Abstract
The stereoselective synthesis of truxillic bis-amino esters from polyfunctional oxazolones is reported. The reaction of 4-((Z)-arylidene)-2-(E)-styryl-5(4H)-oxazolones 2 with Pd(OAc)2 resulted in ortho-palladation and the formation of a dinuclear open-book complexes 3 with carboxylate bridges, where the Pd atom is C^N bonded to the oxazolone. In 3 the two exocyclic C=C bonds of the oxazolone are in a face-to-face arrangement, which is optimal for their [2 + 2] photocycloaddition. Irradiation of dimers 3 in CH2Cl2 solution with blue light (465 nm) promoted the chemo- and stereoselective [2 + 2] photocycloaddition of the exocyclic C=C bonds and the formation of cyclobutane-containing ortho-palladated complexes 4. Treatment of 4 with CO in a MeOH/NCMe mixture promoted the methoxycarbonylation of the palladated carbon and the release of the corresponding ortho-functionalized 1,3-diaminotruxillic bis-amino esters 5 as single isomers.
Highlights
Truxillic acid derivatives (Figure 1a) are a special family of cyclobutanes that have been known since 1888 and show properties of high interest [1]
The synthesis of the oxazolones 2 was carried out following the well-known Erlenmeyer–Plöchl method (Figure 3), by reaction of N-cinnamoylglycine (1) with the corresponding benzaldehyde ArCHO in acetic anhydride as solvent and in the presence of sodium acetate [42,43,44,45,46,47,48]
The oxazolone exocyclic C=C bond has the (Z)-configuration, because the signal assigned to the C=O group appears as a doublet in the proton-coupled 13C NMR spectrum, with a 3JCH coupling constant of 5.6 Hz [50]
Summary
Truxillic acid derivatives (Figure 1a) are a special family of cyclobutanes that have been known since 1888 and show properties of high interest [1]. Signals corresponding to the activation of other C–H bonds present in oxazolones 2 (styryl, aryl) were not observed, so the reaction shows full selectivity towards the ortho-arylidene positions despite the presence of different C–H bonds that could be activated.
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