text: The next part constructed the C ring starting from aldehyde 26, which was extended by one carbon atom to homologue 27 in a Wittig reaction with methoxymethylenetriphenylphosphine (Scheme 3). The acetonide group was removed by dilute hydrochloric acid and sodium iodide in dioxane and one hydroxyl group in the resulting diol 28 was protected as the triethylsilyl ether (TES) 29 with the corresponding silyl chloride and pyridine enabling oxidation of the remaining hydroxyl group to the ketone 30 with the Dess-Martin periodinane. Reaction with Eschenmoser's salt placed a methylene group (C20 in the Taxol framework) in the alpha position of the aldehyde to 31 and the next reaction introduced (the still lacking) C6 and C7 as the Grignard reagent  of allyl bromide in a nucleophilic addition aided by zinc(II) chloride, which blocked the Grignard from attack on carbonate group, to alcohol 32. The newly formed alcohol was protected as the BOM ether 33 with BOMCl and N,N-diisopropylethylamine. After removal of the TES protecting group with ammonium fluoride, the carbonate group in 34 was converted to a hydroxybenzoate group by action of phenyllithium  and the secondary alcohol to the acetate 35 by in situ reaction with acetic anhydride and DMAP. In the next step the acyloin group had its positions swapped by reaction with triazabicyclodecene (other amine bases fail) forming 36 and in the final steps ring closure of ring C was accomplished by ozonolysis at the allyl group to 37 and  Aldol reaction with 4-pyrrolidinopyridine to 38.