PyBOP hexafluorophosphate

PyBOP 0-Benzotriazol-l -yloxytris(pyrrolidino)phosphonium hexafluorophosphate... 

FIGURE 2.19 Pyrrolidino instead of dimethylamino substitutents for the environmental acceptability of phosphonium and carbenium salt-based reagents.56 Tetramethylurea from O-benzotriazol- l -yl/V./V./V./V -tetramethyluronium hexafluorophosphate and tetrafluoroborate is more volatile and is cytotoxic. The product released from PyBOP is not environmentally objectionable. PyBOP = benzotriazol-l-yl-oxytripyrrolidinophosphonium hexafluorophosphate. 

PyBOP benzotriazole-l-yl-oxy-tris-pyrrolidino-phosphonium hexafluorophosphate TBS tert-butyldimethylsilyl... 

Abbreviations DCM, dichloromethane DIC, 1,3-diisopropylcarbodiimide DIEA, diiso-propylethylamine DMAP, 4-dimethylaminopyridine DMF, IVJV-dimethylformamide ELSD, evaporative light scattering detection HOBt, hydroxybenzotriazole IR, infrared LC/MS, high-pressure liquid chromatography/mass spectrometry NMM, V-methylmorpho-line NMR, nuclear magnetic resonance PyBop, benzotriazol-l-yloxytripyrrolidino-phosphonium hexafluorophosphate SAR, structure-activity relationship TFP, tetrafluorophenol THF, tetrahydrofuran. 

Peptide synthesis reagents such as diisopropylcarbodiimide (DIC), benzotriazole-1-yl-oxy-tris-pyrrolidino-phosphonium hexafluorophosphate (PyBOP), bromo-tris-pyrrolidino-phosphonium hexafluorophosphate (PyBroP), 2-(lH-benzotriazole-l-yl)-l,l,3,3-tetramethyluronium hexafluorophosphate (HBTU), 1-hydroxybenzotriazole (FlOBt), piperidine, A-methylmorpholine (NMM), trifluoroacetic acid (TEA), triisopropylsilane (TIS), A -diisopropylethylamine (DIPEA, DIEA), l,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,2-ethanedithiol (EDT), and 4-dimethylaminopyridine (DMAP) may be obtained from Sigma-Aldrich, Chemlmpex, and Novabiochem. 

A second straightforward thioesterification method employs simple condensation conditions of the C-terminal carboxylic acid and thiols by using benzotriazol-l-yl-oxytripyrrolidinophosphonium hexafluorophosphate (PyBOP) or other condensation... 

SCHEME 14.2 Synthesis of lipid IV by Wong, Cheng, and coworkers. ClAc, chloroacetyl NIS, /V-iodosucci n i mide Phth, phthaloyl PyBOP, benzotriazol-l-yl-oxytripyrrolidinophos-phonium hexafluorophosphate RRV, relative reactivity value TBAF, tetrabutylammonium fluoride TBS, fert-butyldimethylsilyl Tf, trillyl TMS, trimethylsilyl Tol, 4-tolyl. 

First, the (/f)-2-methylcysteine compound 5 is N- and S-protected and coupled with the threonine methyl ester to the hydroxyamide 57 with benzotriazole-1 -yloxy-tripyrrolidino-phos-phonium hexafluorophosphate (pyBOP) [16]. Burgess reagent [17] turns out to be the best choice for the conversion of 57 into 58. The use of Burgess reagent for the synthesis of oxazo-line was examined extensively by Wipf et al. [ 18]. For other methods of synthesizing oxazoles, which were developed in connection to the synthesis of calyculin A, see [19]. 

PyBOP = benzotriazol-l-yl-A-oxytris(pyrrolidino)phosphonium hexafluorophosphate (Calbiochem-Novabiochem). 

Bt = benzotriazole At = 7-azabenzotriazole cBt = 6-chlorobenzofriazole, Fig 3 BOP = benzotriazolyl-1-oxy-f/7 s(dimethylamino)phosphonium hexafluorophosphate PyBOP = benzotriazolyl-1-oxy-tripyrrolidinophosphonium hexafluorophosphate HBTU = 0-benzotriazol-1-yl-/V,/ /,A/, / / -tetramethyluronium hexafluorophosphate TBTU = 0-benzotriazol-1-yl-A/,/V,/V, W-tetramethyluronium tetrafluoroborate... 

This reagent, benzotriazol-l-yl-oxytripyrrolidinophosphonium hexafluorophosphate (14, PyBOP)P l (Scheme 4), was designed in order to avoid the formation of toxic HMPA during acylation. As with BOP, it is assumed that the first step is the carboxylic acid activation which involves formation of an acyloxyphosphonium salt.P This initial salt is then attacked by the benzotriazolyloxy anion to form the benzotriazolyl active ester which then reacts with the amino component. PyBOP can easily replace the BOP reagent and is especially suitable for solid-phase peptide synthesis. It is soluble in a wide range of solvents such as DMF, di-chloromethane, THF, and NMP. PyBOP is more useful in peptide synthesis on solid support than in solution. The byproduct, tris(pyrrolidino)phosphine oxide is partially water-soluble and is easily removed by washing. PyBOP is used under the same experimental conditions as BOP. Note that PyBOP is a white, crystalline and non-hygroscopic solid. It can be kept as a solid, but solutions of PyBOP cannot be stored for more than 24 hours. 

Abbreviations used Dde, N-[l-(4,4-dimethyl-2,6-dioxocyclohexadiene)]-ethyl DIEA, diisopropylethylamine DMF, N,N-dimethylformamide EDC, l-Ethyl-3-(3 -dimethylaminopropyl)carbodiimide hydrochloride HOBt, 1-hydroxybenzotriazole NHS, N-hydroxysuccinimide NMP, l-methyl-2-pyrroIidinone PAL, Peptide Amide Linker [5-(4-(9-fluorenylmethyloxycarbonyl) aminomethyl-3,5-dimethoxyphenoxy) valeric acid] PyBOP, Benzotriazole-l-yl-oxy-trispyrrolidino-phosphonium hexafluorophosphate TFA, trifluoroacetic acid. 

PyBOP (benzotriazol-l-yloxy)tripyrrolidinophosphonium hexafluorophosphate SE 2-trimethylsilylethyl... 

PyBOP, proceeding through a presumed benzotriazolyl ester intermediate, was more effective than alternative coupling agents such as DCC, DCC/DMAP, DCC/1 -hydroxybenztriazole, and bromotris(dimethylamino)phosphonium hexafluorophosphate. 

SCHEME 10. Synthesis of H2N-(Thr)2-(LacNAc-a-Thr)-(Thr)2-Gly-OH. Conditions (i) H2, Pd(C), MeOH-AcOH (10 1), 1 h (ii) 20% piperidine in DMF (iii) Fmoc-Thr, (benzotriazol-l-yloxy)tripyrrolidi-nophosphonium hexafluorophosphate (PyBOP), IV-hydroxybenzotriazole (HOBt), ethyldiisopropylamine (DIPEA) (iv) Glyco-amino acid 64, PyBOP, HOBt, DIPEA (v) TFA (vi) NaOMe, MeOH. 

Preparation ofS [1] A solution of N-(tert-butyloxy)carbonyliminodiacetic acid (1) (0.349 g, 1.50 mmol) in dimethyl formamide (DMF) (15 ml) was treated with N -(3-dimefhylaminopropyl)-N-ethyl-carbodiimide hydrochloride (EDC) (0.294 g, 1.54 mmol) at 25 °C. The mixture was stirred at 25 °C for 1 h, and then the amine (1 equiv.) was added and the reaction mixture was stirred for 20 h. It was then poured into 10% aqueous HCl (60 mL) and extracted with ethyl acetate (100 mL). The organic phase was washed with 10% HCl (40 mL) and saturated aqueous NaCl (2 x 50 mL), dried (NazSOr), filtered, and concentrated in vacuo to yield the diacid monoamides 2. Each of the diacid monoamides 2 was dissolved in anhydrous DMF (20 mL/mmol) and the solutions obtained were divided into three equal portions in three separate vials. Each solution was then treated with one of the three amines (1 equiv.), diisopropylefhylamine (2 equiv.), and (benzotriazol-l-yloxy)tripyr-rolidinophosphonium hexafluorophosphate (PyBOP) (1 equiv.). Each solution was stirred at 25 °C for 20 h. The respective mixture was then poured into 10% HCl and extracted with ethyl acetate. The organic phase was washed sequentially with 10% HCl, saturated aqueous NaCl, 5% aqueous NaHCOs, and further saturated aqueous NaCl, then dried (Na2SO4), filtered, and concentrated to yield the diamides 3. Each of the diamides 3 was dissolved in 4 N HCl/dioxane (32 mL/mmol) and the respective mixture was stirred at 25 °C for 45 min. The solvent was then removed in vacuo, the residue was dissolved in anhydrous DMF (28 mL/mmol), and the solution obtained was divided into three equal portions, which were placed in three separate vials. Each solution was treated with one of three carboxylic acids (1 equiv.) followed by diisopropylamine (3 equiv.) and PyBOP (1 equiv.) and the mixtures were stirred for 20 h. Each mixture was then poured into 10% HCl and extracted with ethyl acetate. The organic phase was washed sequentially with 10% HCl, saturated aqueous NaCl, 5% aqueous NaHCOs, and further saturated aqueous NaCl, then dried (Na2SO4), filtered, and concentrated in vacuo to yield the final products 5. 

Independent work of Kenner [57], Castro [58], Hmby [59], and Yamada [60] has shown the potential of applying derivatives of trisdimethylaminophos-phonium salts for the activation of carboxylic acids and subsequent preparation of amides and peptides (Fig. 7). The first phosphonium salt-based reagents commercially available were pi-oxo-bis-[tris(dimethylamino)-phos-phonium]-bis-tetrafluoroborate (22) ( Bates reagent ) [61] and benzotria-zol-1 -yl-iV-oxy-tris(dimethylamino)phosphonium hexafluorophosphate (23) (BOB) [62]. Later, Coste et al. [63] described the pyrrolidino derivative of BOP, benzotriazol-1 -yl-7V-oxy-tris(pyrrolidino)phosphonium hexafluorophosphate (24) (PyBOP), which does not form carcinogenic hexamethylphosphoric triamide (HMPA) as a by-product [64]. Furthermore, phosphonium salts derived from HOAt, such as (7-azabenzotriazol-l-yloxy)-tris(dimethylamino)-phosphonium hexafluorophosphate (25) (AOP) and (7-azabenzotriazol-l-yloxy)-tris(pyrrolidino)phosphonium hexafluorophosphate (26) (PyAOP), have also been prepared and are generally more efficient than BOP and PyBOP [31,65,66]. 

PyBOP) [96], (PyFOP) [100]], [(3,4-dihydro-4-oxo-l,2,3-benzotriazin-3-yl)oxy]-tris(pyrrolidino)phosphonium hexafluorophosphate (49) (PyDOP) [100], [(pentafluorophenyl)oxy]tris(pyrrolidino) phosphonium hexafluorophosphate (50), [(PyPOP) [100], (PfOP) [97], (PyPfpOP) [98]], (pyridyl-2-thio)tris(pyrrolidino) phosphonium hexafluorophosphate (51) (PyTOP) [100], and l,3-dimethyl-2-(l-pyrrolidinyl)-2-(3H-l,2,3-triazolo(4,5-b)-pyridin-3-yloxy)-1,3,2-diazaphospholidinium hexafluorophosphate (AOMP)... 

More recently, the use of phosphonium (e.g., benzotriazole-1-yl-oxy-tris-pyrrolidino phosphonium hexafluorophosphate (PyBOP)) and uronium (e.g., 2-(1H- benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU)) coupling reagents have gained wide use in effecting peptide bond formation, particularly in SPPS (Fig. 7.6). The structures of PyBOP and HBTU both incorporate an equivalent of HOBt, and the final reactive Boc or Fmoc amino acid species is the corresponding OBt active ester (13). 

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