Paricalcitol, an analog of vitamin D, is used as a drug for the prevention and treatment of secondary hyperparathyroidism. In this paper, a new strategy for the synthesis of paricalcitol is described. This approach includes three main improvements: one-pot regioselective ozonization cleavage of the side-chain and methylene at C-19, free-radical reduction removal of the OH group formed at C-19, and side-chain assembly using a Wittig reaction. These are all new strategies for the synthesis of 19-nor-vitamin D 2 compounds. cleavage of the methylene group on ring A of vitamin D 2 were achieved by ozonization/Julia olefination and dihydroxylation/oxidation with NaIO 4 , respectively.Large-scale, efficient, convenient synthesis of paricalcitol is still a challenge. We therefore tried to find an efficient synthetic method. First, we tried to improve the main conversion steps, especially removal of the C-19 methylene group and assembly of the side-chain. It was found that the side-chain and C-19 methylene can be cleaved selectively in

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