Taxane diterpene synthesis studies. Part 2: Towards taxinine - enantiospecific construction of an AB-ring substructure incorporating both quaternary carbon centres and attempts to annulate the C-ring

In connection with efforts to develop an efficient total synthesis of the biologically active natural product taxinine 1, the enzymatically-derived and monochiral cis-1,2-dihydrocatechol 7 was converted, over several steps including a Diels-Alder cycloaddition reaction, into the bicyclo[2.2.2]octan-2-one 18. Reaction of the last compound with the organocerium reagent 22 afforded the 1,5-diene 23 which engaged in an anionic oxy-Cope rearrangement reaction to give, after C-methylation of the product enolate 25, bicyclo[5.3.1]undecenone 27 embodying the AB-ring system of target 1. Two methods for allylic oxidation of such products were developed and several unsuccessful attempts to effect a cyclization reaction so as to establish the taxane C-ring are described.