group on a carbocation generated by acid treatment of exo-Co 2 (CO) 6 -complexed
propargylic alcohols 218 as a method for the preparation of cyclic ethers
(Scheme 45). The practical importance of this method remains in the facile introduction and removal of the cobalt moiety, the extraordinary stabilization of the
propargylic carbocation, and the impact of a number of different factors on the
stereochemistry of the newly created chiral center, as well as on the variety of
synthetic transformations allowed by the presence of the triple bond [158].
Alcaide et al. reported the formation of trans-fused β-lactam oxocines 221 in
good yields in a totally chemo- and regioselective fashion using the PdCl 2 -catalyzed cyclization of α,β-allene diols 220 with allyl halides (Scheme 46) through an
8-endo-trig process by attack of the primary hydroxy group to the terminal allene
carbon, followed by a Heck-type migratory alkene insertion and halide
elimination [159].
Scheme 45 Cobalt-catalyzed heterocyclizations to form medium-sized ethers
Scheme 46 Formation of trans-fused β-lactam oxocines via Pd(II)-mediated heterocyclization
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E. Soriano and J. Marco-Contelles
propargylic alcohols 218 as a method for the preparation of cyclic ethers
(Scheme 45). The practical importance of this method remains in the facile introduction and removal of the cobalt moiety, the extraordinary stabilization of the
propargylic carbocation, and the impact of a number of different factors on the
stereochemistry of the newly created chiral center, as well as on the variety of
synthetic transformations allowed by the presence of the triple bond [158].
Alcaide et al. reported the formation of trans-fused β-lactam oxocines 221 in
good yields in a totally chemo- and regioselective fashion using the PdCl 2 -catalyzed cyclization of α,β-allene diols 220 with allyl halides (Scheme 46) through an
8-endo-trig process by attack of the primary hydroxy group to the terminal allene
carbon, followed by a Heck-type migratory alkene insertion and halide
elimination [159].
Scheme 45 Cobalt-catalyzed heterocyclizations to form medium-sized ethers
Scheme 46 Formation of trans-fused β-lactam oxocines via Pd(II)-mediated heterocyclization
354
E. Soriano and J. Marco-Contelles