Synlett 2011(3): 323-326  
DOI: 10.1055/s-0030-1259318
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Chiral Brønsted Acids and Their Calcium Salts in Catalytic Asymmetric Mannich Reactions of Cyclic 1,3-Diketones

Magnus Rueping*, Teerawut Bootwicha, Erli Sugiono
Institute of Organic Chemistry, RWTH-Aachen University, Landoltweg 1, 52074 Aachen, Germany
Fax: +49(241)8092665; e-Mail: Magnus.Rueping@rwth-aachen.de;
Further Information

Publication History

Received 9 July 2010
Publication Date:
13 January 2011 (online)

Abstract

Asymmetric calcium-catalyzed direct Mannich reactions of 1,3-dicarbonyl compounds with imines have been developed. The reactions proceed under mild conditions and provide the corresponding products with high enantiomeric excess.

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The absolute configuration of the products was obtained from X-ray crystal structure analysis of compound 6c. On the basis of this structure the absolute configuration was assigned to be the S configuration.

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General Procedure for Direct Mannich Reaction: In a typical experiment pyrone 2 or 1,3-cyclohexadione (5; 1.0 equiv), aldimine 3 (1.5 equiv), and the calcium catalyst (5 mol%) were suspended in Bu2O in a screw-capped vial and the resulting mixture was allowed to stir at -40 ˚C for 60-72 h. Purification of the crude product by column chromatography on silica gel afforded the corresponding product 4 or 6.
( R )- tert -Butyl(4-hydroxy-6-methyl-2-oxo-2 H -pyran-3-yl)( p -tolyl)methylcarbamate (4a): Isolated by column chromatography (CH2Cl2-acetone, 10:1) as a white solid in 49% yield and 88% ee. The ee was determined by HPLC using Chiralpak AD-H column [(n-hexane-i-PrOH, 90:10), flow rate: 0.6 mL/min; major enantiomer: t R = 29.59 min; minor enantiomer: t R = 22.07 min]; [α]D -6.1 (c = 1.84, CH2Cl2); mp 173-175 ˚C. ¹H NMR (400 MHz, CDCl3): δ = 7.22 (d, J = 8.0 Hz, 2 H), 7.02 (d, J = 8.0 Hz, 2 H), 6.79 (d, J = 12.0 Hz, 1 H), 6.05 (d, J = 12.0 Hz, 1 H), 5.88 (s, 1 H), 3.41 (s, 1 H), 2.23 (s, 3 H), 2.07 (s, 3 H), 1.39 (s, 9 H). ¹³C NMR (100 MHz, CDCl3): δ = 166.10, 164.94, 161.74, 157.40, 137.67, 136.82, 129.09, 126.12, 102.60, 100.84, 81.11, 28.48, 21.05, 19.85. IR (KBr): 3359, 2980, 1700, 1651, 1530, 1161, 884, 783 cm. MS (EI): m/z (%) = 345 (2) [M+], 339 (30), 251 (41), 230 (58), 200(100), 145 (64), 77 (92).
( R )- tert -Butyl(2-hydroxy-6-oxocyclohex-1-enyl)( p -tolyl)-methylcarbamate (6a): Isolated by column chromatography (CH2Cl2-acetone, 10:1) as a white solid in 47% yield and 73% ee. The ee was determined by HPLC using Chiralpak AD-H column [(n-hexane-i-PrOH, 92:8), flow rate: 0.6 mL/min; major enantiomer: t R = 20.23 min; minor enantiomer: t R = 17.98 min]; [α]D +4.43 (c = 0.75, CH2Cl2); mp 132-134 ˚C. ¹H NMR (400 MHz, CDCl3): δ = 10.60 (br s, 1 H), 7.19 (d, J = 8.0 Hz, 2 H), 7.00 (d, J = 8.0 Hz, 2 H), 6.82 (d, J = 9.9 Hz, 1 H), 5.99 (d, J = 9.9 Hz, 1 H), 2.45 (t, J = 6.0 Hz, 2 H), 2.22 (s, 5 H), 1.79-1.89 (m, 2 H), 1.35 (s, 9 H). ¹³C NMR (100 MHz, CDCl3): δ = 173.73, 157.32, 136.00, 128.80, 125.92, 116.22, 80.60, 48.78, 37.02, 29.64, 28.54, 21.15, 20.62. IR (KBr): 3455, 2922, 1710, 1490, 1165, 1025, 779 cm. MS (EI): m/z (%) = 331 (56) [M+], 266 (58), 239 (92), 194 (100), 116 (58), 78 (52), 70 (98).