PyClock | CAS 893413-42-8 | ≥95%

PyClock — (6-chlorobenzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate — is a phosphonium peptide-coupling reagent and the 6-chloro analogue of PyBOP. In the presence of a tertiary amine base, it converts carboxylic acids to 6-chlorobenzotriazol-1-yl (Cl-OBt) active esters, which then acylate amines to form amide bonds. Because 6-chloro-HOBt is more acidic than HOBt, the Cl-OBt ester is considerably more reactive than the OBt ester formed by PyBOP or HBTU. As a phosphonium salt, PyClock avoids the N-terminal guanidinylation side reaction associated with uronium/aminium reagents such as HBTU and HATU, so excess PyClock can be used in slow cyclisations and fragment condensations without end-capping the peptide chain. PyClock is supplied as a white to off-white crystalline powder, soluble in DMF, NMP and dichloromethane; prolonged preactivation should be avoided because the Cl-OBt active ester is more reactive and less persistent than the HOBt ester.

Common Scientific Applications

• Carboxyl activation: with a tertiary amine base such as DIPEA, NMM or 2,4,6-collidine, PyClock converts carboxylic acids to reactive Cl-OBt esters for amide bond formation.
• Solid-phase and solution-phase peptide synthesis: an effective activator for Fmoc- and Boc-strategy SPPS and solution-phase peptide coupling.
• Sluggish and difficult couplings: the high reactivity of the Cl-OBt ester suits couplings where activation is slow, including difficult sequences.
• Cyclisation and fragment condensation: used for head-to-tail peptide cyclisation and fragment condensation, where tolerance of excess reagent is useful for slow conversions.
• Low racemisation: the 6-Cl-HOBt-derived active ester reduces epimerisation during activation; the weaker base collidine is used for racemisation-prone residues.
• Advantage over uronium and BOP reagents: unlike HBTU, HCTU and HATU it forms no guanidinylated end-capping by-product at the amine, and unlike BOP it generates no HMPA by-product.

Further Reading

For guidance on selecting between carbodiimides, aminium/uronium salts, phosphonium salts and other reagent classes for amide and peptide bond formation, see NorrChemica's Lab Journal guide: Choosing a Coupling Reagent for Amide and Peptide Bond Formation.