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The irradiation of (Z)-4-arylidene-5(4H)-oxazolones 1a–1u with blue light (465 nm) in the presence of the photosensitizer [Ru(bpy)3](BF4)2 (2.5 mol%) and the Lewis acid BF3·OEt2 (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. Characterization of cyclobutanes 2 shows that the photocycloaddition takes place by the coupling of two Z-oxazolones in a head-to-head 1,2-anti way. This change in the orientation of the coupling is promoted by O- or/and N-bonding of the BF3 additive. The δ-cyclobutanes 2 undergo a ring expansion when heated in methanol in the presence of NaOMe (1/1 molar ratio) to give densely substituted pyrrolidine-2,5-dicarboxylates 3 in a regio- and stereoselective way. The mechanism of the cyclobutane-to-pyrrolidine ring expansion has been elucidated using DFT methods. EPU and SS thank the Spanish Government (Grant PID2019-106394GB-I00/AEI/10.13039/501100011033, funded by MCIN/AEI/10.13039/501100011033) and Gobierno de Aragón-FSE (Spain, research group Aminoácidos y Péptidos E19_20R) for funding. S. S. thanks Gobierno de Aragón-FSE for a PhD fellowship. L. R. F. thanks Spanish Ministerio de Ciencia e Innovación (Grant PGC2018-093451-B-I00 funded by MCIN/AEI/10.13039/501100011033/ and FEDER “Una manera de hacer Europa”), the European Union Regional Development Fund (FEDER), and the Diputación General de Aragón, Project M4, E11_20R, for funding. R. L. and E. G. B. thank the Spanish Government (Grant PID2019-110008GB-I00/AEI/10.13039/501100011033, funded by MCIN/AEI/10.13039/501100011033) and SGIker (UPV/EHU) for providing human and computational resources. |
