Methyl acetates with fluorine-containing auxiliaries at their 2 position were demonstrated to react smoothly with various electrophiles in a re face preferential manner (up to 90% de). This was interpreted as the result of an intermediary enolate constructing a bicyclo [3.3. 0] system by the concomitant intramolecular chelation of a metal with α-oxygen and fluorine, resulting in the auxiliary effectively blocking attack from the opposite si face.
