Synthesis of 12β-Methyl-18- nor-bile Acids

Abstract

Decoupling the roles of the farnesoid X nuclear receptor and Takeda G-protein-coupled bile acid receptor 5 is essential for the development of novel bile acid therapeutics targeting metabolic and neurodegenerative diseases. Herein, we describe the synthesis of 12β-methyl-18-nor-bile acids which may serve as probes in the search for new bile acid analogues with clinical applicability. A Nametkin-type rearrangement was applied to protected cholic acid derivatives, giving rise to tetra-substituted Δ^13,14- and Δ^13,17-unsaturated 12β-methyl-18-nor-bile acid intermediates (24a and 25a). Subsequent catalytic hydrogenation and deprotection yielded 12β-methyl-18-nor-chenodeoxycholic acid (27a) and its 17-epi-epimer (28a) as the two major reaction products. Optimization of the synthetic sequence enabled a chromatography-free route to prepare these bile acids at a multi-gram scale. In addition, the first cis-C-D ring-junctured bile acid and a new 14(13 → 12)-abeo-bile acid are described. Furthermore, deuteration experiments were performed to provide mechanistic insights into the formation of the formal anti-hydrogenation product 12β-methyl-18-nor-chenodeoxycholic acid (27a).

Figure 1

Primary and secondary bile acids and a selection of some medically important analogues.

Scheme 1. Previously Reported Syntheses of Δ^13,14- and Δ^13,17-12β-Methyl-18-nor-bile Acid Derivatives

Abbreviations: Tf₂O: trifluoromethanesulfonic anhydride; Mc: chloromethanesulfonate (monochlate); Ms: methanesulfonate; py: pyridine.

Scheme 2. Preparation of Δ^13,14- and Δ^13,17-12β-Methyl-18-nor-bile Acid Intermediates 24a–c and 25a–c

Reagents and conditions: (a) MeOH, p-TsOH·H₂O, Δ or rt; 21b–c: 83–84%; (b) for 22a: Ac₂O, DMAP, py (2.5 equiv), toluene; 90%; for 22b–c: DCM, DMAP, NEt₃, Ac₂O, 0 °C–rt; 2–6 h; 76–86%; (c) for 23a(36)/23c: MsCl, py, 0 °C–rt, o/n; 83–97%; for 23b: McCl, py, 0 °C, 45 min; (d) 24a/c//25a/c: HOAc, NaOAc, 100 °C, 2–3 h; 83–86%; 24b/25b: HOAc, Zn(OAc)₂, 100 °C, 1.5 h; 84%. Abbreviations: DCM: dichloromethane; DMAP: N,N-dimethylaminopyridine; o/n: overnight; rt: room temperature; Ts: para-toluenesulfonate.

Scheme 3. Preparation and Characterization of New 12β-Methyl-18-nor-bile Acids

Scheme 4. Synthesis of 24-nor and 25-homo 12β-Methyl-18-nor-bile Acids 27b,c and 28b,c

Scheme 5. Chromatography-Free, Large-Scale Synthesis of 12β-Methyl-18-nor-alkene Bile Acids 33 and 34 and 12β-Methyl-18-nor-bile Acids 27a and 28a

Reagents and conditions: (a) MeOH, p-TsOH·H₂O, Δ, 2 h, then rt, 3 d; (b) py, DMAP, EtOAc, Ac₂O, 0 °C–rt, o/n; 51% (2 steps, unoptimized); (c) MsCl, py, 0 °C–rt, o/n; (d) NaOAc, HOAc, 100 °C, o/n; (e) 10 M NaOH_aq, MeOH, 80 °C, 72 h; 33: analytical sample: 92.5% pure after fractional crystallization; >99% pure after chromatography; 34: ca. 60 g; 95% pure, following fractional crystallization (HPLC analysis); (f) fractional crystallization; (g) H₂ (5 bar), 10% Pd/C (0.1 equiv), MeOH/H₂O (10:1), rt, 20 h; 27a: 98.8% pure after fractional crystallization; 28a: >99% pure after fractional crystallization (HPLC analysis).

Scheme 6. Deuteration of Δ^13,17-Alkene Bile Acid 33 under Continuous Flow

Figure 2

Carbon NMR analysis of compound 27a–d₃: (A) ¹³C NMR spectrum of 27a (with power-gated ¹H decoupling); (B) ¹³C NMR spectrum of 27a–d₃ (with power-gated ¹H decoupling); and (C) semi-quantitative ¹³C NMR spectrum of 27a–d₃ (inverse-gated ¹H- and ²H-decoupled). ¹³C NMR spectra were recorded at 125 MHz in CD₃OD.

Figure 3

Carbon NMR analysis of the mixture of 28a–d_2–4: (A) ¹³C NMR spectrum of 28a (with power-gated ¹H decoupling); (B) ¹³C NMR spectrum (with power-gated ¹H decoupling) of 28a–d_2–4; and (C) semi-quantitative ¹³C NMR spectrum of the mixture of 28a–d_2–4 (inverse gated, ¹H- and ²H-decoupled; NS 9600). ¹³C NMR spectra were recorded at 125 MHz in CD₃OD. See the Supporting Information for full spectra.