Synlett 2010(8): 1243-1246  
DOI: 10.1055/s-0029-1219782
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Iron- or Cobalt-Catalyzed Nazarov Cyclization: Asymmetric Reaction and Tandem Cyclization-Fluorination Reaction

Motoi Kawatsura*, Koji Kajita, Shuichi Hayase, Toshiyuki Itoh*
Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, Koyama, Tottori 680-8552, Japan
Fax: +81(857)315259; e-Mail: kawatsur@chem.tottori-u.ac.jp; e-Mail: titoh@chem.tottori-u.ac.jp;
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Publikationsverlauf

Received 13 February 2010
Publikationsdatum:
23. März 2010 (online)

Abstract

We succeeded in demonstrating the iron- or cobalt-catalyzed asymmetric Nazarov cyclization of divinyl ketones using iron or cobalt catalysts, which were prepared from Co(ClO4)2˙6H2O, Fe(ClO4)2˙6H2O, or Fe(OTf)2 with a chiral pybox-type ligand. Furthermore, we found that Fe(OTf)2 effectively catalyzed the tandem Nazarov cyclization-fluorination reaction of divinyl ketones.

    References and Notes

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7

Typical Procedure for the Asymmetric Nazarov Cyclization of 1a A solution of Fe(ClO4)˙6H2O (18.1 mg, 0.050 mmol) and (S,S)-tb-pybox (16.5 mg, 0.050 mmol) in anhyd CH2Cl2 (0.50 mL) was stirred at r.t. for 3 h, then 2-benzyliden-3-oxo-4,5-diphenylpent-4-enoic acid ethyl ester (1a, 38.2 mg 0.10 mmol) in anhyd CH2Cl2 (0.50 mL) and hexane (0.50 mL) was added slowly, then stirred at 60 ˚C for 15 h. The reaction mixture was quenched with H2O, and extracted with EtOAc. The combined organic layers was washed with brine, dried over MgSO4, and evaporated. The residue was chromatographed on silica gel (hexane-Et2O = 7:3) to give 26.4 mg (69%) of 2a.
Analytical Data
[α]D ²4 +289 (c 1.0, CHCl3). The enantiomeric purity was determined to be 83% ee by chiral HPLC analysis with a Daicel CHIRALPAK AD-H [hexane-2-PrOH (95:5), flow: 0.5 mL/min, 35 ˚C, t major = 23 min, t minor = 33 min]. The absolute configuration of the major enantiomer is (1R,5S) by the comparison of the reported data. ¹H NMR (400 MHz, CDCl3): δ = 1.31 (t, J = 6.8 Hz, 3 H), 3.64 (m, 1 H), 4.26 (m, 2 H), 5.02 (d, J = 2.8 Hz, 1 H), 7.10-7.34 (m, 15 H). ¹³C NMR (125 MHz, CDCl3): δ = 14.2, 50.9, 61.9, 62.5, 127.2, 127.7, 128.1, 128.2, 128.3, 128.9, 128.9, 129.6, 129.7, 131.2, 134.0, 138.7, 140.5, 168.3, 170.0, 199.4.

8

We also examined the reaction using other chiral pybox-type ligands, such as (S,S)-phe-pybox and (S,S)-inda-pybox, but enantioselectivities were 2% ee (92% yield) and 33% ee (22% yield), respectively.

9

Surprisingly, the reaction of 1a with Fe(OTf)2 in toluene did not proceed.

11

The relative stereochemistry of 3 was estimated by comparison with the product from the reaction of 1 by Cu(OTf)2, see ref. 10a.