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Synlett 2009(14): 2261-2264  
DOI: 10.1055/s-0029-1217808
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Hybrid Diamines Derived from 1,1′-Binaphthyl-2,2′-diamine and α-Amino Acids as Organocatalysts for 1,3-Dipolar Cycloaddition of Aromatic Nitrones to (E)-Crotonaldehyde

Łukasz Weselińskia, Paweł Stępniaka, Janusz Jurczak*a,b
a Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
b Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Fax: +48(22)6326681; e-Mail: jurczak@icho.edu.pl;
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Publication History

Received 4 May 2009
Publication Date:
07 August 2009 (online)

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Abstract

Homochiral derivatives of 1,1′-binaphthyl-2,2′-diamine and various α-amino acids were prepared using a convenient procedure. They were tested as organocatalysts for 1,3-dipolar cyclo­addition of aromatic nitrones to (E)-crotonaldehyde. The l-phenylalanine-based catalyst 10 afforded superior results, with good endo diastereoselectivity, and enantioselectivity of up to 95% ee.

Key words

α-amino acids - asymmetric organocatalysis - binaph­thyl derivatives - 1,3-dipolar cycloaddition - nitrones

10

The reaction was performed using 0 equiv, 3 equiv, 6 equiv and 12 equiv of H2O and no significant difference in yield and diastereoselectivity was observed in all cases, although the enantioselectivity in the absence of H2O was slightly lower; therefore 3 equiv of H2O were always added to the reaction mixture.

12

General Procedure for 1,3-Dipolar Cycloaddition: A solution of the nitrone (0.25 mmol) in nitromethane (1 mL) was placed in a vial, and H2O (3 equiv), the catalyst and the acid co-catalyst were added and the resulting mixture was cooled to 4 ˚C. Then the freshly distilled α,β-unsaturated aldehyde was added (4 equiv, followed by 3 equiv at 24 h intervals) and the mixture was left for a specific period of time. Then it was filtered through a silica gel plug with aid of CH2Cl2, and then concentrated. After purification by flash chromatography (hexane-EtOAc), the residual oils were reduced to the corresponding alcohols with NaBH4 in MeOH. The ee values of purified alcohols were determined by chiral HPLC using CHIRALCEL OD or OD-H columns (95-98% hexane-isopropanol, flow: 1 mL/min).

13

All new compounds obtained here had correct analytical and spectral data.