Oxyfadichalcones A-C: Three chalcone dimers fused through a cyclobutane ring from Tibetan medicine Oxytropis falcata Bunge

Oxyfadichalcones A and B, two unprecedented chalcone dimers fused through a cyclobutane ring by head-to-tail [2+2] cycloaddition of two chalcones that had never been found previously in nature, along with oxyfadichalcone C, a new head-to-head [2+2] cyclized chalcone dimer, were simultaneously obtained from Oxytropis falcata. Structural elucidation was succeeded by spectroscopic and single-crystal synchrotron radiation analysis. Additionally, the photosynthesis of the chalcone dimers was performed and the plausible biosynthesis was discussed.