How am I supposed to know the double bond in structure 2 forms from the retrosynthetic analysis?
So, this is from the solutions manual to Clayden's Organic Chemistry and I'm stuck on this portion of the retrosynthetic analysis (problem 6 part c from chapter 28 problems, I can give more context if needed). They gave us the forward reaction earlier so I know that double bond in the structure labeled 2 existed, and then working backwards through the FGIs isn't too hard. But if I hadn't been given that information and I'm just looking at this retrosynthetically, why would I assume that double bond is there? How can I approach this from a purely retrosynthetic eye?
