A Protection Strategy for High-yield Synthesis of Dimethyl Furan-2,5-dicarboxylate from 5-Hydroxymethylfurfural Using Methanol as an Acetalizing Agent

Developing efficient catalytic processes for obtaining biobased monomers for plastics can significantly contribute to a more sustainable economy. The biomass-derived and chemical platform 5-hydroxymethylfurfural (HMF) can be converted to various key intermediates, which is often hampered by side reactions due to the reactive nature of the formyl group in HMF. Here, we present a stepwise approach involving a protecting agent to obtain dimethyl furan-2,5-dicarboxylate (MFDC), which is a monomer for polyalkylene furanoates, a new class of biobased polyesters. Methanol is used as a solvent, reactant, and protecting agent for HMF's formyl group. In the first step, the oxidative esterification of the hydroxymethyl group to methyl carboxylate in HMF-dimethylacetal catalyzed by Au/CeO₂ affords methyl 5-formylfuran-2-carboxylate dimethylacetal (MFFC-acetal) in high yields (>90 %) from concentrated methanolic solutions (~20 wt %). Without protecting agent, a mixture of methyl-5-hydroxymethylfuran-2-carboxylate (MHMFC), MFDC, and humin byproduct was obtained. The deprotection of MFFC-acetal in the second step proceeded efficiently in acetone with Amberlyst-15, affording MFFC in a >90 % yield. In the final step, oxidative esterification of MFFC in methanol (10 wt %) afforded MFDC in a 93 % yield using Au/CeO₂. The acetal protection strategy with methanol offers an efficient route toward MFDC in two oxidative esterification steps.