Stereoselective Synthesis of Di- and Trisubstituted Aminoalkenes: Tandem Isomerization/Aza-Claisen Rearrangement/Hydride Reduction Sequence

Abstract

The base-induced aza-Claisen rearrangements of N-propynyl-N-β-methallyldimethylammonium salts in EtOH in the presence of NaBH₄ resulted in the formation of enammonium salts, followed by tandem aza-Claisen [3,3]-sigmatropic rearrangement/hydride reduction to give (E)-aminoalkenes stereoselectively in good yield.

Key words

stereoselective aminoalkene synthesis - tandem reactions - N-ylides - base-induced aza-Claisen rearrangements - reductions

References

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9

Optimized Experimental Procedure: To a solution of the quaternary ammonium salt 4 (338 mg, 0.83 mmol) and NaBH₄ (154 mg, 4.15 mmol) in anhyd EtOH (10 mL) was added a solution of NaOEt (1.25 mmol) in anhyd EtOH (2 mL) at 0 °C. After stirring for 24 h at 0 °C, the solution was acidified with 10% aq HCl and then made basic with 10% aq NaOH. EtOH was removed under reduced pressure and the solution was extracted with EtOAc (3 × 30 mL). The combined extract was dried with MgSO₄, filtered and concentrated under reduced pressure. The residue was purified by silica gel column chromatography to afford (E)-aminoalkene 5 (200 mg, 73%). 5: FTIR (neat): 3480, 2850, 1455, 1360, 1090, 1070, 1030, 900, 735, 700 cm^-1. ¹H NMR (270 MHz, CDCl₃): δ = 1.56 (s, 3 H), 1.65 (s, 3 H), 2.0-2.2 (m, 4 H), 2.17 (s, 6 H), 2.72 (s, 4 H), 4.03 (d, 2 H, J = 6.3 Hz), 4.50 (s, 2 H), 4.85 (s, 1 H), 4.94 (s, 1 H), 5.20 (t, 1 H,
J = 6.3 Hz), 5.40 (t, 1 H, J = 6.0 Hz), 7.2-7.7 (m, 5 H). ¹³C NMR (68 MHz, CDCl₃): δ = 15.8, 16.4, 26.4, 39.5, 44.6, 45.4, 64.6, 66.5, 72.0, 113.3, 120.8, 126.4, 127.5, 127.8, 128.3, 132.8, 138.5, 140.3, 145.1.

11

Spectroscopic data for (E)-aminoalkenes 7a: FTIR (neat): 3025, 2940, 2855, 2815, 2765, 1455, 1030, 970, 900, 700 cm^-1. ¹H NMR (270 MHz, CDCl₃): δ = 2.09 (s, 6 H), 2.2-2.4 (m, 2 H), 2.5-2.7 (m, 6 H), 4.7-4.9 (m, 2 H) 5.4-5.5 (m, 2 H), 7.0-7.3 (m, 5 H). ¹³C NMR (68 MHz, CDCl₃): δ = 34.3, 36.0, 37.2, 45.4, 65.4, 112.5, 125.7, 128.2, 128.4, 131.3, 142.0, 146.3.

12

Spectroscopic data for (E)-aminoalkenes 7b (Z- stereo-isomer of trisubstituted olefin for the alkylidene unit): FTIR(neat): 2970, 2810, 1740, 1600, 1495, 1450, 1260, 1030, 970, 860, 745, 700 cm^-1. ¹H NMR (270 MHz, CDCl₃): δ = 1.63 (d, 3 H, J = 6 Hz), 2.17 (s, 6 H), 2.2-2.4 (m, 2 H), 2.6-2.8 (m, 6 H), 5.3-5.5 (m, 3 H), 7.1-7.3 (m, 5 H). ¹³C NMR (68 MHz, CDCl₃): δ = 13.3, 34.4, 36.1, 38.8, 45.4, 57.8, 122.9, 125.6, 128.2, 128.5, 129.1, 130.8, 136.6, 142.1.