I saw the following reaction mechanism in paper Tetrahedron Lett. 1976, 17 (33), 2869–2872.:
I'm not able to understand the following parts:
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.
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?