[4+2] and [3+2] Annulations of α‐Chloroaldehydes and Dithio‐ esters: Synthesis of 1,4‐Oxathiin‐2(3 H )‐ones and 1,3‐Oxathioles

The [4+2] and [3+2] annulations of α‐chloroaldehydes with dithioesters are reported. In the presence of an N‐heterocyclic carbene (NHC) catalyst, the [4+2] annulation went smoothly to afford highly substituted 1,4‐oxathiin‐2(3 H )‐ones, while the [3+2] annulation was observed to give 1,3‐oxathioles in the absence of an NHC.