Fluorination Studies on Glycyrrhetic Acid Derivatives

Attempts to introduce a fluorine atom directly at the C‐3 position of the triterpenoidal glycyrrhetic acid 1 by reacting the tosylate 2 with KF, or methyl glycyrrhetate 5 with Et₂NCF₂CHCIF, failed. The ring A olefin 3 and the rearrangement product 4 were obtained. The homoannular diene 7 was treated with NOF, but only nitroso‐fluorine compounds 8, 9 were isolated. Nitrosyl fluoride was also reacted with olefin 3. A difluoro compound 10 and the expected 2‐keto‐3α‐fluoro compound 11 were obtained. The above two olefins were also reacted with bromine fluoride. Although the diene 7 gave only fluorine‐free compounds 13, 14, the expected fluoro‐bromine compound 16 was obtained from 3. When the 2β‐fluoro‐3α‐bromo compound was treated with Bu₃SnH, the bromine atom was replaced by hydrogen and the 2β‐fluoro compound 17 was formed.