[(2Z)-2-[(3S,5R)-3,5-Bis[[(1,1-dimethylethyl)dimethylsilyl]oxy]-2-methylenecyclohexylidene]ethyl]diphenylphosphine Oxide is used as a reagent in the preparation of hydroxymethylated dihydroxyvitamin D3 analogs which have antitumor and transactivation activity.