Looking for alternative methods to increase the stability of the stereoretentive complexes in the presence of oxygen, we turned our attention to various antioxidants, namely butylhydroxytoluene (BHT), vitamin C, and hydroquinone (HQ) (Table ). As expected, the addition of antioxidants did not influence the yield in reactions carried out in an inert gas atmosphere (Table , entries 1,3,5, and 7), however, when the reaction mixture was exposed to air, a reduction in the destructive effect of oxygen in the presence of BHT was observed. For the reaction carried out under open-air conditions without additive, substrate conversion was 50% (Table , entry 2) while the use of BHT yielded 64% of the desired product (Table , entry 4). The results obtained for two other oxidants, vitamin C and hydroquinone, were rather disappointing (Table , entries 6 and 8). We also investigated whether the positive effect of BHT is limited to dry THF or whether its HPLC-grade equivalent can be used. Unfortunately, only a 12% substrate conversion was observed for both reactions, performed without the additive and in the presence of the antioxidant (Table , entries 9 and 10). Replacing the solvent with more environmentally friendly DMC (dimethyl carbonate), while the reaction was carried out under inert conditions, led to higher yields than in THF, both at room temperature and 45 °C, unfortunately deterioration in selectivity was observed (Table 2, entries 11,12). Under open-air conditions, in contrast to the reactions carried out in THF, the product yield was not affected by the presence of the antioxidant, in both cases 46% of 4aa was obtained (Table , entries 13-14). Next, we studied the reactivity of Ru21 in the cross-metathesis reaction between various terminal alkenes 2a-c and internal alkenes 3a-d (Scheme 2) and compared the results with the benchmark stereoretentive catalysts Ru5 and Ru7. In all cases, regardless of the structure of the ether substituent, we observed very high selectivity toward the Z-isomer, with the worst result 94:6 in the synthesis of 4bb. Additionally, the conversions and yields obtained were similar for all the catalysts tested, except when (Z)-2-butene-1,4-diol (3a) was used as a substrate. In this case, regardless of the terminal olefin used, an advantage of Ru21 over the benchmark complexes Ru5 and Ru7 was observed by at least 12 percentage points (Scheme 2, results for products 4aa and 4ba). For products obtained with 50% yield or lower (Scheme 2, 4ab, 4ad, 4bb, and 4bd), an attempt was made to improve these results by increasing the temperature from 25 °C to 45 or 60 °C. Increasing the temperature by 20 °C indeed improved the yield in most cases while maintaining high selectivity of the reaction, while further heating of the reaction mixture (up to 60 °C) had a negative effect on the obtained results; for product 4bb, a decrease in yield by 14 percentage points was observed (for reactions performed at 45 and 60 °C), which was related to faster catalyst decomposition. Interestingly, the selectivity remained at the same level, regardless of temperature (Z:E = 94:6).