While working on peptides, we protected the amino acids and peptides. Even the very basic Boc protected Valine or Tyrosine is a thick oil which we use in the next step without purification. Even keeping such compounds in fridge for a long time results in thickening of the oil but not solidification (most of the cases).
However, some similar (or even same) marketed compounds are powdered solid or crystals. Is there a process to convert this oily compound to a solid form?
sometimes the oily compound resulted from presence of some impurities . try to extracted your product with a proper solvent . also you can dissolve the impurities with a solvent then decantation to get solidified compound.
TLC shows single spot and I have ensured that there is no solvent is left with the compound it is completely dry.
I do agree with Alaa J Mahrath. Although TLC shows single spot, it does not mean your compound is free from all organic / inorganic compound.
Since amino acids contain both acid and amine group, pH also plays important role. So while work up proper care has to be taken to rid of all traces acidic/basic reagents used in reaction in order to get solid product .
For Boc-protected valine / tyrosine, you can try to stir oily product with 5-10 volumes of ether solvent like diethtyl ether or isopropyl ether to get solid product..
Sometimes you do not get solid because the product is very hygroscopic and the energy cost of losing the water is too high. I had the same issue with a product that would not crystallize unless all the solvents I used were freshly dried.
Also regarding the impurities hypothesis, Nagnnath Kokare . Some impurities are not UV active ( like TEA which is used in many reactions). In those cases, you would not be able to see them on TLC ( unless you stain the TLC plate).
As I deal with those compounds, I found that the best way of dealing with them is to precipitate them from aqueous solution by dissolving your compound in large amount of acidic or basic solution, then try to adjust the pH to neutral.
Sometimes you have to improvise. Some compounds would be easier to prepare as hydrochloride salt, some as a sodium or potassium salt and some as zwitter ions. Explore all avenues!
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