Synlett 2006(6): 837-840  
DOI: 10.1055/s-2006-933146
LETTER
© Georg Thieme Verlag Stuttgart · New York

Synthesis of E-α,β-Unsaturated Ketones with Complete Stereoselectivity via Sequential Aldol-Type/Elimination Reactions Promoted by Samarium ­Diiodide or Chromium Dichloride

José M. Concellón*, Humberto Rodríguez-Solla, Carmen Concellón, Pamela Díaz
Dipartimento Química Orgánica e Inorgánica, Universidad de Oviedo, C/ Julián Clavería 8, 33006 Oviedo, Spain
e-Mail: jmcg@fq.uniovi.es;
Further Information

Publication History

Received 13 January 2006
Publication Date:
14 March 2006 (online)

Abstract

E-α,β-Unsaturated ketones can be obtained with complete E-selectivity by a sequential reaction of dichloroketones with a variety of aldehydes. This transformation was promoted by SmI2, SmI2 in the presence of FeCl3 or CrCl2. The best results were obtained when CrCl2 was used as the metallating agent. A mechanism based on a successive aldol-type reaction and a β-elimination is ­proposed to explain these results.

    References and Notes

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  • 22e

    To see similar chair transition states in the reaction with CrCl2, see ref. 11 and 12.

14

General Procedure for the Synthesis of Compounds 3
SmI2 (3 mmol, 6 equiv) in THF (30 mL), or CrCl2 (3 mmol, 6 equiv) in THF (13 mL), was added to a stirred solution of 1,1-dichloroketone (0.5 mmol, 1 equiv) and the correspond-ing aldehyde (0.5 mmol, 1 equiv) in THF (2 mL) under nitrogen atmosphere. The reaction was refluxed for 2 h, and then the reaction was quenched by the addition of 1.0 M aq HCl. Standard workup afforded the crude α,β-enones 3 which were purified by flash column chromatography on silica gel (hexane-EtOAc, 10:1), or alternatively filtered through a pad of Celite®. When FeCl3·6H2O (0.06 equiv) was required, this salt was added to the 1,1-dichloroketone (1 equiv) and the corresponding aldehyde (1 equiv) in THF prior to SmI2 (6 equiv) addition.

15

When the minor diastereomer was not detected E/Z ratio was assigned >98:2.