Synthesis of amide derivatives for electron deficient amines and functionalized carboxylic acids using EDC and DMAP and a catalytic amount of HOBt as the coupling reagents

Abstract

A convenient protocol for amide bond formation for electron deficient amines and carboxylic acids is described. Amide coupling of aniline derivatives has been investigated with a number of reagents under a variety of reaction conditions. The use of 1 equivalent of EDC and 1 equivalent of DMAP, catalytic amount of HOBt and DIPEA provided the best results. This method is amenable to the synthesis of a range of functionalized amide derivatives with electron deficient and unreactive amines.

Graphical Abstract

Amide functionalities are one of the most common structural features bioactive molecules.^1,2 Not surprisingly, amide coupling reactions are the most frequently used reactions in drug discovery and medicinal chemistry.^3,4 Typically, amide bonds are formed by reaction of an activated carboxylic acid with a particular amine of choice.^5,6 Over the years, a variety of reagents and protocols have been developed for activation of carboxylic acids and coupling of amines. In particular, coupling reagents such as DCC/DMAP, EDC/HOBt, HATU/DIPEA, CDI/Et₃N, and BOPCl/Et₃N are used for many routine coupling reactions.^7–11 However, amide coupling with electron deficient amines is often sluggish and met with less satisfactory results. There are methods developed to deal with these problems.^12–17 As part of our interest in medicinal chemistry optimization involving amide derivatives, we required syntheses of compounds resulting from coupling of aromatic amines and functionalized carboxylic acids. Herein, we report that activation of carboxylic acids with EDC and DMAP in the presence of a catalytic amount of HOBt in acetonitrile provided amide derivatives in good to excellent yields. This reaction protocol also resulted in amide derivatives in excellent yields for electron rich amines and functionalized carboxylic acids.

As shown in Scheme 1, we initially investigated a prototype coupling of Boc-protected valine (1) with 4-amino-N-(4-methoxybenzyl)benzamide (2) with a number of common coupling reagents under a variety of reaction conditions. The results are shown in Table 1. HATU is a widely used reagent for the synthesis of amide derivatives.^18,19 Our coupling reaction of carboxylic acid 1 and amine 2 with 1 equivalent of HATU in the presence of 5 equivalents of DIPEA in DMF at 23 °C for 5 h resulted in 38% yield of amide derivative 3a after silica gel chromatography (entry 1). Reaction 1 and 2 with 1.5 equivalent of BOPCl in the presence of Et₃N (3 equiv) in CH₂Cl₂ at 23 °C was sluggish.^20,21 Coupling product was isolated in 28% yield (entry 2). We examined coupling reaction of 1 and 2 by formation of a mixed anhydride using isobutyl chloroformate (1.5 equiv) in the presence of Et₃N (3 equiv) in CH₂Cl₂ at 23 °C.²² Conversion was slow, however the coupling product 3a was obtained in 65% yield after chromatography (entry 3). Coupling reaction with CDI (1.5 equiv) in the presence of Et₃N (3 equiv) in THF at 23 °C for 48 h, provided only trace amount of coupling product (entry 4). We have also examined coupling reaction with Ph₂SiH₂ (1 equiv) and DIPEA (1 equiv) in the presence of DMAP (0.5 equiv) in acetonitrile at 80 °C for 72 h.¹⁵ However, no coupling product formed under these reaction conditions (entry 5). We investigated coupling using DCC (2 equiv) and DMAP (1 equiv) in CH₂Cl₂ at 23 °C for 4 h. These conditions resulted in product 3a in 28% yield (entry 6). Reaction with DCC (1 equiv) and DMAP (1 equiv) in the presence of DIPEA (5 equiv) in CH₂Cl₂ at 23 °C for 42 h provided only 13% yield of desired coupling product (entry 7). Interestingly, the yield of coupling reaction improved when the reaction was carried out with DCC (1 equiv) and DMAP (1 equiv) in the presence of a catalytic amount (10 mol %) HOBt in acetonitrile at 23 °C for 42 h. This resulted in the coupling product 3a in 51% yield (entry 8). The major drawback for DCC-based coupling reactions has been the problem with chromatographic purification and removal of N,N'-dicyclohexylurea byproduct. We therefore, attempted conditions with EDC for easy removal and work-up. As shown, coupling reaction with EDC (1 equiv) and DMAP (1 equiv) in the presence of DIPEA (5 equiv) in acetonitrile at 23 °C for 42 h resulted in only 11% yield of coupling product 3a. The reaction was sluggish. The corresponding reaction with 2 equiv of EDC improved yield to 19%. The corresponding reaction in the presence of excess of DIPEA (10 equiv) did not improve yield. Reaction with EDC (1 equiv) and HOBt (1 equiv) in the presence of Et₃N (5 equiv) in dichloroethane at 23 °C for 48 h afforded only trace amount of coupling product (entries 9 – 12). The coupling reaction was examined with a catalytic amount (0.1 equiv) of DMAP and a catalytic amount (0.1 equiv) of HOBt, the coupling product 3a was obtained in 38% yield (entry 13). When the coupling reaction was carried out in the presence of DMAP (1 equiv), and a catalytic amount (0.1 equiv) of HOBt, the coupling yield improved significantly to 72% (entry 14). DMAP is essential for efficiency and yields and the role of DMAP may serve as an acyl transfer agent. Thus, this protocol was utilized for coupling of a variety of amines and carboxylic acids. Among various solvents investigated, the coupling reaction in CH₃CN provided the best results. The reaction in CH₂Cl₂ provided comparable results.

Scheme 1.

Amide coupling of acid 1 and amine 2

Table 1.

Coupling of aniline derivative 2 with Boc-valine^a

Entry	Reagents (equiv)	Solvent	Temp (°C)	Time (h)	Yieldb (%)
1	HATU (1), DIPEA	DMF	23	5	38
2	BOPCl (1.5), Et3N (3)	DCM	23	48	28
3	t-BuCOCl (1.5), Et3N (3)	DCM	23	48	65
4	CDI (1.5), Et3N (3)	THF	23	48	Trace
5	Ph2SiH2(1), DMAP (0.5), DIPEA (1)	AcCN	80	72	NP
6	DCC (2), DMAP (1)	DCM	23	4	28
7	DCC (1), DMAP (1), DIPEA (5)	DCM	23	42	13
8	DCC(1), DMAP (1), HOBt (cat)	AcCN	23	42	51
9	EDC (1), DMAP (1), DIPEA (5)	AcCN	23	42	11
10	EDC(2), DMAP (1), DIPEA (5)	AcCN	23	42	19
11	EDC (2), DMAP (1), DIPEA (10)	AcCN	23	72	9
12	EDC (1), HOBt (1), Et3N (5)	DCE	23	48	Trace
13	EDC (1), DMAP (cat), HOBt (cat), DIPEA (5)	AcCN	23	48	38
14	EDC (1), DMAP (1), HOBt (cat), DIPEA (5)	AcCN	40	48	72

^aReactions were carried out 0.2 to 0.5 mmol scale of 1 and 0.1 equiv of HOBt.

^bYields obtained after column chromatography purification.

The role of DMAP is important in this coupling reaction. Presumably, the coupling reaction proceeds as shown in Scheme 2. Reaction of carboxylic acid with EDC in the presence of HOBt results in the formation of reactive HOBt ester 4. DMAP then functions as an acyl transfer agent and forms the highly reactive acyliminium ion intermediate 5 as proposed by Steglich in the esterification reaction.^23,24 The reaction of HOBt ester with electron deficient amines such as aniline is expected to be slow. The reaction of aniline with acyl iminium ion 5 should be relatively faster providing the coupling product 3a with regeneration of DMAP. In principle, only a catalytic amount of DMAP would be necessary. However, coupling requires one equivalent of DMAP for better yield and reaction time.

Scheme 2.

General mechanism of amide coupling

We then investigated the scope and utility of the coupling conditions in entry 14 to a series of carboxylic acids and functionalized amines and the results are summarized in Table 2. Reaction of thiazole carboxylic acid 1b (1.2 equiv) with aniline derivative 2a (1 equiv) in the presence of 1 equivalent EDC, 1 equivalent DMAP, and a catalytic (0.1 equiv) HOBt in CH₃CN at 23 °C for 18 h provided amide derivative 3b in 80% yield after silica gel chromatography using 50% ethyl acetate and hexanes as the solvent system (entry 2).²⁵ Reaction of aniline derivative 2a with 2-(6-methoxynaphthalene-2-yl)propionic acid (naproxen) 1c using conditions in entry 14, resulted in coupling product 3c in 57% yield (entry 3). Reactions of aniline derivative 2a with butanoic acid (1d) and Boc-piperidine carboxylic acid (1e) resulted in amide derivatives 3d and 3e in excellent yields (entries 4 and 5). Coupling of Boc-valine 1a with biphenylamine 2b proceeded well to provide amide derivative 3f in 93% yield (entry 6). This coupling reaction in 1 gram scale, afforded amide derivative 3f with no major change in reaction yield, providing 3f in 91% yield (please see supporting information). Similarly, reaction of Boc-proline 1f and biphenylamine 2b furnished amide derivative 3g in 75% yield (entry7). We have also examined coupling of thiazole carboxylic acid 1b and 4-t-butylaniline derivative 2c which provided amide product 3h in 58% yield (entry 8). We carried out this reaction at higher temperature at 60 °C for 10 h, however, this has resulted only slight improvement of yield to 61%. Reaction of reactive amine such as Boc-piperidine-4-amine 2d with Boc-proline (1f) provided coupling product 3i in excellent yield (entry 9). The corresponding reaction with less reactive carboxylic acid such as naproxen also proceeded well to provide amide product 3j in 70% yield (entry 10). The reaction of amine 2e with naproxen under the above reaction conditions provided coupling product 3k in 62% yield (entry 11). Furthermore, we have examined the efficiency of the above reaction protocol with three secondary amines. As shown, reaction of naproxen 1c with morpholine 2f and ethyl isonipecotate 2g resulted in amide derivatives 3l and 3m in 84% and 66% yields, respectively (entries 12 and 13). Similarly, reaction of thiazole carboxylic acid 1b and proline methyl ester 2h afforded amide derivative 3n in 60% isolated yield (entry 14).

Table 2.

Synthesis of amide derivatives with selected carboxylic acids and primary amines^a,b

Entry	Acids	Amines	Amide Product	Time (h)	Yieldsc (%)
1				48	72
2				18	80
3				22	57
4				48	99
5				24	88
6				12	93d
7				12	75
8				10	58e
9				12	98
10				19	70
11				13	62
12				9	84
13				10	66
14				12	60

^aReactions were carried out 0.5 to 1 mmol scale of acid at 23 °C except entry 1 which was carried out 40 °C

^bAll coupling products were fully characterized by standard spectroscopic techniques.

^cYields are after purification by silica gel chromatography.

^dReaction yield was 91 when the reaction was carried out in 1g scale.

^eProduct 3h was obtained in 61% when the reaction was carried out at 60 °C for 10 h.

We have also investigated coupling reactions of hydroxyethylamine sulfonamide isostere 2i which has been utilized in the design and synthesis of nonpeptide HIV-1 protease inhibitors.²⁶ As shown in Scheme 3, reaction of amine 2i with Boc-valine 1a in CH₃CN at 23 °C for 14 h provided coupling product 3o as the single product in 65% yield. Similarly, reaction of amine 2i with naproxen 1c under similar conditions furnished amide derivative 3p in 62%. Since these derivatives contain dipeptide isosteres embedded in HIV-1 protease inhibitors, we examined protease inhibitory activity using an assay developed by Toth and Marshall.²⁷ Compounds 3o and 3p exhibited HIV-1 protease inhibitory activity with Ki values of 1055 nM and 258 nM, respectively. In comparison, darunavir showed Ki value of 14 pM, in the same assay.²⁸

Scheme 3.

Amide coupling and synthesis of HIV-1 protease inhibitors

In conclusion, we have developed an efficient protocol for the synthesis of amide derivatives with electron deficient and unreactive amines using EDC, and catalytic amount of HOBt in the presence of DMAP. The use of DMAP is critical for reactivity and yields. The reagents are readily available. The reaction is likely to procceed through the acyliminum ion intermediate. The scope and utility of this method has been demonstrated with a variety of amines and carboxylic acids, providing good to excellent yields of amide derivatives. Two of these derivatives 3o and 3p containing hydroxysulfonamide isosteres were assayed for HIV-1 protease inhibitory activity. Both compounds showed low micromolar Ki values. Further use of these methods in the synthesis of bioactive molecules is in progress.