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I'm having trouble finding information on what I believe is called Preparatory Thin Layer Chromatography (this shows you the depth of my ignorance on this subject; I've never taken a chromatography class). I've attached a copy of my synthesis scheme to this message.

The three reactions illustrated were carried out in succession without purificaiton, and I think that I have 150 to 200 mg of total product. I started out with 320 mg of compound I.

I was told that because of the amount of final product that I'm working with that I shouldn't use column chromatography to separate the product from the crude mixture. I was told that I should use preparatory thin layer chromatography to do this, which from my understanding is basically like doing analytical thin layer chromatography but on a larger scale.

Now, for my question. I can't find anything on the internet detailing this kind of procedure. I would really like to see examples, and know something about the theory of how this operates before I start doing stuff to my precious few hundred milligrams of stuff. Does anyone know of any examples, or good sites, or can anyone tell me if the procedure is basically just like doing the little analytical TLC plates that we use to monitor the progress of the reaction, but on a bigger scale?

Thank's in advance for any help/advice/direction,

Hi Mike, this site (http://www.chem.rochester.edu/~nvd/chromatographynotes.html) has a link to a few notes about preparatory TLC. It's just regular TLC on a larger scale, using larger plates with a thicker layer of stationary phase on them, as you surmised. After separation on the plate, you carefully scrape off the band you want and extract the compound from it. I would make a tiny solution of a few milligrams of your mixture and use it with ordinary TLC in order to optimize the separation (by varying the solvent or solvent mixture and stationary phase, alumina or silica gel, for instance) before committing your entire sample. And then you may still want to do half at a time, just in case something goes wrong. You will need to find a solvent that effectively washes the compound from the sationary phase also. Straightforward in principle, but it can be a time-consuming trial and error process! If the spots show on the plate under UV light, that would be nice, otherwise you must run a tiny control sample each time which will have to be treated in some way in order to visualize the spots.

OK Mike, good luck! Just use tiny amounts at first (with ordinary TLC) until you are sure you've got the bugs worked out.

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