Taxol oxetane ring formation

During the first total synthesis of taxol , R. Holton and co-workers installed an exo-methylene group on the C ring in order to set the stage for the D ring (oxetane) formation. The Burgess dehydration reaction was applied to a complex tricyclic tertiary alcohol intermediate (ABC rings) and the desired exocyclic alkene was isolated in 63% yield. 

Standard methods, and this was protected as the cyclic 1,2-carbonate and oxidized to 12.21. Removal of the cinnamoyl side chain was effected by osmylation followed by mild base to give the triol 12.22 and, in a nice touch, the diol 12.23 the latter compound was obtained in optically pure form and could be converted to the taxol side chain by known chemistry. Formation of the oxetane 12.24 from 12.22 followed the same pathway used by Scheeren and his coworkers 406), and ring-opening of the carbonate followed by reduction at C-13 gave 7-deoxybaccatin III (12.25). The synthesis was completed by attachment of the side chain to give 7-deoxytaxol (12.26). 

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