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

The irradiation of 2-aryl-4-(-3'-aryl-allylidene)-5(4)-oxazolones with blue light (456 nm) in the presence of [Ru(bpy)](BF) (bpy = 2,2'-bipyridine, 5% mol) gives the unstable cyclobutane-bis(oxazolones) by [2+2]-photocycloaddition of two oxazolones . Each oxazolone contributes to the formation of 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 with NaOMe/MeOH produces the oxazolone ring opening reaction, affording stable styryl-cyclobutane bis(amino acids) .

Synthesis of 1,2-diaminotruxinic δ-cyclobutanes by BF-controlled [2 + 2]-photocycloaddition of 5(4)-oxazolones and stereoselective expansion of δ-cyclobutanes to give highly substituted pyrrolidine-2,5-dicarboxylates.

The irradiation of ()-4-arylidene-5(4)-oxazolones 1a-1u with blue light (465 nm) in the presence of the photosensitizer [Ru(bpy)](BF) (2.5 mol%) and the Lewis acid BF·OEt (2 equiv.) in deoxygenated methanol at room temperature affords the corresponding 1,2-diaminotruxinic cyclobutane bis-amino esters 2a-2u stereoselectively as the δ-isomer.

Stereoselective, Ruthenium-Photocatalyzed Synthesis of 1,2-Diaminotruxinic Bis-amino Acids from 4-Arylidene-5(4)-oxazolones.

The irradiation of ()-2-phenyl-4-aryliden-5(4)-oxazolones in deoxygenated CHCl at 25 °C with blue light (465 nm) in the presence of [Ru(bpy)](BF) (5% mole ratio) as a triplet photocatalyst promotes the [2+2] photocycloaddition of the C═C bonds of the 4-arylidene moiety, thus allowing the completely regio- and stereoselective formation of cyclobutane-bis(oxazolone)s as single stereoisomers. Cyclobutanes have been unambiguously characterized as the μ-isomers and contain two -oxazolones coupled in an -head-to-head form. The use of continuous-flow techniques in microreactors allows the synthesis of cyclobutanes in only 60 min, compared with the 24-48 h required in batch mode.

Reactivity of ()-4-Aryliden-5(4)-thiazolones: [2 + 2]-Photocycloaddition, Ring-Opening Reactions, and Influence of the Lewis Acid BF.

The irradiation of ()-2-phenyl-4-aryliden-5(4)-thiazolones with blue light (465 nm) in CHCl solution promotes [2 + 2]-photocycloaddition of the exocyclic C═C bonds and the formation of the dispirocyclobutanes . This reaction takes place with high stereoselectivity, given that the ε-isomer (1,3 head-to-tail syn coupling) is formed in more than 90% yield in most of the cases. However, irradiation of 5(4)-thiazolones with blue light (456 nm) in dry MeOH in the presence of BF·OEt leads to the monospirocyclobutanes with full stereoselectivity, also affording the ε-isomer.