From an acyclic, polyunsaturated precursor to the polycyclic taxane ring system: The [4+2]/[2+2+2] and [2+2+2]/[4+2] cyclization strategies

A combination of a cobalt(I)-mediated cyclotrimerization and a [4+2] reaction is described as a new entry into the ABC core of taxanes. The [4+2]/[2+2+2] and the [2+2+2]/[4+2] pathways both lead to the expected ABC core of the title compound. Mechanistic proposals are described to understand the formation of the side products during the [2+2+2] cycloaddition and to apply useful modifications of the highly substituted unsaturated precursors. Indeed, we demonstrate, for the first approach, that the substitution at the propargylic position has a dramatic influence on the cyclization. For the second sequence, we observe that the silicon group at the terminal position of the double bond can alter the chemoselectivity of the cycloaddition. In addition, the use of a temporary silicon tether in the [2+2+2] cycloaddition provides a rapid access to a highly elaborate molecule 38, which exhibits latent functionality for further synthetic transformations. (c) Wiley-VCH Verlag GmbH & Co.