[3,3]-Sigmatropic rearrangements: short, stereocontrolled syntheses of functionalized vitamin D3 side-chain units

Enantiomerically pure C,D-ring allylic alcohol 4 stereospecifically undergoes five types of [3,4]-sigmatropic rearrangements to give C-23 functionalized 16-ene vitamin D3 side-chain units with natural C-20(S) stereochemistry.