Stereochemical course of the reaction between thiocarbonyl compounds and oxiranes: reaction with cis- and trans-2,3-dimethyloxirane

The reactions of thiocarbonyl compounds with cis-2,3-dimethyloxirane (1a) in CH2Cl2 in the presence of BF3.Et2O or SnCl4 led to trans-4,5-dimethyl-1,3-oxathiolanes, whereas with trans-2,3-dimethyloxirane (1b) cis-4,5-dimethyl-1,3-oxathiolanes were formed. With the stronger Lewis acid SnCl4 , the formation of side-products was also observed. In the case of 1,3-thiazole-5(4H)-thione 2, these side-products are the corresponding 1,3- thiazol-5(4H)-one 5 and the 1:2 adduct 8 (Schemes 2-4). Their formation can be rationalized by the decomposition of the initially formed spirocyclic 1,3-oxathiolane and by a second addition onto the C=N bond of the 1 : 1 adduct, respectively. The secondary epimerization by inversion of the configuration of the spiro-C-atom (Schemes 5 - 7) can be explained by a Lewis-acid-catalyzed ring opening of the 1,3-oxathiolane ring and subsequent ring closure to the thermodynamically more stable isomer (Scheme 12). In the case of 2,2,4,4-tetramethyl-3-thioxocyclobutanone (20), apart from the expected spirocyclic 1,3-oxathiolanes 21 and 23, dispirocyclic 1 : 2 adducts were formed by a secondary addition onto the C=O group of the four-membered ring (Schemes 9 and 10)

Computation and Experiment Reveal That the Ring-Rearrangement Metathesis of Himbert Cycloadducts Can Be Subject to Kinetic or Thermodynamic Control

Unusual observations in the ring-rearrangement metathesis (RRM) of Himbert arene/allene cycloadducts to form fused polycylic lactams led to a more in-depth experimental study that yielded conflicting results. Differences in reactivity within related systems and unexpected changes in diastereoselectivity among other similar substrates were not readily explained on the basis of the experimental results. Computational investigations demonstrated substrate-dependent changes in reaction pathways (ring-opening metathesis/ring-closing metathesis [ROM/RCM] cascade vs. ring-closing metathesis/ring-opening metathesis [RCM/ROM] cascade). Furthermore, some reactions were judged to be under thermodynamic control, and others under kinetic control. The greater understanding of the most likely reaction pathways and their energetics provided a reasonable explanation for the previously irreconcilable results