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) (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. Characterization of cyclobutanes 2 shows that the photocycloaddition takes place by the coupling of two -oxazolones in a head-to-head 1,2- way. This change in the orientation of the coupling is promoted by O- or/and N-bonding of the BF 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.