Beckmann Fragmentation and Rearrangement. Part VI thian‐3‐one oximes. Fragmentation reactions no. 26

In 70% aqueous dioxane thian‐3‐one anti ‐oxime p ‐toluenesulfonate (tosylate) ( 8b ) undergoes concerted fragmentation to the methylidenesulfonium ion 14 , part of which recyclizes to 1,3‐thiazepin‐4‐one ( 11 ). With the syn ‐isomer 8b rearrangement to 1,4‐thiazepin‐3‐one ( 10 ) and fragmentation to 14 occur in the ratio 4:1. Analysis of the rate data in 80% ethanol shows that anti fragmentation is 142 times as fast as syn fragmentation, but only 26 times as fast as rearrangement of the ‘homomorphous’ thian‐4‐one oxime tosylate ( 18b ). A comparison of the rates of cyclohexanone oxime tosylate 20 , thian‐3‐ and ‐4‐one oxime tosylates reveals the rate retarding influence of sulfur. – The configurations assigned to the stereoisomeric thian‐3‐one oximes ( 8a ) in the literature have to be reversed in the light of present results.