We were stuck for awhile on this solution.  We wanted to reuse the scaffolding idea that served so well in the simpler puzzle, but this reagent does not nest neatly into itself the way the other one does.  

Our breakthrough was realizing that we only needed a single atom to do the scaffolding job, and that atom could be something we detach and reattach from the product.  After that, we found this solution method fairly quickly.  A little classic OM optimization saved us 400 cycles from the original draft.