Short Synthesis of a Biphenyl-Based Amino Triflamide Catalyst and Its Application in Enamine Catalysis

This paper is dedicated to Professor Hisashi Yamamoto on the occasion of his 80^th birthday.

Abstract

A novel axially chiral biphenyl-based amino triflamide has been designed and successfully synthesized from 1-nitropyrene in 6 steps. This chiral amino triflamide functions as effective catalyst for asymmetric Mannich reaction through enamine intermediates.

Key words

axial chirality - biphenyl - amine organocatalyst - enamine catalysis - Mannich reaction - aminoxylation

• References and Notes


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• 16 Typical Procedure for the Catalytic Asymmetric Mannich Reaction of N-PMP-Protected α-Iminoacetate 9 To a stirred solution of (S)-4 (0.9 mg, 0.0025 mmol) in dioxane (2.5 mL) were added an aldehyde (0.75 mmol) and ethyl (4-methoxyphenylimino)acetate 9 (48.9 μL, 0.25 mmol) in this order at room temperature. After being stirred at the same temperature, the reaction mixture was then quenched with a saturated aqueous NH₄Cl solution and extracted with ethyl acetate. The combined organic extracts were dried over Na₂SO₄, filtered, and concentrated. The residue was purified by flash column chromatography on silica gel (hexane/ethyl acetate = 8:1 to 4:1) to give the corresponding Mannich adduct 10. Ethyl (2S,3R)-3-Formyl-2-(p-methoxyphenylamino)heptanoate (10a) Following the typical procedure, using hexanal (91.6 μL, 0.75 mmol) for 4 h gave the crude product. Purification by flash column chromatography gave the title compound (67 mg, 0.22 mmol, 87% yield, dr = 12:1, 91% ee). ¹H NMR (500 MHz, CDCl₃): δ = 9.65 (d, J = 2.5 Hz, 1 H), 6.80–6.76 (m, 2 H), 6.67–6.63 (m, 2 H), 4.26 (d, J = 6.0 Hz, 1 H), 4.18 (qd, J = 7.1, 3.0 Hz, 2 H), 3.74 (s, 3 H), 2.78–2.71 (m, 1 H), 1.79–1.68 (m, 1 H), 1.67–1.50 (m, 2 H), 1.44–1.28 (m, 3 H), 1.22 (t, J = 7.3 Hz, 3 H), 0.89 (t, J = 7.0 Hz, 3 H). HPLC analysis: Daicel CHIRALPAK IC-3, hexane/i-PrOH (19:1), 1.0 mL/min, λ = 254 nm, t _major = 28.5 min, t _minor = 39.0 min.