Functionalized 1,3‐Diaminotruxillic Acids by Pd‐Mediated C–H Activation and [2+2]‐Photocycloaddition of 5(4H)‐Oxazolones

The orthopalladation of (Z)‐4‐arylidene‐5(4H)‐oxazolones (1a–1o), with electron‐withdrawing substituents (Cl, F, CF3) 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 transoid arrangement, is prevalent. Irradiation of 2a–2n with blue light promotes the C–C coupling by [2+2]‐photocycloaddition of the arylidene C=C bonds and the formation of the dinuclear cyclobutanes (3a–3n). The oxidation of cyclobutane 3d with PhICl2 or Br2 occurs with halogenation of the Pd–C bond and release of the ortho‐halogenated dispirocyclobutane, which can be converted into the ortho‐halo‐1,3‐diaminotruxillic derivatives (5d, 6d, 7d) by oxazolone ring opening in a basic medium. The regio‐ and stereoselective synthesis of 1,3‐diaminotruxillic cyclobutanes has been achieved from 4‐arylidene‐5(4H)‐oxazolones in three steps: (i) Pd‐assisted C–H bond activation; (ii) Pd‐templated photochemical [2+2]‐cycloaddition; (iii) oxidative addition of PhICl2 or Br2 followed by reductive‐elimination by C–X coupling.