Chemical Synthesis of Uncommon Natural Bile Acids: The 9α-Hydroxy Derivatives of Chenodeoxycholic and Lithocholic Acids

Biocatalytic Steroidal 9α-Hydroxylation and Fragmentation Enable the Concise Chemoenzymatic Synthesis of 9,10-Secosteroids

9,10-Secosteroids are an important group of marine steroids with diverse biological activities. Herein, we report a chemoenzymatic strategy for the concise, modular, and scalable synthesis of ten naturally occurring 9,10-secosteroids from readily available steroids in three to eight steps. The key feature lies in utilizing a Rieske oxygenase-like 3-ketosteroid 9α-hydroxylase (KSH) as the biocatalyst to achieve efficient C9-C10 bond cleavage and A-ring aromatization of tetracyclic steroids through 9α-hydroxylation and fragmentation. With synthesized 9,10-secosteroides, structure-activity relationship was evaluated based on bioassays in terms of previously unexplored anti-infective activity. This study provides experimental evidence to support the hypothesis that the biosynthetic pathway through which 9,10-secosteroids are formed in nature shares a similar 9α-hydroxylation and fragmentation cascade. In addition to the development of a biomimetic approach for 9,10-secosteroid synthesis, this study highlights the great potential of chemoenzymatic strategies in chemical synthesis.

Chemical Synthesis of Rare Natural Bile Acids: 11α-Hydroxy Derivatives of Lithocholic and Chenodeoxycholic Acids

A method for the preparation of 11α-hydroxy derivatives of lithocholic and chenodeoxycholic acids, recently discovered to be natural bile acids, is described. The principal reactions involved were (1) elimination of the 12α-mesyloxy group of the methyl esters of 3α-acetate-12α-mesylate and 3α,7α-diacetate-12α-mesylate derivatives of deoxycholic acid and cholic acid with potassium acetate/hexamethylphosphoramide; (2) simultaneous reduction/hydrolysis of the resulting △¹¹ -3α-acetoxy and △¹¹ -3α,7α-diacetoxy methyl esters with lithium aluminum hydride; (3) stereoselective 11α-hydroxylation of the △¹¹ -3α,24-diol and △¹¹ -3α,7α,24-triol intermediates with B₂ H₆ /tetrahydrofuran (THF); and (4) selective oxidation at C-24 of the resulting 3α,11α,24-triol and 3α,7α,11α,24-tetrol to the corresponding C-24 carboxylic acids with NaClO₂ catalyzed by 2,2,6,6-tetramethylpiperidine 1-oxyl free radical (TEMPO) and NaClO. In summary, 3α,11α-dihydroxy-5β-cholan-24-oic acid and 3α,7α,11α-trihydroxy-5β-cholan-24-oic acid have been synthesized and their nuclear magnetic resonance (NMR) spectra characterized. These compounds are now available as reference standards to be used in biliary bile acid analysis.