Oxathiins, rearrangement

A Pummerer rearrangement features in two approaches to 1,3-oxathiins. Refluxing y,8-unsaturated sulfinyl compounds (26) in xylene containing TsOH effects conversion to the 3,1-benzoxathiin (27) via a sulfonium ion intermediate (95CC1197), whilst 2-(hydroxymethyl)phenyl sulfoxides (28), readily cyclise to the benzoxathiin (95T6819). 

The formation of the sultone (160) probably involves addition of the complex across the alkene double bond, a 1,2-hydride shift and an intramolecular nucleophilic substitution reaction. The sultone (161) is formed by addition of sulfur trioxide to give the unstable p-sultone which rearranges to the more stable y-isomer (161). Another useful route to sultones is by metallation of alkanesulfonate esters for example, butane-1,3-dimethylsulfonate (162), prepared from butanel,3-diol, yields the 8-sultone, namely 6-methyl-l,2-oxathiin-2,2-dioxide (163) (Scheme 67). 

Dihydro-2-methyl-l,4-oxathiin-3-carboxanilide (92), known as Carboxin (a fungicide), rearranges to the enamine (93) when refluxed in toluene with / -toluenesulfonic acid. The structural assignment of (93) are made on the basis of crystal x-ray diffraction analysis <84MI 609-02>. 

A synthesis of annulated 1,4-oxathiin-2-ones involves an anionic rearrangement under directed metalation conditions as the key step. A,A-Diethyl o-methylsulfanylaryl carbamates, readily available from phenols, are deprotonated at the SMe group which initiates rearrangement to a thioacetamide from which the oxathiinone can be obtained (Scheme 31) <0415215>. 

Arylimino-l,3-oxathiins (380), obtained from benzoylthioacetanilides (379) and phosgene, undergo slow acid-catalysed rearrangement to the corresponding thioxo-l,3-oxazines (381) (Scheme 147). ... 

---