Alkaloid N-oxide promoted asymmetric cobalt-mediated Pauson-Khand reaction

Intermolecular cobalt-mediated Pauson-Khand reactions of norbornene derivatives 1, 5, 7 and 9 with various alkynes 2a-f were carried out in the presence of chiral amine N-oxides 3. Small amine N-oxides such as (-)-nicotine NI-oxide 3a and (-)-nicotine N1,N1'-bisoxide 3b yielded the cyclopentenones 4 with low enantioselectivities ( < 10 %ee) regardless of the alkyne 2. However, sterically more demanding airline N-oxides with additional hydrogen donor and/or acceptor sites such as ( -)-quinine N-oxide (3c), ( -)-brucine N-oxide (3d), and ( +)-indolizino[3,4-b]quinoline N-oxide (3e) gave enantioselectivities up to 53 %ee for alkynes with tethered hydroxy moieties. (C) 2002 Elsevier Science B.V. All rights reserved.