To extend the scope of this reaction, the carbonylation of β-butyrolactone (BL) to methylsuccinic anhydride (MeSA) was conducted (Table ). Although the catalysts developed by Coates and coworkers were able to carbonylate substituted lactones or epoxides to cyclic anhydrides, our previously optimised conditions for PL proved inefficient for BL (3% MeSA, Table , entry 1). Increasing the temperature to 110 °C improved the yield, but longer reaction times were needed to achieve complete conversion (Table , entries 2-4). The selectivity was however limited, and a maximum yield of 62% of BL was afforded after 72 h. Heating up to 130 °C decreased the selectivity further, similar to what was already observed for PL (Table , entry 5, and see Table ). Increasing the CO pressure improved the selectivity only on the shortterm (Table , entries 6-7), going from >99 to 63% after 16 and 72 h respectively. Combining high pressure and high temperature did not improve the performances of the system. In particular, contrary to what was observed with PL, a higher pressure does not compensate the selectivity loss caused by a high reaction temperature (Table , entries 8-9). This may be explained, at least in part, by a side-reaction of decarboxylation of the BL to form propylene and CO2, that were observed in the gas phase by GC when the reaction was run at temperatures higher than 130 °C (see S.I. for details). a Conditions: BL (0.5 mol‧L -1 ) and mesitylene (internal standard, 10 mol%) with [Co2(CO)8] (5 mol%) in MeCN (2 mL), heated under CO pressure. BL conversions and MeSA yields were measured by GC-MS analysis; the selectivity was calculated as the ratio between yield and conversion. Due to the uncertainty of the analysis method, conversions lower than the yields can be obtained, and in that case, the selectivity value was reported as >99%. Selectivity was not reported and noted "/" when the conversion and/or the yield were <1%. b Traces of glutaric anhydride (GA) detected (1-2%).