Asymmetric Aza-Friedel-Crafts Reaction of 2-Naphthol with Tosylimines Catalyzed by a Dinuclear Zinc Complex

Abstract

The first asymmetric aza-Friedel-Crafts reaction of 2-naphthol with tosylimines was developed via a dinuclear zinc catalyst (up to 98% ee). It provided a new method for the asymmetric synthesis of Betti base derivatives.

Key words

2-naphthol - asymmetric synthesis - Betti base - Friedel-Crafts reaction - tosylimine

References and Notes

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13

The preparative procedure of the dinuclear zinc complex, see Supporting Information for full details.

14

General Procedure for Aza-Friedel-Crafts Reaction
To a solution of ligand 2 (r.t., 192 mg, 0.3 mmol) in toluene (4 mL) in a Schlenk tube was added dropwisely a solution of diethylzinc (0.68 mL, 0.6 mmol) under a nitrogen atmo-sphere. The solution was continued to stir for 1 h to give a solution of complex 2 (0.75 M in toluene). 2-Naphthol (43 mg, 0.3 mmol) and tosylimine (0.9 mmol) were added, and the reaction was stirred for 48 h at 30 ˚C. After the reaction was completed, the mixture was quenched with sat. NH₄Cl solution (5 mL) and EtOAc (5 mL). The organic layer was separated, and the aqueous phase was extracted with EtOAc (3 × 5 mL). The combined organic layers were dried over Na₂SO₄. The solvent was removed under reduced pressure using a rotary evaporator. The residue was purified by column chromatography using CH₂Cl₂-PE-EtOAc = 10:10:1 to give the desired product.
N -[(2-Hydroxynaphthalen-1-yl)(phenyl)methyl]-4-methylbenzenesulfonamide (4a) Yield 90%; light yellow solid; [α]_D ^²0 -55 (c 1.0, CHCl₃); mp 142-144 ˚C. ^¹H NMR (400 MHz, CDCl₃): δ = 7.72 (d, J = 7.2 Hz, 1 H), 7.70 (d, J = 4.4 Hz, 1 H), 7.65-7.50 (m, 1 H), 7.40-7.37 (m, 1 H), 7.33-7.30 (m, 5 H), 7.25-7.20 (m, 3 H), 6.87-6.83 (m, 1 H), 6.64 (d, J = 8.0 Hz, 3 H), 6.48 (s, 1 H), 6.39 (d, J = 8.4 Hz, 1 H), 2.09 (s, 3 H). ^¹³C NMR (75 MHz, CDCl₃): δ = 151.1, 142.8, 139.9, 135.9, 132.3, 129.6, 128.8, 128.7, 128.3, 127.2, 127.1, 126.7, 126.5, 123.3, 121.8, 118.1, 117.3, 54.4, 21.1. ESI-HRMS: m/z calcd for C₂₄H₂₁NO₃S + Na: 426.1140; found: 426.1134. HPLC: ee 96% [Chiralcel OD-H, n-hexane-i-PrOH (95:5), flow rate: 1.0 mL/min]: t _R(minor) = 20.36 min; t _R(major) = 25.71 min.

15

The ee of compounds 4a-l were determined by HPLC using Chiralcel column, see Supporting Information for full details.

16

The molecular structure of product 4k was determined
by X-ray crystallography. CCDC 751078 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallo-graphic data centre via www.ccdc.cam.ac.uk/data_request/cif or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: [phone].

17

The preparative procedure of compound (S)-4a, see Supporting Information for full details.

18

Synthesis of Compound ( R )-5a
To dry degassed THF (2.5 mL) taken in a round-bottomed flask under nitrogen was added Na metal (69 mg, 3 mmol) following naphthalene (40 mg, 2.9 mmol). The mixture was stirred for 1 h at r.t. To this solution was added a concen-trated solution of (R)-4a (58 mg, 0.14 mmol) in dry THF (3 mL). The reaction was stirred at r.t. overnight. The mixture was quenched by addition of a small amount of H₂O care-fully, the solution dried over anhydrous MgSO₄ and filtered. The crude mass was purified by column chromatography to give (R)-5a (21.6 mg, 62%). ^¹H NMR (300 MHz, CDCl₃):
δ = 7.74-7.69 (m, 3 H), 7.49-7.35 (m, 2 H), 7.37-7.15 (m, 6 H), 6.16 (s. 1 H). HPLC: ee 94% [Chiralcel OJ, n-hexane-
i-PrOH (70:30), flow rate: 1.0 mL/min]: t _R(minor) = 22.35 min; t _R(major) = 51.87 min.