New Members of the Cinchona Alkaloids Family: Assembly of the Triazole Heterocycle at the 6′ Position

The substance class of the well-known Cinchona alkaloids is widened by 6′-Amino-cinchonine and 6′-Amino-cinchonidine, novel compounds which incorporate a primary amino function in the quinolinic ring system. These key intermediates open the field for a range of fruitful chemistry. Here is described a short and direct pathway for the synthesis of triazole containing derivatives of the above-mentioned substances using the [3 + 2] Huisgen cycloaddition. For this purpose, the amines were first converted into the corresponding azides. Based on this, non-substituted and silyl-protected triazoles were synthesized as examples. Furthermore, didehydrated derivatives of quincorine and quincoridine were used as addition partners, resulting in compounds that carry the quinuclidine ring of the cinchona alkaloids at both ends. Some of these compounds were examined radiographically to investigate the position of the quinuclidine ring to the triazole. The solid-state structures of compounds 10, 11 and 28 were determined by X-ray diffraction analyses.


Introduction
To the best of our knowledge, the pseudo-enantiomeric compounds 6 -Amino-cinchonine 1 and 6 -Amino-cinchonidine 2 ( Figure 1) are not described in the literature. However, there are a few references to the hydrogenated derivatives, the most recent of which dates from 1960 [1]. It is therefore all the more gratifying that these forgotten derivatives of the Cinchona alkaloids are now rationally available.

Introduction
To the best of our knowledge, the pseudo-enantiomeric compounds 6′-Aminocinchonine 1 and 6′-Amino-cinchonidine 2 ( Figure 1) are not described in the literature. However, there are a few references to the hydrogenated derivatives, the most recent of which dates from 1960 [1]. It is therefore all the more gratifying that these forgotten derivatives of the Cinchona alkaloids are now rationally available. The benefits of Cinchona alkaloids and their derivatives are widely known. Quinine is still used as an anti-malarial in medicine and as a bittering agent in the beverage The benefits of Cinchona alkaloids and their derivatives are widely known. Quinine is still used as an anti-malarial in medicine and as a bittering agent in the beverage industry. Cupreins and cupreidines, hydroxy analogues of amines 1 and 2, are currently very popular in catalyst research [2][3][4]. Derivatives of 6 -Amino-cinchonine 1 and 6 -Amino-cinchonidine

Chemistry
In addition to the aforementioned 6′-Amino-cinchonine 1 and 6′-Amino-cinchonidine 2, these also used the vinyl side chain manipulated derivatives ( Figure 2). Moreover, the chemistry was carried out on the hydrogenated compounds 3 and 4 as well as on the dehydrogenated derivatives 5 and 6. In the case of the latter compounds, the question becomes whether inter-or intramolecular [3 + 2] cycloadditions can be observed.

Introduction of the Azido Functionality
Azide functionality is introduced by a diazotization protocol [24]. Scheme 1 details the conversion of amine 1 to the azido derivative 8. In the first step, the amino group is converted into the diazo group under acidic conditions and the intermediate diazonium species 7 is formed. Without intermediate work-up, the diazo group is substituted by addition of sodium azide to form the 6′-azido-cinchonine 8 in good yield (89%) and purity. The protocol could also be successfully applied to amines 2 to 6 with similarly good yields (Table 1). Under the conditions chosen, alkynes 5 and 6 also gave no indication of any recognizable spontaneous triazole ring closing. The yields were within the range of the other conversions.

Introduction of the Azido Functionality
Azide functionality is introduced by a diazotization protocol [24]. Scheme 1 details the conversion of amine 1 to the azido derivative 8. In the first step, the amino group is converted into the diazo group under acidic conditions and the intermediate diazonium species 7 is formed. Without intermediate work-up, the diazo group is substituted by addition of sodium azide to form the 6 -azido-cinchonine 8 in good yield (89%) and purity. The protocol could also be successfully applied to amines 2 to 6 with similarly good yields (Table 1). Under the conditions chosen, alkynes 5 and 6 also gave no indication of any recognizable spontaneous triazole ring closing. The yields were within the range of the other conversions.
Dihydro-6′-Azido-cinchonidine 11 79% Didehydro-6′-Amino-cinchonine 5 Didehydro-6′-Azido-cinchonine 12 97% Didehydro-6′-Amino-cinchonidine 6 Didehydro-6′-Azido-cinchonidine 13 94% To establish triazole functionality, azides 8 to 11 were reacted with trimethylsilylacetylene according to the protocol of Zhao et al. [18]. The synthesis protocol originally provided a method in which the cleavage of the silyl group is carried out in situ. In order to see the orientation of the Husigen [3 + 2] cycloaddition, the reaction was carried out in two steps. The silylated species was isolated and characterized. As shown in Scheme 2, only the triazole 14 silylated in position 4 is formed from the azide 8 in a yield of 90%. This was clearly detected by NMR experiments. In a final step, compound 14 was converted to the unsubstituted triazole 15 via E1cb mechanism. Here the yield was 81%.

Assembling of the Triazole Unit
Azides 9, 10 and 11 were converted into the corresponding silylated triazoles 16, 18 and 20 and monosubstituted triazoles 17, 19 and 21 using the same protocol. The yields ranged from good to excellent (89 to 94% for the cyclo-addition and 81-87% for the desilylation).

Assembling of the Triazole Unit Containing a Further Quinuclidine Rest
After it was shown that the triazole unit can be, in principle, introduced in good yields, more complex molecules were chosen as alkyne sources in a second phase. The Scheme 1. Synthesis of 6 -Azido-cinchonine 8, starting from 6 -Amino-cinchonine 1 via the diazonium species 7. Table 1. Yields of the transformation of the amins 2 to 6 to the corresponding azides 9 to 13.
Another protocol by Singh et al. was used [19]. The two alkynes 22 and 23 were reacted with the four azides 8-11. Scheme 3 illustrates the two reactions of didehydroquincoridine 22 with the hydrated azides 10 and 11. The synthesis of compounds 24 to 31 was again successful in moderate to good yields (62-82%). Two trends can be observed. The yields in the cinchonidine series (azides 9 and 11, 72-82%) were better than in the cinchonine series (azides 8 and 10, 62-78%). Although the difference is not strikingly large, it cannot be ignored. Furthermore, in three pairs, the yields of the hydrogenated compounds were higher than the yields of the corresponding vinyl species.

X-ray
We investigated three of the synthesized compounds by crystal structure analysis:   The synthesis of compounds 24 to 31 was again successful in moderate to good yields (62-82%). Two trends can be observed. The yields in the cinchonidine series (azides 9 and 11, 72-82%) were better than in the cinchonine series (azides 8 and 10, 62-78%). Although the difference is not strikingly large, it cannot be ignored. Furthermore, in three pairs, the yields of the hydrogenated compounds were higher than the yields of the corresponding vinyl species.

X-ray
We investigated three of the synthesized compounds by crystal structure analysis:  The synthesis of compounds 24 to 31 was again successful in moderate to good yields (62-82%). Two trends can be observed. The yields in the cinchonidine series (azides 9 and 11, 72-82%) were better than in the cinchonine series (azides 8 and 10, 62-78%). Although the difference is not strikingly large, it cannot be ignored. Furthermore, in three pairs, the yields of the hydrogenated compounds were higher than the yields of the corresponding vinyl species.

X-ray
We investigated three of the synthesized compounds by crystal structure analysis: azides 10 and 11 ( Figure 4) and the compound 28 ( Figure 5). The compounds 10 and 11 crystalized in their anti-open conformer. Crystallographic data collection parameter of 10, 11 and 28 can be found in the Supplementary Matericals.

IR Spectroscopy, Melting Point and NMR Experiments
The substances were investigated by infrared spectroscopy. Azides 8 to 13 showed a characteristic band at a wavenumber of ν = 2104 to 2108 cm −1 . The silylated triazoles 14, 16, 18 and 20 had a prominent band at a wavenumber of ν = 837 to 839 cm −1 . When the melting point was determined, all synthesized compounds decomposed and gas evolved. In the 1 H-and 13 C-NMR experiments, the signals were in the range expected for Cinchona alkaloids. Hydroxy protons were not detected at all or only as very broad signals. Triazole 28 Figure 5. The X-ray analysis for compound 28.

IR Spectroscopy, Melting Point and NMR Experiments
The substances were investigated by infrared spectroscopy. Azides 8 to 13 showed a characteristic band at a wavenumber of ν = 2104 to 2108 cm −1 . The silylated triazoles 14, 16, 18 and 20 had a prominent band at a wavenumber of ν = 837 to 839 cm −1 . When the melting point was determined, all synthesized compounds decomposed and gas evolved. In the 1 H-and 13 C-NMR experiments, the signals were in the range expected for Cinchona alkaloids. Hydroxy protons were not detected at all or only as very broad signals.

Further Experiments
We gave special attention to azide-alkyne 12 in order to perform an intramolecular Husigen [3 + 2] cycloaddition. Simple observations on the model showed that this compound can be built up without significant ring stresses. The only condition is that azide 12 turns into a certain possible conformation, however, this compact molecule eluded our grasp. Protocols [25,26] at elevated temperature led to the formation of amine Triazole 28 Figure 5. The X-ray analysis for compound 28.

IR Spectroscopy, Melting Point and NMR Experiments
The substances were investigated by infrared spectroscopy. Azides 8 to 13 showed a characteristic band at a wavenumber of ν = 2104 to 2108 cm −1 . The silylated triazoles 14, 16, 18 and 20 had a prominent band at a wavenumber of ν = 837 to 839 cm −1 . When the melting point was determined, all synthesized compounds decomposed and gas evolved. In the 1 H-and 13 C-NMR experiments, the signals were in the range expected for Cinchona alkaloids. Hydroxy protons were not detected at all or only as very broad signals.

Further Experiments
We gave special attention to azide-alkyne 12 in order to perform an intramolecular Husigen [3 + 2] cycloaddition. Simple observations on the model showed that this compound can be built up without significant ring stresses. The only condition is that azide 12 turns into a certain possible conformation, however, this compact molecule eluded our grasp. Protocols [25,26] at elevated temperature led to the formation of amine

Further Experiments
We gave special attention to azide-alkyne 12 in order to perform an intramolecular Husigen [3 + 2] cycloaddition. Simple observations on the model showed that this compound can be built up without significant ring stresses. The only condition is that azide 12 turns into a certain possible conformation, however, this compact molecule eluded our grasp. Protocols [25,26] at elevated temperature led to the formation of amine 5. In dilute solution [21,27], we observed a non-specific decomposition over time. Using the protocols described in this study, there was mass spectroscopic evidence that oligomeric units were formed.

Discussion
The aim of this work was to investigate the synthetic potential of 6 -Amino-cinchonine 1, 6 -Amino-cinchonidine 2 and their derivatives and whether they could be successfully converted into the azides 8-13. Using the well-established Husigen [3 + 2] cycloaddition method, the construction of the triazoles 14, 16, 18, 20, 24-31 was successfully performed.
Only one addition product was observed in all cases. According to current knowledge, [3 + 2] cycloaddition reaction can proceed via single step-mechanism or stepwise mechanism with a zwitterionic/biradical intermediate. The stepwise mechanism was recently documented for a reaction involving different types of three-atom components, including azides [28]. Since the observed products are stereochemically less hindered, it is likely a main reason for the observed stereochemistry. For considered reactions, the mechanism cannot be proposed a priori since no mechanistic studies were made. The formal additionproducts of ethyne to the azide functionality (Triazoles 15, 17, 19, 21) can be generated by desilylation of the compounds 14, 16, 18, 20. Overall, all reactions proceeded with good purity and good to very good yields. The aim of future work will be to investigate the potential of these compounds in, for example, catalysis or medicinal chemistry. In terms of structural chemistry, the intramolecular cyclisation of azide alkynes 5 and 6 is a challenge for every synthetic chemist.

General Information
All reagents were purchased from commercial sources (Sigma-Aldrich, TCI, Acros or Buchler) and used without further purification. The solvents used were of analytical grade. The 1H and 13C NMR spectra were recorded at room temperature on a Bruker Avance 300, Bruker Avance 400 or Bruker Avance 600, operating at 300 MHz, 400 MHz or 600 MHz for 1H and 75 MHz, 100 MHz or 150 MHz for 13C. Chemical shifts (δ) were reported as relative to the tetramethylsilane peak set at 0.00 ppm. In the case of multiplets, the signals were reported as intervals. The signals were abbreviated as s, singlet; d, doublet; t, triplet; q, quadruplet; and m, multiplet. Mass spectra were recorded on a Finnigan MAT 8400-MSS and Finnigan MAT 4515. High resolution mass spectra were recorded on a Finnigan MAT 95 XP. The reactions were monitored by TLC and performed on silica gel plates 40 × 80 mm Polygram Sil G/UV254 (Macherey-Nagel). Visualization on TLC was achieved by UV light. Column chromatography was performed with Merck silica gel 60 (70-200 mesh).
The numbering of the Cinchona core followed the rules of Paul Rabe, with the side chain numbered based on the IUPAC rules.

Discussion
The aim of this work was to investigate the synthetic potential of 6′-Aminocinchonine 1, 6′-Amino-cinchonidine 2 and their derivatives and whether they could be successfully converted into the azides 8-13. Using the well-established Husigen [3 + 2] cycloaddition method, the construction of the triazoles 14, 16, 18, 20, 24-31 was successfully performed. Only one addition product was observed in all cases. According to current knowledge, [3 + 2] cycloaddition reaction can proceed via single stepmechanism or stepwise mechanism with a zwitterionic/biradical intermediate. The stepwise mechanism was recently documented for a reaction involving different types of three-atom components, including azides [28]. Since the observed products are stereochemically less hindered, it is likely a main reason for the observed stereochemistry. For considered reactions, the mechanism cannot be proposed a priori since no mechanistic studies were made. The formal addition-products of ethyne to the azide functionality (Triazoles 15, 17, 19, 21) can be generated by desilylation of the compounds 14, 16, 18, 20.
Overall, all reactions proceeded with good purity and good to very good yields. The aim of future work will be to investigate the potential of these compounds in, for example, catalysis or medicinal chemistry. In terms of structural chemistry, the intramolecular cyclisation of azide alkynes 5 and 6 is a challenge for every synthetic chemist.

General Information
All reagents were purchased from commercial sources (Sigma-Aldrich, TCI, Acros or Buchler) and used without further purification. The solvents used were of analytical grade. The 1H and 13C NMR spectra were recorded at room temperature on a Bruker Avance 300, Bruker Avance 400 or Bruker Avance 600, operating at 300 MHz, 400 MHz or 600 MHz for 1H and 75 MHz, 100 MHz or 150 MHz for 13C. Chemical shifts (δ) were reported as relative to the tetramethylsilane peak set at 0.00 ppm. In the case of multiplets, the signals were reported as intervals. The signals were abbreviated as s, singlet; d, doublet; t, triplet; q, quadruplet; and m, multiplet. Mass spectra were recorded on a Finnigan MAT 8400-MSS and Finnigan MAT 4515. High resolution mass spectra were recorded on a Finnigan MAT 95 XP. The reactions were monitored by TLC and performed on silica gel plates 40 × 80 mm Polygram Sil G/UV254 (Macherey-Nagel). Visualization on TLC was achieved by UV light. Column chromatography was performed with Merck silica gel 60 (70-200 mesh).
The numbering of the Cinchona core followed the rules of Paul Rabe, with the side chain numbered based on the IUPAC rules.

Synthesis of the Azido-Derivatives 8-13
General procedure: a dropwise a solution of NaNO 2 (1.2 eq) in H 2 O (0.33 g/mL) was added to a mixture of Cinchona amine (1.0 eq) and 15% HCl (0.1 g amine/mL) cooled at −5 • C. After the addition was complete, the reaction mixture was stirred at this temperature for 60 min. A solution of NaN 3 (1.7 eq) in H 2 O (0.2 g/mL) was added dropwise to the reaction mixture at 0 • C. After the addition was finished, the reaction mixture was maintained at 0 • C for 2 h and then stirred at room temperature for 12 h. The product was precipitated by addition of NaOH (20%) until pH 10. Upon filtration, the solid was washed with distilled water and dried. Crystallization from DCM presented the Cinchona azides as yellow solids in good yields (70-90%)     General procedure: in a round-bottom flask the corresponding Cinchona azide (1 eq.) and trimethylsilyl acetylene (1.5 eq.) were suspended in an H 2 O: MeOH (1:1) mixture to deliver a substrate concentration of 0.1 M Cu 2 SO 4 (0.2 eq., 20 mol%) and sodium ascorbate (0.4 eq., 40 mol%). The reaction mixture was stirred at room temperature until complete conversion of the azide (12-24 h). After that, the reaction was quenched by adding an aqueous solution of ammonium chloride and ammonium hydroxide (pH 10/12). The mixture was extracted with DCM (three times), and the combined organic layers were dried over anhydrous sodium sulphate, filtered and concentrated in vacuo. The residues obtained were purified by flash column chromatography using MTBE: MeOH as a gradient elution to afford the corresponding TMS-triazole derivatives.    General procedure: a purified Cinchona TMS-triazole (1.00 eq.) was suspended in a mixture tBu-OH/H 2 O (1:1 ratio) and K 2 CO 3 (2.0 equiv) was added. The reaction mixture was rigorously stirred for 48 h. Upon completion of the reaction, EtOAc was added and the organic layer was separated, washed with water, brine, dried over Na 2 SO 4 , filtered and concentrated in vacuo. The residues obtained were purified by flash column chromatography using MTBE: MeOH as a gradient elution to afford the corresponding triazole products.   partly occupied units of water. The absolute structure parameter suggests numerically a very small contribution of an inversion twined component. As the only heavy atom (Cl1) is only partly occupied, a twin refinement was considered not reliable and was not conducted. During refinement and analysis of the crystallographic data, the programs OLEX2 and Diamond were used [33,34]. Unless noted otherwise, the shown ellipsoids represent the 50% probability level and hydrogen atoms are displayed with an arbitrary radius. Copies of the data can be obtained free of charge from www.ccdc.cam.ac.uk/data_request/cif (accessed on 30 April 2021).