Presented by

Gurudutt Dubey

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• Introduction

• Useful Bases in Peptide Synthesis

• Coupling Agents

1. Carbodiimide reagents

2. Racemization suppressants

3. Phosphonium reagents

4. Aminium/Uronium-Immonium reagents

5. Imidazolium reagents

6. Organophosphorous reagents

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Activation and coupling of a protected amino acid

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Mechanism of base-catalyzed racemization during activation

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1. Carbodiimide Reagents

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• Application of DCC in solid-phase synthesis is limited due to the by-product dicyclohexylurea (DCU)

with is formed simultaneously with the coupling reaction

• DIC is a useful reagent for automated SPPS, because the corresponding urea is soluble in standard

solvents such as isopropanol and can be washed out readily

If base-free conditions are required as to minimize racemization, the combination of DIC and HOBt

(or HOAt, Oxyma Pure) is still one of the best methods e.g. for coupling Fmoc-Cys(Trt)-OH.

• EDC has also been employed in SPPS on highly polar resins compatible with water-containing

solvents. Contrary to DCU, the urea formed from EDC is readily soluble.

In addition to racemization another side reaction with carbodiimides is an O-N migration of

the activated carboxyl function forming an N-acyl urea

• low temperatures are always recommended

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Mechanism of carbodiimide-mediated peptide coupling

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Oxyma Pure®

(Ethyl 2-cyano-2-

(hydroximino)acetate)

HOAt

(1-Hydroxy-7-aza-1H-benzotriazole)

HOSu

(N-Hydroxysuccinimide)

HOOBt (HODhbt)

(Hydroxy-3,4-dihydro-4-oxo-1,2,3-

benzotriazine)

HOBt

(1-Hydroxybenzotriazole)

HOBt-6-sulfonamidomethyl resin · HCl

(200-400 mesh)

(1-Hydroxybenzotriazole-6-

sulfonamidomethyl resin · HCl)

2. Racemization Suppressants

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Role of HOBt in DCC-mediated peptide coupling

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3. Phosphonium Reagents

BOP (Q-1980)(Benzotriazol-1-yloxy-tris(dimethylamino)-

phosphonium hexaflorophosphate)

PyBOP® (Q-2715)(Benzotriazol-1-yloxy-tripyrrolidino-phosphonium hexaflorophosphate)

PyAOP(7-Aza-benzotriazol-1-yloxy-

tripyrrolidinophosphonium hexaflorophosphate)

PyOxim (Q-2760)(Ethyl cyano(hydroxyimino)acetato-O2)-

tri-(1-pyrrolidinyl)-phosphonium hexaflorophosphate)

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Mechanism

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4. Aminium/Uronium-Imonium Reagents

TBTU (BF4-) (Q-1665)/ HBTU (PF6

-)(2-(1H-Benzotriazol-1-yl)-N,N,N’,N’-

tetramethylaminium tetrafloroborate/ hexaflorophosphate)

HCTU(2-(6-Chloro-1H-benzotriazol-1-yl)-

N,N,N’,N’-tetramethylaminiumhexaflorophosphate)

HDMC (Q-2765)(N-[(5-Chloro-1H-benzotriazol-1-yl)-

dimethylamino-morpholino]-uranium hexaflorophosphate N-oxide)

COMU (Q-2735)(1-[1-(Cyano-2-ethoxy-2-

oxoethylideneaminooxy)-dimethylamino-morpholino]-uronium hexaflorophosphate)

TATU (BF4-) (Q-2150)/ HATU (PF6

-) (Q-2780)(2-(7-Aza-1H-benzotriazol-1-yl)-N,N,N’,N’-

tetramethylaminiumtetrafloroborate/hexaflorophosphate)

TOTT (Q-2600)(2-(1-Oxy-pyridin-2-yl)-1,1,3,3-

tetramethylisothiouronium tetrafloroborate)

TFFH (Q-2280)(Tetramethylfloroformamidinium hexaflorophosphate)

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Mechanism of HBTU-mediated peptide coupling

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5. Imidazolium Reagents

CBMIT1,1′-Carbonylbis(3-

methylimidazolium) triflate

CDI (N,N'-Carbonyldiimidazole)

CIP/CIB2-Chloro-1,3-dimethylimidazolinium

hexafluorophosphate/tetrafluoroborate

BOI2-(1H-Benzotriazol-1-yloxy)-1,3-dimethylimidazolidium

Hexafluorophosphate

CMBI2-chloro-1,3-dimethyl-1H-benzo[d]imidazol-3-ium

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Mechanism of CDI-mediated peptide coupling

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DPPADiphenylphosphoryl azide

MPTADimethylphosphinothioylazide

DPP-ClDiphenylphosphinic chloride

BOP-ClBis(2-oxo-3-oxazolidinyl)phosphinic

chloride

MPTO3-Dimethylphosphinothioyl-

2(3H)-oxazolone

FDPPPentafluorophenyl

diphenylphosphinate

6. Organophosphorous Reagents

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DPPA, NaHCO3

DMF, 0-5 oC, 3 days

77%

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References

1. So-Yeop Han, S. Y.; Kim, Y. A. Tetrahedron 2004, 60, 2447–2467.

2. Valeur, E.; Bradley, M. Chem. Soc. Rev. 2009, 38, 606-631.

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