(TPAP, NMO) to ketone 41. The ring A diol group was next removed in a combined elimination reaction and Barton deoxygenation with 1,1'-thiocarbonyldiimidazole forming alkene 42. Finally the C15 hydroxyl group was introduced by oxidation at the allyl position with in two steps PPC and sodium acetate (to the enone) and with K-selectride to alcohol 43 which was protected as a TES ether in 44. Synthesis D ring The synthesis of the D ring (scheme 6) started from 44 with allylic bromination with copper(I) bromide and benzoyl tert-butyl peroxide to bromide 45. By adding even more bromide, another bromide 46 formed (both compounds are in chemical equilibrium) with the bromine atom in an axial position. Osmium tetroxide added two hydroxyl groups to the exocyclic double bond in diol 47 and oxetane ring-closure to 48 took place with DBU in a nucleophilic substitution. Then, acylation of the C4 hydroxyl group (acetic anhydride, DMAP, pyridine) resulted in acetate 49. In the final steps phenyllithium opened the ester group to form hydroxy carbonate 50, both TES groups were removed (HF, pyr) to triol 51 (baccatin III) and the C7 hydroxyl group was back-protected to 52. Tail synthesis The amide tail synthesis (scheme 7) was based on an asymmetric Aldol reaction. The starting compound is the commercially available Benzyloxyacetic acid 53 which was converted to the thio ester 55 (Ethanethiol) through the acid chloride 54 (thionyl chloride, pyridine). This formed the silyl enol ether 55 (n-butyllithium, trimethylsilyl chloride, Diisopropylamine) which reacted with chiral amine catalyst 58, tin triflate and nBu2(OAc)2 in a Mukaiyama aldol addition with benzaldehyde to alcohol 59 with 99% anti selectivity and 96% ee. The next step converting the alcohol group to an amine in 60 was a Mitsunobu reaction (hydrogen azide, diethyl azodicarboxylate, triphenylphosphine with azide reduction to amine by Ph3P). The amine group was benzoylated with benzoyl chloride (61) and hydrolysis removes the thioether group in 62. Tail addition In the final synthetic steps (scheme 8) the amide tail 62 was added to ABCD ring 52 in an esterification catalysed by o,o'-di(2-pyridyl) thiocarbonate (DPTC) and DMAP forming ester 63. The Bn protecting group was removed by hydrogenation using palladium hydroxide on carbon (64) and finally the TES group was removed by HF and pyridine to yield Taxol 65. See also Danishefsky Taxol total synthesis Holton Taxol total synthesis Kuwajima Taxol total synthesis Nicolaou Taxol total synthesis Paclitaxel total synthesis