1. A convoluted extraction series.
  2. Change the limiting reagent.
  3. Distillation.
  4. Trituration / Sohxlet.
  5. Prolonged high vacuum treatment.
  6. Recrystallization.
  7. The bisulfite/DNPH trick (aldehydes only).
  8. Secondary reactions [1].
  9. HPLC.
  10. The next reaction in the series.

[1] Protections, oxidations, etc.  Ex. Contaminant an amine?  Treat with acetyl chloride and base.  RF shoots up, and the column is no longer difficult.  Transient protections are also useful for 1,3-diketones and other tautomeric structures.

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