Superbase-Promoted Acylation of Hindered Alcohols

The commercially available nonionic superbase P(MeNCH₂CH₂)₃N (1a) is very useful for the acylation of unreactive hindered alcohols as well as acid-sensitive alcohols. The reactions proceed in high yields using an acid anhydride, and 1a can be regenerated in a single step. The relative rates for benzoylation of (±)-menthol in C₆D₆ using conventional acylation reagents and strong nonionic bases are compared. In general, acetylation with 1a is accelerated in the polar solvent CH₃CN whereas benzoylation is faster in the nonpolar solvent C₆H₆. The benzoylation intermediate RC(O)P(MeNCH₂CH₂)₃N⁺ was found to be in equilibrium with 1a, with lower temperatures favoring the intermediate. The relative stabilities of several known acylating intermediates are compared.