Friedel-Crafts catalysts as assistants in the tritylation of less reactive hydroxyls

Less reactive hydroxyls, such as those present in secondary alcohols and in some primary alcohols, phenols and carboxylic acids, were easily tritylated with the homogeneous assistance of equimolar quantities of chlorides of di- and trivalent metals in aprotic solvents. The metal ions allowed both high concentration of the effective reagent triphenylmethylcarbenium ion and mobilisation of the hydroxyl proton, thus giving rise to rapid, room temperature substitution reactions. The experimental conditions were so mild that other protected groups, such as alkyl- or trityl-protected carboxyls and (9-fluorenylmethoxycarbonyl)- or trityl-protected amino groups, remained unaffected.

A new, mild, high rate and simple procedure for O-tritylation of less reactive hydroxyls in primary and secondary alcohols, phenols and carboxylic acids is described, based on the use of stoichiometric Friedel-Crafts catalysts. Results concerning the use of ZnCl2 as assistant in acetonitrile are reported.