O-Benzotriazol-1-yl-N,N,N,N-tetramethyluronium tetrafluoroborate

[125700-67-6]  · C11H16BF4N5OP  · (MW 321.09)

(reagent used for peptide coupling)

Alternate Name: TBTU.

Physical Data: mp 205 °C.

Solubility: soluble in DMF (0.6 mol L-1).

Form Supplied in: white solid; widely available.

Purification: can be purified by crystallization from a mixture of MeOH and i-PrOH.

Handling, Storage, and Precautions: very stable, not hygroscopic and can be stored more than 2 years. Violent decomposition can occur when dried at elevated temperature.1

Peptide Coupling Reagent

O-Benzotriazol-1-yl-N,N,N,N-tetramethyluronium tetrafluoroborate (TBTU) and hexafluorophosphate (HBTU) have been found to behave similarly in solution and in solid-phase peptide synthesis.1,2,3 This reagent gives lower racemization in solid-phase than in solution-phase1 and has been implemented in automated stepwise peptide synthesis on different resins by means of Fmoc/t-Bu4 and Boc/Bn5 strategies employing a combination of TBTU:HOBt:DIEA (1:1:1.5, threefold excess) in DMF. Thus, coupling yields are higher than 95% by using coupling times of ca. 12 h.6 The protected segments 74-99 of HIV 1-protease have been prepared using TBTU:NMM and Fmoc/t-Bu strategy on TentaGel resin.7 However, in conventional Fmoc solid-phase peptide synthesis (SPPS) with TBTU:HOBt:DIEA, considerable racemization has been detected for Fmoc-Cys (Trt) residue, which requires a base-free activation step.8

The preparation of cyclic peptides is focused on the total synthesis of natural products with important biological activities, being carried out both in solid- or solution-phase by a lactamization process with condensing reagents.9 Thus, in a comparison study of different reagents during the cyclization of hexapeptides, TBTU was 5 to 70 times faster than diphenylphosphoryl azide (DPPA) or HBTU.4a,4b The solid-phase head-to-tail cyclization using TBTU:DIEA activation gave quantitative yields for cyclic peptides containing Asp, Asn or Glx residues.10 The solution-phase macrolactamization of a cyclopeptide analog of neuropeptide Y has been carried out with TBTU:HOBt:DIEA under high dilution conditions.4a TBTU has shown to be faster than diisopropylcarbodiimide (DIC): HOBt in the synthesis of tyrosine-containing cyclic 10 to 12-membered cyclopeptides.11 For the solid-phase cyclization of the sponge origin cyclopeptide stylopeptide 1, TBTU: HOBt:DIEA in dichloromethane gave higher yields than HBTU, N,N-bis(2-oxo-3-oxazolidinyl)phosphinic chloride (BOP-Cl), or diethyl phosphorocyanidate (DEPC).12 Moreover, 1-hydroxy-7-azabenzotriazole (HOAt)containing uronium or phosphonium salts and TBTU were used as coupling reagents in the cyclization of thymopentin-derived pentapeptides, the later giving lower degrees of epimerization on the C-terminal Tyr residue.13 The combination TBTU:HOAt afforded better yields in the cyclization of the (40R,45R)-isomer of the marine metabolite trunkamide A than the usually used reagent O-(7-azabenzotriazol-1-yl)-N,N,N,N-tetramethyluronium hexafluorophosphate (HATU).14 Another selected application of TBTU was in the total synthesis of the sponge origin cyclotheonamide B.15 This coupling reagent was used in the convergent synthesis of the segments Pro-hArg-D-Phe and vinylogous Tyr-Dpr to give the linear pentapeptide, as well as in its lactamization step under dilution conditions (1). In addition, for the synthesis of the antineoplastic constituent of marine sponge phakellistatin 2 (cyclo-Tyr-Pro-Phe-Pro-Ile-Ile-Pro), the final cyclization has been performed using several coupling reagents, such as TBTU, BOP-Cl, PyBroP [bromotris(pyrrolidino)phosphonium hexafluorophosphate], and TBTU:HOAt, the former giving the highest yield (55%).16 However, in the case of phakellistatin 5 (cyclo-Phe-Asn-Ala-Met-Ala-Ile-Pro), the cyclization step gave better results with HATU, whereas only a 15% yield was obtained employing TBTU:DIEA.17 Cyclic hexapeptides composed by alternating L-proline and substituted 3-aminobenzoic acid subunits, able to bind monosaccharides, have been prepared by means of TBTU:DIEA.18

In the solution-phase synthesis of sterically hindered aminoisobutyric acid (Aib)containing thiopeptides, TBTU has been employed as coupling reagent in the presence of HOBt and DIEA at room temperature (2).19

Amidation

In the preparation of a library of the tumor-promoting teleocidin, the synthesis of the N13-des(methyl)indolactam V core structure has been carried out by means of TBTU: HOBt in N-methylpyrrolidone (NMP) in solution-phase and in solid-phase (3).20 In the synthesis of seco forms of the peptide metabolite diazonamide A, a potent tumor-growth inhibitor, the amide bond of the ring and the valine appendage have been incorporated by means of the combination TBTU:DIEA.21

Peptide nucleic acid (PNA) oligomers replace the oligonucleotide backbone of DNA with a poly[N-(2-aminoethyl)glycine] backbone. For solid-phase synthesis of dithiasuccinoyl (Dts)-protected PNA oligomers, the corresponding monomer has been incorporated by means of HBTU in NMP, whereas TBTU gave similar results by reducing DIEA:coupling reagent ratio to 1:3.22 The TBTU:TEA combination is a very fast reagent (10 min) for the anchoring of nucleoside-3-O-succinates to a widely used alkylamine-controlled pore glass (LCAA-CPG) for solid-phase synthesis of oligonucleotides.23 Another resin for solid-phase C-glycopeptide synthesis, polyethylene glycol polyacrylamide (PEGA), has been derivatized with p(hydroxymethyl)benzoic acid linker by means of TBTU:NEM (N-ethylmorpholine), which has also been employed in the incorporation step of the unprotected C-glycosyl azido acid building block.24

Iron(III) hexadentate 3-hydroxy-2(1H)pyridinone ligands have been prepared under solution-phase conditions by coupling of its benzyl ether with tris(2-aminoethyl)amine (TREN), 1,3,5-tris(aminomethyl)benzene (TRAM) or 1,3,5-tris(methylaminomethyl)benzene (TRMAM) in the presence of TBTU and NMM as base.25 Moreover, (E)-Nisopropyl-5-tosyl-4-pentenamide, a precursor of a d-acyldienyl anion equivalent, has been prepared in 80% yield from the corresponding acid by means of TBTU, whereas it was obtained in 70% overall yield through the acyl chloride generated with oxalyl chloride.26 The tosylated crotonamide (E)-N-isobutyl-4-tosyl-2-butenamide (1) has been obtained in 80% yield from the acid by means of TBTU, but only in 50% by using the acyl chloride.27 In the preparation of the chiral auxiliaries (R)- and (S)-aaminoisovalerophenone hydrochlorides (2), which have been used for the construction of pyrazinone alanine28a and glycine28b templates, their N,N-dimethylamides of N-BocD- and L-valine precursors have been synthesized by using TBTU.

Weinreb amides can be prepared in high yield and without purification by treatment of carboxylic acids with TBTU:DIEA and MeONHMe-HCl in acetonitrile at room temperature.29 However, this coupling with 1-(3-dimethylaminopropyl)-3ethylcarbodiimide hydrochloride (WSC or EDC) gave lower yields of the corresponding hydroxamates, chromatographic purification being necessary.29a

A new application of TBTU is the use as cleavage reagent for tetrahydropyranyl, silyl, and 4,4-dimethoxytrityl ethers in a mixture of acetonitrile and water. THP and DMT ethers can be removed in the presence of the TBDMS group.30

The polymer-bound TBTU (P-TBTU, 3) has been prepared31 from polystyrene-bound 1hydroxybenzotriazole (P-HOBt)32 and has been used for di and tripeptide synthesis with pyridine as base, even in wet acetonitrile. Yields are similar or slightly lower than those using TBTU, lower racemization ratios than with HATU being obtained.

Related Reagents.

HBTU.


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Carmen Nájera

Universidad de Alicante, Alicante, Spain



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