Addition of allylstannanes to an oxy-stabilized carbenium ion on a 1,7-dioxaspiro[5.5]undecane ring system

The nucleophilic addition of allylstannanes to (2R*,5S*,6S*)-2-acetoxy-5-benzyloxy-1,7-dioxaspiro[5.5]undecane 1 has been studied. The optimum conditions involve the use of trimethylsilyl trifluoromethanesulfonate in dichloromethane at -78 degrees C. In the examples studied, substitution of the acetoxy group at C-2 proceeds from an axial direction, however, subsequent ring flipping of the substituted ring occurs as well affording allylated prod.,ts in which the substituents at both C-2 and C-5 are equatorial.