At this point the system was still not efficient enough. The sideproducts isolated, γ-butyrolactone the saturated Nacyloxazolidinone product as well as cinnamic acid, indicated a slow oxidation step of the Breslow intermediate (Scheme 1c). Further optimization with different bases and ETM-systems were investigated. The most effective base proved to be DBU (entry 9) capable of delivering 3 in 90% yield, while the weaker organic base, triethylamine (TEA) (entry 10) failed to generate any product at all. We also noted that it was possible to lower the amount of NHC catalyst (from 5 mol% to 1 mol%), base (from 0.5 eq. to 0.2 eq.). Furthermore, the reaction seemed to be sensitive to any changes in concentration; doubling the amount of EtOAc (3.2 ml, 77% yield) give lower yields in comparison with the optimized amount (1.6 ml, 90% yield). Different ETM-systems (entry 11-12) were tested and showed that 8 together with FePc were the most suited combination. With a stoichiometric amount of 8 the product was obtained in 84% yield, less efficient in comparison with our developed aerobic method (entry 13). Reaction conditions: a cinnamaldehyde (1 eq.), 2-oxazolidinone (1.5 eq.), cat (5 mol%), base (0.5 eq.), ETM (5mol%), ETM' (3 mol%), solvent (0.16 M). As footnote a, but with cat (1 mol%), base (0.2 eq.), ETM (3 mol%), ETM' (2 mol%), solvent (0.31 M). As footnote a, but with cat (1mol%). Reactions performed by excluding the different ETMs and O2 showed that they are essential for an efficient reaction (for full optimization of reaction conditions, see ESI).