# Ring expansion of two fused rings to a larger ring

I saw the following reaction mechanism in paper Tetrahedron Lett. 1976, 17 (33), 2869–2872.:

I'm not able to understand the following parts:

1. How did the conversion of 27 to 29 take place? I've never seen that kind of a ring expansion, and a detailed explanation would help - maybe including the necessary transition state(s) and why it happens.

2. 27 to 28 and 30 to 31 aren't clear as well; why should such rearrangements happen and how do they happen? What is the mechanism?

• A key thing to remember is that 4-membered rings are highly strained and not very stable. – Waylander Mar 21 '18 at 14:51
• Alright, I was aware of that. What's next? – arya_stark Mar 21 '18 at 14:52
• @schrodinger_16 The bridge C-C bond of 27 has just shifted to the right and created that big ring. It is interesting to note though, that there are two ways for it to shift: one creates this 8-member ring, the other creates two five member rings; I'm curious how we can even predict theoretically which combination will have lesser strain. – Gaurang Tandon Mar 21 '18 at 15:10

I think the possible mechanism for the intermidiate conversions can be as following.Here thr $\ce{R}$ is actually $\ce{-OCHO}$.