Peptide synthesis carbonyl chemistry in action

We hope that you can see from this example that all the steps in this important synthesis use chemistry that you have already met. The art in synthesis is to put the steps together in the right order, and we aren t asking you to think about that (that can wait until Chapter 30)—at this stage see these syntheses as a way of revising what you have already learned. 

In fact, the use of acyl chlorides in peptide synthesis is rare because of the potenlial for unwanted side-reactions, including racemization and dimerization to form by-products known as diketopiperazines. We will represent our activated acid as RCOX, where X is some other good leaving group such as a phenol. 

The main problem, though, is that there is another free C02H, which could react with the COX group to form an anhydride, and two different free amines, either of which might react, giving both LeuLeu (which we don t want) and LeuGly (which we do). 

C-termlnus means the end of the peptide that carries the terminal G02H group. The other end, carrying die NH2 group, isthefV-- terminus. By convention, we write the W-terminus on the left and the O-terminus on the right. The structure of oxytocin is represented here in terms of the three-letter codes for amino acids—there is a full list in Chapter 49. 

We introduced the dipeptide LeuGly as an example H2N-Cys-Tyr-lle-Gln Asn-Cy8-Pro-Leu-Gly-CONH2 because it forms the C-terminus of the peptide hor- oxytocin 

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