Synthesis of Indole-Coupled KYNA Derivatives via C–N Bond Cleavage of Mannich Bases

KYNAs, a compound with endogenous neuroprotective functions and an indole that is a building block of many biologically active compounds, such as a variety of neurotransmitters, are reacted in a transformation building upon Mannich bases. The reaction yields triarylmethane derivatives containing two biologically potent skeletons, and it may contribute to the synthesis of new, specialised neuroprotective compounds. The synthesis has been investigated via two procedures and the results were compared to those of previous studies. A possible alternative reaction route through acid catalysis has been established.

Because of their biological potential, DIMs have received much attention in organic synthesis, and several highly efficient methods have been developed for their production [18][19][20][21][22].
Another compound with biological relevance is kynurenic acid (KYNA). Among the important features of KYNA, it is known to be one of the few known endogenous excitatory amino acid receptor blockers with a broad spectrum of antagonistic properties in supraphysiological concentrations. It is well established that KYNA has high affinity toward N-methyl-D-aspartate (NMDA) receptors. Moreover, it has recently been disclosed that KYNA shows an even higher affinity towards the positive modulatory binding site at the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor [23].

Results and Discussion
The work of Baruah et al. [42][43][44] includes both pathways of TRAM (3) synthesis starting either from indole-based Mannich products (Scheme 1, route i) or naphthol-/phenolbased Mannich products (route ii). Thus, the investigation of similar reaction routes is outlined for the ethyl ester of KYNA as well, starting from the Mannich base of indole (route iii) or the Mannich base of KYNA (route iv).

Results and Discussion
The work of Baruah et al. [42][43][44] includes both pathways of TRAM (3) synthesis starting either from indole-based Mannich products (Scheme 1, route i) or naphthol-/phenol-based Mannich products (route ii). Thus, the investigation of similar reaction routes is outlined for the ethyl ester of KYNA as well, starting from the Mannich base of indole (route iii) or the Mannich base of KYNA (route iv).  First, based on route (iii), a reaction between the Mannich base of indole and KYNA ethyl ester was planned. To synthesize the Mannich base of indole (4), several literature methods have been explored. Reactions utilizing catalysts, such as ferric phosphate [47] and iodine [48], or protic solvents, such as ethanol, methanol [49], and ethylene glycol [50], resulted mainly in bisindole derivative 5 mentioned in the corresponding literature as a byproduct. However, Mannich base 4 could be synthesized under neat conditions applying only indole, benzaldehyde, and pyrrolidine (with or without L-proline as catalyst [51,52]). Surprisingly, the highest yield was achieved by using the application of the surfactant sodium dodecyl sulfate (SDS, Scheme 2) [53].
First, based on route (iii), a reaction between the Mannich base of indole and KYNA ethyl ester was planned. To synthesize the Mannich base of indole (4), several literature methods have been explored. Reactions utilizing catalysts, such as ferric phosphate [47] and iodine [48], or protic solvents, such as ethanol, methanol [49], and ethylene glycol [50], resulted mainly in bisindole derivative 5 mentioned in the corresponding literature as a byproduct. However, Mannich base 4 could be synthesized under neat conditions applying only indole, benzaldehyde, and pyrrolidine (with or without L-proline as catalyst [51,52]). Surprisingly, the highest yield was achieved by using the application of the surfactant sodium dodecyl sulfate (SDS, Scheme 2) [53].

Scheme 2. The synthesis of aminoalkylated indole derivatives.
With the starting material in our hands, in the first C-C bond-forming reactions, 4 was reacted with the ethyl ester of KYNA (6) in MeCN at 100 °C (under MW conditions) with thiourea as the catalyst, based on the work of Baruah et al. [42] (Scheme 3). The conversion determined by NMR spectrometry was low; thus, the best conditions used for the alternative reaction route [43] were investigated, showing promising results (Table 1). Fortunately, the still low yield of 7 could be further improved by raising the reaction temperature to 160 °C. However, any further increase caused a decomposition of the starting materials.  With the starting material in our hands, in the first C-C bond-forming reactions, 4 was reacted with the ethyl ester of KYNA (6) in MeCN at 100 • C (under MW conditions) with thiourea as the catalyst, based on the work of Baruah et al. [42] (Scheme 3). The conversion determined by NMR spectrometry was low; thus, the best conditions used for the alternative reaction route [43] were investigated, showing promising results (Table 1). Fortunately, the still low yield of 7 could be further improved by raising the reaction temperature to 160 • C. However, any further increase caused a decomposition of the starting materials.
First, based on route (iii), a reaction between the Mannich base of indole and KYNA ethyl ester was planned. To synthesize the Mannich base of indole (4), several literature methods have been explored. Reactions utilizing catalysts, such as ferric phosphate [47] and iodine [48], or protic solvents, such as ethanol, methanol [49], and ethylene glycol [50], resulted mainly in bisindole derivative 5 mentioned in the corresponding literature as a byproduct. However, Mannich base 4 could be synthesized under neat conditions applying only indole, benzaldehyde, and pyrrolidine (with or without L-proline as catalyst [51,52]). Surprisingly, the highest yield was achieved by using the application of the surfactant sodium dodecyl sulfate (SDS, Scheme 2) [53]. With the starting material in our hands, in the first C-C bond-forming reactions, 4 was reacted with the ethyl ester of KYNA (6) in MeCN at 100 °C (under MW conditions) with thiourea as the catalyst, based on the work of Baruah et al. [42] (Scheme 3). The conversion determined by NMR spectrometry was low; thus, the best conditions used for the alternative reaction route [43] were investigated, showing promising results (Table 1). Fortunately, the still low yield of 7 could be further improved by raising the reaction temperature to 160 °C. However, any further increase caused a decomposition of the starting materials.  The use of a catalyst was crucial, as the desired TRAM did not form without the use of a base or an acid catalyst. It is interesting to mention that although acid catalysis resulted in somewhat higher conversion, both acid and base catalysis could enhance the synthesis of 7. Baruah et al. hypothesized that the reaction taking place between the indole derivative and varied electron-rich aromatic structures involves the formation of intermediate 10 [42,43]. In their proposed elimination-addition pathway starting from a Mannich base of indole, thiourea activates the amine moiety of the aminoalkyl function through double hydrogen bonding and converts it into a better leaving group (Scheme 4, I). Concerning triethylamine (TEA) used in our reactions, a hydrogen bond is unable to form; therefore, a more direct form of catalysis is proposed. Through the application of high temperature and TEA, the deprotonation of the indole moiety takes place followed by a subsequent rearrangement of the indole anion into benzylidene intermediate 10. Then, the latter is attacked by a molecule of the electron-rich KYNA yielding compound 7. It is also surmised that the C-N bond cleavage of the indole derivative could also take place through the elimination of pyrrolidine via the protonation of the amine moiety, making it a better leaving group and leading to intermediate 10 (Scheme 4, II). The use of a catalyst was crucial, as the desired TRAM did not form without the use of a base or an acid catalyst. It is interesting to mention that although acid catalysis resulted in somewhat higher conversion, both acid and base catalysis could enhance the synthesis of 7. Baruah et al. hypothesized that the reaction taking place between the indole derivative and varied electron-rich aromatic structures involves the formation of intermediate 10 [42,43]. In their proposed elimination-addition pathway starting from a Mannich base of indole, thiourea activates the amine moiety of the aminoalkyl function through double hydrogen bonding and converts it into a better leaving group (Scheme 4, I). Concerning triethylamine (TEA) used in our reactions, a hydrogen bond is unable to form; therefore, a more direct form of catalysis is proposed. Through the application of high temperature and TEA, the deprotonation of the indole moiety takes place followed by a subsequent rearrangement of the indole anion into benzylidene intermediate 10. Then, the latter is attacked by a molecule of the electron-rich KYNA yielding compound 7. It is also surmised that the C-N bond cleavage of the indole derivative could also take place through the elimination of pyrrolidine via the protonation of the amine moiety, making it a better leaving group and leading to intermediate 10 (Scheme 4, II). Further optimization of the reaction involved the change of solvent from the aprotic and apolar toluene to solvents representing a wider range of the aprotic-protic and apolar-polar scale (Table 1). It is hypothesized that toluene may be the best solvent because of the lack of H-bridge bonds and polarity of the solvent can contribute to a more unstable, Further optimization of the reaction involved the change of solvent from the aprotic and apolar toluene to solvents representing a wider range of the aprotic-protic and apolarpolar scale (Table 1). It is hypothesized that toluene may be the best solvent because of the lack of H-bridge bonds and polarity of the solvent can contribute to a more unstable, and thus more reactive, intermediate [54].
After successfully optimizing the reaction through route (iii), the synthesis of 7 was planned through the reaction of the KYNA Mannich base with indole (Scheme 1, route iv). KYNA Mannich derivatives synthesized previously are abundant [41]; however, compounds containing the crucial phenol structure were narrowed down only to a single compound (11,Scheme 5). Unfortunately, using this derivative in the reaction under conditions optimized previously did not result in the desired compound. It is presumed that this may be due to the N,N-dimethylaminoethyl moiety being a bad leaving group. In order to fully support this hypothesis, a synthetic procedure was applied. Unfortunately, a Mannich base of KYNA containing a secondary amine function could not be synthesized, which is probably due to steric hindrance. Thus, considering the similarity of 1-naphthol to KYNA [41], the synthesis of Mannich bases 16a and 16b was carried out, as shown in Scheme 6. A comparison of the reaction of 1-naphthol with 6 and the reactions of 16a and 16b with indole (Scheme 7) allows us to arrive at two conclusions: (a) Mannich bases containing secondary amines (e.g., 11 and 16b) are less prone to undergo the transformation because of a bad leaving group character; (b) reactions through intermediate 10 are more preferable compared to reactions via possible ortho-quinone methide intermediates 12 and 17 derived either from 11 or from the Mannich bases of 1-naphthol (16a,b). This may be due to a possible hydrogen bridge between the hydroxy/oxo group in 16a,b and 11 in addition to the amine moiety, making the protonated form a more stable intermediate. It is presumed that this may be due to the N,N-dimethylaminoethyl moiety being a bad leaving group. In order to fully support this hypothesis, a synthetic procedure was applied. Unfortunately, a Mannich base of KYNA containing a secondary amine function could not be synthesized, which is probably due to steric hindrance. Thus, considering the similarity of 1-naphthol to KYNA [41], the synthesis of Mannich bases 16a and 16b was carried out, as shown in Scheme 6. It is presumed that this may be due to the N,N-dimethylaminoethyl moiety being a bad leaving group. In order to fully support this hypothesis, a synthetic procedure was applied. Unfortunately, a Mannich base of KYNA containing a secondary amine function could not be synthesized, which is probably due to steric hindrance. Thus, considering the similarity of 1-naphthol to KYNA [41], the synthesis of Mannich bases 16a and 16b was carried out, as shown in Scheme 6. A comparison of the reaction of 1-naphthol with 6 and the reactions of 16a and 16b with indole (Scheme 7) allows us to arrive at two conclusions: (a) Mannich bases containing secondary amines (e.g., 11 and 16b) are less prone to undergo the transformation because of a bad leaving group character; (b) reactions through intermediate 10 are more preferable compared to reactions via possible ortho-quinone methide intermediates 12 and 17 derived either from 11 or from the Mannich bases of 1-naphthol (16a,b). This may be due to a possible hydrogen bridge between the hydroxy/oxo group in 16a,b and 11 in addition to the amine moiety, making the protonated form a more stable intermediate. A comparison of the reaction of 1-naphthol with 6 and the reactions of 16a and 16b with indole (Scheme 7) allows us to arrive at two conclusions: (a) Mannich bases containing secondary amines (e.g., 11 and 16b) are less prone to undergo the transformation because of a bad leaving group character; (b) reactions through intermediate 10 are more preferable compared to reactions via possible ortho-quinone methide intermediates 12 and 17 derived either from 11 or from the Mannich bases of 1-naphthol (16a,b). This may be due to a possible hydrogen bridge between the hydroxy/oxo group in 16a,b and 11 in addition to the amine moiety, making the protonated form a more stable intermediate. To further investigate the scope and limitations of the reaction, the synthesis of TRAMs containing different KYNA derivatives was planned. The reactions were carried out applying the optimized conditions (see Table 1, Entry #4) starting from KYNA derivatives 19a-h substituted at the B ring (Scheme 8). The reactions resulted in a diverse range of compounds (20a-h). In the case of derivative 19e, the reaction applying microwaves as a heat source resulted in an exceptionally low conversion. To test whether a kinetic control takes place during the transformation, a longer reflux treatment was carried out. As the result with an almost full conversion was promising, reflux conditions were applied to the other derivatives as well, showing a general increase in conversions and supporting our hypothesis.
It is interesting to mention that the type of substituents on the B ring influenced the reactions to a lesser extent (e.g., Table 2, Entry #2, and #10) compared to the position of the substituents (e.g., Table 2, #10, and #16). Both chloro-and methoxy-KYNA derivatives, To further investigate the scope and limitations of the reaction, the synthesis of TRAMs containing different KYNA derivatives was planned. The reactions were carried out applying the optimized conditions (see Table 1, Entry #4) starting from KYNA derivatives 19a-h substituted at the B ring (Scheme 8). The reactions resulted in a diverse range of compounds (20a-h). To further investigate the scope and limitations of the reaction, the synthesis of TRAMs containing different KYNA derivatives was planned. The reactions were carried out applying the optimized conditions (see Table 1, Entry #4) starting from KYNA derivatives 19a-h substituted at the B ring (Scheme 8). The reactions resulted in a diverse range of compounds (20a-h). In the case of derivative 19e, the reaction applying microwaves as a heat source resulted in an exceptionally low conversion. To test whether a kinetic control takes place during the transformation, a longer reflux treatment was carried out. As the result with an almost full conversion was promising, reflux conditions were applied to the other derivatives as well, showing a general increase in conversions and supporting our hypothesis.
It is interesting to mention that the type of substituents on the B ring influenced the reactions to a lesser extent (e.g., Table 2, Entry #2, and #10) compared to the position of the substituents (e.g., Table 2, #10, and #16). Both chloro-and methoxy-KYNA derivatives, In the case of derivative 19e, the reaction applying microwaves as a heat source resulted in an exceptionally low conversion. To test whether a kinetic control takes place during the transformation, a longer reflux treatment was carried out. As the result with an almost full conversion was promising, reflux conditions were applied to the other derivatives as well, showing a general increase in conversions and supporting our hypothesis.
It is interesting to mention that the type of substituents on the B ring influenced the reactions to a lesser extent (e.g., Table 2, Entry #2, and #10) compared to the position of the substituents (e.g., Table 2, #10, and #16). Both chloro-and methoxy-KYNA derivatives, with substituents at C-5 and C-7, showed somewhat lower reactivity compared to the ethyl ester of KYNA (longer reaction times were needed). However, the same substituents in positions C-6 and C-8 caused a significant decrease in reactivity of the KYNA skeleton.