Because a number of important musk molecules possess Z-configured double bonds-profiting from well-established and recently commercially available Z-selective molybdenum and ruthenium catalysts-we decided to test whether the above high-temperature polymerisationdepolymerisation process can be conducted in stereoselective fashion allowing the synthesis of musk macrocycles as valuable Z-isomers (Scheme 1A). As a model transformation in our study we selected mRCM of diene 1 leading to 16-membered lactone 2-a macrocyclic musk that was previously obtained under classical high-dilution conditions. Initial tests showed that diene 1 placed under vacuum in a distillation apparatus and heated to 110 °C in presence of 2.6 mol% of non Z-selective molybdenum catalyst Mo1 30 in PAO-6 (a type of synthetic paraffin oil used in car engines) used as a diluent, produced a distillate containing (Z/E)-2 (92% of yield) as a 18:82 mixture of Z/E-isomers (Scheme 1A). Encouraged by this result, we decided to test the Z-selective molybdenum monoaryloxide pyrrolide (MAP) catalyst Mo2, in hope of obtaining selectively (Z)-2 pg. 2 of 19 using the same reactive distillation method. Using the previously elaborated conditions (PAO-6, vacuum, 110 °C, 8 h), we were pleased to see that lactone 2 was produced in a high isolated yield of 93%. Unfortunately, despite the use of the Z-selective catalyst, the E-cycloalkene was again formed with a significant preference (Z/E = 30:70), which is rather typical to standard, not Z-selective catalysts, such as Mo1 and not to Mo2. Similarly, another commercially available Z-selective catalyst, Mo3, led in high-concentration mRCM distillation to macrocycle 2 in very satisfactory yield of 87%, albeit again with a rather disappointing Z-stereoselection (Z/E = 26:74). Given the highly unsatisfactory selectivity of commercial Z-selective molybdenum complexes, we opted to test the ruthenium family of Z-selective catalysts.