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Highly Enantio‐ and s ‐trans CC Bond Selective Catalytic Hydrogenation of Cyclic Enones: Alternative Synthesis of (−)‐Menthol
Author(s) -
Ohshima Takashi,
Tadaoka Hiroshi,
Hori Kiyoto,
Sayo Noboru,
Mashima Kazushi
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200701505
Subject(s) - chemistry , enantioselective synthesis , cationic polymerization , asymmetric hydrogenation , catalysis , menthol , moiety , enone , medicinal chemistry , double bond , stereochemistry , organic chemistry
A highly enantioselective catalytic hydrogenation of cyclic enones was achieved by using the combination of a cationic Rh I complex, ( S )‐5,5′‐bis{di(3,5‐di‐ tert ‐butyl‐4‐methoxyphenylphosphino)}‐4,4′‐bi‐1,3‐benzodioxole (DTBM‐SEGPHOS), and (CH 2 CH 2 PPh 3 Br) 2 . The presence of an s ‐cis CC bond isopropylidene moiety on the cyclic enone influenced the enantioselectivity of the hydrogenation. Thus, the hydrogenation of 3‐alkyl‐6‐isopropylidene‐2‐cyclohexen‐1‐one, which contains both s ‐cis and s ‐trans enones, proceeded in excellent enantioselectivity (up to 98 % ee ). To obtain high enantio‐ and s ‐trans selectivities, the addition of a halogen source to the cationic Rh complex was the essential step. With the key step of the s ‐trans selective asymmetric hydrogenation of piperitenone, we demonstrated a new synthetic method for optically pure (−)‐menthol via three atom‐economical hydrogenations. Moreover, we found that the complete s ‐trans and s ‐cis CC bond selective reactions were also realized by the proper choice of both the chiral ligands and halides.