Azole antifungals — imidazoles such as ketoconazole and triazoles such as voriconazole and posaconazole — attach an imidazole or 1,2,4-triazole ring to a hydrophobic side chain through an N–C bond. That coupling is an SN2 alkylation of an azolide salt, a reaction class unusually sensitive to solvent. Dimethyl sulfoxide (DMSO) is the preferred medium because it dissolves the ionic azolide salt while keeping the azole anion bare and highly nucleophilic.
Where DMSO Is Used in Azole APIs
Ketoconazole uses DMSO for imidazole N-alkylation, generating the imidazolide with NaH to attach its dioxolane–piperazine side chain. Voriconazole and posaconazole run their 1,2,4-triazole N-alkylation in DMSO with NaH or KOH, forming the triazolide before adding the side-chain electrophile. Because the side chains are enantiopure and thermally sensitive, the coupling is held at 50–100 °C rather than pushed to higher temperature, so that racemization is avoided.
The azole heterocycle is first deprotonated by a strong base to give an imidazolide or triazolide salt, which then attacks the side-chain electrophile. This step determines both yield and the correct attachment nitrogen, since azoles carry several ring nitrogens and mis-alkylation produces regioisomeric impurities. In a protic solvent the azolide anion is hydrogen-bonded and weakly nucleophilic, whereas DMSO solvates only the metal cation and leaves the anion exposed.
| API | Azole ring | DMSO N-alkylation |
|---|---|---|
| Ketoconazole | Imidazole | Imidazolide + dioxolane side chain (NaH) |
| Voriconazole | 1,2,4-Triazole | Triazolide + chiral side chain |
| Posaconazole | 1,2,4-Triazole | Triazolide + piperazine side chain |
Table 1: DMSO N-alkylation across three azole antifungal APIs
Process Conditions
The azole is charged into dry DMSO, the azolide is formed at 0–25 °C, and the side-chain electrophile is added before warming to coupling temperature. Anhydrous DMSO (≤0.05% water) is required because trace moisture protonates the anion back to the neutral azole and hydrolyzes the halide or tosylate electrophile. On completion the batch is quenched into water and the azole product is crystallized or extracted, with a final recrystallization upgrading enantiomeric purity for chiral members such as voriconazole.
Why DMSO over DMF and Acetonitrile
DMSO dissolves alkali-metal azolide salts that acetonitrile cannot, giving a homogeneous and faster reaction. It solvates the sodium or potassium cation through its sulfoxide oxygen while leaving the azolide anion bare and highly nucleophilic. Unlike DMF, DMSO does not decompose to release dimethylamine under the basic, warmed conditions these couplings require, and its 189 °C boiling point delivers the needed warmth without pressure equipment.
References
- Parker, A.J. “Protic-dipolar solvent effects and organic reactivity,” Chemical Reviews, 1969, 69(1), 1–32.
- ICH Harmonised Tripartite Guideline Q3C(R6): Impurities — Guideline for Residual Solvents, 2019.
- Pfizer Inc., voriconazole process patents covering triazole N-alkylation of chiral side chains.