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

Acid-mediated cyclization reactions are described of seven methyl 2-ac etoxy-2-(3-alken-1-oxy)acetates with different chain substitution. The major product of the tin tetrachloride-induced cyclization reaction i s in most cases a tetrahydropyran containing an equatorial carbomethox y function at C2 and an axial chlorine atom at C4. The mechanism of it s formation involves a net cis-addition of the intermediate alpha-este r oxycarbenium ion to the carbon-carbon double bond, most likely cause d by a quasi axial orientation of the ester function in a chair-like t ransition state. The results are interpreted by invoking (1) the occur rence of a 2-oxonia-Cope rearrangement and (2) the participation of th e ester function in the mechanism of cyclization by trapping the cycli c cationic intermediate. The cyclizations of two methyl 2-acetoxy-2-(4 -alken-1-oxy)acetates and two methyl 2-acetoxy-2-(alkynoxy)acetates ar e also described .