Mitsunobu reaction leaving group formation

Among them, the first compound prepared and studied was 3,5-anhydro-1,2-0-isopropylidene-a-D-xylofuranose (87)347 349 and its 1,2-O-cyclohexy-lidene analogue.350 Formation of the oxetane ring is based on the internal substitution of an conventional leaving group X at C-5, or surprisingly, also phthalimido group in 86 351 for application of the Mitsunobu reaction, see Ref. 349. 

Figure 6.32 Successes and failure of carbonate protection in an attempt to make aryl P-mannobiosides. The formation of the cyclic orthocarbonate must be entirely due to the complex phosphine oxide leaving group in the Mitsunobu reaction, since 4,6-di-O-acetyl-a-D-mannopyranosyl bromide 2,3-carbonate gave the expected aryl P-mannosides on reaction -with phenoxides. ...

Since all the methods used for substituting a hydroxy function with a nucleophilic reagent transformed the alcohol into a good leaving group, attempts at substitution reactions led to formation of the trienone (47). For example, under Mitsunobu reaction conditions [113] the only product identified was the trienone (47). 

Another interesting example of an inversion reaction is the Mitsunobu reaction, which displaces a hydroxyl with a carboxylate group. Reaction of a chiral alcohol with triphenylphosphine and diethyl azodicarboxylate (PhsP, DEAD) in the presence of a carboxylic acid results in the formation of a good leaving group followed by the Sn2 replacement of that group by the carboxylate nucleophile to give an ester product with an inverted stereochemistry. 

---