Thermal decomposition of a sulfoximide in the presence of a carboxylic acid; an interesting rearrangement

5-Hexyl-7-(S-methylsulfonimidoyl)xanthone-2-carboxylic acid 1a , when heated above its melting point gives a number of products including the corresponding methyl ester 1b , which arises by a novel methyl group transfer. A similar methyl transfer reaction is observed when the sulfoximide and carboxylic acid groups are present in different molecules.