The direct [2+2]-photocycloaddition of (Z)-2-phenyl-4-aryliden-5(4H)-oxazolones 1 to give 1,3-diaminotruxillic cyclobutane-derivatives 2 in very good yields (75-100%) is reported. The reaction takes place by irradiation of CH2Cl2 solutions of 1 with the blue light (465 nm) provided by LED lamps of low power (around 1 W) for 72 h. Four isomers of the 1,3diaminotruxillic cyclobutanes 2 were obtained, all of them fully characterized by a combination of NMR spectroscopy and X-ray diffraction analysis. The reaction shows a certain selectivity, since one of the isomers (the epsilon) is obtained preferentially, and works for electron-releasing and electron-withdrawing substituents at the arylidene ring. A novel setup is presented for the in-line monitoring of the continuous flow photo-assisted synthesis of the cyclobutane derivatives 2 by NMR spectroscopy, with the microreactor dramatically reducing reaction times to only 30 minutes with clear product distribution of up to four isomers. The mechanism of this [2+2]photocycloaddition has been calculated by DFT methods, explaining all experimental findings. The reaction takes place through a stepwise formation of two new CC bonds through a transient diradical singlet intermediate. The isomeric distribution of the final products is not due to equilibration processes, but instead reflects the kinetic preference during the rate limiting CC bond formation step.

The orthopalladation of (Z)-4-arylidene-5(4H)-oxazolones (1a-1o), with electron-withdrawing substituents (Cl, F, CF 3 ) in the 4-arylidene ring has been carried out by C-H bond activation. The process is regioselective and only the ortho C-H bond of the 4-arylidene ring is activated. The orthopalladated complexes (2a-2o) have different structures (mono-, di-and trinuclear), although the dinuclear open-book scaffold, in which the C=C bonds of the arylidene group are in a face-to-face tran- [a] Scheme 4. Different reaction pathways observed in the oxidation of 3d: synthesis of functionalized halogenated cyclobutanes 5d and 6d.