HBTU

Technical Specifications

Product Description & Scientific Applications

HBTU (commonly named O-(1H-benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate; structurally the guanidinium N-oxide salt, 1-[bis(dimethylamino)methylene]-1H-benzotriazolium 3-oxide hexafluorophosphate) is a benzotriazole-based uronium/guanidinium coupling reagent for Fmoc solid-phase and solution-phase peptide synthesis. X-ray and solution studies confirm the guanidinium N-oxide structure. HBTU activates carboxylic acids to reactive 1-hydroxybenzotriazolyl (OBt) active esters under mild conditions, giving high coupling yields with low racemisation.

Common Scientific Applications

Fmoc solid-phase peptide synthesis (SPPS): a standard coupling reagent for the dominant methodology for assembling peptides of up to roughly fifty residues on polymeric resin supports. The Fmoc-amino acid is pre-activated with HBTU and a tertiary-amine base (DIPEA or NMM) in DMF to form a benzotriazolyl active ester that acylates the free α-amino group of the resin-bound chain. It is compatible with standard Fmoc-amino acid building blocks on manual and automated synthesisers; its fast, mild activation suits high-throughput parallel synthesis.

Racemisation suppression with HOBt: adding 1-hydroxybenzotriazole (HOBt) as an auxiliary nucleophile further suppresses racemisation at the α-carbon during activation of sterically demanding or racemisation-prone residues such as cysteine and histidine.

Solution-phase amide-bond formation: activates a broad range of carboxylic acids — N-protected amino acids, aromatic acids, and sterically hindered substrates — for coupling with primary and secondary amines under mild conditions. The water-soluble tetramethylurea by-product is removed by aqueous extraction, unlike the insoluble urea precipitates from carbodiimide reagents such as DCC and EDC.

Macrolactamisation and cyclic peptides: drives head-to-tail intramolecular ring closure at high dilution, suppressing oligomerisation, to give cyclic peptides with distinct conformational and metabolic-stability properties relative to linear analogues.

Fragment condensation: enables convergent peptide synthesis by coupling protected fragments; it carries a risk of C-terminal epimerisation via oxazolone formation unless the activated residue is glycine or proline.

Acyl azides, ureas, and carbamates: HBTU activates carboxylic acids toward reaction with an azide source such as sodium azide to give acyl azides; on warming, these undergo the Curtius rearrangement to isocyanates, trapped by amines or alcohols to furnish ureas and carbamates — including dipeptidyl urea esters and symmetric and asymmetric ureas in one-pot procedures. The chemistry tolerates Fmoc, Boc, and Cbz protecting groups across diverse functional groups.

Quinoxaline synthesis: serves as a non-metal condensing catalyst for quinoxaline derivatives from 1,2-diamines and 1,2-diketones such as benzil, under mild, short-reaction-time conditions.

Comparison with related coupling reagents: HBTU and HATU share the same tetramethyl uronium/guanidinium cation but differ in the heteroaromatic leaving group — benzotriazol-1-yl (OBt) for HBTU, 7-azabenzotriazol-1-yl (OAt) for HATU. HATU couples faster and gives lower epimerisation at higher cost; HBTU is the routine choice, HATU reserved for difficult sequences, hindered residues, and fragment condensations. Related reagents include TBTU (tetrafluoroborate counter-ion), HCTU (6-chloro-1-hydroxybenzotriazole-derived), and COMU (morpholino-based, improved safety profile).

Further Reading

Choosing a Coupling Reagent for Amide and Peptide Bond Formation.

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