The construction of the AB taxane ring system through a [4+2] reaction and Co(I)-[2+2] cyclization is presented. For the first time, CpCo(CO)2 catalyzes the ring closure of strained polyunsaturated triynic compound into eight-memberedring.
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
[GRAPHICS]A new and efficient sequence of two consecutive cyclizations, a cobalt(I)-mediated [2 + 2 + 2] cyclotrimerization and a Diels-Alder reaction, is proposed to allow the formation of the ABC core of the taxoids in 65% overall yield, starting from an acyclic polyunsaturated precursor.