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An Improved Synthesis of (24R)-24,25-Dihydroxyprovitamin D3 1
17 September 2002 (online)
(24R)-24,25-Dihydroxyprovitamin D3 was prepared in 33% overall yield by a six-step procedure starting from the Diels-Alder adduct of ergosteryl benzoate and 1,4-dihydrophthalazine-1,4-dione. The yield thus obtained is more than four times as high as that of an eight-step synthesis based on the Diels-Alder adduct of ergosteryl acetate and 4-phenyl-1,2,4-triazoline-3,5-dione. The combination of the benzoyl group and the 1,4-dioxo-1,2,3,4-tetrahydrophthalazin-2,3-diyl group for the protection of the 3ß-hydroxy group and the 5,7-diene system, respectively, did not only raise the yield of the crucial ozonization step to 87% but was also advantageous in that it diminished the air and light sensitivity of the intermediates in the following reaction steps.