Rapid and efficient synthesis of aliphatic ester dendrons and dendrimers

A divergent approach to synthesize dendritic aliphatic polyester structures based on 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) is described. The key building block is the anhydride of isopropylidene-2,2-bis(methoxy)propionic acid which is synthesized in high yields through self-dehydration, utilizing N,N'-dicyclohexylcarbodiimide (DCC) as reagent. The high reactivity of the anhydride toward hydroxyl groups makes the divergent synthesis of dendrimers and dendrons viable. Dendritic growth occurs in the presence of protecting groups sensitive toward hydrogenolysis, such as benzyl esters and ethers. The acetonide-protecting group is easily removed under acidic conditions using DOWEX 50W-X2 resin in methanol. Fourth-generation dendrons and dendrimers were successfully synthesized in high yields utilizing 1.3-1.5 equiv of anhydride per hydroxyl group. Common characteristics of the esterification reaction were short reaction time, mild reaction conditions, easy monitoring by NMR analysis, and simple workup. This synthetic approach opens up the possibility to utilize orthogonal protecting groups of acetonide-protected 2,2-bis(hydroxymethyl)propionic anhydride as a novel building block.