Process for making esters of 2-acetoxyalkanoic acids using a 3,6-dialkyl-1,4-dioxane-2,5-dione or poly-(alpha-hydroxyalkanoic acid) as a starting material
2-Acetoxyalkanoic acid esters are made in a reaction of a 3,6-dialkyl-1,4-dioxane-2,5-dione or a poly(α-hydroxyalkanoic acid), an acetate ester and an alcohol or phenol in the presence of a transesterification catalyst. Unlike previous methods for making 2-acetoxyalkanoic acid esters, this process proceeds in high yield and high selectivity to the desired product.
1. A process for making a 2-acetoxyalkanoic acid ester comprising heating a mixture consisting of a 3,6-dialkyl-1,4-dioxane-2,5-dione, in which the alkyl groups at the 3 and 6 position may be unsubstituted or substituted with substituents that do not react under the conditions of the process, an excess of acetate ester having the structure
and at least 0.1 mole per mole of the 3,6-dialkyl-1,4-dioxane-2,5-dione of an alkanol or phenol having the structure R 1 —OH to a temperature of at least 150° C. under superatmospheric pressure in a reaction vessel in the presence of a transesterification catalyst while maintaining the water content in the reaction vessel to below 0.5% by weight to convert at least a portion of the 3,6-dialkyl-1,4-dioxane-2,5-dione to a 2-acetoxyalkanoic acid ester having the structure
wherein R is an unsubstituted or inertly substituted alkyl group corresponding to the alkyl groups at the 3 and 6 positions of the starting 3,6-dialkyl-1,4-dioxane-2,5-dione and R 1 in each instance is alkyl or aryl, the process being characterized by a selectivity of at least 40% to the 2-acetoxyalkanoic acid ester and a yield of 2-acetoxyalkanoic acid ester of at least 40%, based on the starting amount of the 3,6-dialkyl-1,4-dioxane-2,5-dione; and
wherein the transesterification catalyst is selected from the group consisting of an alkyl or aryl sulfonic acid compound, hydrochloric acid, sulfuric acid, phosphoric acid, an oligomer of phosphoric acid, tin chloride, tin oxide, a dialkyl tin oxide, an alkyltinalkoxide, an alkyltincarboxylate, and boron trifluoride.
2. The process of claim 1 , wherein the 3,6-dialkyl-1,4-dioxane-2,5-dione is lactide.
3. The process of claim 1 , wherein the mixture contains at least 0.8 mole per mole of the 3,6-dialkyl-1,4-dioxane-2,5-dione of an alkanol or phenol having the structure R 1 —OH.
4. The process of claim 2 , wherein the acetate ester is methyl acetate and the alkanol is methanol.
5. The process of claim 2 , wherein the acetate ester is phenyl acetate and the phenol is phenol.
6. The process of claim 2 , wherein the acetate ester is n-butyl acetate and the alkanol is n-butanol.
7. The process of claim 1 , further comprising recovering the 2-acetoxyalkanoic acid ester.
8. The process of claim 7 , wherein the 2-acetoxyalkanoic acid ester is recovered by crystallization or distillation.
9. The process of claim 1 , wherein the conversion of the starting 3,6-dialkyl-1,4-dioxane-2,5-dione is at least 90%.
10. The process of claim 1 , wherein the selectivity to the 2-acetoxyalkanoic acid ester is at least 60%.
11. The process of claim 1 wherein the transesterification catalyst is selected from the group consisting of para-toluene sulfonic acid and tin chloride.