Mitsunobu with 4-nitrobenzoic acid

The Mitsunobu reaction is usually used to introduce an ester with inversion of configuration. The use of this methodology on an anomeric hydroxyl was found to give only the /3-benzoate, whereas other methods gave mixtures of anomers. Improved yields are obtained in the Mitsunobu esterification when p-nitrobenzoic acid is used as the nucleophile/ Bis(dimethylamino) azodicarboxylate as an activating agent was... 

The C-2 hydroxyl derivative with the unnatural configuration (2-Epi-Sl or 62) was synthesized using Mitsunobu conditions of triphenylphosphine (PPI13), diisopropyl azodicarboxylate (DIAD), and 4-nitrobenzoic acid as the nucleophile, which was subsequently hydrolyzed with K2C03 in MeOH to afford 62 in 64% yield over two steps [34]. Compounds 57 and 2-epi-Sl have been exploited as common intermediates in diversification of the C-2 position. Hydroxyl 57 has been utilized... 

A larger C-ll-C-25 fragment of (-f )-milbemycin P is prepared by a somewhat similar strategy (Scheme 98) [154]. Addition of phenylthiomethyllithium to 590 gives a 1 3 mixture of diastereomers 678 and 679 that is difficult to separate. Furthermore, the desired diastereomer 678 represents the minor component of the mixture. Consequently, the mixture is transformed to epoxide 680, which is then allowed to react with aluminate 681 to provide a more easily separable mixture of alcohols. After separation, the major (undesired) alcohol is converted to the desired 682 by Mitsunobu inversion with 4-nitrobenzoic acid. 

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