Divergent Chemo- and Biocatalytic Route to 16ß-Methylcorticoids: Asymmetric Synthesis of Betamethasone Dipropionate, Clobetasol Propionate, and Beclomethasone Dipropionate.

ABSTRACT

16ß-Methylcorticoids are among the most important glucocorticoid steroids for the treatment of various dermatological disorders, respiratory infections, and other allergic reactions elicited during inflammatory responses of the human body. Betamethasone dipropionate, clobetasol propionate, and beclomethasone dipropionate are particularly noteworthy for their synthetic intractability. Despite five decades of research, these 16ß-methylcorticoids have remained challenging synthetic targets owing to insurmountable issues of reactivity, selectivity, and cost efficiency associated with all previously explored strategies. We herein report our practicability-oriented strategy toward the unified stereoselective synthesis of 16ß-methylcorticoids in 12.6-14.0 % overall yield from commercially available 9α-hydroxyandrost-4-ene-3,17-dione (9α-OH-AD). In this approach, the chiral C16ß-Me and C17α-OH groups of the corticosteroid D ring were installed via a substrate-controlled diastereo- and enantioselective Mn-catalyzed oxidation-reduction hydration of Δ4,9(11),16 -triene-3,20-dione. The C1-C2 double bond of the corticosteroid A ring was constructed using an unprecedented engineered 3-ketosteroid-Δ1 -dehydrogenase (MK4-KstD)-catalyzed regioselective Δ1 -dehydrogenation of Δ4,9(11) -diene-3,21-dione. This strategy provides a general method and a key precursor for the divergent synthesis of a variety of glucocorticoids and related steroidal drugs.