Mitsunobu reaction fluorination

Fluoroalkyl Glycosides (RFn-(CH2)2-n-0-sugar) and Perfluor-oalkylidene Acetals Derived from Sugars The very low nucleophilicity of fluoroalcohols makes it difficult to substitute of a hydroxyl (anomeric or not). ° This is the reason why this type of ether is not very common. Such ethers have only been isolated in very small quantities in solvolysis reactions, or in carben insertions, performed in fluorous alcohols.Preparation of these ethers has been solved by means of the Mitsunobu reaction. This reaction is known to be dependent on the pA a of the acceptor of the glycosyl the acidity of fluorous alcohols allows a much easier deprotonation than with non fluorinated alcohols." ... 

In the synthesis of 3-fluoroamino acid 256 (Scheme 9.55), fluorination of alcohol 251 (R2 = H) with morpho-DAST afforded the desired alcohol only in low yield (10-25%). The same reaction of trimethylsilyl ether 251 (R2 = TMS) improved the yield (50%). The Mitsunobu amination of the secondary hydroxyl group in 254 successfully gives 255 in 89% yield. Starting from 250, several fluorinated a- and P-amino acids 257-259 have been prepared [31]. 

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