Crude extracts from the ginkgo tree, Ginkgo biloba, have been used for centuries to alleviate symptoms associated with asthma. There are four principal components of Ginkgo extracts, called ginkgolide A, B, D, and M. During E. J. Corey’s classic synthesis of ginkgolide B, compound 1 was converted into compound 5 (J. Am. Chem. Soc. 1988, 110, 649–651):
(b) Draw a mechanism for the conversion of 1 to 3 and explain why this step is irreversible.