(a) In the first stage of step (i) a reaction between DMSO and oxalyl chloride takes place to generate the active intermediate 12. Provide a curly arrow mechanism to show how the reaction of 12 with alcohol 8 gives aldehyde 9.
(Hint: remember triethylamine is also present in the reaction mixture.)
(b) Demonstrate your understanding of retrosynthetic analysis by redrawing step (ii) in the retrosynthetic direction. Show the disconnection, retrosynthetic arrow, and synthons corresponding to the starting materials.
(c) Propose a mechanism for step (ii). Note that n-BuLi is a strong base and assume the alkoxide intermediate is protonated upon workup.
(d) Compare the reactivity of aldehyde 9 with that of the dithiane 10 (a masked aldehyde) and use this to explain the concept of umpolung.
