Chemical Synthesis of Selected Xanthine Derivatives
4.3. Results and Discussion
4.3.2. Development of new routes for synthesis of xanthine derivatives
In this study, two novel synthetic schemes (Scheme-I and Scheme-II) have been developed to synthesize 1, 3, 8-trisubstituted xanthine derivatives. Scheme-I is a six step synthesis procedure whereas, Scheme-II is of eight steps. Diverse set of compounds were developed using Scheme I and II. Both schemes begin with the use of common
‘xanthine’ molecule as a starting material. These schemes share two common initial steps and the final step is also common to both. In scheme-I, the three intermediate steps are exclusive while in scheme-II, five intermediate steps are exclusive.
denotes scheme – II)
Reagents and Conditions: (a) Br2, H2O, 100°C, 3h; (b) benzyl chloride, Anhydrous K2CO3, Anhydrous DMF, 2.5 h, 70°C; (c) Alkyl iodide, Anhydrous K2CO3, Anhydrous DMF, 6h, 70°C; (d) Alkyl iodide, Anhydrous K2CO3, Anhydrous DMF, 12 h, 70°C; (e) Pd (PPh3)4, Anhydrous K2CO3, DMF, 48 h, 110°C, inert argon atmosphere; (g) 4- methoxy benzyl chloride, Anhydrous K2CO3, Anhydrous DMF, 2.5 h, 70°C; (h) Alkyl iodide, Anhydrous K2CO3, Anhydrous DMF, 12 h, 70°C; (i) Pd (PPh3)4, Anhydrous K2CO3, DMF, 48 h, 110°C, inert argon atmosphere; (j) TFA, conc. H2SO4, reflux, 22h (k) Alkyl iodide, Anhydrous K2CO3, Anhydrous DMF, 12 h (f or l) H2, 10% Pd/H, Methanol, 48 h, rt.
The reaction starts in both the schemes with bromination reaction at C8 position of xanthine. Bromination reaction at C8 position in the first step and was essential because original xanthine has only one hydrogen atom at C8 position with –CH group.
The bromination reaction at C position was required for C arylation reaction to be
methylene (-CH2-) group. Therefore, bromination of xanthine was carried out first in both the schemes. In xanthine, presence of three –NH groups at N1, N3 and N7 positions was the most challenging part in understanding the nature of the whole xanthine molecule. The three -NH sites of xanthine showed different reactivity because of their different atomic environment. In the development of schemes for synthesizing xanthine derivative, it was essential to go for selective reactions at different positions of xanthine.
In this study, substitution at N1, N3 and C8 positions were selected for synthesis of xanthine derivatives. Substitution at these three positions (N1, N3 and C8) of xanthine scaffold was to be selective. However, the presence of -NH groups at three different positions of xanthine and their different atomic environment was the major hinderance.
Understanding the reactivity of three -NH positions of xanthine was imperative. In this study, by concentration optimization of reactant, it was found that -NH group at N7
position was most reactive. This was because it faced less steric hinderance as compared to other –NH groups. Thus, to make the selective substitution reaction at N3 and N1
positions of xanthine, it was imperative to protect N7 position first. This was because – NH group at N7 position possessed highest reactivity among all -NH groups. For protecting –NH group at N7 position, benzyl chloride was used in both scheme-I and scheme-II. With different concentration analysis of benzyl chloride to protect N7 position it was found that –NH at N7 position was protected first by benzyl group in all xanthine scaffold. When concentration of benzyl chloride was higher than the required concentration for selective protection of N7 position, benzyl chloride acted at N3 position because NH at N3 position showed second higher reactivity among three –NH groups.
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This was because the reaction centre of -NH at N1 position is surrounded by carbonyl groups at C2 and C6 positions. Due to these reasons, a sequential selectivity was generated at xanthine scaffold in order of N7 substitution>N3 substitution>N1 substitution as shown in figure 4.3. Therefore, during synthesis, reactant gets attracted first to the N7
position and uses all the available N7 sites. Then it goes to the relatively lower reactive site i.e. N3 site. After occupancy of the N3 sites reactant attacked the N1 site. The protection of N7 position was carried out by SN2 mechanism where concentration of both the reactants have equal role in determining the product formation. The selective protection of –NH group at N7 position was dependent on the concentration of both reactants (8-bromoxanthine and benzyl chloride). Multiple products were obtained when benzyl chloride was in higher concentration than the concentration actually needed for the occupancy of N7 position only. When concentration was higher than the needed concentration for protection at N7 position, the rest of the reactant (benzyl chloride) subsequently attacked the –NH group at N3 position and occupied all N3 position with formation of 3, 7- dibenzyl 8-bromo xanthine. After occupying all N3 positions, rest of the reactant attacked the –NH group at N1 position. The protection followed the order of 7-benzyl 8-bromoxanthine > 3,7–dibenzyl 8-bromoxanthine > 1,3,7- tribenzyl 8- bromoxanthine. Therefore, selective protection was completely dependent on the concentration of the benzyl reactant used. Hence, optimization of concentration of benzyl chloride for selective protection at N7 position was carried out. The selective protection at N7 position was achieved only when concentration of benzyl chloride was reduced to 0.5 equivalents of 8-bromoxanthine. Selective protection was necessary to achieve greater yield in shortest possible time bypassing the tedious workup and use of costly
protection at N7 position was followed by three independent steps while in scheme-II, it was followed by five independent steps, finally both schemes merge and shared the last common step. Figure 4.3 represents the reaction affinity of different –NH positions of xanthine.
Figure 4.3 Reactivity pattern of –NH groups at N1, N3 and N7 positions of xanthine Scheme-I: Synthesis of compound 6a1-6a7
2 3 2 3
Anhydrous DMF, 12 h, 70°C, 90-95%; (e) Pd (PPh3)4, Anhydrous K2CO3, DMF, 48 h, 110°C, inert argon atmosphere, 63-99%; (f) H2, 10% Pd/H, Methanol, 48 h, rt, 50-93%.
In scheme-I, the N7 protection step was followed by substitution. First at N3 position and then at N1 position of xanthine by alkylation reaction. Alkyl groups such as methyl, ethyl, propyl, n-butyl and iso-butyl groups were selected for substitution reaction because of their positive biological implications such as inotropic effect, higher blood- brain barrier permeability level and plasma protein binding efficiency (Sanae et al., 1995). Alkylation reaction both at N3 and N1 positions followed the SN2 reaction mechanism. Therefore, in both schemes, alkylation reaction was dependent on the concentration of both reactants (xanthine intermediates and alkyl halides). Alkylation reaction was performed in anhydrous DMF in the presence of anhydrous K2CO3. In scheme-I, after selective protection at N7 position with benzyl group, the first alkylation took place at N3 position followed by alkylation reaction at N1 position. Both were carried out with the optimized concentration of the respective alkyl halides. For N3
substitution, propyl, butyl and isobutyl groups were used whereas, for N1 substitution, methyl and ethyl groups were used. After sequential substitution at N3 and N1 positions, the intermediate product obtained were subjected to Suzuki coupling reaction for C8 arylation to install phenyl ring with various functional groups and side chains at C8 position. Suzuki coupling reaction was carried out with respective aryl-boronic acid in presence of palladium catalyst to give aryl substituted xanthine derivatives, 5a1-5a7. For aryl substitution at C8 position, meta-substituted phenyl ring was used. The substituent used at meta position was isopropyl, methyl and floro groups. After substitution at all selected positions (N , N and C positions) deprotection of N position was carried out
scheme-I were 6a1-6a7.
Scheme-II Synthesis of compound 8b1-8b2
Reagents and Conditions: (a) Br2, H2O, 100°C, 3h, 71 %; (b) benzyl chloride, Anhydrous K2CO3, Anhydrous DMF, 2.5 h, 70°C, 67 %; (g) 4-methoxy benzyl chloride, Anhydrous K2CO3, Anhydrous DMF, 2.5 h, 70°C, 70%; (h) Alkyl iodide, Anhydrous K2CO3, Anhydrous DMF, 12 h, 70°C, 88 %; (i) Pd (PPh3)4, Anhydrous K2CO3, DMF, 48 h, 110°C, inert argon atmosphere; 72%; (j) TFA, conc H2SO4, reflux, 22h, 73%; (k) Alkyl iodide, Anhydrous K2CO3, Anhydrous DMF, 12 h, 84-95%; (l) H2, 10% Pd/H, Methanol, 48 h, rt, 71-77% .
Scheme-II is an alternate scheme designed for the synthesis of analogous
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protection strategies at N7 and N3 position of 8-bromoxanthine was followed by alkylation reaction at N1 position which was then followed by Suzuki coupling reaction for arylation reaction at C8 position. The sequential alkylation at N1 position and arylation at C8 position were followed by deprotection of N3 position by acid catalyzed method. The deprotection reaction was carried out selectively at N3 position to make N3 site available for various substituents to be used. After selective deprotection at N3
position, alkylation reactions were carried out for N3 substitutions. In scheme-II, after substitution at N1, N3 and C8 positions of xanthine, another deprotection reaction was carried out to deprotect the N7 position. This was similar to catalytic deprotection method which was employed in scheme-I. Thus, by using scheme-II, compound 8b1-8b2 were synthesized. Compound 8b1 (C5) was same as compound 6a5 (C5) obtained from scheme-I. Compound 8b2 (C6) was synthesized by exclusively using scheme-II.
Rational behind development of scheme-I and scheme-II
The foundation for the design and development of the above two schemes was their protection and deprotection strategies. In both scheme-I and scheme-II, the final step of the scheme was the deprotection of benzyl group at N7 position. For this deprotection, different deprotection methods were attempted. Both acid deprotection method and catalytic hydrogenation method were used for deprotection at N3 and N7 position, but these methods worked differently at N3 and N7 position of xanthine derivatives. Acid deprotection method did not work for benzyl deprotection at N7 position of xanthine derivatives. In both scheme-I and scheme-II, the catalytic hydrogenation was proved as the best method for selective deprotection at N7 position of the xanthine derivatives when benzyl group was used as the protecting group. However, in scheme-II benzyl group was
7 3
was observed that when benzyl group was used as protecting group at both N3 and N7
positions, catalytic deprotection occurred only at N7 position and did not affect the N3 position. When 3,7-dibenzyl protected xanthine derivative was subjected to acidic deprotection, then it was found that acidic deprotection occurred selectively at N3
position with very less product formation. From this study, it was analyzed that there was a strong possibility of selective deprotection at the N3 position if a more appropriate protecting group was used. Hence, in scheme-II, p-methoxy benzyl chloride was used to protect N3 position because of the presence of methoxy (an electron releasing) group that facilitated deprotection. The selective deprotection of p-methoxy benzyl group at N3
position was carried out by acid deprotection method. The acid deprotection was selective for the N3 position both in the case of benzyl and p-methoxy benzyl protecting groups. However, this method was more suitable for deprotection of p-methoxy benzyl group. The acid deprotection method was non-reactive to deprotection of benzyl protecting group at N7 position. Therefore, this method was used for selective N3
deprotection in scheme-II.
Catalytic deprotection of benzyl group at N7 position of xanthine derivatives in scheme-I and scheme-II
Scheme-II Step-6b: Acid deprotection of p-methoxy benzyl (PMB) group at N3 position of xanthine derivative in scheme-II
Thus, the whole synthesis work was divided into two schemes; scheme-I and scheme-II, which simplified the nature of xanthine derivatives by giving clear view on the reactivity and substitution pattern of different –NH groups of the xanthine scaffold.
Scheme-I can be applied for those compounds which have common N3 position with diverse N1 and C8 substitutions. Likewise scheme-II can be applied for synthesis of compounds having common N1 and C8 substituent but different N3 substituent. It was apparent from the synthesis of two compounds (8b1 and 8b2) using one single synthesis pathway of scheme-II. These two compounds varied only at N3 position. Compound 8b1 consists of iso-butyl group at N3 position whereas, compound 8b2 consisted of n-butyl group. Thus, these two separate schemes are designed to construct diverse library of
effectiveness and readily available reagents, use of non-hazardous chemicals, no need for chromatographic clean up in most of the steps, shorter reaction time, multiple compound generation using single scheme and better product yield. Due to these advantages, the above proposed schemes are better alternatives as compared to existing methods which has been used worldwide till date for the synthesis of xanthine derivatives.