Superbase-Promoted Acylation of Hindered Alcohols

1996-05-03T00:00:00Z (GMT) by Bosco A. D'S John G. Verkade
The commercially available nonionic superbase P(MeNCH<sub>2</sub>CH<sub>2</sub>)<sub>3</sub>N (<b>1a</b>) 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 <b>1a</b> can be regenerated in a single step. The relative rates for benzoylation of (±)-menthol in C<sub>6</sub>D<sub>6</sub> using conventional acylation reagents and strong nonionic bases are compared. In general, acetylation with <b>1a</b> is accelerated in the polar solvent CH<sub>3</sub>CN whereas benzoylation is faster in the nonpolar solvent C<sub>6</sub>H<sub>6</sub>. The benzoylation intermediate RC(O)P(MeNCH<sub>2</sub>CH<sub>2</sub>)<sub>3</sub>N<sup>+</sup> was found to be in equilibrium with <b>1a</b>, with lower temperatures favoring the intermediate. The relative stabilities of several known acylating intermediates are compared.