SOLID PHASE CHEMISTRY

Contents- Introduction Solid Support Linkers, Anchors Protecting Groups Coupling Reagents Deprotecting Groups Application In peptide synthesis Advantages Disadvantages

Introduction Solid phase synthesis was developed by B. Merrifield, It is of the particular interest in the field of Peptide synthesis. Solid-Phase Synthesis focuses on chemical reactions of substrates attached to solid supports. Including methods for attachment and detachment from the supports.

Eg. Solid phase Synthesis of a dipeptide Z-NH-CH-COOH + Cl-CH2 R Z-NH-CH-COO-CH2 NH2-CH-COO-CH2 Z-NH-CH-COX + NH2-CH-COO-CH2 R1 R

CONT……. Z-NH-CH-CO-NH-CH-COO-CH2 R1 R NH2-CH-NH-CH-COOH + R1 R solid support Z protecting group

Problems with Traditional Synthesis 1 chemist 1 molecule Can only make one molecule at a time. Each synthesis very time consuming Multistep syntheses have loss at each step Purification of products very time-consuming between steps. Yields can be low and produces very few molecules at a time for testing.

Solid Support Resin act as a solid support for a solid phase synthesis. The term solid support used to denote the matrix upon which chemical reaction is performed. It is a copolymer of styrene and divinylbenzene.

Types Crosslinked Polystyrene -Styrene+Divinylbenzene -Most widely used -Hydrophobic

PEG Grafted Supports -Tentagel - 1-2% crosslinked polystyrene - More flexible -Hydrophilic

Inorganic Supports- -Glass beads as a inorganic supports - High mechanical strength -Negligible swelling

Properties:- Chemical stability Mechanical stability It must swell extensively in the solvents used for the synthesis; Peptide-resin bond should be stable during the synthesis; Peptide-resin bond can be cleaved effectively at the end of the synthesis.

Linker, Anchors The linker that is bound to the resin is called anchor. Remains at the resin after cleavage. Resin Linker Resin Anchor

linker Bifunctional spacer molecules which contain on one end a functional group for attachment to the solid support and on the other end a selectively cleavable group onto which the starting molecule is attached.

Anchors Types : 1) Benzylic Halide Type Resin- i. Merrifield resin. =

Anchor ii. Trityl chloride resin.

Anchor 2) Benzylic alcohols Type Resin - Wang resin

Anchor 3)Benzylic amines Type Resin- Rink amide resin

Anchor 4) Aromatic aldehyde Type Resin- Bal resin

Protecting Groups A fragment bound to functional group that blocks the reactivity of that group. Good protecting groups are easily attached and removed using mild reaction condition.

Protecting Groups Acid Protection

The most common acid protecting group used is the methyl ester. It is stable in most coupling reaction and deprotection reaction conditions. Alanine methyl ester Depending on what type of coupling reaction and with what amino acids will be used other acid protecting groups can be used, such as the allyl ester. Alanine allyl ester

Protecting Groups Amine Protection

There are two standard types of N-protecting groups used, the Boc and Fmoc group. N-Boc cleavage N-Fmoc cleavage

Protecting Groups Protection of the R-group

hydroxyl groups (Ser), thiol groups (Cys), amines (Lys) and Some of the different R-groups that must be protected before coupling are hydroxyl groups (Ser), thiol groups (Cys), amines (Lys) and carboxylic acids (Asp).

Coupling Reagents In the coupling reactions of peptide synthesis the carboxyl group of the acylating amino acid is activated. The care should be taken in selecting the activation method to avoid racemization.

Coupling Reagents HOBt- (N- Hydroxy benztriazole) HOAt- (1- Hydroxy-7-aza- benztriazole) HBTU- (O-benztriazole-N,N,N’,N’-tetramethyl-uronium-hexafluoro phosphate) HATU- 2-(1H-7-Azabenzotriazol-1-yl)--1,1,3,3-tetramethyl uronium hexafluorophosphate 27

Deprotecting Groups It is the cleavage of the side-chain protecting groups. For protecting group such as Boc using Trifluoroacetic acid (TFA) the deprotecting groups like HF and Trifluoromethanesulfonic acid (TFMSA) have been used.

Deprotecting Groups Mild Acids : e.g. Trifluoroacetic acid. Hydrochloric acid. Methanesulfonic Acid. Alkaline condition : e.g. Piperidine in dimethylformamide.

Solid Phase Synthesis Mechanism & Application R. Bruce Merrifield 7/15/1921–5/14/2006 Nobel prize in chemistry 1984

Peptide Synthesis CH2 CH Treating the polymeric support with chloromethyl methyl ether (ClCH2OCH3) and SnCl4 places ClCH2 side chains on some of the benzene rings.

Peptide Synthesis CH2 CH CH2Cl The side chain chloromethyl group is a benzylic halide, reactive toward nucleophilic substitution.

Peptide Synthesis CH2 CH CH2Cl The chloromethylated resin is treated with the Boc-protected C-terminal amino acid. Nucleophilic substitution occurs, and the Boc-protected amino acid is bound to the resin as an ester.

Peptide Synthesis CH2 CH CH2Cl R O – BocNHCHCO

Peptide Synthesis Next, the Boc protecting group is removed with HCl. CH2 CH BocNHCHCO R O Next, the Boc protecting group is removed with HCl.

Peptide Synthesis HOBt-promoted coupling adds the second amino acid H2NCHCO R CH2 CH O HOBt-promoted coupling adds the second amino acid

Peptide Synthesis Remove the Boc protecting group. NHCHCO R O CH2 CH BocNHCHC R' Remove the Boc protecting group.

Peptide Synthesis Add the next amino acid and repeat. CH2 CH NHCHCO R H2NCHC R' Add the next amino acid and repeat.

Peptide Synthesis CH2 CH NHCHCO R O NHCHC R' C + H3N peptide Remove the peptide from the resin with HBr in CF3CO2H

Peptide Synthesis CH2 CH CH2Br NHCHCO R O NHCHC R' C + H3N peptide

Advantages of Solid Phase Synthesis Synthetic intermediates don’t have to be isolated. Quick process. Reagents simply washed away each step. Can be automated with robots!!

Disadvantages to Solid Phase Synthesis All the synthesis can’t be done on solid phase. Typically, kinetics not the same. Unsuitable for solvent assisted chemical reaction. High viscosity in reactant system. Insufficient purity if reaction steps are incomplete.

REFERENCES www.google.co.in www.discoveryofsps.com www.wikipedia.org Introduction to medicinal chemistry 3rd edition by Patric.

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