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

Abstract

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 N1-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 amine 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.