Asymmetric iodocyclization catalyzed by salen-CrIIICl: its synthetic application to swainsonine |
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Authors: | Kwon Hyo Young Park Chul Min Lee Sung Bae Youn Joo-Hack Kang Sung Ho |
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Institution: | Center for Molecular Design and Synthesis, Department of Chemistry, School of Molecular Science (BK21), KAIST, Daejeon 305-701, Korea. |
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Abstract: | The previously developed enantioselective iodocyclization of gamma-hydroxy-cis-alkenes required 30 mol% of (R,R)-salen-Co(II) complex as chiral catalyst and 0.75 equivalent of N-chlorosuccinimide (NCS) as activator to produce 2-substituted tetrahydrofurans with 61 to 90% ee. Due to the considerable loading amount of the Co(II) complex, another more effective catalyst was pursued by screening (R,R)-salen-transition metal complexes. When 10 mol% of the catalysts were applied with 0.5 equivalent of NCS, a higher level of stereoselectivity was attained with the corresponding Cr(III)Cl (84% ee), Mn(II)Cl (52% ee) and Co(II) complexes (66% ee). Refinement of the conditions established a novel catalytic enantioselective iodocyclization protocol using iodine in the presence of 7 mol% of (R,R)-salen-Cr(III)Cl complex activated by 0.7 equivalent of NCS in toluene to induce 74 to 93% ee. |
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