Literature Survey and Further Studies on the 3-Alkylation of N-Unprotected 3-Monosubstituted Oxindoles. Practical Synthesis of N-Unprotected 3,3-Disubstituted Oxindoles and Subsequent Transformations on the Aromatic Ring

The paper provides a comprehensive review of the base-catalysed C3-alkylation of N-unprotected-3-monosubstituted oxindoles. Based on a few, non-systematic studies described in the literature using butyllithium as the deprotonating agent, an optimized method has now been elaborated, via the corresponding lithium salt, for the selective C3-alkylation of this family of compounds. The optimal excess of butyllithium and alkylating agent, and the role of the halogen atom in the latter (alkyl bromides vs. iodides) were also studied. The alkylation protocol has also been extended to some derivatives substituted at the aromatic ring. Finally, various substituents were introduced into the aromatic ring of the N-unprotected 3,3-dialkyloxindoles obtained by this optimized method.


Introduction
The biological activity of 1,3-dihydro-2H-indol-2-one (oxindole (1), Figure 1) derivatives and their structural relationship to indoles render these compounds important targets in medicinal and synthetic organic chemistry. Launched drugs possessing an oxindole skeleton are summarized in Figure 1: the dopamine agonist ropinirole (2) for the treatment of Parkinson's disease and restless legs syndrome; the atypical antipsychotic ziprasidone (3) and two oncology drugs from the tyrosine kinase inhibitor family, sunitinib (4) and the recently launched nintedanib (5). Several other compounds have reached human Phase III [1], Phase II [2] or Phase I [3] clinical trials, and thousands of further oxindole derivatives are or were studied in preclinical testing in various therapeutic fields.
According to the literature, N-unprotected 3-alkyloxindoles 6 can be prepared by condensation of oxindole (1) with ketones or aromatic aldehydes and subsequent reduction of the primarily formed 3-alkylideneoxindoles, 7. However, in the case of aliphatic aldehydes the yields are low because of aldol-type side reactions [4][5][6]. In order to avoid these difficulties, we disclosed an efficient method for the regioselective synthesis of N-unprotected 3-alkyloxindoles 6, based on the Raney nickel (Ra-Ni) induced 3-alkylation of oxindole (1) with primary and secondary alcohols (Scheme 1) [7][8][9]. This reaction involves a reductive alkylation as the key step: Raney nickel acts as the oxidizing agent in the transformation of the alcohol to the corresponding carbonyl compound, then as the catalyst during the reduction of the in situ-formed 3-alkylideneoxindole. Next we set ourselves the task to develop an efficient method for the 3-alkylation of N-unprotected 3-alkyloxindoles (8,Scheme 2) to give N-unprotected 3,3-dialkyloxindoles 9. Scheme 2. C3-Alkylation of 3-alkyloxindoles 8. This reaction involves a reductive alkylation as the key step: Raney nickel acts as the oxidizing agent in the transformation of the alcohol to the corresponding carbonyl compound, then as the catalyst during the reduction of the in situ-formed 3-alkylideneoxindole. This reaction involves a reductive alkylation as the key step: Raney nickel acts as the oxidizing agent in the transformation of the alcohol to the corresponding carbonyl compound, then as the catalyst during the reduction of the in situ-formed 3-alkylideneoxindole. Next we set ourselves the task to develop an efficient method for the 3-alkylation of N-unprotected 3-alkyloxindoles (8,Scheme 2) to give N-unprotected 3,3-dialkyloxindoles 9. Scheme 2. C3-Alkylation of 3-alkyloxindoles 8. Scheme 1. Reductive alkylation reactions of oxindole (1) to give 3-alkyloxindoles 6.
Next we set ourselves the task to develop an efficient method for the 3-alkylation of N-unprotected 3-alkyloxindoles (8,Scheme 2) to give N-unprotected 3,3-dialkyloxindoles 9. This reaction involves a reductive alkylation as the key step: Raney nickel acts as the oxidizing agent in the transformation of the alcohol to the corresponding carbonyl compound, then as the catalyst during the reduction of the in situ-formed 3- Next we set ourselves the task to develop an efficient method for the 3-alkylation of N-unprotected 3-alkyloxindoles (8,Scheme 2) to give N-unprotected 3,3-dialkyloxindoles 9.
3-Alkyloxindoles 8 have two regiochemically distinct and easily removable protons (N-H, C3-H), thus rendering possible the formation of N-and C3-alkylated products in a deprotonation-alkylation sequence (i.e. alkylation by nucleophilic substitution). In order to find optimal reaction conditions for the regioselective alkylation of 3-alkyloxindoles 8 we first analyzed the results of systematic studies on the alkylation of oxindole (1) described in the literature.
Use of a lithium base, first described by Kende et al., proved to be a better approach towards selective C3-alkylation [25], Treatment of oxindole (1) with butyllithium (BuLi, 2.0 eq) in the presence of N,N,N ,N -tetramethylethylenediamine (TMEDA, 2.0 eq) in THF at −75 • C followed by reaction with various alkyl halides at a temperature between −20 • C and room temperature gave varying yields of the 3-monosubstituted (8, from <20% to 72% yield) and 3,3-disubstituted (9, from <20% to 66% yield) products (Scheme 5), the ratio of which depended also on the excess of alkylating agent and on the reaction conditions. Besides the pleasing lack of N-alkylation, the major drawbacks of this BuLi-TMEDA protocol are the limited mono/di selectivity, the need for chromatographic purification, and the fact that bromides (except for benzyl bromide and ethyl bromoacetate) were unreactive, therefore the corresponding iodides, which are less easily available and more expensive, had to be used. Two examples are given also for the second alkylation in position 3 under the same conditions: 3-butyloxindole was alkylated with iodomethylcyclohexane (74% yield) and 3-benzyloxindole with benzyl bromide (87%). It is noteworthy that omission of TMEDA gave poor results in all these reactions. The 3-acetyl protecting group increases the acidity of the C3 position, thereby improving selectivity vs. the N1 atom. 3-Alkyl(alkenyl/ alkynyl)-3-acetyloxindoles 14 thus obtained were then hydrolysed with Na2CO3 in EtOH to give 3-monosubstituted derivatives 8, thus rendering the introduction of a second alkyl group into position 3 possible. However, only one representative of this family was described: 3-propargyloxindole was reacted with 3,3-dimethylallylchloride in the presence of NaOEt to give the disubstituted congener 9b in 45% yield. The low yield of 9b may be due to an incomplete selectivity in this step. Nevertheless, this method has several drawbacks: numerous reaction steps, very long reaction times, chromatographic purifications and low overall yield.
Use of a lithium base, first described by Kende et al., proved to be a better approach towards selective C3-alkylation [25], Treatment of oxindole (1) with butyllithium (BuLi, 2.0 eq) in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA, 2.0 eq) in THF at −75 °C followed by reaction with various alkyl halides at a temperature between −20 °C and room temperature gave varying yields of the 3-monosubstituted (8, from <20% to 72% yield) and 3,3-disubstituted (9, from <20% to 66% yield) products (Scheme 5), the ratio of which depended also on the excess of alkylating agent and on the reaction conditions. Besides the pleasing lack of N-alkylation, the major drawbacks of this BuLi-TMEDA protocol are the limited mono/di selectivity, the need for chromatographic purification, and the fact that bromides (except for benzyl bromide and ethyl bromoacetate) were unreactive, therefore the corresponding iodides, which are less easily available and more expensive, had to be used. Two examples are given also for the second alkylation in position 3 under the same conditions: 3-butyloxindole was alkylated with iodomethylcyclohexane (74% yield) and 3-benzyloxindole with benzyl bromide (87%). It is noteworthy that omission of TMEDA gave poor results in all these reactions. from <20 to 72% from <20 to 66% Scheme 5. Direct alkylation of oxindole (1) via its lithium salt.
Our comprehensive literature search did not reveal further systematic studies (other than those demonstrated in Schemes 3-5) on the synthesis of N-unprotected 3,3-disubstituted oxindoles 9 starting from 3-monosubstituted oxindoles (8). As shown above, due to the biological importance of oxindole derivatives, several research groups applied the above alkylation reactions for the introduction of a second substituent in position 3 of a 3-monosubstituted oxindole 8, albeit, in quite an erratic way.
Our comprehensive literature search did not reveal further systematic studies (other than those demonstrated in Schemes 3-5) on the synthesis of N-unprotected 3,3-disubstituted oxindoles 9 starting from 3-monosubstituted oxindoles (8). As shown above, due to the biological importance of oxindole derivatives, several research groups applied the above alkylation reactions for the introduction of a second substituent in position 3 of a 3-monosubstituted oxindole 8, albeit, in quite an erratic way.
For the sake of completeness it is worth mentioning that, due to the ambident nucleophile character of oxindoles, deprotonation and alkylation can take place not only on C3 or N1, but also on the O2 atom. To the best of our knowledge, C,O-or C,N-dialkylations are not described. On the other hand, selective O-alkylation can take place under certain conditions, although the occurrence of this reaction in the scientific literature is very rare. It can only be carried out using special alkylating agents, e.g., trialkyloxonium tetrafluoroborates [38][39][40][41][42][43][44][45][46].
Apart from regioselectivity issues, a further difficulty during the alkylation of oxindole derivarives is caused by the observation that position 3 of 3-monoalkyloxindoles 8 is prone to oxidation under basic conditions. Bai et al. described the synthesis of a wide range of 1-acetyl-3-hydroxy-3-phenacyloxindole derivatives 16 starting from 1-acetyloxindole (17, Scheme 7) and α-tosyloxyacetophenone (18,R=Ph,X=OTs) in an open vial [47]. In a control experiment, the reaction of 17 and α-tosyloxyacetophenone was carried out under nitrogen atmosphere for 8 h, and the 3-monosubstituted oxindole 19 was obtained (yield is not disclosed). Then the reaction was continued (8 h) by opening the flask, leading to the formation of 3-hydroxy derivative 16, presumably via the corresponding hydroperoxide, the presence of which was proved by electrospray ionization mass spectrometry (ESI-MS). Since the key factor of the suppression of side reactions is the exclusion of atmospheric oxygen, the reductive method elaborated by our research group (Scheme 1) [9] for the synthesis of 3-monoalkyloxindoles 8 is particularly advantageous. For the sake of completeness it is worth mentioning that, due to the ambident nucleophile character of oxindoles, deprotonation and alkylation can take place not only on C3 or N1, but also on the O2 atom. To the best of our knowledge, C,O-or C,N-dialkylations are not described. On the other hand, selective O-alkylation can take place under certain conditions, although the occurrence of this reaction in the scientific literature is very rare. It can only be carried out using special alkylating agents, e.g., trialkyloxonium tetrafluoroborates [38][39][40][41][42][43][44][45][46].
Apart from regioselectivity issues, a further difficulty during the alkylation of oxindole derivarives is caused by the observation that position 3 of 3-monoalkyloxindoles 8 is prone to oxidation under basic conditions. Bai et al. described the synthesis of a wide range of 1-acetyl-3-hydroxy-3-phenacyloxindole derivatives 16 starting from 1-acetyloxindole (17, Scheme 7) and α-tosyloxyacetophenone (18,R=Ph,X=OTs) in an open vial [47]. In a control experiment, the reaction of 17 and α-tosyloxyacetophenone was carried out under nitrogen atmosphere for 8 h, and the 3-monosubstituted oxindole 19 was obtained (yield is not disclosed). Then the reaction was continued (8 h) by opening the flask, leading to the formation of 3-hydroxy derivative 16, presumably via the corresponding hydroperoxide, the presence of which was proved by electrospray ionization mass spectrometry (ESI-MS). Since the key factor of the suppression of side reactions is the exclusion of atmospheric oxygen, the reductive method elaborated by our research group (Scheme 1) [9] for the synthesis of 3-monoalkyloxindoles 8 is particularly advantageous.

Results and Discussion
In the present work we set ourselves the task to carry out a deeper study on the scope and limitations of the deprotonation of 3-monosubstituted oxindoles with BuLi and subsequent alkylation. The advantages of using lithium bases (alkyllithiums, lithium dialkylamides) instead of other alkali metal bases in C-alkylation reactions for deprotonation of C-H acids (e.g., ketones, esters, amides) is well documented [48]. Lithium cation, as the smallest alkali metal ion has a stronger tendency to form O-Li and N-Li bonds with increased covalent character [49], thus inhibiting undesirable Oand N-alkylations. 3-Monosubstituted oxindoles 8, optionally substituted on the aromatic ring, as the starting materials of the present study were synthesized from the corresponding isatins [50].

Results and Discussion
In the present work we set ourselves the task to carry out a deeper study on the scope and limitations of the deprotonation of 3-monosubstituted oxindoles with BuLi and subsequent alkylation. The advantages of using lithium bases (alkyllithiums, lithium dialkylamides) instead of other alkali metal bases in C-alkylation reactions for deprotonation of C-H acids (e.g., ketones, esters, amides) is well documented [48]. Lithium cation, as the smallest alkali metal ion has a stronger tendency to form O-Li and N-Li bonds with increased covalent character [49], thus inhibiting undesirable O-and N-alkylations. 3-Monosubstituted oxindoles 8, optionally substituted on the aromatic ring, as the starting materials of the present study were synthesized from the corresponding isatins [50].

Results and Discussion
In the present work we set ourselves the task to carry out a deeper study on the scope and limitations of the deprotonation of 3-monosubstituted oxindoles with BuLi and subsequent alkylation. The advantages of using lithium bases (alkyllithiums, lithium dialkylamides) instead of other alkali metal bases in C-alkylation reactions for deprotonation of C-H acids (e.g., ketones, esters, amides) is well documented [48]. Lithium cation, as the smallest alkali metal ion has a stronger tendency to form O-Li and N-Li bonds with increased covalent character [49], thus inhibiting undesirable Oand N-alkylations. 3-Monosubstituted oxindoles 8, optionally substituted on the aromatic ring, as the starting materials of the present study were synthesized from the corresponding isatins [50].
The reactions summarized in Table 1 were strictly performed under inert atmosphere. Prior to the reactions, the flask was made inert by using three consecutive vacuum-argon cycles, and an argon atmosphere was maintained until quenching the reaction. However, during the first series of experiments where a less strict pre-inertization was used, significant amounts of the 3-hydroxy side products were isolated. When starting from 3-isopropyloxindole (8f), the alkylation using 2.5 eq BuLi and 2.5 eq EtBr provided, after work-up a significant amount (23%) of 3-hydroxy-3-isopropyloxindole (21a, Scheme 9), besides the expected 3-ethyl-3-isopropyloxindole (9i, 36%). This finding is particularly surprising in the light of the paper of Kende et al. that does not describe N-alkylation side-reactions with alkyl iodides [25]. When using decreased excesses of the reagents (2.2 eq BuLi, 1.2 eq MeI), 3,3-dialkyl product 9c was obtained in 71% yield, while the formation of 1,3,3-trialkyl derivative 20a could not be detected (entry 2). Unexpectedly, alkylation of 8b occurred regioselectively even with 2.5 eq of EtI (73%, entry 3). Change of EtI (2.5 eq) to EtBr (1.2 eq) led to even better results (90%, entry 4), and the reaction was also performed with BnBr (80%, entry 5). Extension of the ethylation (with 1.2 eq EtBr) to derivatives 8b-e substituted on the aromatic ring was also successful (entries 6-8).
The reactions summarized in Table 1 were strictly performed under inert atmosphere. Prior to the reactions, the flask was made inert by using three consecutive vacuum-argon cycles, and an argon atmosphere was maintained until quenching the reaction. However, during the first series of experiments where a less strict pre-inertization was used, significant amounts of the 3-hydroxy side products were isolated. When starting from 3-isopropyloxindole (8f), the alkylation using 2.5 eq BuLi and 2.5 eq EtBr provided, after work-up a significant amount (23%) of 3-hydroxy-3-isopropyloxindole (21a, Scheme 9), besides the expected 3-ethyl-3-isopropyloxindole (9i, 36%). The targeted synthesis of the 3-hydroxy derivatives 21a,b was carried out with BuLi (2.5 eq) without alkylating agent and under non-inert conditions with good yields (Scheme 10). The presence of the hydroxy moiety in products 21a,b renders further functionalizations possible thereby making these compounds valuable synthetic building blocks. The targeted synthesis of the 3-hydroxy derivatives 21a,b was carried out with BuLi (2.5 eq) without alkylating agent and under non-inert conditions with good yields (Scheme 10). The presence of the hydroxy moiety in products 21a,b renders further functionalizations possible thereby making these compounds valuable synthetic building blocks. This finding is particularly surprising in the light of the paper of Kende et al. that does not describe N-alkylation side-reactions with alkyl iodides [25]. When using decreased excesses of the reagents (2.2 eq BuLi, 1.2 eq MeI), 3,3-dialkyl product 9c was obtained in 71% yield, while the formation of 1,3,3-trialkyl derivative 20a could not be detected (entry 2). Unexpectedly, alkylation of 8b occurred regioselectively even with 2.5 eq of EtI (73%, entry 3). Change of EtI (2.5 eq) to EtBr (1.2 eq) led to even better results (90%, entry 4), and the reaction was also performed with BnBr (80%, entry 5). Extension of the ethylation (with 1.2 eq EtBr) to derivatives 8b-e substituted on the aromatic ring was also successful (entries 6-8).
The reactions summarized in Table 1 were strictly performed under inert atmosphere. Prior to the reactions, the flask was made inert by using three consecutive vacuum-argon cycles, and an argon atmosphere was maintained until quenching the reaction. However, during the first series of experiments where a less strict pre-inertization was used, significant amounts of the 3-hydroxy side products were isolated. When starting from 3-isopropyloxindole (8f), the alkylation using 2.5 eq BuLi and 2.5 eq EtBr provided, after work-up a significant amount (23%) of 3-hydroxy-3-isopropyloxindole (21a, Scheme 9), besides the expected 3-ethyl-3-isopropyloxindole (9i, 36%).

Scheme 9. Ethylation of 3-isopropyloxindole (8f) under insufficiently inert conditions.
The targeted synthesis of the 3-hydroxy derivatives 21a,b was carried out with BuLi (2.5 eq) without alkylating agent and under non-inert conditions with good yields (Scheme 10). The presence of the hydroxy moiety in products 21a,b renders further functionalizations possible thereby making these compounds valuable synthetic building blocks. As demonstrated among others by two marketed drugs, ziprasidone (3) and sunitinib (4), and some further drug candidates [51], substitution at position 5 of the aromatic ring is of importance in the oxindole family. A possible approach for the synthesis of 5-substituted 3,3-dialkyloxindoles is 3-alkylation of a 5-subsituted 3-monoalkyloxindole. Nevertheless, several moieties (e.g., Br, NO 2 , ortho-directing groups, etc.) can be incompatible with BuLi-mediated 3-alkylation. Therefore, aromatic substitution reactions were carried out, starting from 3,3-diethyloxindole (9d) as the model compound (Scheme 11). Moreover, further modifications of the primarily obtained compounds 9l-o have also been envisaged.

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General Information
All melting points were determined on a Büchi 535 capillary melting point apparatus (Büchi, Flawil, Switzerland) and on an OptiMelt Automated Melting Point System by Stanford Research Systems (Sunnyvale, CA, USA). IR spectra were obtained on a IFS-113v FT spectrometer (Bruker, Billerica, MA, USA). 1 H-NMR and 13 C-NMR spectra were recorded on a Bruker Avance III (400 and 100 MHz for 1 H-and 13 C-NMR spectra, respectively) or a Bruker Avance III HD 600 (600 and 150 MHz for 1 H and 13 C-NMR spectra, respectively) spectrometer (Bruker, Billerica, MA, USA). CDCl 3 , DMSO-d 6 or CD 3 CN was used as the solvent and tetramethylsilane (TMS) as internal standard. Chemical shifts (δ) and coupling constants (J) are given in ppm and in Hz, respectively. The electron ionization (EI) mass spectra were recorded on a Clarus 560 D mass spectrometer coupled with a Clarus 500 gas chromatograph (Perkin-Elmer, Waltham, MA, USA). The ESI+ mass spectra (MS) were recorded on a LTQ XL mass spectrometer (Thermo Fisher, Waltham, MA, USA) coupled with an Acquity TM UPLC (Waters, Milford, MA, USA). Elemental analyses (EA) were performed on a 2400 analyzer (Perkin-Elmer, Waltham, MA, USA) on a VARIO EL III Model CHN elemental analyzer (Elementar, Langenselbold, Germany) or on an Elementar Vario MICRO cube (CHNS) elemental analyzer (Elementar, Langenselbold, Germany). The chloride and bromide contents were determined by titration. The reactions were followed by analytical thin layer chromatography on silica gel 60 F 254 (Merck, Darmstadt, Germany). All unspecified reagents were purchased from commercial sources . Compounds 9a-b, 9d, 20a-b, 21a are known in the literature . Compounds 9c, 9m, 9n, 9p, 21b are mentioned but poorly characterized in the literature, therefore their full characterization is given below. Compounds 9e-l, 9o, 9q-u are new and characterized below.

General Procedure I for the Synthesis of Compounds 9c-h (and By-Product 20a)
To a mixture of butyllithium in hexane (2.2-2.5 eq, 1.6 M) and THF, the solution of the appropriate 3-alkyloxindole 8b-e in THF was added dropwise at −78 • C under argon atmosphere. Then the appropriate alkyl halide (1.2-2.5 eq) in THF was added dropwise, the acetone-dry ice bath was removed and the reaction mixture was allowed to warm to room temperature. The stirring was continued for further 4 h, the mixture was quenched with ethanol (EtOH) and the solvents were evaporated. The residue crystallized upon treatment with water. It was triturated in water, filtered, washed with water until the pH was adjusted to 7, then washed twice with diisopropyl ether (DIPE) and dried. Analytical samples were obtained by recrystallization from the indicated solvents.

General Procedure II for the Synthesis of Compounds 9i-k (and By-Product 20b)
To a mixture of BuLi in hexane (2.2-2.5 eq, 1.6 M) and THF, the solution of 3-isopropyloxindole (8f) in THF was added dropwise at −78 • C, under argon atmosphere. Then the appropriate alkyl halide (1.2-2.5 eq) in THF was added dropwise, the acetone-dry ice bath was removed and the reaction mixture was allowed to warm to room temperature. The stirring was continued for further 4-6 h. The mixture was quenched with EtOH (2 mL), saturated ammonium chloride solution (10 mL) was added, then it was stirred for 30 min. The layers were separated and the aqueous layer was extracted with EtOAc (3 × 10 mL). The combined organic layer was washed with brine (10 mL), dried over anhydrous MgSO 4 , filtered and the solvent was removed in vacuo at 40 • C. The residue crystallized upon treatment with hexane (5 mL), then it was filtered. Analytical samples were obtained by recrystallization from the indicated solvents.

Conclusions
A systematic study of regioselective 3-alkylation reaction of N-unprotected-3-monosubstituted oxindoles was carried out after summarizing the literature of the numerous, albeit sporadic, precedents giving mostly unsatisfactory results. We have now elaborated an optimized method for the 3-alkylation of N-unsubstituted 3-alkyloxindoles by applying butyllithium as the base and alkyl bromides as the alkylating agents. The method has been extended to various alkyl groups in position 3 and various substituents on the aromatic ring. Introduction of new substituents into the aromatic ring of 3,3-diethyloxindole is disclosed. The formation of 3-hydroxylated side-products was investigated and the targeted synthesis of these compounds is also described. Owing to the presence of the unsubstituted nitrogen atom N1 in the title products (and the 3-hydroxy moiety in certain compounds), further functionalizations can also be carried out, thereby making these compounds valuable building blocks in synthetic organic or medicinal chemistry.