A two-phase bromination process using tetraalkylammonium hydroxide for the practical synthesis of α-bromolactones...

A simple and efficient method for α-brominating lactones that affords α-bromolactones under mild conditions using tetraalkylammonium hydroxide (R₄N⁺OH^-) as a base was developed. Lactones are ring-opened with Br₂ and a substoichiometric amount of PBr₃, leading to good yields of the corresponding α-bromocarboxylic acids. Subsequent intramolecular cyclization over 1 h using a two-phase system (H₂O/CHCl₃) containing R₄N⁺OH^- afforded α-bromo lactones in good yields. This method can be applied at the 10 mmol scale using simple operations. α-Bromo-δ-valerolactone, which is extremely reactive and difficult to isolate, could be isolated and stored in a freezer for about one week using the developed method. Optimizing the solvent for environmentally friendly large-scale syntheses revealed that methyl ethyl ketone (MEK) was as effective. In addition, in situ-generated α-bromo-δ-valerolactone was directly converted into a sulfur-substituted functional lactone without difficulty by reacting it with a sulfur nucleophile in one pot without isolation. This new bromination system is expected to facilitate the industrial use of α-bromolactones as important intermediates.