Development of a Chemoenzymatic Route for the Preparation of 6-Keto Estradiol: A Key Intermediate for Fulvestrant

A novel synthetic route was developed to synthesize 6-keto estradiol (2), an intermediate in the synthesis of fulvestrant and analogues. Using norandrostenedione as a starting material, the target 2 was obtained through five step reactions including esterification, hydroxylation, oxidation, enzyme-catalyzed dehydrogenative aromatization, and enzyme-catalyzed reduction. In hydroxylation, phthalic anhydride (PA)/H2O2/pyridine was developed as a catalytic system to give the desired 6a and 6b in 90% yield under mild conditions. The enzyme 3-ketosteroid-Δ1-dehydrogenase (Δ1-KstD) was utilized for the dehydrogenative aromatization of the A ring in 7, while 17β-hydroxysteroid dehydrogenases (17β-HSDs) were employed for the stereoselective reduction of 17-keto in 8. The related impurities and process parameters were studied in detail. In addition, the scale-up of this synthetic route was successfully executed on a 300.00 g scale, yielding 159.04 g of 6-keto estradiol (2) with a purity of 99.6% and an overall yield of 50% in five steps. Other notable features of this synthetic route included high yields, mild reaction conditions, and inexpensive and commercially available starting materials.