酰胺的合成方法（英语）

2.4.2.4.2. Acyloxyboron intermediates . . . . . . . . . . . . . . . . . . . . . . . . . . 10836 Tetrahedron 61 (2005) 10827–10852 * Corresponding author. Tel.:C44 1235 86 15 61; fax:C44 1235 44 15 0 2.4.2.4.3. O-acylisourea using carbodiimides as coupling reagents . . . . . . 10837 2.5. Esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10838 2.5.1. Alkyl esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10838 2.5.2. Active esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10839 2.5.2.1. Multistep procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10840 2.5.2.1.1. Succinimidyl esters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10840 2.5.2.1.2. Use of 1,2,2,2-tetrachloroethyl chloroformate as intermediate . . 10840 2.5.2.1.3. Isoxazolium salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10840 2.5.2.2. One-pot solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10841 2.5.2.2.1. Phosphonium salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10841 2.5.2.2.2. Uronium salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10843 2.5.2.2.3. Ammonium salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10844 2.5.2.2.3.1. Triazinyl esters . . . . . . . . . . . . . . . . . . . . . . . . . 10844 2.5.2.2.3.2. Mukaiyama’s reagent . . . . . . . . . . . . . . . . . . . . . 10844 Tetrahedron report number 740 Amide bond formation and peptide coupling Christian A. G. N. Montalbetti* and Virginie Falque Evotec, 112 Milton Park, Abingdon OX14 4SD, UK Received 2 August 2005 Available online 19 September 2005 Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10828 2. Amide bond formation: methods and strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10828 2.1. Acyl halides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10829 2.1.1. Acyl chlorides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10829 2.1.1.1. Acyl chloride formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10829 2.1.1.2. Coupling reactions with acyl chlorides . . . . . . . . . . . . . . . . . . . . . . . . . . . 10831 2.1.1.3. Limitations of acyl chlorides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10831 2.1.2. Acyl fluorides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10831 2.1.3. Acyl bromides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10832 2.2. Acyl azides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10832 2.3. Acylimidazoles using CDI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10833 2.4. Anhydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10834 2.4.1. Symmetric anhydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10834 2.4.2. Mixed anhydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10834 2.4.2.1. Mixed carboxylic anhydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10834 2.4.2.2. Mixed carbonic anhydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10834 2.4.2.3. N-carboxy anhydrides or Leuch’s anhydrides . . . . . . . . . . . . . . . . . . . . . . 10835 2.4.2.4. Broadened concept of mixed anhydrides . . . . . . . . . . . . . . . . . . . . . . . . . . 10836 2.4.2.4.1. Ethoxyacetylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10836 0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. Keywords: Amide; Carboxamide; Peptide; Coupling; Condensation; Ligation; Amidation; Aminolysis; Acyl halide; Acyl chloride; Acyl azide; CDl; Acylimidazole; Anhydride; Mixed anhydride; Ester; Activated ester; Activated acid; Phosphonium salt; Uranium salt; Ammonium salt; Protease; Amidase; Lipase; Enzyme; N-Carboxyanhydride; Acylboron; Coupling reagent; Polymer-supported; Solid-phase. 9; e-mail: christian.montalbetti@evotec.com doi:10.1016/j.tet.2005.08.031 2.6. Other coupling methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10845 2.6.1. Staudinger ligation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10845 2.6.2. Using proteases and amidases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10845 2.6.3. Microwave activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10846 2.6.4. Solid-phase strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10846 rted s nts . y . . . . . . . . . . . . . . lies on the 3 The direct temperatu patible wi Section 2. attack by the amino group is necessary (Scheme 2). the yield, to reduce the amount of by-products, to improve selectivity, to facilitate the final purification, to define a scalable process or to exploit more economical reagents. In the last two decades, the combined rapid development of solid-phase technologies and coupling methods has enabled parallel synthesis to become a tool of choice to produce vast amounts of diverse compounds for early discovery in the pharmaceutical industry. here are tives with isolated Scheme 1. E C. A. G. N. Montalbetti, V. Falque / Tetrahedron 61 (2005) 10827–1085210828 side of hydrolysis rather than synthesis. condensation of the salt can be achieved at high re (160–180 8C),4 which is usually quite incom- th the presence of other functionalities (see also 6.3). Therefore, activation of the acid, attachment azides, acylimidazoles, anhydrides, esters etc. T different ways of coupling reactive carboxy deriva an amine: † an intermediate acylating agent is formed and then subjected to aminolysis the amide bond formation has to fight against adverse thermodynamics as the equilibrium shown in Scheme 1 and Carboxy components can be activated as acyl halides, acyl 2.6.4.1. Classical polymer-suppo 2.6.4.2. Polymer-supported reage 2.6.4.3. Catch and release strateg 3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . References and notes . . . . . . . . . . . . . . . . . . . . 1. Introduction The amide functionality is a common feature in small or complex synthetic or natural molecules. For example, it is ubiquitous in life, as proteins play a crucial role in virtually all biological processes such as enzymatic catalysis (nearly all known enzymes are proteins), transport/storage (haemo- globin), immune protection (antibodies) and mechanical support (collagen). Amides also play a key role for medicinal chemists. An in-depth analysis of the Comprehensive Medicinal Chemistry database revealed that the carboxamide group appears inmore than 25% of known drugs.1 This can be expected, since carboxamides are neutral, are stable and have both hydrogen-bond accepting and donating properties. In nature, protein synthesis involving a sequence of peptide coupling reactions (amide bond formation between two a-amino acids or peptides) is very complex, probably to safeguard the unique and precisely defined amino acid sequence of every protein. This barrier is overcome in vivo by a selective activation process catalysed by enzymes,where the required amino acid is transformed into an intermediate amino ester. This intermediate is then involved in a process mediated by the coordinated interplay of more than a hundred macromolecules, including mRNAs, tRNAs, activating enzymes and protein factors, in addition to ribosomes.2 Amide or ester bond formation between an acid and, respectively, an amine or an alcohol are formally conden- sations. The usual esterifications are an equilibrium reaction, whereas, on mixing an amine with a carboxylic acid, an acid– base reaction occurs first to form a stable salt. In other words, ster bond versus amide bond formation. 2. Amide bond formation: methods and strategies Hence, a plethora of methods and strategies have been developed and these are now available for the synthetic, medicinal or combinatorial chemist. Relevant examples of these methods are indicated in this report. The chemist might have to screen a variety of such conditions to find the method best adapted to his situation. For example, due to poor reactivity or steric constraints in some extreme cases, the challenge will be to get the amide formed at all. In other situations, the chemist will require the reaction to avoid racemisation. In general, the aim could also be to optimise Scheme 2. Acid activation and aminolysis steps. ynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 10847 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10848 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10848 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10848 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10848 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10848 of a leaving group to the acyl carbon of the acid, to allow † a reactive acylating agent is formed from the acid in a separate step(s), followed by immediate treatment with the amine † the acylating agent is generated in situ from the acid in the presence of the amine, by the addition of an activating or coupling agent. As illustrated in the Section 1, amide bond formation can often present difficulties such as low yields, racemisation, degradation, difficult purification etc. To face these challenges, numerous mild coupling reagents and methods have been developed that not only are high yielding, but that potentially help to prevent racemisation of neighbouring chiral centres. A classical example of racemisation is encountered in peptide synthesis when the terminal acid peptide is activated, leading to the formation of the corresponding oxazolone 1a. Under mild basic conditions, the oxazolone undergoes racemisation via the formation of conjugated anionic intermediate 2. The resulting oxazolone 1a, 1bmixture reacts then with a nucleophile, explaining the loss of chiral integrity of the coupled material 3a, 3b (Scheme 3). Therefore, peptides are usually grown at the N-terminus and mild activation conditions are needed. In this latter approach, the activation is advantageously performed on an N-protected a-amino acid, thus avoiding the oxazolone formation. 2.1. Acyl halides chlorides) are one of the easiest methods to activate an acid and numerous acyl chlorides are commercially available. This is usually a two-step process, involving first the conversion of the acid into the acyl halide followed by the coupling itself. 2.1.1.1. Acyl chloride formation. Thionyl chloride SOCl2 4, 5 oxalyl chloride (COCl)2 5, 6,7 phosphorus trichloride PCl3, 8 phosphorus oxychloride POCl3 9 and phosphorus pentachloride PCl5 10 are commonly used to generate acyl chlorides from their corresponding acids. Phoshonium pentachloride is generally used for aromatic acids, which contains electron-withdrawing substituents and which do not react readily with thionyl chloride 4.11 The mechanism of acid chloride formation using thionyl chloride 4 or oxalyl chloride 5 is illustrated in Scheme 4. Caution: it is important to note that the use of oxalyl chloride 5 is accompanied by the stoichiometric production of two molecules of gas, one of which is carbon monoxide.12 The generated volume of gas and resulting chemical or safety hazards should always be taken into consideration before setting up these reactions.13 These reactions are often promoted by the addition of a drop of dimethylformamide (DMF).14 The catalytic role of DMF is described in Scheme 5. One of the major disadvantages of the previously cited pling. C. A. G. N. Montalbetti, V. Falque / Tetrahedron 61 (2005) 10827–10852 10829 Scheme 3. Oxazolone-mediated racemisation occurring during peptide cou 2.1.1. Acyl chlorides. Acyl chlorides (also called acid Scheme 4. Mechanism for acyl chloride formation using oxalyl chloride 5 or thi chlorinating agents is the production of HCl. Some substrates (e.g., those containing Boc-protected amines) onyl chloride 4. Scheme 5. Activation with DMF: catalytic cycle. C. A. G. N. Montalbetti, V. Falque / Tetrahedron 61 (2005) 10827–1085210830 are acid sensitive and require non-acidic conditions. For example, cyanuric chloride (2,4,6-trichloro-1,3,5-triazine) 6 is used to carry out acyl chloride formation in the presence of triethylamine.15 The presence of this organic base maintains the basic pH conditions throughout the reaction. The proposed mechanism involves an initial aromatic nucleophilic substitution that generates the corresponding activated aromatic ester 7 and the chlorine anion. The following step is the nucleophilic attack of the chlorine anion on the activated ester to generate the desired acyl chloride (Scheme 6). Cyanuric chloride 6 is a suitable activating agent for the large-scale manufacture of amides.16 The process presents many advantages. It involves only 0.33 equiv of triazine promoter, which minimises reagent utilisation and by- product generation. Inexpensive inorganic bases may be used instead of amine bases and the reaction tolerates water. The resulting cyanuric acid by-product can be easily removed by filtration and with a basic wash. Neutral conditions have also been developed and provide mild conversion of carboxylic acid into acyl chloride. For example, triphenylphosphine (TPP) and a source of chloride Scheme 6. Acyl chloride formation using cyanuric chloride 6. Scheme 7. Acyl chloride formation using TPP and carbon tetrachloride. have been studied. Carboxylic acids are converted by TPP and carbon tetrachloride into the corresponding acyl chloride,17 analogous to the conversion of alkyl alcohols into alkyl chlorides.18 It is suggested that initial formation of triphenyltrichloromethylphosphonium chloride 8 occurs with further reaction yielding chloroform and triphenyl- acyloxyphosphonium chloride (Scheme 7). Difficulties to separate the product from the phosphorus- containing by products can be avoided by the use of polymer-supported phosphine–carbon tetrachloride reagent. Caution: the toxicity and environmental risks19 associated with carbon tetrachloride render this procedure less attractive. Carbon tetrachloride can be substituted by hexachloroacetone.20 Villeneuve has demonstrated that carboxylic acids could be converted by hexachloroacetone and TPP at low temperature into the corresponding acyl chloride. This method was also applied to generate highly reactive formyl chloride. Alternatively, trichloroacetonitrile and TPP also provide mild and efficient conditions.21 Other neutral conditions are described by Ghosez et al. using tetramethyl-a-chloroenamine 9.22 During this pro- cess, the formation of hydrogen halides is avoided. Thus, this method is extremely useful when acid-labile protecting groups are present (Scheme 8). 2.1.1.2. Coupling reactions with acyl chlorides. The amide bond is formed by reacting the acyl chloride with the desired amine (Scheme 9). An additional base is usually required to trap the formed HCl and to avoid the conversion of the amine into its unreactive HCl salt. Couplings are usually performed in inert dry solvents, in the presence of a non-nucleophilic tertiary amine (NEt3, 23 iPr2NEt [also called Hu¨nig’s base], or N-methylmorpholine). Having said that, acyl chlorides are often robust enough to be coupled to amines under aqueous conditions, for example, in the presence of NaOH24 (Schotten–Baumann conditions). These reactions can be accelerated with a catalytic amount of pyridine or N,N-dimethylaminopyridine (DMAP).25 In some cases, pyridine is used as the solvent. The formation of an intermediate acylpyridinum salt 10 is stipulated (Scheme 10). The use of metallic zinc can also accelerate the coupling at room temperature. The method is applicable to alkyl, aryl, heterocycles, carbohydrates and amino acids and leads to high yields.26 2.1.1.3. Limitations of acyl chlorides. Nevertheless, acyl chlorides have limited value in peptide coupling because of the danger of hydrolysis, racemisation, cleavage of protecting groups and other side reactions (e.g., N-carboxy anhydride formation, see Section 2.4.2.3). The tendency of acyl chlorides to racemise under basic conditions can be illustrated by the standard synthesis of ketenes.27 Ketenes 11 are formed by reacting an acyl chloride containing an a proton with NEt3. The ketene 11 can further react with a nucleophile such as an amine to yield the corresponding addition product with an obvious they are useful in peptide chemistry. They react in the Scheme 8. Use of Ghosez chlorination agent 9. C. A. G. N. Montalbetti, V. Falque / Tetrahedron 61 (2005) 10827–10852 10831 Scheme 11. Potential racemisation via ketene formation. Scheme 10. Catalytic role of pyridine. Scheme 9. Aminolysis. Scheme 12. Acyl fluoride formation using cyanuric fluoride 12. same way as acyl chlorides. loss of chiral integrity (Scheme 11). 2.1.2. Acyl fluorides. Racemisation and side reaction problems can sometimes be avoided by using acyl fluorides as active intermediates.28 Acyl fluorides are, indeed, less moisture sensitive than acyl chlorides and more reactive towards amines. Another advantage is that they are compatible with Fmoc or Cbz N-protections and even with tBu esters or other acid-labile ester groups, and thus 29 Administrator 线条 Administrator 线条 / Tetrahedron 61 (2005) 10827–10852 Acyl fluorides are commonly formed using cyanuric fluoride 1230 in the presence of pyridine and react in a similar way to cyanuric chloride 6 (Scheme 12). Alternatively, N,N-tetramethylfluoroformamidinium hexa- Scheme 14. Acyl bromide formation using TPP and NBS 14. Scheme 13. Acyl fluoride formation using TFFH 13. C. A. G. N. Montalbetti, V. Falque10832 fluorophosphate (TFFH) 13 can be used in the presence of Hu¨nig’s base.31 This salt is advantageous in being non- hygroscopic and stable to handling under ordinary con- ditions. The postulated two-step mechanism is described in Scheme 13. The TFFH 13 activation uses the urea formation as the driving force. Very similar reagents are used as one- pot coupling reagents and do not require the isolation of the intermediate acyl chloride.32 Diethylaminosulphur trifluoride (DAST) Et2NSF3 33,34 and deoxofluor (MeOEt)2NSF3 35 have been used to convert carboxylic acid or acyl chloride into carbonyl fluoride. These fluorinating agents have the advantage of reacting in the absence of a base. Differential scanning calorimetry (DSC) studies suggest that deoxofluor is safer to use on a large scale than DAST, as its exotherm is gradual over a wider temperature range and easier to control. 2.1.3. Acyl bromides. Acyl bromides are used on some rare occasions to generate amide bonds. a-Bromoacetyl bromide is one of the most common examples. Acid bromides prepared with phosphorus pentabromide usually also undergo a-bromination.36 O