Phase Transfer Catalysed Asymmetric Epoxidation of Chalcones Using Chiral Crown Ethers Derived from d-Glucose and d-Mannose

Abstract

New chiral monoaza-15-crown-5 lariat ethers synthesized from d-mannose and the glucose-based crown ethers of similar type generated significant asymmetric induction as phase transfer catalysts in the epoxidation of chalcones with tert-butyl-hydroperoxide (80-92% ee).

Key words

chiral crown ethers - asymmetric phase transfer catalysis - lariate ether - asymmetric epoxidation

References

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11

Selected data for 5: [α]_D ²⁰ +18.0 (c = 1, CHCl₃). ¹H NMR:
δ = 7.47 (d, 2 H, ArH), 7.36 (t, 3 H, ArH), 5.59 (s, 1 H, benzylidene-CH), 4.78 (s, 1 H, anomer-H), 4.24 (q, J = 10.1 Hz, 1 H, H-6), 4.06 (t, J = 9.6 Hz, 1 H, H-6), 3.66-4.00 (m, 16 H, OCH₂, H-2, H-3, H-4, H-5), 3.38 (s, 3 H, OCH₃), 3.26 (t, 2 H, CH₂I), 3.18 (t, 2 H, CH₂I). For 2a: [α]_D ²⁰ +16.0 (c = 1, CHCl₃). ¹H NMR: δ = 7.48 (d, 2 H, ArH), 7.35 (t, 3 H, ArH), 5.30 (s, 1 H, benzylidene-CH), 4.75 (s, 1 H, anomer-H), 4.24 (q, J = 10.1 Hz, 1 H, H-6), 4.11 (t, J = 9.6 Hz, 1 H, H-6), 3.55-3.98 (m, 18 H, OCH₂, H-2, H-3, H-4, H-5), 3.37 (s, 3 H, OCH₃), 2.78 (t, 6 H, CH₂N). FAB-MS: 484 [M⁺ + H], 506 [M⁺ + Na]. For 2b: [α]_D ²⁰ +15.0 (c = 1, CHCl₃). FAB-MS: 498 [M⁺ + H], 520 [M⁺ + Na]. For 2c: [α]_D ²⁰ +19.6 (c = 1, CHCl₃). ¹H NMR: δ = 7.39 (d, 2 H, ArH), 7.27 (t, 3 H, ArH), 5.52 (s, 1 H, benzylidene-CH), 4.67 (s, 1 H, anomer-H), 4.16 (q, J = 10.1 Hz, 1 H, H-6), 4.02 (t, J = 9.6 Hz, 1 H, H-6), 3.45-3.95 (m, 18 H, OCH₂, H-2, H-3, H-4, H-5), 3.31 (s, 3 H, OCH₃), 3.24 (t, 3 H, OCH₃), 2.72 (t, 4 H, CH₂N), 2.56 (t, 2 H, CH₂N), 1.69 (m, 2 H, CH₂). FAB-MS: 512 [M⁺ + H], 534 [M⁺ + Na]. For 2d: [α]_D ²⁰ +18.8 (c = 1, CHCl₃). ¹H NMR: δ = 7.71 (d, 2 H, tosyl-ArH), 7.49 (d, 2 H, tosyl-ArH), 7.38 (d, 2 H, ArH), 7.32 (t, 3 H, ArH), 5.62 (s, 1 H, benzylidene-CH), 4.74 (s, 1 H, anomer-H), 4.26 (q, J = 10.1 Hz, 1 H, H-6), 4.12 (t, J = 9.6 Hz, 1 H, H-6), 3.51-4.00 (m, 16 H, OCH₂, H-2, H-3, H-4, H-5), 3.40 (s, 3 H, OCH₃), 3.20-3.26 (m, 4 H, CH₂N), 2.44 (s, 3 H, CH₃). FAB-MS: 594 [M⁺ + H], 616 [M⁺ + Na]. For 2e: [α]_D ²⁰ +26.3 (c = 1, CHCl₃). ¹H NMR: δ = 7.47 (d, 2 H, ArH), 7.35 (t, 3 H, ArH), 5.60 (s, 1 H, benzylidene-CH), 4.78 (s, 1 H, anomer-H), 4.25 (q, J = 10.1 Hz, 1 H, H-6), 4.14 (t, J = 9.6 Hz, 1 H, H-6), 3.55-4.08 (m, 16 H, OCH₂, H-2, H-3, H-4, H-5), 3.39 (s, 3 H, OCH₃), 2.85 (t, 2 H, CH₂N), 2.76 (t, 2 H, CH₂N), 2.55 (m, 1 H, NH). FAB-MS: 440 [M⁺ + H], 462 [M⁺ + Na].

12

General Procedure for the Epoxidation of Chalcones: Chalcone (1.44 mmol) and the crown ether (0.1 mmol) were dissolved in 3 mL of toluene and 1 mL of 20% aq NaOH was added maintaining the temperature at 5 °C with ice water. Then 0.5 mL of tert-butylhydroperoxide (5.5 M decane solution, 2.88 mmol) was added and the mixture stirred at 5 °C. After completing the reaction (1-48 h), a mixture of 7 mL of toluene and 10 mL of water was added. The organic phase was dried (Na₂SO₄) and concentrated in vacuo. The crude product was purified on silica gel by preparative TLC with hexane-EtOAc (10:1) as eluent, for 7a [α]_D = -196 (c = 1, CH₂Cl₂, 20 °C) with 92% ee (lit., [α]_D -214 for the pure enantiomer); [10] mp 64-66 °C (EtOH). ¹H NMR (CDCl₃): δ = 8.02 (d, 2 H, o-COPh-H), 7.63 (t, 1 H, p-COPh-H), 7.50 (t, 2 H, m-COPh-H), 7.38-7.44 (m, 5 H, CHPh-H), 4.30 (d, J = 1.9 Hz, 1 H, COCH), 4.09 (d, J = 1.9 Hz, 1 H, PhCH). For 7c: [α]_D = -167.9 (c = 1, CH₂Cl₂, 20 °C) with 82% ee; mp 81 °C (EtOH). ¹H NMR: δ = 8.01 (d, 2 H, o-COPh-H), 7.39 (m, 5 H, CHPh-H), 6.95 (d, 2 H, m-COPh-H), 4.25 (d, J = 1.7 Hz, 1 H, COCH), 4.07 (d, J = 1.3 Hz, 1 H, PhCH), 3.87 (s, 3 H, OCH₃). For 7d: [α]_D = -156.1 (c = 1, CH₂Cl₂, 20 °C) with 80% ee; mp 121 °C (EtOH). ¹H NMR: δ = 7.96 (d, 2 H, o-COPh-H), 7.46 (d, 2 H, m-COPh-H), 7.40 (t, 3 H, m,p-CHPh-H), 7.36 (d, 2 H, o-CHPh-H), 4.23 (d, J = 1.6 Hz, 1 H, COCH), 4.07 (d, J = 1.3 Hz, 1 H, PhCH).