Science of Synthesis

Jain, N. F.; Masse, C. E., Science of Synthesis, (2007) 20, 711.

A common approach to the synthesis of esters involves activation of the corresponding alcohols as their alkoxyphosphonium salts followed by displacement with a carboxylate anion via an SN2 mechanism to afford the desired ester. The alkoxyphosphonium salts are typically generated in situ using triphenylphosphine and diethyl azodicarboxylate. The carboxylate anion is generated by protonation of diethyl azodicarboxylate and the overall process is commonly referred to as the Mitsunobu reaction. A representative example of this procedure utilizes standard Mitsunobu conditions with 3-α-hydroxy-5-β-pregnan-20-one (‌1‌) to generate a stereochemically inverted trifluoroacetate ester ‌2‌ (‌Scheme 1‌).[‌1‌] In particular, treatment of alcohol ‌1‌ with trifluoroacetic acid in the presence of triphenylphosphine, diethyl azodicarboxylate, and sodium benzoate in anhydrous tetrahydrofuran at ambient temperature cleanly affords the inverted ester ‌2‌ as a single diastereomer in 91% yield.

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References

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• 8.Meeeee-Meee, (8888) 8/8e, 888.
• 8.Meeeee-Meee, (8888) M 88e, 888.