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January 3, 2007
You'll Be Safe Under Here. Maybe.
I see that there's a new edition of the book that organic chemists just call "Greene"; otherwise known as "Protective Groups in Organic Synthesis". This is the fourth, and I'm of an age to remember the first one back in about 1981, when I was still an undergraduate. The new volume could swallow the old one whole - it must be five times the length. Back then the book had some competition, but now it has none at all.
For the non-chemists in the crowd, a protecting group is the molecular equivalent of masking tape. It's a temporary group that you tack on to some reactive part of a molecule to keep it from messing things up while you work on the rest. Ideally, this would be something totally selective for the functionality you're trying to protect, which goes on easily and comes off just as lightly without affecting anything else. Oh, and it has to stand up to every other kind of reaction known to science along the way. That protecting group does not yet exist, or if it does, no one's told me about it. But there are some pretty decent ones, depending on what you're asking them to put up with before their removal.
Everyone who does organic synthesis has to use these things at some point. Because I did carbohydrate-based synthesis in my grad school work (all those hydroxyl groups), I had to use the things constantly, and the trick was to keep them straight and removable in each other's presence. But it's safe to say that no one likes using them. There's a persistent feeling of. . .well, inelegance that attaches to them, no matter how nifty the conditions used to apply and remove them. After all, that's two more steps added to your synthesis, and two which more or less cancel each other out after all that work.
You can't help but think that really advanced organic chemistry will find a way around such problems. We aren't there. For now, it's duct-taped umbrellas, cement-spotted plastic domes, and jammed-on crash helmets for our molecules, and not much we can do about it. Greene's book has all the ugly but necessary details anyone needs.
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