With Unprotected Amino Acids to Tetrazolo Peptidomimetics
Here we describe the direct usage of C,N-unprotected amino acids in the Ugi-tetrazole reactions to produce a novel class of acid-tetrazole compounds. Surprisingly, only tetrazole Ugi product is found and not traces of other possible Ugi type reactions. Based on this reaction pathway we have designed the synthesis of novel tetrazole-peptidomimetics. A high level of structural diversity can be achieved with this isocyanide based multicomponent reactions (IMCRs), providing a platform for the production of functionalized building blocks for novel bioactive molecules and nontraditional scaffolds which previously were not accessible.
<p>There have been enormous developments in both the Ugi- and Ugi-tetrazole reactions since 1959, which provides a general route to diverse peptide, peptidomimetics and other complex molecules.1 In multicomponent reactions (MCR), isocyanide based multicomponent reactions (IMCRs) are highly convergent processes which have had a great impact in pharmaceutical and drug discovery research.2 Although IMCRs have proven to be a solution for many common problems in synthetic chemistry such as diversity, complexity and harsh conditions, novel scaffolds, for example by the use of bi-functional substrates are of high interest.3</p><p>N-substituted amino acids are interesting scaffolds as their parent play a key role in biochemistry and are the structural units of peptides, proteins and enzymes. Moreover protected amino acid derived Ugi/Ugi-tetrazole products can efficiently act as biomimetics undergoing competing interactions at the active site of enzymes and cell receptors. In 1996 Ugi reported the use of α-amino acids as bifunctional educts in the Ugi-5-center-4-component reaction (U-5C-4CR) instead of the amine and acid components in the familiar Ugi-4-component reaction (U-4CR).4 Later on very effective variations of the Ugi-reaction were reported by using amino acids as one of the starting component.5 Yudin et.al., Orru et.al. and Paixão et.al. have shown the great impact of the direct use of amino acid in Ugi-type reactions.5d, 5c & 5h Our group also reported a "truly" four-component reaction for the synthesis of iminodicarboxamides by employing amino acids directly in an U-5C-4CR.5e In 2013, Lämmerhofer et.al reported the synthesis of phosphopeptidomimetics by utilizing aminophosphonic acids in U-5C-4CR.5g Even though unprotected amino acids and aminophosphonic acids are utilized in various Ugi-type reactions, to the best of our knowledge, this is the first study on the use of unprotected amino acids in the Ugi-tetrazole reaction (Scheme 1).</p><p>This finding is of importance since the tetrazole is a known bioisostere of the carboxylic acid with often superior pharmacokinetic/pharmacodynamic (PKPD) properties.6 Even so the Ugi tetrazole reaction has developed tremendously in recent years the use of unprotected amino acids is unprecedented and surprising.7</p><p>Therefore we report herein a mild and efficient technology for the synthesis of highly substituted N,C-unprotected but N-substituted acid-tetrazoles 4 and peptido-tetrazoles 6 using Ugi-tetrazole/Ugi reactions from natural N,C-unprotected amino acids, di and tripeptides and commercially available aldehydes/ketones, amines and isocyanides [Scheme 1, b]. In continuation of our previous tetrazole work, we decided to explore the direct usage of amino acids in Ugi-tetrazole reaction.8 We expected that the reaction of L-proline 1a with paraformaldehyde 2a, benzyl isocyanide 3a and sodium azide would lead to the Ugi-tetrazole (4a) or U-5C-4CR or the U-4CR product, all in principal possible reaction pathways for these reactants. Initial attempts in DCM interestingly furnished the Ugi-tetrazole product (4a) without any other possible by-product, however in only 15% yield, the remainder was recovered starting materials. A subsequent solvent optimization exercise of this amino acid based Ugi-tetrazole reaction is summarized in Table 1.</p><p>We started optimization by screening different solvents for Ugi-tetrazole reaction of the 1.0 equivalent of L-proline, 1.0 equivalent of paraformaldehyde, 1.0 equivalent of benzyl isocyanide 3a and 1.2 equivalent sodium azide. As expected, non-polar solvents and polar-aprotic solvents gave us very poor yield of Ugi-tetrazole product due to the low solubility of starting materials. Interestingly, polar-protic solvents like trifluoroethanol (TFE) and isopropanol gave only 35% and 20% yield of the Ugi-tetrazole product 4a, respectively. Similar related reactions of C,N-unprotected dipeptides toward diketopiperazine by Ugi reaction explicitly require the protic polar solvent TFE. When we used methanol as solvent, it furnished the desired product in 78% yield after 12 h. We also tested zinc chloride as Lewis-acid catalyst (10 mol %) but without increase of the yield. When we used methanol and water in a 5:1 ratio, the Ugi-tetrazole product was furnished in 71% yield after 12 h. Thus 1.0 equivalent of amino acid, 1.0 equivalent of aldehyde, 1.0 equivalent of isocyanide and 1.2 equivalent of sodium azide in 1.0 M of methanol or methanol : H2O (5:1) for 12 h was the optimized condition for the Ugi-tetrazole reaction.</p><p>With the optimized reaction conditions in hand, the scope of the "Ugi-tetrazole" reaction was further investigated by reacting a library of natural/unnatural α-amino acids, β-amino acids and γ/ω-amino acids with different aldehydes, isocyanides and sodium azide in methanol or methanol : H2O (5:1) solvent system to furnish an amino acid-tetrazole library 4b–p. All the substrates 1, 2 and 3 furnished the expected Ugi-tetrazole products 4b–p in 27–74% yields, almost irrespective of the electronic and steric factors of the substituents present. L-Proline reacted with paraformaldehyde 2a, glycine methyl ester isocyanide 3b to furnished the expected Ugi-tetrazole product 4b in 74% yield (see Table 2). Similarly, L-alanine, L-phenylalanine, 4-hydroxy-L-proline, L-isoleucine, L-valine, glycine, L-tyrosine, L-methionine and L-selenomethionine also furnished the Ugi-tetrazole products in good yields as diastereomeric mixtures, respectively. Interestingly histidine is also a valid substrate with modest yield (27% 4k). Surprisingly, higher non α-amino acids worked as well. Thus, the reaction of β-alanine with piperonal, phenylethyl isocyanide also furnished the expected Ugi-tetrazole product 4n in 48% yield with very low yield of Ugi side product (β-lactam, only observed in MS, not isolatable). Next, γ-aminobutyric acid was reacted with 2-methoxy benzaldehyde, and tert-butyl isocyanide furnishing the Ugi-tetrazole product 4o in good 53% yield. ω-Aminovaleric acid is also valid substrate in this reaction, and it produce the tetrazole product 4p in 66% yield with great purity.</p><p>After synthesizing a library of acid-tetrazole in an Ugi-tetrazole reaction, we further transformed them into biologically important peptidomimetics 6 through another Ugi-4CR. Reaction of acid-tetrazole product 4b with 1.0 equivalent of paraformaldehyde, 1.0 equivalent tert-butyl isocyanide and 1.0 equivalent of 2-(4-chlorophenyl)ethanamine in methanol at room temperature for 24 h furnished for example Ugi-product 6a in 50% yield. This compound 6 has a unique and defined structure which consist of an amino acid inside the chain, a small peptidomimetic chain but with potentially enhanced stability or biological activity and at the end a tetrazole unit which is a bioisostere for cis-amide groups. This interesting structural unite in one molecule was achieved only two synthetic steps. In a similar manner, the selective two-step sequential Ugi-tetrazole/Ugi-reaction strategy was demonstrated with five more substrates in good overall yields (see Table 2; 6b–f). Lastly, we investigated the synthetic utility of this methodology by reacting the N,C-unprotected dipeptide, Gly-Gly and tripeptide, Gly-Gly-Gly. To our great delight also these reactions worked accordingly and afforded the products 4p and 4q in 69% and 58% yields, respectively (see Scheme 2).</p><p>A plausible mechanism of this Ugi-tetrazole reaction is shown in scheme 3 and involves Schiff base formation, which leads to the intermediate nitrilium ion after addition of isocyanide, followed by azide attack, and a final irreversible sigmatropic rearrangement to the tetrazole product.9 In case of amino acids, the Ugi-tetrazole reaction may also process through pathway B. There, after addition of the isocyanide 3 to the Schiff base, an O-acylamide (II) is formed. By the nucleophilic attack of the azide at the isocyanide carbon and it leads to the original intermediate (III) of Ugi-tetrazole reaction. the subsequent rearrangement leads to the acid-tetrazole product 4.</p><p>We have performed an appropriate control experiment to investigate the competitive nature of azide anion in Ugi-tetrazole reaction with C,N-unprotected amino acid as showed in scheme 4. In the presence of sodium azide and an external acid (propionic acid) along with the 2-amino-2-methylpropanoic acid, cyclopropanecarbaldehyde and benzyl isocyanide under standardized reaction conditions, After 12 h, we have carefully examined the NMR and ESI-HRMS spectrums of the crude reaction mixture. (see SI). As expected, the crude NMR spectrum and ESI-HRMS experiment showed the major Ugi-tetrazole product (4d) along with very trace amount of Ugi-product (7). These results tell us that nucleophilic nature of azide anion is higher than the acid, which is the reason to lead the model reaction towards Ugi-tetrazole product formation.</p><p>Chemistry at its best, by the 'lego' approach is the great advantage of MCR to afford outstanding scaffold diversity. Here, we have identified a novel 'lego-like' approach towards the synthesis of substituted amino acid-tetrazoles and tetrazoles-peptidomimetics. This robust and air- and moisture-tolerant procedure is a valuable addition to MCR chemistry and expands the substrate scope to include amino acids, peptide like chains and 1,5-disubstituted tetrazoles in one molecule.</p><p>The work was financially supported from the NIH (NIH 2R01GM097082-05) and by the Innovative Medicines Initiative Joint Undertaking under grant agreement no. 115489, resources of which are composed of financial contribution from the European Union's Seventh Framework Programme (FP7/2007-2013) and EFPIA companies' in-kind contribution. Funding has from the European Union's Horizon 2020 research and innovation programme under MSC ITN "Accelerated Early stage drug dIScovery" (AEGIS, grant agreement No 675555) and CoFund ALERT (grant agreement No 665250). We thank Dr. Marcel de Vries for his help in HRMS analysis. Qian Wang was supported by the China Scholarship Council.</p><p>Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x</p><p>Reported Direct Use of Amino Acid in Ugi-type Reactions and Our Methodology.</p><p>Di- and tripeptides in Ugi-tetrazole reaction.</p><p>Anticipated Mechanistic Pathways.</p><p>Investigation of the competitive nature of azide anion.</p><p>Optimization for the Ugi-tetrazole reaction of l-proline.a,b</p><p>The reactions were run using 1a (1.0 mmol), 2a (1.0 mmol), 3a (1.0 mmol) and NaN3 (1.2 mmol) in the indicated solvent for 12 h.</p><p>Isolated yield.</p><p>Two MCRs on one substrate; scope and limitations.a,b,c & d</p><p>Ugi-tetrazole reaction was carried out in MeOH or MeOH:H2O (5:1)(1.0 M) with 1.0 equiv. of amino acid 1 relative to the aldehyde 2, isocyanide 3 and 1.2 equiv. of NaN3 at 25 °C for 12 h.</p><p>Ugi-reactions was carried out in MeOH (1.0 M) with 1.0 equiv. of aldehyde 2, amine 5 after 10 mints addition of 1.0 equiv. of acid-tetrazole 4 and isocyanide 3 (1.0 equiv.) at 25 °C and stirred for 24 h.</p><p>Yield refers to the column-purified product.</p><p>dr was determined by NMR analysis.</p><p>reaction was carried out in MeOH:H2O (5:1)(1.0 M).</p>