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):
(c) Upon treatment with 4, anion 3 is converted into compound 5, and byproduct 6 is formed. Describe the factors that render 6 a particularly stable anion.