Azobisisobutyronitrile → 1alpha,3beta-bis(tert-butyldimethylsilyloxy)-20(S)-hydroxypregna-5,7,16-triene

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Reaction Steps

To (1S,3R,17R,20S)-1,3-bis(tert-butyldimethylsilyloxy)-17,20-epoxypregna-5-ene (100 g) obtained in Example 1, 2,2′-azobis(isobutyronitrile) (7.98 g) and γ-collidine (82.4 mL) were added, and the resulting mixture was stirred in n-heptane (1000 mL) at 75° C. for 15 minutes. After cooling the mixture to room temperature, a precipitate was removed by filtration, and the filtrate which was a solution in n-heptane was concentrated under reduced pressure. To the obtained concentrated residue containing γ-collidine, toluene (650 mL) and aluminum isopropoxide (3.64 g) were added, and the resulting mixture was heated to reflux for 2 hours. The reaction solution was cooled, and the organic layer was sequentially washed with aqueous hydrochloric acid, aqueous sodium bicarbonate and with water, followed by concentration under reduced pressure. After suspending the obtained concentrated residue in a mixed solvent of ethyl acetate, acetonitrile and triethylamine, a precipitate was collected by filtration, washed with acetonitrile, heat-dried under reduced pressure to obtain (1S,3R,20S)-1,3-bis(tert-butyldimethylsilyloxy)-20-hydroxypregna-5,7,16-triene as slightly yellow powder (37.5 g; yield: 37.7%).

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