Jeyakannu Palanirajaa,
Selvaraj Mohana Roopan*a and
G. Mokesh Rayalub
aChemistry of Heterocycles & Natural Product Research Laboratory, Department of Chemistry, School of Advanced Sciences, VIT University, Vellore 632 014, Tamilnadu, India. E-mail: mohanaroopan.s@gmail.com; mohanaroopan.s@vit.ac.in; Fax: +91 4162245544/+91 4162245766; Tel: +91 98656 10356/+91 04162 202336
bDepartment of Mathematics, School of Advanced Sciences, VIT University, Vellore 632 014, Tamilnadu, India
First published on 29th February 2016
An efficient synthesis of nitrogen ring junction pyrimido-indazoles has been developed. This is a metal catalyzed transformation that proceeds via A3 coupling reaction between 1H-indazol-3-amine, aromatic aldehydes and alkynes, which undergoes 6-endo-dig cyclization leading to the highly functionalized pyrimido[1,2-b]indazoles. Response surface methodology (RSM) was used to investigate the effect of catalyst (A1), reaction temperature (B1) and time (C1). The fluorescence quantum yields of the pyrimido[1,2-b]indazoles were calculated.
These compounds have immense importance in pharmaceutical industry owing to their wide range of interesting biological activity. The anxiolytic drugs such as fasiplon, taniplon and divaplon having nitrogen ring junction core motifs are currently used in clinics (Fig. 1).25 Camptothecin and mappicine are recently approved by FDA (Food and Drug Administration), USA having nitrogen ring junction structure. These two drugs have possible uses in antimicrobial, anticancer, antibiotic and antiparasitic action. Therefore, there is a demand for the efficient and practical synthetic pathway to generate heterocyclic units for the synthesis of natural and biomimetic compounds.26,27
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Fig. 1 Clinical and FDA approved drugs with ring junctioned nitrogen heterocycles as core structure. |
Current researchers in organic synthesis focus on the discovery of the methods that taken into the account of sustainable chemistry. The multi component assembly reactions (MCARs) are considered as rapid assembling of more than two reactants into higher mass compounds in a single pot. It became very interesting in the discovery of biologically active compounds due to their atom economy, ease to handle and higher yield.28–30 A3 coupling is a type of MCARs involves an aldehyde, alkyne and amine as reactants which give propargylamine as product.31 This reaction was described as a direct dehydrative condensation. It requires a transition metal catalyst like ruthenium, copper, silver or gold to get a desired product. Domino reactions also fall under the category of MCARs which allow the formation of complex compounds starting from simple reactants.32 Recently, copper mediated organic transformation had an important role for the product formation. Naresh et al. reported a microwave assisted one pot multi component transformation to synthesis furoquinoxalines in good to excellent yields via copper catalysed A3 coupling. The reactions proceeds via 5-endo-dig cyclization.33 Similarly, the A3 coupling through 5-endo-dig cyclization to synthesized N-fused heterocycles were described by Chernyak et al.34 and Guchhait et al.35 In earlier report, we have reported an overview of synthetic arylation via transition metal catalyst.36 Now, our intent to offer an one pot synthesis of highly functionalized pyrimido[1,2-b]indazoles via 6-endo-dig cyclization from simple reaction conditions, easy to access, low cost starting materials.
Response surface methodology (RSM) is a collection of mathematical and statistical techniques for empirical model building. By careful design of experiments, the objective is to optimize a response (output variables) which is influenced by several independent variables (input variables). An experiment series of tests, runs, in which changes are made in the output variables in order to identify the reasons for changes in the output response.37–42
Entry | Metal catalyst (mol%) | Acid catalyst (mol%) | Solvent | Time (h) | Yieldc (%) |
---|---|---|---|---|---|
a Note: reactions were carried out with 1 mmol of 1, 2a and 3a in 5 mL of solvent at the reflux temperature of the solvents (except neat, DMSO or DMF reactions). NR – no reaction.b Reaction temperature – 120 °C.c Isolated yield. The optimized condition was mentioned in bold letter. | |||||
1 | — | — | Neatb | 16 | NR |
2 | — | — | ACN | 16 | NR |
3 | CuI (10) | TFA(10) | ACN | 4 | 15 |
4 | CuBr (10) | TFA(10) | ACN | 4 | 20 |
5 | CuCl (10) | TFA(10) | ACN | 4 | Traces |
6 | CuO (10) | TFA(10) | ACN | 4 | 45 |
7 | CuSO4·5H2O (10) | TFA(10) | ACN | 4 | 65 |
8 | Cu(OAC)2 (10) | TFA(10) | ACN | 4 | 30 |
9 | CuSO4·5H2O (10) | Iodine(10) | ACN | 16 | 50 |
10 | CuSO4·5H2O (10) | PTSA(10) | THF | 16 | 73 |
11 | CuSO4·5H2O (10) | PTSA(10) | DMSOb | 16 | 45 |
12 | CuSO4·5H2O (10) | PTSA(10) | DMFb | 16 | 25 |
13 | CuSO4·5H2O (10) | PTSA(10) | 1,4-Dioxane | 16 | 70 |
14 | CuSO4·5H2O (10) | PTSA(10) | Toluene | 16 | 80 |
15 | CuSO4·5H2O (10) | PTSA(10) | Benzene | 16 | 70 |
16 | CuSO4·5H2O (5) | PTSA(10) | Toluene | 16 | 63 |
17 | CuSO4·5H2O (15) | PTSA(10) | Toluene | 16 | 77 |
18 | CuSO4·5H2O (20) | PTSA(10) | Toluene | 16 | 87 |
19 | CuSO4·5H2O (30) | PTSA(10) | Toluene | 16 | 84 |
20 | CuSO4·5H2O (20) | PTSA(10) | Toluene | 4 | 70 |
21 | CuSO4·5H2O (20) | PTSA(10) | Toluene | 8 | 90 |
22 | CuSO4·5H2O (20) | PTSA(5) | Toluene | 8 | 78 |
23 | CuSO4·5H2O (20) | PTSA(15) | Toluene | 8 | 83 |
24 | CuSO4·5H2O (20) | PTSA(10) | Toluene | 12 | 88 |
Reaction | Variables | Code | Units | Levels | ||
---|---|---|---|---|---|---|
−1 | 0 | +1 | ||||
Metal reaction | Catalyst used (mol%) | A1 | mg | 10 | 20 | 30 |
Reaction temperature | B1 | °C | 110 | 120 | 130 | |
Reaction time | C1 | h | 4 | 8 | 12 |
The general reaction pathway was mentioned in Scheme 1 which implies, two possible product formation of 2,4-diphenylpyrimido[1,2-b]indazole 4a and 3-benzyl-2-phenyl-5H-imidazo[1,2-b]indazole 5a. Here we intent to report the synthesis of 2,4-diphenylpyrimido[1,2-b]indazole via multicomponent assembly reaction through 6-endo-dig cyclization. In optimization, we have set up this scheme by investigating the reaction of 1H-indazol-3-amine 1, phenyl acetylene 2a and benzaldehyde 3a in the absence of catalyst and solvent at 120 °C for 16 h, but the desired compound 4a was not achieved (Table 1, entry 1 and 2).
When this reaction was carried out in the presence of copper catalyst such as CuI, CuBr, CuCl, CuO, CuSO4·5H2O and Cu(OAC)2 (10 mol%) along with TFA (10 mol%) at 85 °C, the desired product 4a was observed with reasonable yield (Table 1, entry 7). Further, the reaction conditions were fine-tuned by changing the solvents, acid catalysts, and reaction time (Table 1, entry 8–22). While we used DMSO (Table 1, entry 11) and DMF (Table 1, entry 12) as a solvent the temperature maintained at 120 °C. After the optimization, we have found that 20 mol% CuSO4·5H2O and 10 mol% p-toluenesulfonic acid (PTSA) in the presence of toluene at 120 °C for 8 h has been considered as an optimized condition for synthesis of 2,4-diphenylpyrimido[1,2-b]indazole 4a (Table 1, entry 21). The synthesized product was fully characterized by its melting point, 1H NMR, 13C NMR and HRMS data.
A Box–Behnken design (BBD) center-united design was employed to design the experiments and the results obtained after running the seventeen experiments are represented in Table 3.38,39 The three components such as the catalyst loading (A1), reaction temperature (B1) and response time (C1) were utilized for metal mediated reaction. The best – fitting models were determined by multiregression and backward elimination. The experimental yield (actual) was obtained as an average of triplicate determinations. The yield was increased from 22 to 91%, depending on the reaction conditions.
Run | Metal condition | ||||
---|---|---|---|---|---|
A1 | B1 | C1 | Y1 (%) | X1 (%) | |
a Where Y1 – experimental yield, X1 – predicted yield. | |||||
1 | 30 | 120 | 12 | 80.10 | 81.50 |
2 | 20 | 110 | 12 | 81.40 | 79.88 |
3 | 10 | 120 | 12 | 72.31 | 72.75 |
4 | 30 | 130 | 8 | 85.22 | 84.63 |
5 | 30 | 120 | 4 | 40.11 | 39.25 |
6 | 20 | 120 | 8 | 90.33 | 89.60 |
7 | 20 | 110 | 4 | 33.13 | 34.13 |
8 | 20 | 120 | 8 | 88.01 | 89.60 |
9 | 20 | 130 | 4 | 35.03 | 36.13 |
10 | 10 | 120 | 4 | 22.05 | 20.50 |
11 | 20 | 130 | 12 | 86.32 | 84.88 |
12 | 10 | 130 | 8 | 75.03 | 75.38 |
13 | 20 | 120 | 8 | 91.22 | 89.60 |
14 | 20 | 120 | 8 | 89.11 | 89.60 |
15 | 20 | 120 | 8 | 90.10 | 89.60 |
16 | 10 | 110 | 8 | 67.08 | 67.38 |
17 | 30 | 110 | 8 | 86.00 | 85.63 |
On the basis of the BBD analysis, the quadratic polynomial model relationship between the experimental yield (Y1) and the process variables in coded units is obtained from eqn (1).
Y1 = 89.60 + 6.87A1 + 1.75B1 + 23.63C1 − 2.25A1B1 − 2.50A1C1 + 0.75B1C1 − 8.30A12 − 3.05B12 − 27.80C12 | (1) |
Analysis of variance (ANOVA) for the response surface quadratic model was used to investigate the fitness, signification of the mannequin, precision of the mannequin, result of the private variables and interactive effect on the answer. The ANOVA for response surface quadratic models are presented in Table 4. The model F-value of 410.40 implies the model is significant. There may be an increase in F value of about 0.01%, which may be due to noise. If the values of “prob” is less than 0.050 of “F” then the model terms are significant. From the Table 2, A1, B1, C1, A1B1, A1C1, A12, B12, C12 are significant model terms. If the values greater than 0.1000 then the model terms are not significant. If there are many insignificant model terms (not counting those required to support hierarchy) and model reduction it may improve your model. In case “Lack of Fit F-value” is 2.88 then it implies the lack of fit is not significant relative to the pure error. Around 16.63% probability that a “Lack of Fit F-value” this large could occur due to interference. Non-significant lack of fit is good and we want the model to fit. The “Pred R-Squared” of 0.9784 is in reasonable agreement with the “Adj R-Squared” of 0.9957; i.e., the deviation is less than 0.2. “Adeq Precision” measures the signal to interference ratio (Fig. 2). A ratio greater than 4 is desirable. The ratio of 58.772 indicates an adequate signal. This model can be used to navigate the design space.
Source | Sum of squares | DF | Mean square | F value | p-Value prob > F | |
---|---|---|---|---|---|---|
a Where S – significant, NS – non-significant, DF – degree of freedom. | ||||||
Model | 8680.02 | 9 | 964.45 | 410.40 | <0.0001 | S |
A1 | 378.13 | 1 | 378.13 | 160.90 | <0.0001 | |
B1 | 24.50 | 1 | 24.50 | 10.43 | 0.0145 | |
C1 | 4465.13 | 1 | 4465.13 | 1900.05 | <0.0001 | |
A1B1 | 20.25 | 1 | 20.25 | 8.62 | 0.0219 | |
A1C1 | 25.00 | 1 | 25.00 | 10.64 | 0.0138 | |
B1C1 | 2.25 | 1 | 2.25 | 0.96 | 0.3604 | |
A12 | 290.06 | 1 | 290.06 | 123.43 | <0.0001 | |
B12 | 39.17 | 1 | 39.17 | 16.67 | 0.0047 | |
C12 | 3254.06 | 1 | 3254.06 | 1384.71 | <0.0001 | |
Residual | 16.45 | 7 | 2.35 | |||
Lack of fit | 11.25 | 3 | 3.75 | 2.88 | 0.1663 | NS |
Pure error | 5.20 | 4 | 1.30 | |||
Correlation total | 8696.47 | 16 |
The results in Table 4 shows that the interaction between variables has a significant effect on the yield of the product. The interaction between the relevant variables would be less when the contour of the response surface is circular.45–47 On the other hand, the interaction between the corresponding variables would be higher when the contour of the response surface is elliptical.48,49 The effect of interaction between catalyst loading (A1) and reaction temperature (B1) on constant reaction time (C1) of 8 h is shown in Fig. 3A. The combined effect of A1 and C1 on B1 of 120 °C is shown in Fig. 3B. The effect of interaction between B1 and C1 on A1 of 20 mol% is shown in Fig. 3C. The contour plots of A1 and B1 (Fig. 3Ab) are almost circular, which indicates that there is less interaction between A1 and B1. The contour plots of A1 and C1 (Fig. 3Bb), B1 and C1 (Fig. 3Cb) are elliptical, which indicates a perfect interaction between A1, C1 and B1, C1.
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Fig. 3 Combined effect of different parameters on the yield for metal mediated reaction. (A) Effect of temperature and catalyst. (B) Effect of time and catalyst. (C) Effect of time and temperature. |
From these experiments, the optimum conditions for predicted and experimental yields are shown in Table 5. To confirm the adequacy of the model for predicting the maximum yield, three confirmation runs was performed using optimized condition; the results are 93.1%, 91.2% and 91.7% isolated yields for the compound 4a. The good agreement between the experimental and predicted yield has been tested by RSM with the statistical design of experiments. It has been effectively used to optimize parameters which has been involved in the reaction.
Parameter | Catalyst mol% (A1) | Reaction temperature °C (B1) | Reaction time h (C1) | Yield (%) |
---|---|---|---|---|
Predicted | 21.33 | 121.33 | 8.53 | 93.1 |
Experimental | 20.00 | 120.00 | 8.00 | 90.0 |
To demonstrate the positive impact of this reaction, the scope of the reaction substrate was explored under optimized conditions. A wide range of substituted aromatic aldehydes 3(a–p) which includes electron neutral, electron releasing and electron withdrawing groups could be reacted with 1H-indazol-3-amine 1(a–d) and alkynes 2(a–e) to get corresponding pyrimido[1,2-b]indazoles 4(a–w) in moderate to good yields (Table 6). All aromatic aldehydes including hetero aldehydes were easily converted into the desired products which showed that steric nature did not affect the reactivity. We have explored the gram scale preparation of 5.69 g (85% yield) of the compound 4c (Scheme 2) from 20 mmol of 1H-indazol-3-amine 1, 20 mmol of phenyl acetylene 2a, and 20 mmol of 4-methoxybenzaldehyde 3c.
The plausible mechanism for the formation 2,4-diphenylpyrimido[1,2-b]indazole 4a via A3 coupling reaction was illustrated in Scheme 3. The first step involved the formation of imine with the elimination of one water molecule from amine and aldehyde. The formed imine I in situ was attacked by copper acetylide (Cu-A) resulting in copper complex intermediate II followed by isomerization to form intermediate III (Scheme 3). Then it undertook intramolecular N–H bond activation and regioselective attack at the electron deficiency center of the triple via 6-endo-dig-cyclization offering the intermediate IV. The intermediate IV consequently undergoes demetalation and autoxidation leading to the formation of 2,4-diphenylpyrimido[1,2-b]indazole 4a.
The synthesized ring junction compounds 4(a–t) are yellow to red powder with good solubility in most common organic solvents. The compounds are showing higher fluorescence properties in ethyl acetate medium.
The solvatochromism spectra of the 2,4-diphenylpyrimido[1,2-b]indazole 4a, was studied with 15 different solvents (Fig. S1†). Among all the solvents, ethyl acetate exhibits good UV/Vis absorbance. Similarly, we have utilized ethyl acetate for remaining synthesized compounds 4(a–t). The UV/Vis absorbance and fluorescence emission spectra of the synthesized ring junction compounds 4(a–t) were recorded (Fig. S2†) in ethyl acetate (10−5 M). Among these compounds, compound 4h shows the maximum fluorescence intensity (Fig. S3†).
We have calculated the fluorescence quantum yield50 (ΦF) of the fluorescence active compounds. The ΦF was calculated by using following formula (2),
ΦF = (ΦRISODRηS)/(IRODSηR) | (2) |
Entry | λmax (abs, nm) | λmax (em, nm) | Stokes shift (nm) | OD | I | ΦF |
---|---|---|---|---|---|---|
a Sh – shoulder; abs – absorbance; em – emission; OD – excited absorbance; I – integral area; ΦF – fluorescence quantum yield. | ||||||
Tryptophan | 280 | 355 | 75 | 0.384 | 158![]() |
0.130 |
4a | 306 | 533 | 227 | 0.774 | 46![]() |
0.019 |
4b | 308 | 535 | 227 | 0.540 | 38![]() |
0.023 |
4c | 306 | 499 | 193 | 1.176 | 59![]() |
0.016 |
268(Sh) | 499 | 231 | 0.827 | 59![]() |
0.023 | |
4d | 270 | 543 | 273 | 1.761 | 29![]() |
0.005 |
4e | 308 | 535 | 227 | 1.794 | 70![]() |
0.013 |
4f | 272 | 530 | 258 | 0.449 | 54![]() |
0.040 |
322(Sh) | 530 | 208 | 0.439 | 54![]() |
0.041 | |
4g | 322 | 536 | 214 | 1.562 | 42![]() |
0.008 |
4h | 362 | 527 | 165 | 1.019 | 79![]() |
0.025 |
260(Sh) | 527 | 267 | 0.703 | 79![]() |
0.036 | |
4i | 308 | 542 | 234 | 0.440 | 47![]() |
0.035 |
242(Sh) | 542 | 300 | 0.210 | 47![]() |
0.073 | |
4j | 310 | 553 | 223 | 0.612 | 51![]() |
0.027 |
4k | 302 | 530 | 228 | 1.132 | 79![]() |
0.023 |
4l | 310 | 532 | 222 | 1.712 | 27![]() |
0.005 |
4m | 310 | 533 | 233 | 1.973 | 71![]() |
0.012 |
4n | 296 | 531 | 235 | 0.870 | 39![]() |
0.014 |
4o | 296 | 528 | 232 | 0.566 | 26![]() |
0.015 |
4p | 296 | 532 | 236 | 0.678 | 65![]() |
0.031 |
4q | 284 | 532 | 248 | 0.272 | 75![]() |
0.090 |
270 | 532 | 262 | 0.209 | 75![]() |
0.117 | |
4s | 306 | 534 | 228 | 1.883 | 73![]() |
0.013 |
270(Sh) | 534 | 264 | 1.251 | 73![]() |
0.019 | |
4t | 308 | 553 | 245 | 1.064 | 31![]() |
0.010 |
270(Sh) | 553 | 283 | 0.838 | 31![]() |
0.012 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02596j |
This journal is © The Royal Society of Chemistry 2016 |