Synlett 2008(19): 2997-3000  
DOI: 10.1055/s-0028-1087300
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Organocatalytic Asymmetric 1,3-Dipolar Cycloaddition of Nitrones to Nitroolefins

Wei Du, Yan-Kai Liu, Lei Yue, Ying-Chun Chen*
Key Laboratory of Drug-Targeting and Drug Deliver System of Education Ministry, West China School of Pharmacy, Sichuan University, Chengdu 610041, P. R. of China
Fax: +86(28)85502609; e-Mail: ycchenhuaxi@yahoo.com.cn; e-Mail: ycchen@scu.edu.cn;
Further Information

Publication History

Received 1 August 2008
Publication Date:
23 October 2008 (online)

Abstract

The asymmetric 1,3-dipolar cycloaddition of nitrones to nitroolefins was investigated by employing novel thiourea-containing organocatalysts. This transformation exhibited excellent diastereoselectivities (generally >99:1 dr) and moderate to high enantioselectivities (up to 88% ee). A 2,3-diaminopropanol derivative with three contiguous chiral centers was efficiently prepared from one cycloaddition adduct.

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β-Nitrostyrene exhibited very sluggish reactivity in the catalytic 1,3-dipolar cycloaddition even at much higher temperature (>50 ˚C).

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Thiourea-pyrrole catalysts 1f-k were prepared in a similar procedure as 1e, see ref. 14.

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General Procedure for the Asymmetric 1,3-Dipolar Cycloaddition Reaction
Catalyst 1j (6.6 mg, 0.01 mmol, 10 mol%), nitrone 2a (20.0 mg, 0.1 mmol) and 4 Å MS (50 mg) were stirred in redistilled MTBE (0.4 mL) at 0 ˚C. Then nitroolefin 3a (14.0 mg, 0.12 mmol) in MTBE (0.1 mL) was added. After 6 d, product 4a was isolated by FC on SiO2 eluted with EtOAc-PE as an oil; 24.5 mg, 79% yield; R f  = 0.5 (PE-EtOAc, 15:1); [α]D ²0 -101.3 (c 1.00 in CH2Cl2); 82% ee, determined by HPLC analysis [Daicel chiralcel OD, n-hexane-i-PrOH (95:5), 1.0 mL/min, λ = 254 nm, t R(major) = 7.70 min, t R(minor) = 11.16 min]. ¹H NMR (400 MHz, CDCl3): δ = 7.44-7.41 (m, 2 H), 7.35-7.33 (m, 3 H), 7.21-7.17 (m,
2 H), 7.04-6.98 (m, 3 H), 5.29 (dd, J = 9.2, 7.2 Hz, 1 H), 4.80 (t, J = 7.2 Hz, 1 H), 4.74 (d, J = 9.2 Hz, 1 H), 2.08-2.03 (m, 1 H), 1.14 (d, J = 6.8 Hz, 3 H), 1.02 (d, J = 6.8 Hz, 3 H) ppm. ¹³C NMR (50 MHz, CDCl3): δ = 148.1, 133.3, 129.2, 128.8, 128.7, 128.2, 124.1, 116.4, 94.5, 84.7, 73.0, 30.7, 19.0, 17.9 ppm. ESI-HRMS: m/z calcd for C18H20N2O3 + Na: 335.1372; found: 335.1320.

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General Procedure for the Synthesis of 2,3-Diamino-propanol 5 Compound 4a (31 mg, 0.1 mmol, 99% ee) and NiCl2˙6H2O (100 mg, 0.4 mmol) were stirred in MeOH (1 mL) and THF (0.5 mL) at r.t. for 10 min. Then NaBH4 (33 mg, 0.9 mmol) was added in portions at 0 ˚C. After stirring for 5 min, EtOAc (5 mL) and H2O (5 mL) were added. After filtration, the filtrate was extracted with EtOAc (2 × 10 mL). The combined organic layers were dried over Na2SO4 and concentrated. The residue and (Boc)2O (26 mg, 0.12 mmol) were dissolved in CH2Cl2 and stirred for 2 h. The solvent was then removed in vacuo, and the residue was purified by flash chromatography on SiO2 (EtOAc-PE) to give N-Boc-diaminopropanol 5 as an oil; 35 mg, 91% yield for two steps; R f  = 0.4 (PE-EtOAc, 5:1); [α]D ²0 +18.6 (c 0.70 in CH2Cl2); 99% ee, determined by HPLC analysis [Daicel chiralcel AD, n-hexane-i-PrOH (90:10), 1.0 mL/min, λ = 254 nm, t R(minor) = 5.48 min, t R(major) = 9.32 min]. ¹H NMR (400 MHz, CDCl3): δ = 7.33-7.28 (m, 4 H), 7.23-7.21 (m, 1 H), 7.11-7.07 (m, 2 H), 6.70-6.67 (m, 1 H), 6.59-6.57 (m, 2 H), 5.46 (s, 1 H), 5.12 (d, J = 8.4 Hz, 1 H), 4.84-4.77 (m, 1 H), 4.03 (s, 1 H), 3.54 (s, 1 H), 1.86-1.84 (m, 1 H), 1.30 (s, 9 H), 1.06 (d, J = 6.8 Hz, 3 H), 1.02 (d, J = 6.4 Hz, 3 H) ppm. ¹³C NMR (50 MHz, CDCl3): δ = 155.4, 147.0, 140.6, 129.1, 128.5, 127.1, 126.9, 118.3, 114.8, 79.6, 78.8, 58.6, 56.1, 29.9, 28.2, 19.4, 18.2 ppm. ESI-HRMS: m/z calcd for C23H32N2O3 + H: 385.2491; found: 385.2453.

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CCDC-700901 (6) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.