Asymmetric Pauson−Khand Reactions Using Camphor-Derived Chelating Thiols as Chiral Controllers
journal contributionposted on 24.08.2001, 00:00 by Iolanda Marchueta, Elvira Montenegro, Dmitri Panov, Marta Poch, Xavier Verdaguer, Albert Moyano, Miquel A. Pericàs, Antoni Riera
A convenient procedure for the preparation of enantiopure 10-(R-thio)-2-exo-bornanethiols from (1S)-camphor-10-thiol has been developed. The ethynyl derivatives of these thiols gave excellent diastereoselectivities (up to 98:2) in Pauson−Khand reactions with norbornene and norbornadiene through the intermediacy of a chelated dicobalt pentacarbonyl complex. Thermal reaction conditions starting from the preformed chelated complex gave better results than N-oxide-promoted runs with in situ generation of the chelated intermediate. The corresponding adducts have been elaborated through a protocol consisting of conjugate addition, samarium iodide-promoted cleavage of the chiral auxiliary, and retro-Diels−Alder reaction to afford 4-substituted 2-cyclopentenones in high enantiomeric purity.