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

Abstract

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.

2-Benzoyl-4-phenyl-1,2,5-thiadiazol-3(2H)-one 1,1-Dioxide

“…5) [8]. More recently, we reported the photochemically mediated oxidative ring contraction of 1,2,6-thiadiazines 6 to 1,2,5-thiadiazol-3(2H)-one 1-oxides 7 under ambient conditions [9]. The reaction mechanism was revealed as a chemoselective [3 + 2] cycloaddition forming an endoperoxide, followed by ring contraction with selective carbon atom excision and complete atom economy.…”

“…Scheme 1. Ring contractions of 4H-1,2,6-thiadiazines to various 1,2,5-thiadiazole scaffolds [7][8][9].…”

“…Subsequently, we discovered that heating a solution of benzo[e][1,2,6]thiadiazino[3,4b][1,4]diazepin-10(11H)-one (4) led to the formation of 2-(4-chloro-1,2,5-thiadiazol-3-yl)quinazolin-4(3H)-one (5)[8]. More recently, we reported the photochemically mediated oxidative ring contraction of 1,2,6-thiadiazines 6 to 1,2,5-thiadiazol-3(2H)-one 1-oxides 7 under ambient conditions[9]. The reaction mechanism was revealed as a chemoselective[3 + 2] cycloaddition forming an endoperoxide, followed by ring contraction with selective carbon atom excision and complete atom economy.…”