Synthesis and structural study of benzamarones by X-ray and NMR spectroscopy

In this work only one diastereomer of benzamarone (1,2,3,4,5-pentaphenylpentan-1,5-dione) 1 was isolated from tandem condensation/addition reaction between deoxybenzoin and benzaldehyde in alkaline medium and by Michael addition of deoxybenzoin to (E)-2-phenylchalcone. Compound 1 was investigated by single crystal X-ray diffraction: C35H28O2, Mr = 480.57, triclinic system, space group P1 with unit cell parameters a = 11.489(2), b = 11.669(2), c = 11.855(2) Å, α = 104.31(4), β = 116.91(3), γ = 98.96(3)°, V = 1305.6(4) Å3, Z = 2, T = 25(2) °C, ρcalc = 1.222 gr/cm3, μ = 0.074 mm−1. This study allowed us to assign the configuration 2R,4R/2S,4S corresponding to the diastereomer racemate C (Fig. 1) due to the fact that the crystal structure exhibits two independent molecules in the unit cell. Data obtained using NMR spectroscopy [1D and 2D NMR] are agreement with those obtained by X-ray diffraction. By comparison of NMR data, the configuration corresponding to the diastereomer racemate C was assigned to other substituted benzamarones synthesized by us, which present two and three stereocenters in their structure.