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Synlett 2009(20): 3303-3306  
DOI: 10.1055/s-0029-1218378
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Insights into the Conversion of Propargylic Tosylates into Bromoallenes

Ya-Jun Jian, Yikang Wu*
State Key Laboratory of Bioorganic & Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. of China
e-Mail: yikangwu@mail.sioc.ac.cn;
Further Information

Publication History

Received 22 September 2009
Publication Date:
18 November 2009 (online)

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Abstract

All aspects of the title reaction, essentially the only entry into optically active 3-monosubstituted-1-bromoallenes up to now, have been probed systematically for the first time. The new results show that: (1) there is no need to purify CuBr or to exclude air or moisture, (2) addition of Cu(II) led to even higher enantiopurity, and (3) Et2O is the best solvent. Much better selectivities were obtained under the new conditions.

Key words

allenes - substitutions - bromine - alkynes - stereoselective synthesis

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Unlike CuBr, CuBr2 is almost insoluble in Et2O or THF. The fluctuating results might be caused by the heterogeneity of such reaction system.

8

Their syntheses are given in Supporting Information.

9

General Procedure for the Conversion of Propargylic Tosylates into the Corresponding Bromoallenes
To commercially available anhyd Et2O (7.0 mL) were added in turn CuBr (2.0 mmol), LiBr (1.0 mmol), and Cu(OTf)2 (0.1 mmol). The mixture was stirred at ambient temperature 15 min. To this purple solution was added a solution of the propargylic tosylate (i.e., 7 or 10, 1.0 mmol) in Et2O (3.0 mL). The mixture was then stirred at ambient temperature for 6 h before being diluted with Et2O, washed with aq sat. NH4Cl, H2O, and brine, and dried over anhyd Na2SO4. Removal of the drying agent by filtration and the solvent by rotary evaporation left the crude product oil, on which ¹H NMR was run to determine the 8/9 (or 11/12) molar ratios. Column chromatography on silica gel gave 8/9 (or 11/12, in most cases almost inseparable from each other) for calculation of the yields. After removal of the protecting groups (e.g., the TBS in 8) the ee values could be determined by chiral HPLC.