Synthesis of 1,2-diaminotruxinic δ-cyclobutanes by BF 3 -controlled [2 + 2]-photocycloaddition of 5(4 H )-oxazolones and stereoselective expansion of δ-cyclobutanes to give highly substituted pyrrolidine-2,5-dicarboxylates.

The irradiation of ( Z )-4-arylidene-5(4 H )-oxazolones 1a-1u with blue light (465 nm) in the presence of the photosensitizer [Ru(bpy) 3 ](BF 4 ) 2 (2.5 mol%) and the Lewis acid BF 3 ·OEt 2 (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 BF 3 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.