1-(3-Dimethylaminopropyl)-3-Ethylcarbodiimide in the Synthesis of Glycyrrhizic Acid Amino-Acid Conjugates

Abstract

Glycyrrhizic acid (GA, 1) is the main triterpene glycoside from roots of Glycyrrhiza glabra L. and G. uralensis Fisch. and a leading natural glycoside with potential as a medicine for developing new immunomodulators and antiviral agents [1,2]. Several conjugates of GA with amino acids and dipeptides displayed pronounced antiviral activity against human immunodeficiency (HIV) [3,4], atypical pneumonia of SARS-CoV [5], Epstein-Barr [6], and flu A/H1N1 viruses [7]. Previously, we proposed methods for preparing conjugates of GA with alkyl and benzyl esters of L-and D-amino acids via activation of the GA carboxylic acids using N-hydroxybenzotriazole (HOBt) and N,N′-dicyclohexylcarbodiimide (DCC) [8], N-hydroxyphthalimide and DCC [9], or N-hydroxysuccinimide (HOSu) and DCC [3]. The main drawback of these methods for preparing amino-acid conjugates of GA is their two steps (preparation of GA activated ester and subsequent reaction of it with an amine). Furthermore, the dicyclohexylurea formed if DCC is used contaminates the target conjugates. Therefore, water-soluble carbodiimides, in particular 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (DEC), which does not form dicyclohexylurea, are more attractive as condensing reagents.The present communication reports the selective synthesis of amino-acid conjugates of GA with free amino acids in the glycoside carbohydrate part using DEC and L-amino-acid t-butyl ester hydrochlorides.Condensation of GA with amino-acid t-butyl ester hydrochlorides (amino-component, AC) was carried out at room temperature in DMF in the presence of DEC with GA-AC/DEC mole ratios 1:2.5-3.0:3.0-3.2 in the presence of an excess of Et 3 N. The ACs were L-amino-acid t-butyl esters (valine, leucine, isoleucine, phenylalanine, methionine). The obtained carboxy-substituted conjugates were deblocked by CF 3 COOH in CH 2 Cl 2 . The target GA conjugates (2-6) were isolated by column chromatography in 55-60% yields. Compounds 2 and 3 were prepared first. The physicochemical properties of 4-6 agreed with those reported earlier [3]. The structures of 3-5 were confirmed by high-resolution PMR (500 MHz) and 13 C NMR spectra (125 MHz). 13 C NMR spectra of the obtained GA conjugates showed additional resonances for the amino-

Glycyrrhizic Acid Derivatives Bearing Amino Acid Residues in the Carbohydrate Part as Dengue Virus E Protein Inhibitors: Synthesis and Antiviral Activity

“…The GL derivatives used in this study may be divided into three groups: Compounds 2 – 8 are GL conjugates containing two L- or D-amino acids methyl esters; group II (Compounds 9 – 16 ) is presented by GL conjugates containing two residues of ethyl esters L- or D-amino acids, and dipeptide Gly-PheOEt; group III includes GL conjugates with free L-amino acids 17 – 21. Compounds 2 [ 42 ] , 3 [ 43 ] , 5 [ 42 ] , 6 [ 42 , 46 ] , 8 [ 47 ] , 9 [ 46 ], 11 [ 46 ], 16 [ 46 ] , 17 – 20 [ 46 , 48 , 49 ] , and 21 [ 46 , 50 ] were previously described and their analytical and spectral data were identical to the published data. Conjugates 2 – 5 with methyl esters of amino acids were produced using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) [ 48 ] in dimethylformamide (DMF) in the presence of an excess of triethyl amine at room temperature (20–22 °C), showing yields of 52–55% after purification by column chromatography (CC).…”

“…Compounds 2 [ 42 ] , 3 [ 43 ] , 5 [ 42 ] , 6 [ 42 , 46 ] , 8 [ 47 ] , 9 [ 46 ], 11 [ 46 ], 16 [ 46 ] , 17 – 20 [ 46 , 48 , 49 ] , and 21 [ 46 , 50 ] were previously described and their analytical and spectral data were identical to the published data. Conjugates 2 – 5 with methyl esters of amino acids were produced using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) [ 48 ] in dimethylformamide (DMF) in the presence of an excess of triethyl amine at room temperature (20–22 °C), showing yields of 52–55% after purification by column chromatography (CC). Compounds 6 – 16 were prepared by the selective activation of COOH groups of the GL glucuronide part with HOSu/DCC [ 46 , 49 , 51 ] in DMF with yields of 55–60%.…”

“…GL conjugates with free amino acids 17 – 21 were synthesized by a new simplified procedure using amino acids as Na salts in a mixture of 1N aqueous NaOH and DMF with yields of 54–56% after the purification by CC. Earlier, we prepared GL conjugates with free amino acids by using tert-butyl esters of amino acids with the subsequent release with CF 3 COOH [ 48 , 49 ] with similar yields. All compounds produced had a purity of ≥95% according to HPLC analysis ( Supplementary Figures S1–S20 ).…”

“…All compounds produced had a purity of ≥95% according to HPLC analysis ( Supplementary Figures S1–S20 ). Analytical and spectral data for GL derivatives 2 [ 42 ] , 3 [ 43 ] , 5 [ 42 ] , 6 [ 42 , 46 ] , 8 [ 47 ] , 9 [ 46 ], 11 [ 46 ], 16 [ 46 ] , 17 – 20 [ 46 , 48 , 49 ] , and 21 [ 46 , 50 ] were identical to the previously published data but their 1 H NMR spectra (500 MHz) are given firstly in this paper. The structures of the novel Compounds 4, 7, 10, 12 – 14, and 15 were confirmed by IR and NMR ( 1 H-500 MHz and 13 C-125 MHz) spectra and elemental analysis.…”

“…Lit. [ 48 ]: [α] D 20 + 58° ( c 0.06; MeOH). HPLC (95.8 ± 0.8%, τ 2.78 min) ( Supplemental Figure S43 ); IR (ν, cm −1 ): 3500-3200 (OH, NH), 1716 (COOH), 1662 (C=O), 1539 (CONH).…”