Continuous Flow Synthesis of 2H-Thiopyrans

  • Author: European Journal of Organic Chemistry
  • Published Date: 22 November 2020
  • Source / Publisher: European Journal of Organic Chemistry/Wiley-VCH
  • Copyright: Wiley-VCH GmbH
thumbnail image: Continuous Flow Synthesis of 2<i>H</i>-Thiopyrans

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Thioaldehydes are highly reactive species that are difficult to handle due to their strong tendency to polymerize. However, if they are reacted directly after in-situ generation, thioaldehydes can be effectively converted into 2H-thiopyrans via thia-Diels–Alder reactions. These sulfur heterocycles have applications in natural product synthesis and medicinal chemistry.


Florian Sachse and Christoph Schneider, University of Leipzig, Germany, have developed a process for the photochemical generation of thioaldehydes in a continuous flow process, which were reacted in situ with electron-rich 1,3-butadienes in thia-Diels–Alder reactions (pictured below, D = electron-donating group). The approach uses the known photochemical generation of thioaldehydes from phenacyl sulfides via a Norrish II reaction under UV light. The phenacyl sulfides were synthesized via reactions of α-bromo-acetophenone with the corresponding thiol, or via reactions of the thiol 2-mercapto-1-phenylethan-1-one with the corresponding bromides. Using these precursors and electron-rich 1,3-butadienes, a broad range of cycloadducts was obtained in good yields, short reaction times, and up to gram amounts.

 


The established process is very robust and allows for the scalable and high-yielding synthesis of 2H-thiopyrans across a broad substrate range. In addition, the process performs significantly better than the corresponding batch reaction in terms of productivity and yield.


 

 

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