Building block is reported by Huw Davies and coworkers as a precursor to the 2013 EROS Best Reagent; Methyl (E)-4-Phenyl-2-diazo-3-butenoate. Methyl (E)-4-Phenyl-2-diazo-3-butenoate is a donor-acceptor precursor to the corresponding Ru-carbenoid that is utilized in C-H insertion and cyclopropanation reactions.