Mitsunobu macrolactonization

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Here is a standard procedure for a Mitsunobu macrolactonization.

Procedure

Mitsunobu macrolactonization.png


To a solution of the seco-acid (0.1 mmol) and PPh3 (0.4 mmol) in PhMe (70 mL) under N2 at -10 °C was added a solution of DIAD (0.4 mmol) in PhMe (30 mL) via syringe pump over 1 h. The resulting mixture was slowly allowed to warm to rt. After disappearance of the starting material by TLC, the reaction was concentrated and the crude material was purified by flash chromatography on silica gel.

Notes:

  • PhMe is the most common solvent for this reaction. Other useful ones are: PhH, THF and CH2Cl2.
  • Very dilute conditions and slow addition of the diazo compound are critical to avoid formation of the diolide (dimerization of the seco-acid).
  • Slow addition of the seco-acid to the azo/phosphine mixture can also prevent diolide formation.
  • If addition of the azo compound onto the acid occurs, other diazo compounds can be used: DEAD (1972-28-7), DBAD (212-796-9), ADDP (10465-81-3) depending on the bulkiness needed.
  • Changing the triphenylphosphine to a less bulky one (PBu3, PMe3) can accelerate slow reactions.


Mechanism

Mitsunobu macrolactonization mech.png