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)](BF4)(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 mu s), 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.