Lexacalcitol can be obtained by two related ways: 1) The decarboxylative oxidation of 1(S),3(R)-bis(tert-butyldimethylsilyloxy)-20(S)-formyl-9,10-secopregna-5(E),7(E),10(19)-triene (I) by bubbling air and catalyzed by cupric acetate and 1,4-diazabicyclo[2.2.2]octane in DMF gives compound (II) with a keto group at C20. The reduction of (II) with NaBH4 in THF/methanol yields the corresponding alcohol (III) as the 20(R)-isomer, which is condensed with 6-bromo-3-ethyl-3-(trimethylsilyloxy)hexane (IV) by means of potassium tert-butoxide in THF, affording 1(S),3(R)-bis(tert-butyldimethylsilyloxy)-20(R)-[4-ethyl-4-(trimethylsilyloxy)hexyl]-9,10-secopregna-5(E),7(E),10(19)-triene (V). Isomerization of (V) by irradiation with a UV lamp with anthracene as photosensitizer in dichloromethane gives the corresponding 5(Z),7(E),10(19)-isomer (VI), which is finally deprotected with HF in ethyl acetate/acetonitrile/water. 2) The condensation of alcohol (III) with 6-bromo-3-ethyl-3-(tetrahydropyran-2-yloxy)hexane (VII) with potassium tert-butoxide as before gives 1(S),3(R)-bis(tert-butyldimethylsilyloxy)-20(R)-[4-ethyl-4-(tetrahydropyran-2-yloxy)hexyl]-9,10-secopregna-5(E),7(E),10(19)-triene (VIII), which is isomerized with UV light and anthracene as before, affording the corresponding 5(Z),7(E),10(19)-isomer (IX). Finally, this compound is also deprotected with HF in the same mixture of solvents.