Renewable Cyclobutane-1,3-dicarboxylic Acid (CBDA) Building Block Synthesized from Furfural via Photocyclization

A novel renewable building block with a semirigid structure, (1 alpha,2 alpha,3 beta,4 beta)-2,4-di(furan-2-yl) cyclobutane-1,3-dicarboxylic acid (CBDA-5), was synthesized from furfural. The synthesis started with a Knoevenagel condensation between furfural and malonic acid, and a subsequent decarboxylation in the same pot, which produced 2-furanacrylic acid. The corresponding ester, ethyl-2-furanacrylate, was prepared by the Fisher or Steglich esterification of 2-furanacrylic acid with ethanol. Solvent-free [2 + 2] photodimerization of crystalline ethyl 2-furanacrylate was then carried out at -20 degrees C under blacklight to give diethyl 2,4-di (furan-2-y1) cyclobutane-1,3-dic arb oxyl at e (CBDE-5). Afterward, CBDE-5 was hydrolyzed to give the corresponding dicarboxylic acid, CBDA-5. Both CBDE-5 and CBDA-5 were confirmed by NMR spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, high-resolution mass spectrometry (HRMS), and single-crystal X-ray diffraction (XRD). A preliminary study showed that CBDE-5 and CBDA-5 could be used as renewable building blocks to produce fully biobased polyesters. In this study, several "green" techniques were applied to prepare renewable building blocks, including solvent-free crystallization, solvent- and metal-free photodimerization, and the use of residential blacklight as an energy-efficient, cost-effective, and operator-friendly UV (ECO-UV) irradiation source in the stereoregular photocydoaddition.