In this section of the Holton Taxol synthesis (Scheme 4), the oxetane D ring was completed and ring B was functionalized with the correct substituents. Allylic alcohol 34, obtained from deprotection of silyl enol ether 33 with hydrofluoric acid, was oxidized with osmium tetroxide in pyridine to give triol 35. After protection of the primary hydroxyl group, the secondary hydroxyl group in 36 was converted to a good leaving group using p-toluenesulfonyl chloride. Subsequent deprotection of the trimethylsilyl ether 37 gave tosylate 38, which underwent cyclization to give oxetane 39 by nucleophilic displacement of the tosylate that occurred with inversion of configuration. The remaining unprotected tertiary alcohol was acylated, and the triethylsilyl group was removed to give allylic alcohol 41. The carbonate ester was cleaved by reaction with phenyllithium in tetrahydrofuran at -78 °C to give alcohol 42. The unprotected secondary alcohol was oxidized to ketone 43 using  tetrapropylammonium perruthenate (TPAP) and N-methylmorpholine N-oxide (NMO). This ketone was deprotonated with potassium tert-butoxide in tetrahydrofuran at low temperature and further oxidized by reaction with benzeneseleninic anhydride to give α-hydroxyketone 44. Further treatment of 44 with potassium tert-butoxide furnished α-hydroxyketone 45 through a  Lobry-de Bruyn-van Ekenstein Rearrangement.  Substrate 45 was subsequently acylated to give α-acetoxyketone 46.