Corey–Nicolaou macrolactonization

Corey–Nicolaou macrolactonization is a named reaction of organic chemistry, for the synthesis of lactones from hydroxy acids, found in 1974. The reaction uses 2,2'-dipyridyldisulfide and triphenylphosphine as reagents and runs in polar aprotic solvent under mild conditions.
 * Corey-Nicolaou-Makrolactonisierung_ÜV2.svg

Mechanism
The hydroxy acid first reacts with the 2,2'-Dipyridyldisulfide to form the corresponding 2-pyridinethiol ester, and after a proton transfer, the alkoxide attacks the carbonyl carbon, forming a tetrahedral transition state, before resolving back to the desired lactone and 2-pyridinethione.
 * Corey-Nicolaou_Macrolactonization_Mechanism.tif