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The [4+2]‐Cycloaddition of α‐Nitrosoalkenes with Thiochalcones as a Prototype of Periselective Hetero‐Diels–Alder Reactions—Experimental and Computational Studies

2019; Wiley; Volume: 26; Issue: 1 Linguagem: Inglês

10.1002/chem.201903385

1521-3765

Grzegorz Mlostoń, Katarzyna Urbaniak, Marcin Jasiński, E.‐U. Würthwein, Heinz Heimgartner, Reinhold Zimmer, Hans‐Ulrich Reißig,

Synthesis and Reactions of Organic Compounds

Abstract The [4+2]‐cycloadditions of α‐nitrosoalkenes with thiochalcones occur with high selectivity at the thioketone moiety of the dienophile providing styryl‐substituted 4 H ‐1,5,2‐oxathiazines in moderate to good yields. Of the eight conceivable hetero‐Diels–Alder adducts only this isomer was observed, thus a prototype of a highly periselective and regioselective cycloaddition has been identified. Analysis of crude product mixtures revealed that the α‐nitrosoalkene also adds competitively to the thioketone moiety of the thiochalcone dimer affording bis‐heterocyclic [4+2]‐cycloadducts. The experiments are supported by high‐level DFT calculations that were also extended to related hetero‐Diels–Alder reactions of other nitroso compounds and thioketones. These calculations reveal that the title cycloadditions are kinetically controlled processes confirming the role of thioketones as superdienophiles. The computational study was also applied to the experimentally studied thiochalcone dimerization, and showed that the 1,2‐dithiin and 2 H ‐thiopyran isomers are in equilibrium with the monomer. Again, the DFT calculations indicate kinetic control of this process.