heterocycle 6 (Scheme 6) in situ, via the reaction of the corresponding azophosphine (MesN2P i Pr2) with ethyl 2-butynoate (H3CC≡CCO2Et). Addition of an excess (10 equiv.) of H2O allowed for conversion to the new product 7 within two hours (Scheme 4). Slow evaporation of an n-hexane solution yielded single crystals of 7 suitable for SXRD (Figure ). To our satisfaction, 7 is structurally analogous to the proposed intermediate I3, with one -CO2Et replaced by a -CH3 group. 7 exists as the computationally favoured E-isomer. In line with the structure proposed by DFT, 7 possesses a longer than idealised C14=C15 bond [1.362(4) Å], and a shorter than idealised N1-C14 bond [1.380(3) Å], reflective of the weakened C=C bond that allows isomerism to occur. In the solution phase, only one set of peaks was observed by multinuclear NMR spesctroscopy, supporting the presence of only one isomer in solution. 2D-NOESY NMR spectroscopy also supported the assignment of 7 as the E-isomer, with a clear cross-peak between the methyl group on the alkene and the ortho-methyl groups on the mesityl substituent.