Synthesis of Bis(amino acids) Containing the Styryl-cyclobutane Core by Photosensitized [2+2]-Cross-cycloaddition of Allylidene-5(4H)-oxazolones

The irradiation of 2-aryl-4-(E-3′-aryl-allylidene)-5(4H)-oxazolones 1 with blue light (456 nm) in the presence of [Ru(bpy)(3)](BF(4))(2) (bpy = 2,2′-bipyridine, 5% mol) gives the unstable cyclobutane-bis(oxazolones) 2 by [2+2]-photocycloaddition of two oxazolones 1. Each oxazolone contributes to the formation of 2 with a different C=C bond, one of them reacting through the exocyclic C=C bond, while the other does so through the styryl group. Treatment of unstable cyclobutanes 2 with NaOMe/MeOH produces the oxazolone ring opening reaction, affording stable styryl-cyclobutane bis(amino acids) 3. The reaction starts with formation of the T(1) excited state of the photosensitizer (3)[Ru*(bpy)(3)](2+), which reacts with S(0) of oxazolones 1 through energy transfer to give the oxazolone T(1) state (3)(oxa*)-1, which is the reactive species and was characterized by transient absorption spectroscopy. Measurement of the half-life of (3)(oxa*)-1 for 1a, 1b and 1d shows large values for 1a and 1b (10–12 μs), while that of 1d is shorter (726 ns). Density functional theory (DFT) modeling displays strong structural differences in the T(1) states of the three oxazolones. Moreover, study of the spin density of T(1) state (3)(oxa*)-1 provides clues to understanding the different reactivity of 4-allylidene-oxazolones described here with respect to the previously reported 4-arylidene-oxazolones.