Synthesis of sinapine and its unprecedented ruthenium-catalyzed [2+2] photodimerization

Sinapine was easily synthesized from commercially available starting materials in an overall yield of 32% while 0.47% were obtained by its extraction from white mustard. Irradiation of 1 in the presence of Ru(dndp) 3 Cl 2 furnished in a [2+2] cycloaddition cyclobutanoid dimer, a d-truxinic acid. The putative mechanism of this reaction was supported by DFT calculations.