Peptide Syntheses. Illustrative Protection: BOC/ t Bu. A. Introduction. do not acid

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Kevin Burgess, May 3, 2017 1 Peptide yntheses from chapter(s) in the recommended text A. Introduction do not acid -Met-e- -Met-Met- -e-e- -e-met- dipeptide dipeptide dipeptide dipeptide diketopiperazine symmetrical diketopiperazine unsymmetrical diketopiperazine would also impractical synthesis - protect one of the fragments and C- protect the other. Reactions f Protected Amino Acids P-Met-Cl -Gly-P' P-Met-Gly-P' P-e-Cl -Pro-P' P-e-Pro-P' Illustrative Protection: BC/ -BC Protected Amino Acids amines

Kevin Burgess, May 3, 2017 2 amines. Cl 2 Cl -Leu- -Met- -Val- (BC) 2 with trifluoroacetic () acid.

Kevin Burgess, May 3, 2017 3 carbon dioxide. - BC-e- -C 2 2 unstable carbamate amino acid carbocation Give the products of the following reactions CF 3 C 2 2 BC-Val-Ala- -Val-Ala-

Kevin Burgess, May 3, 2017 4 undesirable iet 3 Tyr / Trp Ac C 2 2.0 equiv. - Ac C 2 Ac C 2 1.0 equiv. - Ac C 2 usually

Kevin Burgess, May 3, 2017 5 - Bn Bn Bn BC-Tyr(Bn)- unstable carbamate -C 2 2 Bn amino acid carbocation iet 3 carbocation by-product C-Protection f Amino Acids With -Groups Ac Ac Ac-Met- by-product cation

Kevin Burgess, May 3, 2017 6 1-Adamantyl esters cannot are BC-Gly-Cl -Met- BC-Gly-Met- BC-Pro-Cl -e- BC-Pro-e- BC-Asp(Bn)-Cl -Thr(Bn)- BC-Asp(Bn)-Thr(Bn)- BC-Asp(Bn)-Cl -is(bn)-thr(bn)- BC-Asp(Bn)-is(Bn)-Thr(Bn)- Activation f -Protected Amino Acids too reactive for using carbodiimide reagents ie dicyclohexylurea, because the by-products can be protonated and are water-soluble.

Kevin Burgess, May 3, 2017 7 Let be represented as R 1 R 2 BC - R 2 2 R 1 R 2 BC R 1 BC-e- - BC R 1 R 2 BC-e-Ala- by-product The Epimerization Problem epimerize) epimeric products. difficult to separate

Kevin Burgess, May 3, 2017 8 azlactone. Me X -X Me X -X azlactone forms rapidly azlactone forms slowly is driven by aromatic stabilization in the product and simultaneous loss carbamate. more Ac-Ala- BC-Gly-Ala-

Kevin Burgess, May 3, 2017 9 trategies In olution ase yntheses That Avoid Epimerization will will tend to BC-Ile-Val-Ala- less prone to racemization BC-Ile-Val-Ala- more prone to racemization circle the one amino acid in one of these structures that is most vulnerable to epimerization are C- to - direction B. olid ase Peptide yntheses are mixed with is usually required easier to purify advantages of are not optimally C-terminus. m n n styrene polystyrene "4-chloromethylpolystyrene" Cl 2 reaction

Kevin Burgess, May 3, 2017 10 Cl BC BC BC-Pro- often in the presence of a scavenger; this does not -terminus C 2 Bn 2 C 2 Bn BC-Ala- BC-Asp(Bn)-support C 2 Bn 2 C 2 Bn

Kevin Burgess, May 3, 2017 11 2 BC-Leu- 2 Cl BC-er( )- 2 BC-Leu- 2

Kevin Burgess, May 3, 2017 12 F and scavengers BC-Gly-Ala-Met-Pro F iet 3 2 draw peptide Cl BC-M- BC-M-support -M-support BC-e- BC-FM-support -FM-support BC-Gly- BC-GFM-support -GFM-support BC-Gly- BC-GGFM-support

Kevin Burgess, May 3, 2017 13 -GGFM-support BC-Leu- BC-LGGFM-support F iet 3 -LGGFM- C. ide-chain Protection f Amino Acids may is required. undesirable desirable 2 2 Lys Arg Glu Asp

Kevin Burgess, May 3, 2017 14 Tyr Trp er Thr 2 2 no Cys Gln Asn e D. The FMC Approach F base labile via. FMC-Glu( )- FMC-Lys(BC)- FMC-er( )-

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