A Stereoselective Process for the Manufacture of a 2′-Deoxy-β-d-Ribonucleoside Using the Vorbrüggen Glycosylation

Abstract

A practical and scalable process for the manufacture of cladribine (1) is described. Vorbrüggen glycosylation of doubly silylated 2-chloroadenine 2 with protected 1-O-acetyl-2-deoxy-α,β-d-ribofuranose 3 under reversible conditions in the presence of 20 mol % triflic acid in a solvent that selectively precipitated the desired β-anomer β-4a whilst leaving the unwanted α-anomer α-4a in solution to isomerise allowed good overall stereoselectivity with exclusive regioselectivity. An aging step allowed anomerisation of α-4a to β-4a, thereby improving the isolable yield of the β-anomer. Direct filtration of the product mixture without a catalyst quench or aqueous workup furnished the crude β-anomer β-4a in good yield (up to 68%) and purity (>95% by HPLC) with no regioisomers detected and only ∼1–3% (by HPLC) of the undesired α-anomer. Deprotection of the crude, unpurified intermediate β-4a followed by recrystallisation provided drug-grade cladribine (1). The process includes three isolation steps and was demonstrated on kilogram scales using cGMP providing 99.8–99.9% pure cladribine in up to an overall 43% yield based on 2-chloroadenine (5). In contrast to previous methods, column chromatography and/or bulky directing groups were not required in the glycosylation step, a high pressure vessel was not needed in the deprotection step, and only one dedicated recrystallisation step was necessary.

Anion exchange resins in phosphate form as versatile carriers for the reactions catalyzed by nucleoside phosphorylases

“…The dual function of QAE Sephadex A-25 noted above was tested in the enzymatic synthesis of cladribine using dRib-1Pi as a barium salt and bound to the resin. Chemical synthesis of this nucleoside, which has a wide spectrum of pharmacological activities [ 6 , 63 – 65 ], is wearisome [ 5 , 66 ]. This evidence prompted us to engage in the design and development of an efficient biotechnological synthetic route.…”

Electrochemical Synthesis of Imino‐C‐Nucleosides by “Reactivity Switching” Methodology for in situ Generated Glycoside Donors

“…The N ‐α proton of dichloroacetic acid (DCA) adducts appeared at medium range (6.9–6.7 ppm), while the anomeric ratio shifted to α‐anomer. It indicated the DCA adduct is more reactive than AcOH adduct, thus thermodynamically favored α‐anomer was enriched due to the instability of β‐anomer [9] …”

“…It indicated the DCA adduct is more reactive than AcOH adduct, thus thermodynamically favored α-anomer was enriched due to the instability of βanomer. [9] Fraser-Reid and co-workers developed the "armed-disarmed" concept as a rule of thumb to evaluate the anomeric reactivity of O-protected carbohydrates. [10a] This methodology relied on an equilibrium between protected glycoside donor and oxocarbenium ion intermediate (Figure 2C), and recently Bennett and Kwan provided mechanistic insight into these phenomena by detailed KIE analysis.…”

Cladribine Analogues via O6-(Benzotriazolyl) Derivatives of Guanine Nucleosides

“…The synthesis of cladribine has primarily relied on three major methods: (a) glycosylation reactions of a nucleobase with a sugar [ 12 , 13 , 14 , 15 , 16 , 17 , 18 ], and its variations; (b) deoxygenation of the C2′ hydroxyl group of a suitable nucleoside derivative [ 12 , 15 , 19 , 20 ]; (c) enzymatic glycosyl transfer reactions [ 21 , 22 , 23 , 24 ]; and (d) conversion of readily available nucleoside precursors (some utilizing nucleosides for glycosyl transfer reactions) [ 21 , 22 , 24 , 25 , 26 ]. Each of these methods has been used with varying levels of convenience and success.…”