Abstract
The reaction between carboxylic acids (RCOOH) and dialkyl dicarbonates [(R1 OCO)2 O], in the presence of a weak Lewis acid such as magnesium chloride and the corresponding
alcohol (R1 OH) as the solvent, leads to the esters RCOOR1 in excellent yields. The mechanism involves a double addition of the acid to the
dicarbonate, affording a carboxylic anhydride [(RCO)2 O], R1 OH and carbon dioxide. The esters arise from the attack of the alcohols on the anhydrides.
Exploiting the lesser reactivity of tert -butyl alcohol in comparison with other alcohols, a clean synthesis of both carboxylic
anhydrides and esters has been set up. In the former reaction, an acid/Boc2 O molecular ratio of 2:1 leads to the anhydride in good to excellent yields, depending
on the stability of the resulting anhydride to the usual workup conditions. In the
latter reaction, stoichiometric mixtures of the acid and Boc2 O are allowed to react with a twofold excess of a primary alcohol, secondary alcohol
or phenol (R2 OH) to give the corresponding esters (RCOOR2 ). Purification of the products is particularly easy since all byproducts are volatile
or water soluble. A very easy chromatography is required only in the case of nonvolatile
alcohols. A broad variety of sensitive functional groups is tolerated on both the
acid and the alcohol, in particular a high chemoselectivity is observed. In fact,
no transesterification processes occur with the acid-sensitive acetoxy group and methyl
esters.
Key words
synthetic methods - anhydrides - esters - Lewis acids - magnesium salts
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