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Hello everyone,
Trying to understand the total synthesis of bryostatin, I have some difficulties for few steps.Once they obtained the alcohol and aldehyd intermediate, they realised in situ cyclodehydration and enol ether epoxidation to yield pyran.The reaction looks very easy and obvious but I tried a lot of times but no way.I will attach a picture of the reaction wich correponds to the step 9 to 10, if someone can help me to understand the mechanism.

Thank you for your time!