However, Ru21 demonstrated a 10 percentage point improvement over Ru5 (Scheme 3). Subsequently, the indole derivative 2e, an analogue of the synthetic cannabinoid 5F-PB-22, a designer drug that acts as a cannabinoid agonist, was successfully converted to product 4eb with 51-64% yields and complete selectivity (>99 Z:E) for both complexes tested. In the reaction involving the Tadalafil derivative 2f (which is an analogue of an active pharmaceutical ingredient used in the treatment of erectile dysfunction and marketed as Cialis™ , by Lilly Eli & Company) and (Z)-butene-1,4-diol (3a) high selectivity (>99:1) and moderate isolated yields (45-60%) of the target product 4fa were observed. In particular, in all cases, Ru21 demonstrated an efficiency of approximately 10 percentage points higher compared to the benchmark Ru5. To further expand the scope of our catalysts, we conducted a cross-metathesis reaction between two internal alkenes: the Sildenafil analogue 2g and (Z)-1,4-bis(benzyloxy)but-2-ene (3c) at 60 °C (Scheme 4). While the catalyst Ru5 afforded a marginally higher yield of 4gc (64%) compared to Ru21 (61%), the latter complex exhibited superior selectivity for the desired reaction. Encouraged by the results obtained so far, we have drawn our attention to a high concentration ringclosing metathesis (HC-RCM) that transforms long-carbon-chained dienes such as 5 into an unsaturated musk compound, a valuable ingredient for the flavour and fragrance industry. The process is based on the reversibility of the metathesis reaction (the oligomers and cyclic compounds formed are in thermodynamic equilibrium), and the possibility of shifting this equilibrium toward the expected musk product by distilling it from the reaction mixture. As a result, macrocyclisation can be carried out at concentrations of 0.2-0.7 M, rather than, as for decades, at concentrations of 1-20 mM, which favour cyclic products over oligomers. Unfortunately, although HC-RCM can be performed effectively with nonselective complexes, its stereoselective version suffers from low yield, 31%, (Z):(E)= 95:5. Therefore, we decided to check if known Ru7 or Ru21 can provide better results in macrocyclisation of nonenyl oleate (5) under previously elaborated conditions (PAO-6 engine oil as diluent, 110 °C, 1×10 -6 mbar, 8 h) (Scheme 5). Although Ru7 provided the desired musk 6 with a higher isolated yield than Ru24 (52 versus 31%, respectively), at the same time a significant erosion of selectivity was observed (for Ru7 (Z):(E) = 58:42, for Ru24 (Z):(E) = 95:5). Unfortunately, the catalyst with modified ether moiety, Ru21, provided only traces of product 6 under the same conditions. Lowering the temperature to 100 °C and adding 10 mol% of BHT improved the result of the reaction to a 20% yield while maintaining very good selectivity, (Z):(E) = 95:5, although still worse than the result obtained in the presence of Ru24. Scheme 5. Z-Stereoretentive HC-RCM of nonenyl oleate ( ) with Ru7, Ru21, and Ru24. During our study, we verified the stability of the selected complexes over time (Figure ). The tests were conducted at room temperature in deuterated THF for 4 days. As expected, Ru7 with the SIPr ligand exhibited the highest stability, with 90% of the catalyst remaining intact after 24 hours. The catalysts with the SIMes ligand were less stable and the most fragile were Ru5 and Ru21, which decayed by almost 50-60% after 24 hours.