Asymmetric iodocyclization catalyzed by salen-CrIIICl: its synthetic application to swainsonine.
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.
Kwon
Hyo Young
HY
Center for Molecular Design and Synthesis, Department of Chemistry, School of Molecular Science (BK21), KAIST, Daejeon 305-701, Korea.
Park
Chul Min
CM
Lee
Sung Bae
SB
Youn
Joo-Hack
JH
Kang
Sung Ho
SH
D002384:Catalysis; D002712:Chlorides / Q000737:chemistry*; D002857:Chromium / Q000737:chemistry*; D003500:Cyclization; D005029:Ethylenediamines / Q000737:chemistry*; D007455:Iodine / Q000737:chemistry*; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D013237:Stereoisomerism; D017026:Swainsonine / Q000138:chemical synthesis* / Q000737:chemistry
D002712:Chlorides; D005029:Ethylenediamines; D009942:Organometallic Compounds; D002857:Chromium; C011452:disalicylaldehyde ethylenediamine; D007455:Iodine; D017026:Swainsonine