Syntheses of New Multisubstituted 1-Acyloxyindole Compounds

The syntheses of novel 1-acyloxyindole compounds 1 and the investigations on reaction pathways are presented. Nitro ketoester substrate 2, obtained in a two-step synthetic process, underwent reduction, intramolecular addition, nucleophilic 1,5-addition, and acylation to afford 1-acyloxyindoles 1 in one pot. Based on the systematic studies, we established the optimized reaction conditions for 1 focusing on the final acylation step of the intermediate 1-hydroxyindole 8. With the optimized conditions, we succeeded in synthesizing 21 examples of new 1-acyloxyindole derivatives 1 in modest yields (Y = 24 − 35%). Among the 1-acyloxyindole compounds, 1-acetoxyindole compounds 1x were generally unstable, and their yields were relatively lower than the other 1-acyloxyindoles. We expect that a bulkier alkyl or aromatic group on R2 could stabilize the 1-acyloxyindole compounds. Significantly, one-pot reactions of a four-step sequence successfully generated compounds 1 that are all new and might be difficult to be synthesized otherwise.

In this study, we aimed to create novel derivatives of multisubstituted 1-ac doles ( Figure 2) with improved chemical stability and meaningful biological activi the nitro ketoester substrate obtained in a two-step sequence, we devised a con one-pot synthetic method of consecutive four-step sequence to afford the de acyloxyindole compounds 1. In addition, we expect that the compounds could useful prodrugs for valuable medicinal agents or pharmacokinetic structural com for drug delivery.

Synthesis of Conjugate Nitro Ketoester 2
At first, we prepared the substrate 2 [25,[32][33][34] in two-step reactions using 2 6-nitrotoluene as a starting material. For this purpose, we applied our previou dures [32] with minor modifications. As shown in Scheme 1, 2-chloro-6-nitrotoluen reacted with dimethyloxalate in the presence of excess sodium hydride to afford an excellent yield (Y = 96%). Subsequently, 4 was reacted with dimethylmethylene ium chloride to add a methylene group at α-carbon in 4, affording conjugate ket [32] in a good yield (Y = 85%). The result of synthesis of 2 was slightly improved co to the previous results [32].
In this study, we aimed to create novel derivatives of multisubstituted 1-acyloxyindoles ( Figure 2) with improved chemical stability and meaningful biological activity. With the nitro ketoester substrate obtained in a two-step sequence, we devised a convenient one-pot synthetic method of consecutive four-step sequence to afford the desired 1acyloxyindole compounds 1. In addition, we expect that the compounds could serve as useful prodrugs for valuable medicinal agents or pharmacokinetic structural components for drug delivery.

Synthesis of Conjugate Nitro Ketoester 2
At first, we prepared the substrate 2 [25,[32][33][34] in two-step reactions using 2-chloro-6-nitrotoluene as a starting material. For this purpose, we applied our previous procedures [32] with minor modifications. As shown in Scheme 1, 2-chloro-6-nitrotoluene 3 was reacted with dimethyloxalate in the presence of excess sodium hydride to afford 4 [32] in an excellent yield (Y = 96%). Subsequently, 4 was reacted with dimethylmethyleneimminium chloride to add a methylene group at α-carbon in 4, affording conjugate ketoester 2 [32] in a good yield (Y = 85%). The result of synthesis of 2 was slightly improved compared to the previous results [32].

Optimization for Formation of 1-Acyloxyindoles 1
The reactions to generate 1-acyloxyindoles 1 consist of two main parts: formation of 1-hydroxyindole intermediates 8 and formation of 1-acyloxyindoles 1 by acylation of 8. Although we previously established the reaction conditions for 1-hydroxyindoles 8 [32], re-optimization for synthesis of 1-acyloxyindoles 1 is required because the whole process, including the acylation step, needs to be carried out in one pot. With substrate 2, we first attempted to perform systematic studies on the reaction conditions suitable for formation of 1-acyloxyindole 1. As indicated in Scheme 2, the substrate 2 was reduced to generate hydroxylamine 5, cyclized to provide hydroxyindoline 6, and dehydrated to produce conjugate nitrone 7. Then, nucleophilic 1,5-addition of alcohol to 7 produced the intermediate 1-hydroxyindole 8, and, finally, acylation of 8 afforded 1-acyloxyindole 1.

Optimization for Formation of 1-Acyloxyindoles 1
The reactions to generate 1-acyloxyindoles 1 consist of two main parts: formation o 1-hydroxyindole intermediates 8 and formation of 1-acyloxyindoles 1 by acylation of 8 Although we previously established the reaction conditions for 1-hydroxyindoles 8 [32] re-optimization for synthesis of 1-acyloxyindoles 1 is required because the whole process including the acylation step, needs to be carried out in one pot. With substrate 2, we firs attempted to perform systematic studies on the reaction conditions suitable for formation of 1-acyloxyindole 1. As indicated in Scheme 2, the substrate 2 was reduced to generate hydroxylamine 5, cyclized to provide hydroxyindoline 6, and dehydrated to produce con jugate nitrone 7. Then, nucleophilic 1,5-addition of alcohol to 7 produced the intermediate 1-hydroxyindole 8, and, finally, acylation of 8 afforded 1-acyloxyindole 1. Scheme 2. Proposed scheme for synthesis of multisubstituted 1-acyloxyindoles 1.
In particular, as a base for the acylation reaction, we tested several reagents, such a K2CO3, triethylamine (TEA), N,N-diisopropylethylamine (DIEA), 4-dimethylamino pyridine (DMAP), and 1,8-diazabicyclo [5.4.0]undece-7-ene (DBU). Among them, DBU provided the best results (data not shown), which are consistent with our previous report [25]. Thus, we chose DBU for our purpose. Optimization of the reaction conditions wa performed by varying the amount of SnCl2•2H2O, DBU, alcohol (R 1 OH), and acylating agent (R 2 COX). SnCl2•2H2O, an appropriate reducing agent for aromatic nitro group [35] was applied to convert 2 to 7 (2 → 5 → 6 → 7). We used benzyl alcohol (BnOH) as a tem plate nucleophile and pivaloyl chloride as a template acylating agent in dimethoxyethane (DME) to produce 1dy (Table 1). Considering our previous procedure for synthesis of 1 alkoxyindoles [25], we applied the range of reagents as such: SnCl2•2H2O 2.5-3.7 eq and DBU 10.6-15.7 eq for synthesis of 1. Here, we used an increased amount of DBU due to expected extra consumption by carboxylic acids that could be generated by partial hydrol ysis of the acylating agents. At lower or higher amounts than 3.3 eq for SnCl2•2H2O and Scheme 2. Proposed scheme for synthesis of multisubstituted 1-acyloxyindoles 1.
In particular, as a base for the acylation reaction, we tested several reagents, such as K 2 CO 3 , triethylamine (TEA), N,N-diisopropylethylamine (DIEA), 4-dimethylaminopyridine (DMAP), and 1,8-diazabicyclo [5.4.0]undece-7-ene (DBU). Among them, DBU provided the best results (data not shown), which are consistent with our previous reports [25]. Thus, we chose DBU for our purpose. Optimization of the reaction conditions was performed by varying the amount of SnCl 2 ·2H 2 O, DBU, alcohol (R 1 OH), and acylating agent (R 2 COX). SnCl 2 ·2H 2 O, an appropriate reducing agent for aromatic nitro group [35], was applied to convert 2 to 7 (2 → 5 → 6 → 7). We used benzyl alcohol (BnOH) as a template nucleophile and pivaloyl chloride as a template acylating agent in dimethoxyethane (DME) to produce 1dy (Table 1). Considering our previous procedure for synthesis of 1-alkoxyindoles [25], we applied the range of reagents as such: SnCl 2 ·2H 2 O 2.5-3.7 eq and DBU 10.6-15.7 eq for synthesis of 1. Here, we used an increased amount of DBU due to expected extra consumption by carboxylic acids that could be generated by partial hydrolysis of the acylating agents. At lower or higher amounts than 3.3 eq for SnCl 2 ·2H 2 O and 14.0 eq for DBU, the product 1dy was obtained in relatively poor yields (entries 1, 2, and 7, Table 1). We also compared the yields of 1-hydroxyindole intermediate 8d by varying the amount of SnCl 2 ·2H 2 O and found that the isolated yield of 1-hydroxyindole 8d with 3.3 eq of SnCl 2 ·2H 2 O was better (Y = 48%) than 3.7 eq (Y = 33%) and 2.5 eq (Y = 32%) in the case of 2.0 eq of BnOH. The orders of yields for intermediate 1-hydroxyindole 8d and 1-pivaloyloxyindole 1dy were generally correlated. The amount of SnCl 2 ·2H 2 O might be an important factor for construction of 1-acyloxyindole as well as 1-hydroxyindole intermediate by triggering the reduction of nitro group in 2. We further tested the amount of BnOH (1.5-3.0 eq) and pivaloyl chloride (1.5-3.0 eq). When using BnOH less than 1.5 eq, the product 1dy was obtained in poor yields (entries 3 and 4). More than 2.0 eq of BnOH and pivaloyl chloride did not seem to improve the yield (entry 6). Taken together, we chose the optimized condition for 1dy (entry 5): 1.0 eq of 2, 3.3 eq of SnCl 2 ·2H 2 O, 2.0 eq of BnOH at 40 • C, and then 14.0 eq of DBU and 2.0 eq of pivaloyl chloride at room temperature, which was applied to all other reactions unless otherwise noted.  14.0 eq for DBU, the product 1dy was obtained in relatively poor yields (entries 1, 2, and 7, Table 1). We also compared the yields of 1-hydroxyindole intermediate 8d by varying the amount of SnCl2•2H2O and found that the isolated yield of 1-hydroxyindole 8d with 3.3 eq of SnCl2•2H2O was better (Y = 48%) than 3.7 eq (Y = 33%) and 2.5 eq (Y = 32%) in the case of 2.0 eq of BnOH. The orders of yields for intermediate 1-hydroxyindole 8d and 1pivaloyloxyindole 1dy were generally correlated. The amount of SnCl2•2H2O might be an important factor for construction of 1-acyloxyindole as well as 1-hydroxyindole intermediate by triggering the reduction of nitro group in 2. We further tested the amount of BnOH (1.5-3.0 eq) and pivaloyl chloride (1.5-3.0 eq). When using BnOH less than 1.5 eq, the product 1dy was obtained in poor yields (entries 3 and 4). More than 2.0 eq of BnOH and pivaloyl chloride did not seem to improve the yield (entry 6). Taken together, we chose the optimized condition for 1dy (entry 5): 1.0 eq of 2, 3.3 eq of SnCl2•2H2O, 2.0 eq of BnOH at 40 °C, and then 14.0 eq of DBU and 2.0 eq of pivaloyl chloride at room temperature, which was applied to all other reactions unless otherwise noted.

Synthesis of New Derivatives of 1-Acyloxyindole 1
Under the optimized condition (entry 5, Table 1), we synthesized new 1-acyloxyindole derivatives by employing various nucleophiles and several acylating reagents (acetic anhydride and acyl chlorides) (Scheme 3). First, SnCl2•2H2O and 4Å molecular sieves were stirred in DME for 30 min at room temperature. We added alcohol and substrate 2, and then the reaction mixture was stirred at 40 °C for 1.5-3 h. After confirming that the starting material 2 was converted to 1-hydroxyindole 8 by checking TLC, we slowly added 14.0 eq of DBU with vigorous stirring. The reaction mixture was stirred for 30 min at room temperature and then acetic anhydride or acyl chloride was added in an ice bath. We kept stirring the reaction mixture at room temperature for 1.5-4 h, leading to formation of targeted 1-acyloxyindoles 1.

Synthesis of New Derivatives of 1-Acyloxyindole 1
Under the optimized condition (entry 5, Table 1), we synthesized new 1-acyloxyindole derivatives by employing various nucleophiles and several acylating reagents (acetic anhydride and acyl chlorides) (Scheme 3). First, SnCl 2 ·2H 2 O and 4Å molecular sieves were stirred in DME for 30 min at room temperature. We added alcohol and substrate 2, and then the reaction mixture was stirred at 40 • C for 1.5-3 h. After confirming that the starting material 2 was converted to 1-hydroxyindole 8 by checking TLC, we slowly added 14.0 eq of DBU with vigorous stirring. The reaction mixture was stirred for 30 min at room temperature and then acetic anhydride or acyl chloride was added in an ice bath. We kept stirring the reaction mixture at room temperature for 1.5-4 h, leading to formation of targeted 1-acyloxyindoles 1.
As acylating agents of 1-hydroxyindole intermediate 8, acetic anhydride, pivaloyl chloride, benzoyl chloride, butanoyl chloride, hexanoyl chloride, and hydrocinnamoyl chloride were employed ( Table 2). For acetylation reactions, we used acetic anhydride instead of acetyl chloride due to the high reactivity and instability of acetyl chloride. For example, both acetic anhydride and acetyl chloride provided 1dx in similar yields (Y =~30%), so we chose acetic anhydride. The yields for acetylation were generally lower than those for pivaloylation and benzoylation. For example, among 1dx, 1dy, and 1dz (entries [13][14][15], the yield of 1-acetoxyindole 1dx was lower than those for 1dy and 1dz with bulkier alkyl and aromatic group, respectively. Moreover, the yield of 1dw with phenethyl group (entry 12) was higher than that of 1dx. We expected that low yields of 1acetoxyindoles might be due to the instability of the compounds and that a bulkier alkyl or aromatic group on R 2 could stabilize the 1-acyloxyindole compounds. Interestingly, when we analyzed the spectroscopic features of these compounds, we found some consistence. For example, we found that the δ values ( 13 C NMR) of carbonyl carbons of N-OC(O)CH 3 in 1-acetoxyindoles 1x were~168.5, which means an upfield shift (~2) compared with those of carbonyl carbons in corresponding esters (R-OC(O)CH 3 ). In addition, the λ max values in UV-Vis were in the range of 229-236 nm. We also performed some of the reactions for 1du, 1dx, and 1dz in a larger scale (1.1 mmol of 2) and confirmed robust reproducibility of the established optimized conditions. Consequently, we successfully synthesized 21 new 1-acyloxyindole compounds 1 in modest yields (Y = 24-35%). As acylating agents of 1-hydroxyindole intermediate 8, acetic anhydride, pivaloyl chloride, benzoyl chloride, butanoyl chloride, hexanoyl chloride, and hydrocinnamoyl chloride were employed ( Table 2). For acetylation reactions, we used acetic anhydride instead of acetyl chloride due to the high reactivity and instability of acetyl chloride. For example, both acetic anhydride and acetyl chloride provided 1dx in similar yields (Y = ~30%), so we chose acetic anhydride. The yields for acetylation were generally lower than those for pivaloylation and benzoylation. For example, among 1dx, 1dy, and 1dz (entries [13][14][15], the yield of 1-acetoxyindole 1dx was lower than those for 1dy and 1dz with bulkier alkyl and aromatic group, respectively. Moreover, the yield of 1dw with phenethyl group (entry 12) was higher than that of 1dx. We expected that low yields of 1-acetoxyindoles might be due to the instability of the compounds and that a bulkier alkyl or aromatic group on R 2 could stabilize the 1-acyloxyindole compounds. Interestingly, when we analyzed the spectroscopic features of these compounds, we found some consistence. For example, we found that the δ values ( 13 C NMR) of carbonyl carbons of N-OC(O)CH3 in 1acetoxyindoles 1x were ~168.5, which means an upfield shift (~2) compared with those of carbonyl carbons in corresponding esters (R-OC(O)CH3). In addition, the λmax values in UV-Vis were in the range of 229-236 nm. We also performed some of the reactions for 1du, 1dx, and 1dz in a larger scale (1.1 mmol of 2) and confirmed robust reproducibility of the established optimized conditions. Consequently, we successfully synthesized 21 new 1-acyloxyindole compounds 1 in modest yields (Y = 24-35%).  As acylating agents of 1-hydroxyindole intermediate 8, acetic anhydride, pivaloyl chloride, benzoyl chloride, butanoyl chloride, hexanoyl chloride, and hydrocinnamoyl chloride were employed ( Table 2). For acetylation reactions, we used acetic anhydride instead of acetyl chloride due to the high reactivity and instability of acetyl chloride. For example, both acetic anhydride and acetyl chloride provided 1dx in similar yields (Y = ~30%), so we chose acetic anhydride. The yields for acetylation were generally lower than those for pivaloylation and benzoylation. For example, among 1dx, 1dy, and 1dz (entries [13][14][15], the yield of 1-acetoxyindole 1dx was lower than those for 1dy and 1dz with bulkier alkyl and aromatic group, respectively. Moreover, the yield of 1dw with phenethyl group (entry 12) was higher than that of 1dx. We expected that low yields of 1-acetoxyindoles might be due to the instability of the compounds and that a bulkier alkyl or aromatic group on R 2 could stabilize the 1-acyloxyindole compounds. Interestingly, when we analyzed the spectroscopic features of these compounds, we found some consistence. For example, we found that the δ values ( 13 C NMR) of carbonyl carbons of N-OC(O)CH3 in 1acetoxyindoles 1x were ~168.5, which means an upfield shift (~2) compared with those of carbonyl carbons in corresponding esters (R-OC(O)CH3). In addition, the λmax values in UV-Vis were in the range of 229-236 nm. We also performed some of the reactions for 1du, 1dx, and 1dz in a larger scale (1.1 mmol of 2) and confirmed robust reproducibility of the established optimized conditions. Consequently, we successfully synthesized 21 new 1-acyloxyindole compounds 1 in modest yields (Y = 24-35%). Scheme 3. Synthesis of various 1-acyloxyindoles 1.
As acylating agents of 1-hydroxyindole intermediate 8, acetic anhydride, pivaloyl chloride, benzoyl chloride, butanoyl chloride, hexanoyl chloride, and hydrocinnamoyl chloride were employed ( Table 2). For acetylation reactions, we used acetic anhydride instead of acetyl chloride due to the high reactivity and instability of acetyl chloride. For example, both acetic anhydride and acetyl chloride provided 1dx in similar yields (Y = ~30%), so we chose acetic anhydride. The yields for acetylation were generally lower than those for pivaloylation and benzoylation. For example, among 1dx, 1dy, and 1dz (entries [13][14][15], the yield of 1-acetoxyindole 1dx was lower than those for 1dy and 1dz with bulkier alkyl and aromatic group, respectively. Moreover, the yield of 1dw with phenethyl group (entry 12) was higher than that of 1dx. We expected that low yields of 1-acetoxyindoles might be due to the instability of the compounds and that a bulkier alkyl or aromatic group on R 2 could stabilize the 1-acyloxyindole compounds. Interestingly, when we analyzed the spectroscopic features of these compounds, we found some consistence. For example, we found that the δ values ( 13 C NMR) of carbonyl carbons of N-OC(O)CH3 in 1acetoxyindoles 1x were ~168.5, which means an upfield shift (~2) compared with those of carbonyl carbons in corresponding esters (R-OC(O)CH3). In addition, the λmax values in UV-Vis were in the range of 229-236 nm. We also performed some of the reactions for 1du, 1dx, and 1dz in a larger scale (1.1 mmol of 2) and confirmed robust reproducibility of the established optimized conditions. Consequently, we successfully synthesized 21 new 1-acyloxyindole compounds 1 in modest yields (Y = 24-35%).   Furthermore, some degree of decomposition of 1-acyloxyindoles (1du, 1dv, and 1dx) with linear alkyl groups (R 2 = n − Pr, n − Pen, and Me) on a TLC plate was observed. Partial degradation was observed for 1-acetoxyindole 1dx within 30 min, and, for 1-butanoyloxyindole 1du and 1-hexanoyloxyindole 1dv, within 2 h. However, 1-pivaloyloxyindole 1dy and 1-benzoyloxyindole 1dz were not easily decomposed on TLC. Consequently, we found that these 1-acyloxyindole compounds seem to exhibit significantly different stabilities depending on the R 2 in the acyl group (R 2 CO). Furthermore, these observations prompted us to test the stability of these compounds under hydrolysis conditions. We found that 1-butanoyloxyindole 1du and 1-acetoxyindole 1dx were easily hydrolyzed to provide 1-hydroxyindole under mildly basic conditions (data not shown). It is expected that this instability is due to the labile ester bond of NO-C(O)R 2 . This bond seems easily cleavable in even weakly acidic or basic conditions, resulting in 1-hydroxyindole and carboxylic acids (Scheme 4). We believed that this labile ester bond might provide us with an interesting possibility of its application in a prodrug strategy, which aims to explore drug delivery by lowering the polarity of the compounds by acylation of 1-hydroxyindole. Thus, further application studies on stability are in progress. 26 17 PhCH 2 CH 2 OH pivaloyl chloride  Furthermore, some degree of decomposition of 1-acyloxyindoles (1du, 1dv, and 1dx) with linear alkyl groups (R 2 = n − Pr, n − Pen, and Me) on a TLC plate was observed. Partial degradation was observed for 1-acetoxyindole 1dx within 30 min, and, for 1-butanoyloxyindole 1du and 1-hexanoyloxyindole 1dv, within 2 h. However, 1-pivaloyloxyindole 1dy and 1-benzoyloxyindole 1dz were not easily decomposed on TLC. Consequently, we found that these 1-acyloxyindole compounds seem to exhibit significantly different stabilities depending on the R 2 in the acyl group (R 2 CO). Furthermore, these observations prompted us to test the stability of these compounds under hydrolysis conditions. We found that 1-butanoyloxyindole 1du and 1-acetoxyindole 1dx were easily hydrolyzed to provide 1-hydroxyindole under mildly basic conditions (data not shown). It is expected that this instability is due to the labile ester bond of NO-C(O)R 2 . This bond seems easily cleavable in even weakly acidic or basic conditions, resulting in 1-hydroxyindole and carboxylic acids (Scheme 4). We believed that this labile ester bond might provide us with an interesting possibility of its application in a prodrug strategy, which aims to explore drug delivery by lowering the polarity of the compounds by acylation of 1-hydroxyindole. Thus, further application studies on stability are in progress. 27 18 PhCH 2 CH 2 OH benzoyl chloride  Furthermore, some degree of decomposition of 1-acyloxyindoles (1du, 1dv, and 1dx) with linear alkyl groups (R 2 = n − Pr, n − Pen, and Me) on a TLC plate was observed. Partial degradation was observed for 1-acetoxyindole 1dx within 30 min, and, for 1-butanoyloxyindole 1du and 1-hexanoyloxyindole 1dv, within 2 h. However, 1-pivaloyloxyindole 1dy and 1-benzoyloxyindole 1dz were not easily decomposed on TLC. Consequently, we found that these 1-acyloxyindole compounds seem to exhibit significantly different stabilities depending on the R 2 in the acyl group (R 2 CO). Furthermore, these observations prompted us to test the stability of these compounds under hydrolysis conditions. We found that 1-butanoyloxyindole 1du and 1-acetoxyindole 1dx were easily hydrolyzed to provide 1-hydroxyindole under mildly basic conditions (data not shown). It is expected that this instability is due to the labile ester bond of NO-C(O)R 2 . This bond seems easily cleavable in even weakly acidic or basic conditions, resulting in 1-hydroxyindole and carboxylic acids (Scheme 4). We believed that this labile ester bond might provide us with an interesting possibility of its application in a prodrug strategy, which aims to explore drug delivery by lowering the polarity of the compounds by acylation of 1-hydroxyindole. Thus, further application studies on stability are in progress. 32 19 c-HxOH acetic anhydride 1fx  Furthermore, some degree of decomposition of 1-acyloxyindoles (1du, 1dv, and 1dx) with linear alkyl groups (R 2 = n − Pr, n − Pen, and Me) on a TLC plate was observed. Partial degradation was observed for 1-acetoxyindole 1dx within 30 min, and, for 1-butanoyloxyindole 1du and 1-hexanoyloxyindole 1dv, within 2 h. However, 1-pivaloyloxyindole 1dy and 1-benzoyloxyindole 1dz were not easily decomposed on TLC. Consequently, we found that these 1-acyloxyindole compounds seem to exhibit significantly different stabilities depending on the R 2 in the acyl group (R 2 CO). Furthermore, these observations prompted us to test the stability of these compounds under hydrolysis conditions. We found that 1-butanoyloxyindole 1du and 1-acetoxyindole 1dx were easily hydrolyzed to provide 1-hydroxyindole under mildly basic conditions (data not shown). It is expected that this instability is due to the labile ester bond of NO-C(O)R 2 . This bond seems easily cleavable in even weakly acidic or basic conditions, resulting in 1-hydroxyindole and carboxylic acids (Scheme 4). We believed that this labile ester bond might provide us with an interesting possibility of its application in a prodrug strategy, which aims to explore drug delivery by lowering the polarity of the compounds by acylation of 1-hydroxyindole. Thus, further application studies on stability are in progress. 24 20 c-HxOH pivaloyl chloride  Furthermore, some degree of decomposition of 1-acyloxyindoles (1du, 1dv, and 1dx) with linear alkyl groups (R 2 = n − Pr, n − Pen, and Me) on a TLC plate was observed. Partial degradation was observed for 1-acetoxyindole 1dx within 30 min, and, for 1-butanoyloxyindole 1du and 1-hexanoyloxyindole 1dv, within 2 h. However, 1-pivaloyloxyindole 1dy and 1-benzoyloxyindole 1dz were not easily decomposed on TLC. Consequently, we found that these 1-acyloxyindole compounds seem to exhibit significantly different stabilities depending on the R 2 in the acyl group (R 2 CO). Furthermore, these observations prompted us to test the stability of these compounds under hydrolysis conditions. We found that 1-butanoyloxyindole 1du and 1-acetoxyindole 1dx were easily hydrolyzed to provide 1-hydroxyindole under mildly basic conditions (data not shown). It is expected that this instability is due to the labile ester bond of NO-C(O)R 2 . This bond seems easily cleavable in even weakly acidic or basic conditions, resulting in 1-hydroxyindole and carboxylic acids (Scheme 4). We believed that this labile ester bond might provide us with an interesting possibility of its application in a prodrug strategy, which aims to explore drug delivery by lowering the polarity of the compounds by acylation of 1-hydroxyindole. Thus, further application studies on stability are in progress. Furthermore, some degree of decomposition of 1-acyloxyindoles (1du, 1dv, and 1dx) with linear alkyl groups (R 2 = n − Pr, n − Pen, and Me) on a TLC plate was observed. Partial degradation was observed for 1-acetoxyindole 1dx within 30 min, and, for 1-butanoyloxyindole 1du and 1-hexanoyloxyindole 1dv, within 2 h. However, 1-pivaloyloxyindole 1dy and 1-benzoyloxyindole 1dz were not easily decomposed on TLC. Consequently, we found that these 1-acyloxyindole compounds seem to exhibit significantly different stabilities depending on the R 2 in the acyl group (R 2 CO). Furthermore, these observations prompted us to test the stability of these compounds under hydrolysis conditions. We found that 1-butanoyloxyindole 1du and 1-acetoxyindole 1dx were easily hydrolyzed to provide 1-hydroxyindole under mildly basic conditions (data not shown). It is expected that this instability is due to the labile ester bond of NO-C(O)R 2 . This bond seems easily cleavable in even weakly acidic or basic conditions, resulting in 1-hydroxyindole and carboxylic acids (Scheme 4). We believed that this labile ester bond might provide us with an interesting possibility of its application in a prodrug strategy, which aims to explore drug delivery by lowering the polarity of the compounds by acylation of 1-hydroxyindole. Thus, further application studies on stability are in progress. Furthermore, some degree of decomposition of 1-acyloxyindoles (1du, 1dv, and 1dx) with linear alkyl groups (R 2 = n − Pr, n − Pen, and Me) on a TLC plate was observed. Partial degradation was observed for 1-acetoxyindole 1dx within 30 min, and, for 1-butanoyloxyindole 1du and 1-hexanoyloxyindole 1dv, within 2 h. However, 1-pivaloyloxyindole 1dy and 1benzoyloxyindole 1dz were not easily decomposed on TLC. Consequently, we found that these 1-acyloxyindole compounds seem to exhibit significantly different stabilities depending on the R 2 in the acyl group (R 2 CO). Furthermore, these observations prompted us to test the stability of these compounds under hydrolysis conditions. We found that 1-butanoyloxyindole 1du and 1-acetoxyindole 1dx were easily hydrolyzed to provide 1hydroxyindole under mildly basic conditions (data not shown). It is expected that this instability is due to the labile ester bond of NO-C(O)R 2 . This bond seems easily cleavable in even weakly acidic or basic conditions, resulting in 1-hydroxyindole and carboxylic acids (Scheme 4). We believed that this labile ester bond might provide us with an interesting possibility of its application in a prodrug strategy, which aims to explore drug delivery by lowering the polarity of the compounds by acylation of 1-hydroxyindole. Thus, further application studies on stability are in progress.

Mechanistic Investigations on Reaction Pathways
We investigated the reaction mechanisms and pathways based on the observed products, as shown in Scheme 5. We suggest that three pathways, A 1 , A 2 , and B, are involved in the reaction mechanism, which derives some support from our previous work [34]. The nitro group of conjugate ketoester derivative 2 was reduced to afford hydroxylamine compound 5 (or conformer 5 ). The pathways A 1 and A 2 proceeded through conformer 5, and pathway B through conformer 5 . The intramolecular addition of N-H of two conformers, 5 Molecules 2022, 27, 6769 8 of 17 and 5 , provided two different indoline derivatives, 6 and 10, respectively. Following dehydration of 6, it was possible to generate conjugate nitrone 7. Nucleophilic 1,5-addition of alcohol (R 1 OH) produced 1-hydroxyindole 8 (Path A 1 ); subsequent acylation of the hydroxy group with R 2 COX provided 1-acyloxyindole 1. However, instead of alcohol, H 2 O as a nucleophile could be added to conjugate nitrone 7 to produce dihydroxy species 9 (Path A 2 ). Dihydroxy compound 9 could be acylated with R 2 COX to provide diacylated compound 12.
In the process of synthesizing 1dy, dipivaloylated compound 12 (R 2 = t-Bu) was obtained and identified by mass analysis (446 [M + Na] + ). On the other hand, conformer 5 produced enolic compound 10 through intramolecular conjugate addition (aza-Michael addition) (Path B). Then, subsequent oxidative aromatization provided 1-hydroxyindole 11 [34]. Although we expected that acylation of 11 could produce 13, the acylated product 13 was difficult to be isolated and even identified. In most of the reactions in Table 2, we believed that substrate 2 proceeded through not only Path A 1 but also Path A 2 and Path B, which might explain the low yields of the products 1. Furthermore, some degree of decomposition of 1-acyloxyindoles (1du, 1dv, and 1dx) with linear alkyl groups (R 2 = n − Pr, n − Pen, and Me) on a TLC plate was observed. Partial degradation was observed for 1-acetoxyindole 1dx within 30 min, and, for 1-butanoyloxyindole 1du and 1-hexanoyloxyindole 1dv, within 2 h. However, 1-pivaloyloxyindole 1dy and 1-benzoyloxyindole 1dz were not easily decomposed on TLC. Consequently, we found that these 1-acyloxyindole compounds seem to exhibit significantly different stabilities depending on the R 2 in the acyl group (R 2 CO). Furthermore, these observations prompted us to test the stability of these compounds under hydrolysis conditions. We found that 1-butanoyloxyindole 1du and 1-acetoxyindole 1dx were easily hydrolyzed to provide 1-hydroxyindole under mildly basic conditions (data not shown). It is expected that this instability is due to the labile ester bond of NO-C(O)R 2 . This bond seems easily cleavable in even weakly acidic or basic conditions, resulting in 1-hydroxyindole and carboxylic acids (Scheme 4). We believed that this labile ester bond might provide us with an interesting possibility of its application in a prodrug strategy, which aims to explore drug delivery by lowering the polarity of the compounds by acylation of 1-hydroxyindole. Thus, further application studies on stability are in progress.

General
Reagents were obtained from Sigma-Aldrich (Darmstad, Germany), Thermo Fisher (Waltham, MA, USA), and TCI (Tokyo, Japan). They were of commercial quality and used without further purification unless otherwise stated. Reactions were periodically moni-Scheme 5. Proposed pathways for 1, 12, and 13.

General
Reagents were obtained from Sigma-Aldrich (Darmstad, Germany), Thermo Fisher (Waltham, MA, USA), and TCI (Tokyo, Japan). They were of commercial quality and used without further purification unless otherwise stated. Reactions were periodically monitored by thin-layer chromatography (TLC) carried out on 0.25 mm Merck silica gel plates (20 × 20 cm; Merck F 254 ) (Darmstad, Germany) and visualized by UV light. Purifications were performed by preparative TLC (PTLC) and column chromatography. PTLC separations were carried out on the same silica gel plates. Column chromatography was performed using Merck silica gels (230-400 mesh) (Zvornik, Bosnia and Herzegovina). Melting points (uncorrected) were determined in Deckgläser microscope cover glasses (Lauda-Königshofen, Germany) using a Thermo Scientific 00590Q apparatus (Dubuque, Iowa, USA). 1 H (300 MHz) and 13 C (75 MHz) NMR spectra were obtained by a Bruker DRX 300 spectrometer (Zürich, Switzerland), and chemical shifts (δ) are expressed with respect to tetramethylsilane (TMS). NMR spectra are presented in the Supplementary Materials. Mass spectra were obtained in EI or ESI ionization modes (Agilent, Santa Clara, CA, USA). High resolution mass spectra were obtained using JEOL apparatus (Tokyo, Japan) at the Korea Basic Science Institute, Republic of Korea. HPLC analyses were performed using the following Waters Associate Units: 515 A pump, 515 B pump, dual λ absorbance 2487 detector, and COSMOSIL 5C 18 -AR-II Packed Column (4.6 × 250 mm) (Worcester, MA, USA). The products were analyzed using a linear gradient: from 70% A (aqueous) and 30% B (acetonitrile) for 3 min (isocratic) to 10% A and 90% B over 30 min at a flow rate of 1 mL/min with eluent monitoring at 254 nm. HPLC solvents were filtered (aqueous solution with PALL FP-450, 0.45 µm, 47 mm; acetonitrile with PALL TF-450, 0.45 µm, 47 mm) and degassed before use.

Conclusions
We reported the studies on one-pot synthesis of new 1-acyloxyindoles 1 through fourstep reactions. With substrate 2 obtained by a two-step synthetic sequence, we performed the reactions using SnCl 2 ·2H 2 O as a reducing agent and alcohol (R 1 OH) as a nucleophile through reduction, intramolecular addition, and nucleophilic 1,5-addition, affording intermediate 1-hydroxyindole 8. Subsequent acylation of 8 using acetic anhydride or acyl chlorides (R 2 COX) in a basic condition provided target compound 1-acyloxyindoles 1. Optimization of the reaction conditions was established as follows: 1) conjugate ketoester 2 (1.0 eq), SnCl 2 ·2H 2 O (3.3 eq), and ROH (2.0 eq) in DME at 40 • C; and 2) DBU (14.0 eq) and acetic anhydride or acyl chloride (2.0 eq) at room temperature. Consequently, using the optimized conditions, 21 examples of new 1-acyloxyindole derivatives were successfully synthesized in modest yields (Y = 24-35%) through one-pot reaction of a four-step sequence.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27196769/s1, The charts for 1 H-and 13 C-NMR spectroscopies are available online.

Data Availability Statement:
The data presented in this study are available in insert article or Supplementary Materials here. Samples of compounds 1ax-1fz are available from the authors.

Conflicts of Interest:
The authors declare no conflict of interest.