Acylsulfonium ylide

The mechanism of the Pummerer reaction can essentially be broken down into four steps (equations 21-24). "" The initial step involves acylation of the sulfoxide oxygen to form an acyloxysulfonium salt (83). In the following step, removal of a proton from the ot-carbon of (83) produces an acylsulfonium ylide (84). This ylide subsequently undergoes concurrent electron reorganization and S—O bond cleavage to give the ion pair (85). In the final step, condensation of acetate ion with the sulfur-stabilized car-bocation gives the a-acyloxy sulflde product (86). 

The mechanism of the Pummerer rearrangement consists of four steps 1) acylation of the sulfoxide oxygen to form an acyloxysulfonium salt 2) loss of a proton from the a-carbon to afford an acylsulfonium ylide 3) cleavage of the sulfur-oxygen bond to give sulfur-substituted carbocation (RDS) and 4) capture of the nucleophile by the carbocation. 

Nitrosocyclohexene, derived by dehydrochlorination of a-chlorocyclo-hexanone oxime, suffers nucleophilic attack by acylsulfonium ylides to give ultimately 3-acyl-3,3a,4,5,6,7-hexahydroanthranils (190) in practicable yields (Eq. II)272... 

---