a. Barbiturikills, a Spanish graphic designer.
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