New synthesis of carboxin and oxycarboxin pesticides: application to the preparation of their new analogues substituted at the C-2 methyl group

A new synthesis of carboxin la and its 4,4-dioxide derivative, oxycarboxin 2a, has been devised via N-bromosuccinimide-promoted oxidative rearrangement of acetoacetanilide 1,3-0xathiolane 3. The replacement of N-bromosuccinimide with molecular bromine leads to the formation, from compound 3, of a C-2 bromomethylcarboxin derivative lb. The latter is conveniently exploited to prepare a new class of carboxins and oxycarboxins (after oxidation of the sulfur) substituted at the C-2 methyl, by replacement of the bromine atom with various nucleophiles.