Biomimetic enantioselective synthesis of β,β-difluoro-α-amino acid derivatives

Abstract

Although utilization of fluorine compounds has a long history, synthesis of chiral fluorinated amino acid derivatives with structural diversity and high stereoselectivity is still very appealing and challenging. Here, we report a biomimetic study of enantioselective [1,3]-proton shift of β,β-difluoro-α-imine amides catalyzed by chiral quinine derivatives. A wide range of corresponding β,β-difluoro-α-amino amides were achieved in good yields with high enantioselectivities. The optically pure β,β-difluoro-α-amino acid derivatives were further obtained, which have high application values in the synthesis of fluoro peptides, fluoro amino alcohols and other valuable fluorine-containing molecules.

Fig. 1. Representative β,β-difluoro-α-amino acid derivatives.

I: β,β-difluorophenylalanyl puromycin; II: 3,3-difluoro-3,4-dideoxy-KRN7000 analog; III:CCRF-CEM folypoly-γ-glutamate synthetase.

Fig. 2. Strategies for asymmetric synthesis of β,β-difluoro-α-amino acid derivatives.

Strategies, a enzymatic hydrolysis; b asymmetric hydrogenation; c chiral auxiliaries. Valuable molecules: difluoro peptides, difluoro amino alcohols, and difluoro alkaloid.

Fig. 3. Scope of β,β-difluorinated imines^[a,b,c].

a Conditions: 1a (0.1 mmol), cat. G (5 mol%), toluene (1.0 mL), room temperature, 4–16 h. b Isolated yield after flash-column chromatography. c Enantiomeric excess (ee) determined via chiral-phase HPLC analysis. d Recrystallization from hexane/ethyl acetate (20/1). e After hydrolysis. f dr was determined via crude ¹H NMR. g Cat. H (5 mol%) was used.

Fig. 4. Mechanistic studies.

Deuterization and cross deuterization experiment.

Fig. 5. KIE from parallel experiments.

The parallel experiments of 1a and 1a-d₂.

Fig. 6. Proposed transition model.

The possible mechanistic of the enantioselective [1,3]-proton-shift reaction.

Fig. 7. Gram-scale synthesis and transformations.

The gram-scale synthesis of 1i. The transformation of 2i to II and 2t to 5.

Fig. 8. Solid-phase peptide synthesis of difluorinated oxytocin.

a About 20% piperidine/DMF, 10 min, double; b 1.5 equiv. fluorinated leucine, 1.5 equiv. HOBt, 1.5 equiv. DIC, minimum DMF, rt, 16 h; c 10% piperidine/DMF, 3 mins, double; d 3 equiv. amino acid, 2.9 equiv. HBTU, 6 equiv. DIPEA, minimum DMF, rt, 1.5 h; e 10% piperidine/DMF, 3 mins, double; f TFA cocktail, global deprotection, rt, 3 h; g GSH/GSSG folding, rt, over-night. Fmoc = fluorenylmethoxycarbonyl, HOBt = hydroxybenzotriazole, DIC = N,N’-diisopropylcarbodiimide, HBTU = N,N,N’,N’-tetramethyl-O-(1H-benzotriazol-1-yl)-uronium hexafluorophosphate, DIPEA = N,N-diisopropylethylamine, GSSG = glutathione disulfide.

Fig. 9. Folded difluoro-oxytocin stable than folded WT oxytocin.

s4: fold WT-oxytocin, s5: linear WT oxytocin; s4′: fold difluoro-oxytocin, s5′: linear difluoro-oxytocin, s1–s3/s6–s9 scrambling, s1′–s3′/s6′–s9′ scrambling.