Benzo[d]imidazole and Aliphatic α-Amino Acid Derived Primary Amines in Asymmetric Aldol Reactions

Pravinkumar Hansraj Mohite; Pavel Drabina; Filip Bureš

doi:10.1055/s-0033-1340598

Synlett, Inhaltsverzeichnis

Synlett 2014; 25(4): 491-494
DOI: 10.1055/s-0033-1340598

letter

Benzo[d]imidazole and Aliphatic α-Amino Acid Derived Primary Amines in Asymmetric Aldol Reactions

Pravinkumar Hansraj Mohite

Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice, 53210, Czech Republic Fax: +420(46)6037068 eMail: filip.bures@upce.cz

,

Pavel Drabina

Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice, 53210, Czech Republic Fax: +420(46)6037068 eMail: filip.bures@upce.cz

,

Filip Bureš*

Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice, 53210, Czech Republic Fax: +420(46)6037068 eMail: filip.bures@upce.cz

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Abstract

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Abstract

Starting from essential α-amino acids, four new benzo[d]imidazole and alkyl-chain-substituted primary amines were synthesized. The reaction sequence involves activation of the Boc-amino acid carboxylic acid, reaction with o-phenylenediamine, and subsequent cyclization to benzo[d]imidazole. N-Methylation and final Boc group removal afforded four new primary amines. The synthesized amines were preliminarily applied as organocatalysts in an asymmetric version of the aldol reaction between 4-nitrobenzaldehyde and acetone/cyclohexanone, achieving chemical yields of 40–64% and ee and de values up to 65 and 96%, respectively.

Key words

benzo[d]imidazole - primary amine - α-amino acid - asymmetric aldol reaction - organocatalysis

Volltext

Referenzen

References and Notes
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8 Synthesis of (S)-4; Typical Procedure: Methyl chloroformate (1.6 mL, 21.2 mmol) was added to a mixture of (S)-N-Boc-ILe (4.9 g, 21.2 mmol), triethylamine (3.0 mL, 21.2 mmol), and DMF (18 mL) at –20 °C. After 15 min stirring, o-phenylenediamine (2.3 g, 21.2 mmol) was added and the reaction was stirred at 20 °C for 4 h. The solvent was evaporated and the residue was partitioned between H₂O and EtOAc. The organic layer was washed with NaHCO₃ (5% aq.), brine, H₂O, dried (Na₂SO₄), and the solvent was evaporated to afford amino-amide 8 (5.7 g, 80%). A solution of 8 (5.7 g, 20.0 mmol) in glacial AcOH (10 mL) was heated at 65 °C for 1 h, then the solvent was evaporated and the residue was partitioned between H₂O and EtOAc. The organic layer was washed with H₂O, dried (Na₂SO₄), and the solvent was evaporated. Crystallization of the residue from Et₂O–hexane afforded benzo[d]imidazole derivative 12 (3.9 g, 76%). Derivative 12 (1.2 g, 3.8 mmol) dissolved in anhydrous THF (20 mL) was treated with LHMDS (1 M in THF, 3.8 mL, 3.8 mmol) at 0 °C for 30 min, whereupon iodomethane (0.25 mL, 4.0 mmol) was added and the reaction was stirred at 20 °C for 3 h. The reaction was diluted with H₂O and extracted with EtOAc. The organic layer was dried (Na₂SO₄), the solvents were evaporated, and the residue was purified by column chromatography (SiO₂; EtOAc–hexane, 1:1) to gave N-methyl derivative 16 (772 mg, 64%). Boc derivative 16 (730 mg, 2.3 mmol) was treated with TFA (1 mL) at 20 °C for 1 h, then Et₂O–hexane (1:1) was added to the reaction mixture until the product precipitated. The crude product was filtered and purified by column chromatography (SiO₂; EtOAc–CH₂Cl₂–MeOH, 1:1:0.2) to afford (S)-4 (180 mg, 36%) as a viscous oil. [α]_D ²⁰ –21.8 (c 1, MeOH). ¹H NMR (400 MHz, DMSO-d ₆): δ = 0.85 (d, J = 6.4 Hz, 3 H, CH₂CH ₃), 0.90 (t, J = 7.2 Hz, CHCH ₃), 1.19 and 1.75 (2 × m, 2 × 1 H, CH ₂), 1.90 (m, 1 H, CH), 3.85 (s, 3 H, NCH ₃), 4.03 (d, J = 7.6 Hz, 1 H, NH₂CH), 7.20–7.55 (m, 2 H, CH _Ar), 7.56 (d, J = 7.0 Hz, 1 H, CH _Ar), 7.62 (d, J = 7.0 Hz, 1 H, CH _Ar). ¹³C NMR (100 MHz, DMSO-d ₆): δ = 11.20, 15.78, 24.06, 29.80, 39.92, 52.30, 110.02, 118.51, 121.35, 121.62, 135.68, 141.92, 157.67. MALDI-HRMS (DHB): m/z [M+H]⁺ calcd for C₁₃H₂₀N₃ ⁺: 218.1652; found: 218.1645.
9 Balboni G, Guerrini R, Salvadori S, Bianchi C, Rizzi D, Bryant SD, Lazarus LH. J. Med. Chem. 2002; 45: 713
10 Lacoste E, Landais Y, Schenk K, Verlhac J.-B, Vincent J.-M. Tetrahedron Lett. 2004; 45: 8035
11 Aldol Reaction; General Procedure: A solution of catalyst 1–4 (0.09 mmol) in acetone or cyclohexanone (7.5 mL) was treated with TFA (3.5 μL, 0.045 mmol) at 20 °C for 5 min, whereupon 4-nitrobenzaldehyde (151 mg, 1.0 mmol) was added and the reaction was stirred for 24 h. The solvent was evaporated and the residue was purified by column chromatography (SiO₂; EtOAc–hexane, 1:1).
12 Enantiomeric excesses were determined by chiral phase HPLC. For 17: Daicel Chiralpak AS-H; n-hexane–i-PrOH, 70:30; flow rate 0.5 mL/min; λ = 254 nm, t _R = 25.32, 33.65 min. For 18: Daicel Chiralpak AD-H; n-hexane–i-PrOH, 80:20; flow rate 0.5 mL/min; λ = 254 nm; t _R = 22.69 (syn), 24.34 (syn), 26.35 (anti), 33.26 (anti) min.

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