The Photochemical Mediated Ring Contraction of 4H-1,2,6-Thiadiazines To Afford 1,2,5-Thiadiazol-3(2H)-one 1-Oxides

• Emmanouil Broumidis^a, Christopher G. Thomson^a, Brendan Gallagher^a, Lia Sotorríos^a, Kenneth G. McKendrick^a, Stuart A. Macgregor*^a, Martin J. Paterson^a, Janet E. Lovett^b, Gareth O. Lloyd^c, Georgina M. Rosair^a, Andreas S. Kalogirou^d,e, Panayiotis A. Koutentis*^e, and Filipe Vilela*^a,f

• ^aInstitute of Chemical Sciences, School of Engineering & Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom
• ^bSUPA School of Physics and Astronomy and BSRC, University of St Andrews, St. Andrews, KY16 9SS, United Kingdom
• ^cJoseph Banks Laboratories, School of Chemistry, University of Lincoln, Brayford Pool, Lincoln LN6 7TS, United Kingdom
• ^dDepartment of Life Sciences, School of Sciences, European University Cyprus, 6 Diogenes Str., Engomi, P.O. Box 22006, 1516 Nicosia, Cyprus
• ^eDepartment of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia Cyprus
• ^fContinuum Flow Lab, School of Engineering & Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom

1,2,6-Thiadiazines treated with visible light and 3O2 under ambient conditions are converted into difficult-to-access 1,2,5-thiadiazole 1-oxides (35 examples, yields of 39–100%). Experimental and theoretical studies reveal that 1,2,6-thiadiazines act as triplet photosensitizers that produce 1O2 and then undergo a chemoselective [3 + 2] cycloaddition to give an endoperoxide that ring contracts with selective carbon atom excision and complete atom economy. The reaction was optimized under both batch and continuous-flow conditions and is also efficient in green solvents.

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