Reactivity study of 4-hydroxy-2 <i>H</i> -chromene-2-thiones with symmetrical 1,4-acetylene dicarboxylates and methyl propiolate using ytterbium( <scp>iii</scp> ) triflate as catalyst: stereoselective synthesis of substituted fumarate and ( <i>Z</i>

Stereoselective hydrothiolation of symmetrical 1,4-acetylene dicarboxylates with 4-hydroxy-2<i>H</i>-chromene-2-thione and its derivatives was carried out using 10 mol% Yb(OTf)₃ in toluene at 100 °C to yield (58-83%) substituted fumarate derivatives. In addition, under the same reaction conditions, 4-hydroxy-2<i>H</i>-chromene-2-thiones react with methyl propiolate to provide Michael addition products, substituted acrylate derivatives, exclusively with <i>Z</i>-selectivity in 74-90% yields. It was observed that the sulfur atom of the 4-hydroxy-2<i>H</i>-chromene-2-thione at the C-2 position reacts with acetylene rather than either the C-3 position or the oxygen atom present on the C-4 position. Moreover, a partial interaction can be seen in the single-crystal XRD structures of the desired products 3fb, 3fc and 7da between the ester oxygen and the sulfur at the C-2 position, which may be one of the reasons for the <i>Z</i>-selectivity of the desired product. These protocols offer broad substrate scopes, high selectivity, and functional group tolerance, demonstrating excellent atom economy, good yields, and shorter reaction times.