Regioselectivity, intermolecular Pauson-Khand reaction

Schore introduced the first examples of intramolecular Pauson-Khand reaction (Scheme 2-13), resolving the regioselectivity issue observed in the intermolecular Pauson-Khand reaction. The intramolecular version employs a carbon tether, linking the alkene and alkyne moieties, leading to the formation of a bicyclic product. Since the debut of this reaction, it has been extensively used as the key step in natural product synthesis. ... 

The Pauson-Khand reaction is especially useful for the intramolecular cycliza-tion of 1,6-enynes to give bicyclo[3.3.0]octenones. The intermolecular reaction has problems of regioselectivity when unsymmetrical alkynes or alkenes are used, although some substrates give especially good selectivity for various reasons. 

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