The rearrangement of 2-benzyl-5-mesitoyl-3(2H)-isothiazolone to 2-phenyl-6-mesitoyl-3,4-dihydro-1,3-thiazin-4(2H)-one

2-Benzyl-5-mesitoyl-3(2H)-isothiazolone ( 8 ) has been prepared from 3-mesitoylpropionic acid ( 5 ). Reaction of the isothiazolone 8 with sodium ethoxide in ethanol has been found to yield an isomeric rearranged compound, which was characterized as 2-phenyl-6-mesitoyl-3,4-dihydro-1,3-thiazin-4(2H)-one ( 9 ). This unexpected rearrangement is attributed to the abstraction of a benzylic hydrogen atom from the N-benzyl group, followed by ring enlargement through cleavage of the isothiazolone S? N bond.