Chrysomycin A Derivatives for the Treatment of Multi-Drug-Resistant Tuberculosis

Abstract

Tuberculosis (TB) is a life-threatening disease resulting in an estimated 10 million new infections and 1.8 million deaths annually, primarily in underdeveloped countries. The economic burden of TB has been estimated as approximately 12 billion USD annually in direct and indirect costs. Additionally, multi-drug-resistant (MDR) and extreme-drug-resistant (XTR) TB strains resulting in about 250 000 deaths annually are now widespread, increasing pressure on the identification of new anti-TB agents that operate by a novel mechanism of action. Chrysomycin A is a rare C-aryl glycoside first discovered over 60 years ago. In a recent high-throughput screen, we found that chrysomycin A has potent anti-TB activity, with minimum inhibitory concentration (MIC) = 0.4 μg/mL against MDR-TB strains. However, chrysomycin A is obtained in low yields from fermentation of Streptomyces, and the mechanism of action of this compound is unknown. To facilitate the mechanism of action and preclinical studies of chrysomycin A, we developed a 10-step, scalable synthesis of the isolate and its two natural congeners polycarcin V and gilvocarcin V. The synthetic sequence was enabled by the implementation of two sequential C-H functionalization steps as well as a late-stage C-glycosylation. In addition, >10 g of the advanced synthetic intermediate has been prepared, which greatly facilitated the synthesis of 33 new analogues to date. The structure-activity relationship was subsequently delineated, leading to the identification of derivatives with superior potency against MDR-TB (MIC = 0.08 μg/mL). The more potent derivatives contained a modified carbohydrate residue which suggests that further optimization is additionally possible. The chemistry we report here establishes a platform for the development of a novel class of anti-TB agents active against drug-resistant pathogens.

Figure 1

C-aryl glycoside natural products and synthetic plans for chrysomycin A and its analogues. (a) Representative bioactive C-aryl glycosides. (b) Structures of gilvocarcin family natural products. (c) Our bond disconnections of chrysomycin A. In our retrosynthetic analysis of chrysomycin A, both the sugar moiety and vinyl group were assembled at the late stage. The core structure of the chromophore was constructed through sequential regioselective C–H functionalizations. (d) Late-stage diversification of the natural product at multiple sites.

Figure 2

Concise and scalable syntheses of aglycon and glycosyl donors. (a) Regioselective C–H functionalizations enabled scalable preparation of the aglycon 21. (b) Synthesis of glycosyl donors 23a–c. Reagents and conditions are as follows: (i) NBS, MeCN, r.t.; i-PrI, NaH, DMF, 0 to 70 °C. (ii) [Ir(cod)OMe]₂, dtbpy, HBpin, hexane, 80 °C. (iii) Pd(OAc)₂, KF, PMHS, THF, H₂O, r.t. (iv) [Pd(dppf)Cl₂]·CH₂Cl₂, KOH, MTBE/H₂O, 80 °C, then 40% NaOH (aq). (v) K₂S₂O₈, AgNO₃, MeCN/H₂O, 50 °C. (vi) Pd/C, H₂, MeOH, r.t.; Tf₂O, Et₃N, CH₂Cl₂, −78 °C; potassium vinyltrifluoroborate, [Pd(dppf)Cl₂]·CH₂Cl₂, Et₃N, n-PrOH, reflux. (vii) LiAlH₄, THF, 50 °C. (viii) Ac₂O, AcOH, H₂SO₄, r.t. (ix) DMAPA, THF, 20 °C. (x) CCl₃CN, DBU, 4 Å MS, CH₂Cl₂, r.t. (xi) N-Phenyltrifluoroacetimidoyl chloride, K₂CO₃, acetone, r.t.

Figure 3

Total syntheses of chrysomycin A, polycarcin V, and gilvocarcin V through late-stage C-glycosylation. Reagents and conditions are as follows: (i) SnCl₄, 4 Å MS, DCE, r.t. (ii) BCl₃, CH₂Cl₂, −20 °C. (iii) 1.5 M H₂SO₄ in MeOH, 70 °C. (iv) AlCl₃, CH₂Cl₂, −20 °C. (v) NaOMe, MeOH, r.t.

Figure 4

Methods used for late-stage diversification of chrysomycin A.

Figure 5

Late-stage diversification of chrysomycin A. (a) Synthesis of the C2 glycosylated derivatives. (b) Synthesis of the C4 hybrid derivatives. (c) Synthesis of the C3 hybrid derivatives via meta-selective C–H functionalization. Reagents and conditions are as follows: (i) SnCl₄, 4 Å MS, DCE, r.t. (ii) 1.5 M H₂SO₄ in MeOH, 70 °C. (iii) BCl₃, CH₂Cl₂, −78 °C. (iv) NaOMe, MeOH, r.t. (v) POCl₃, DMF, CHCl₃, 70 °C. (vi) NaOClO, NaH₂PO₄, 2-methyl-2-butene, THF/tBuOH/H₂O, r.t. (vii) 2-Propynylamine, HBTU, HOBt, DIPEA, DMF, r.t. (viii) AlCl₃, DCM, r.t. (ix) CuSO₄, TBTA, sodium ascorbate, DMF/tBuOH/H₂O, r.t. (x) tert-butyl 2-iodobenzoate, L₁, L₂, Pd(OAc)₂, AgOAc, r.t. (xi) Pd/C, H₂(3 MPa), THF/MeOH, r.t.; PhNTf₂, TEA, DCM, .r.t.; potassium vinyltrifluoroborate, [Pd(dppf)Cl₂]·CH₂Cl₂, Et₃N, n-PrOH, reflux. (xii) 7.5% TFA, DCM, r.t.; prop-2-yn-1-amine, HOBT, HBTU, DIPEA, DMF, r.t. (xiii) β-Glycosyl azides, CuSO₄, TBTA, Na ascorbate, DMF/tBuOH/H₂O, r.t.