Trituration is a solid/liquid extraction technique, used to separate soluble impurities from a finely ground solid product (the inverse of what goes on in a Soxhlet extractor).  One of the best ways of separating ions from non-ions, it’s my go-to technique for purifying the product of Boc deprotections, and works passably well for any situation where you have an extreme difference in polarity between a compound and its impurities.  Solvent selection is driven by the nature of the impurities, and I find that 2% MeOH in Et2O works very well for ionic species.  Similar to a standard liquid-liquid extraction, trituration can’t separate closely related compounds, but the technique scales well and three consecutive triturations can be completed in about ten minutes (less if samples are run in parallel).

Here’s a test case:

In these images I’m deprotecting a fully Boc protected aminoglycoside (5 or 6 Boc groups), using a 9:1 TFA:water mixture.  This is a final compound, so I have approximately 35 mg of starting material (eventually the product will go off for biological testing).

The vial is chilled to 0℃ and roughly 1 mL of the TFA mixture is added.  After three minutes of shaking on ice the vial is transferred to the rotovap, where the acid is removed under high vacuum (dry ice trap).  A yellow oil is produced, visible here as a thick film on the side of the vial.

About 1 mL of 49:1 Et2O:MeOH is added to the vial, and the walls are scraped with a microspatula.  After some preliminary grinding the off-white powder is allowed to settle, and the solvent is removed via pipette [1].  A fresh aliquot of methanol/ether is added, and the process is repeated.

When trituration is complete the cap is removed and the vial is held in a 45℃ oil bath for about twenty seconds.  This boils off any residual ether, and prevents bumping when the sample is transferred to the high-vac line.  For aminoglycosides an overnight stay on the hivac line is required to remove the last equivalent or so of diethyl ether, which coordinates strongly to the various amines.

Once dry, a small amount of the product is removed and analyzed via HPLC and ESI-MS.  If the deprotection proceeded cleanly the compound is generally pure; otherwise prep-HPLC is performed.

[1] If I were running several triturations simultaneously the scraping would also be done via pipette.  Any powder that becomes trapped in the pipette can be rinsed out after trituration with a small amount of methanol.  This saves time since I’m not washing the same spatula 4+ times.

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