RESUMO
[reaction: see text] The Pauson-Khand reaction of cyclopropene with a variety of terminal alkynes has been studied. The best reaction conditions involve NMO activation in CH(2)Cl(2) at -35 degrees C. In this way, 3-substituted-bicyclo[3.1.0]hex-3-en-2-ones have been obtained in good to excellent yields. As a synthetic application, several types of substituted cyclopentenones have been prepared from these cycloadducts by protocols involving conjugate addition and reductive ring opening.
RESUMO
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.
