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

A novel
renewable building block with a semirigid structure, (1α,2α,3β,4β)-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 °C under blacklight
to give diethyl 2,4-di­(furan-2-yl)­cyclobutane-1,3-dicarboxylate (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 photocycloaddition.