Payne Rearrangement
Payne rearrangement is the isomerization under basic conditions of 2,3-epoxy alcohols to isomeric 1,2-epoxy alcohols with inversion of configuration. Overall, the Payne rearrangement represents a migration of the epoxide.
Under aprotic conditions, nucleophilic opening of epoxide isomers can be achieved with hydrides or organocuprates. Nucleophilic attack generally takes place at the least substituted carbon, yielding the more substituted diol product.
In the given example, inversion of three contiguous stereocenters results after two epoxide migrations, opening of the epoxide by carboxylate, and hydrolysis of the resulting lactone.
The thia-Payne rearrangement has only been observed in the forward direction (epoxide to thiiranium) with in situ opening of the thiiranium. Invertive nucleophilic opening at C-2 is possible through the use of trialkylaluminum reagents.
Under aprotic conditions, nucleophilic opening of epoxide isomers can be achieved with hydrides or organocuprates. Nucleophilic attack generally takes place at the least substituted carbon, yielding the more substituted diol product.
Stereochemistry of Payne Rearrangement
Payne rearrangement occurs with inversion of stereochemistry at C-2. Substrates containing multiple adjacent hydroxyl groups may undergo "cascade" epoxide migrations with inversion at each site of nucleophilic attack.In the given example, inversion of three contiguous stereocenters results after two epoxide migrations, opening of the epoxide by carboxylate, and hydrolysis of the resulting lactone.
Mechanism of Payne Rearrangement
The possible mechanism is given below-Aza- and Thia-Payne Rearrangements
The aza-Payne rearrangement may be effected in either the "forward" (epoxide to aziridine) or "reverse" (aziridine to epoxide) direction depending on the conditions employed. Electron-poor aziridines undergo the reverse rearrangement in the presence of hydride base, while the corresponding epoxy amines undergo the forward rearrangement in the presence of boron trifluoride etherate.The thia-Payne rearrangement has only been observed in the forward direction (epoxide to thiiranium) with in situ opening of the thiiranium. Invertive nucleophilic opening at C-2 is possible through the use of trialkylaluminum reagents.
