PI-CYCLIZATIONS OF ALPHA-METHOXYCARBONYL OXYCARBENIUM IONS - SYNTHESIS OF OXACYCLIC CARBOXYLIC ESTERS

Acid-mediated cyclization reactions are described of seven methyl 2-acetoxy-2-(3-alken-1-oxy)acetates with different chain substitution. The major product of the tin tetrachloride-induced cyclization reaction is in most cases a tetrahydropyran containing an equatorial carbomethoxy function at C2 and an axial chlorine atom at C4. The mechanism of its formation involves a net cis-addition of the intermediate alpha-ester oxycarbenium ion to the carbon-carbon double bond, most likely caused by a quasi axial orientation of the ester function in a chair-like transition state. The results are interpreted by invoking (1) the occurrence of a 2-oxonia-Cope rearrangement and (2) the participation of the ester function in the mechanism of cyclization by trapping the cyclic cationic intermediate. The cyclizations of two methyl 2-acetoxy-2-(4-alken-1-oxy)acetates and two methyl 2-acetoxy-2-(alkynoxy)acetates are also described .