The first construction of the B,C,D-ring fragment of pinnatoxins via highly stereocontrolled acetallization

The synthesis of the B,C,D-ring fragment of pinnatoxins is described. The highly stereocontrolled construction of the 6,5,6-tris-spiroacetal core was achieved on treatment of compound 3 with aqueous HF in CH 3 CN. The anomeric effect at C 16 enhanced by neighboring carbonyl group led to form the tris-spiroacetal having the natural configuration.