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

Abstract

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 T1 excited state of the photosensitizer 3[Ru*(bpy)3]2+, which reacts with S0 of oxazolones 1 through energy transfer to give the oxazolone T1 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 T1 states of the three oxazolones. Moreover, study of the spin density of T1 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.

Orthopalladated imidazolones and thiazolones: synthesis, photophysical properties and photochemical reactivity

“…Given the interesting photochemical reactivity previously observed in 4-arylidene-5(4 H )-oxazolones to yield bis-amino acids of diaminotruxillic and diaminotruxinic types, 16–19 and considering the continued interest in this type of cyclobutanes due to their exceptional pharmacological properties in the treatment of type 2 diabetes, 63–66 we have attempted to extend this reactivity to free imidazolone substrates ( 2 ). However, irradiation of ( 2a )–( 2e ) in a variety of solvents with blue light (465 nm) does not lead to any detectable transformation (Fig.…”

“…15 Additionally, from a photochemical standpoint, they undergo 2 + 2 photocycloadditions and other light-induced processes, resulting in diaminotruxillic and diaminotruxinic-type cyclobutanes that are highly interesting due to their pharmacological properties. 16–19…”