The following scheme is adapted from the total synthesis of EBC-23 by Williams and co-workers. i) n-BuLi TMSO S. S. ii) ? TMS 2 3 4 | iv) v) NABH4 Et,BOMe TSOH 12 12 7 6 5 b) Identify two types of selectivity that arise during the reduction of 5 using NABH4 in step v), and briefly state what alternative reaction products could have formed. (4 Marks) c) Suggest reagents for step iv), the conversion of the dithiane 4 to the ketone 5. (Ignore the loss of the TMS group.) (1 Mark) d) Draw a plausible structure for intermediate 3, explain the regioselectivity in step ii), and briefly state how the reaction is an example of umpolung reactivity. (4 Marks) e) Identify what reaction is occurring to generate 7 from 6 in step vi), and draw a mechanism. (4 Marks)