Microwave-assisted synthesis of base-modified fluorescent 1,4-dihydropyridine nucleosides: photophysical characterization and insights

Abstract

A synthesis of a small library of fluorescent 1,4-dihydropyridine nucleoside analogues has been successfully carried out under solvent-free conditions via a one-pot three-component Hantzsch condensation reaction. The process involved a Ba(NO₃)₂ catalyzed solvent-free reaction between 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine, differently substituted β-keto ester and ammonium acetate under microwave irradiation. This facile methodology yielded the desired products with very high yields (86–96%) under solvent-free reaction conditions in a short reaction time, which was followed by a simple workup. Yields obtained under microwave and conventional heating were compared, with the microwave irradiation condition displaying superior results. The synthesized compounds were characterized by IR, ¹H-NMR, ¹³C-NMR, ¹H–¹H COSY, ¹H–¹³C HETCOR, 2D NOESY NMR and HRMS analysis. These nucleoside analogues exhibited significant fluorescence, with a prominent emission band around 460 nm (excitation at 235 nm). Photophysical studies revealed strong fluorescence intensity, excellent Stokes shifts (70–162 nm), and high quantum yields (0.022–0.579), outperforming other pyrimidine-based fluorescent nucleosides. Notably, 5-(diethyl 2′′,6′′-propyl-1′′,4′′-dihydropyridine-3′′,5′′-dicarboxylate)-4′′-yl-2′-deoxyuridine demonstrated a quantum yield as high as 0.579 in DMSO during solvatochromic studies, highlighting their potential for probing local nucleic acid structure and dynamics. Additionally, we demonstrated the scalability of the synthesis protocol by producing one of the compounds on a gram scale, confirming its practical viability for large-scale production. This study underscores the potential of these fluorescent nucleoside analogues for various biochemical applications.

---

1. Introduction

DNA, the “molecule of life”, stores genetic instructions vital for biological processes such as cell division and viral replication.¹ DNA's structure is constituted by nucleotides, which contains nucleobase, carbohydrate and phosphate backbone, whereas nucleoside monomers are composed of a nucleobase and carbohydrate moiety. Over recent decades, chemists have synthesized numerous modified nucleosides to evaluate their biological activities, including antiviral,^2–5 anti-HIV,^6,7 anticancer,^8,9 antimetabolites,^10,11 antisense properties¹² and many more.^13,14 The worldwide clinical success of these nucleosides inspires scientists to advance the synthesis of modified nucleosides (Fig. 1).^15–20 Among the modified nucleosides, the synthesis of modified fluorescent nucleoside analogues has undergone rapid growth due to the non-emissive nature of the canonical bases in nucleic acids.²¹

Fig. 1 Structures of some FDA approved sugar modified nucleosides.

Several base analogues, such as 2-aminopurine, pyrrolo-dC (pyrrolo-deoxycytidine), 1,3-diaza-2-oxophenoxazine (tCO), bodipy-labelled bases, and cyanine-labelled bases, which possess intrinsic fluorescence or can be chemically modified to exhibit fluorescent properties, have been used in the synthesis of fluorescent nucleosides.^22–25 When fluorescent nucleobases are incorporated synthetically into DNA and RNA, they serve as potent tools, providing new insights into DNA and RNA and leading to advancements in chemistry, biology, and medicine.²⁶ The fascination with base-modified fluorescent nucleosides lies in their exceptional sensitivity to the microenvironment. This characteristic feature positions them as potent tools for delving into the intricate structure and functions of nucleic acids, offering promising avenues for further research and applications.^27–30

In the last few decades, structural modifications at the fifth position of the uracil base of uridine have gained attention for exploration of their photophysical properties (Fig. 2).^21,31–43 Modifications at this position exhibited Watson–Crick type hydrogen bonding as similar as their native counterparts.⁴⁴ Additionally, 5-substituted 2′-deoxyuridines are known for their antiviral activity against a range of viruses, including Herpes simplex virus (HSV-1 and HSV-2),⁴⁵ Hepatitis B virus (HBV),⁴⁶ Vaccinia virus (VV),⁴⁷ Varicella-zoster virus (VZV),⁴⁸ and Cowpox virus (CV)⁴⁶ and these nucleoside monomers have also demonstrated notable antibacterial activities.^46,49,50

Fig. 2 Some representative base modified fluorescent nucleosides.

Over the decades, our research group has developed numerous facile and fruitful methodologies to synthesize sugar-modified or nucleobase modified nucleosides.^32,51–60 Recently, we have focused our attention on modifying the C-5 position of the uracil base of uridine by introducing different heterocyclic functionalities at that position to develop nucleosides with enhanced fluorescence properties (Fig. 2).^32,59,60 As part of our ongoing efforts to enhance the fluorescence properties of the uridine moiety, a green methodology to incorporate 1,4-dihydropyridine (DHP) at the C-5 position of uridine has been reported herein. 1,4-Dihydropyridines (DHPs) stand out as crucial heterocyclic compounds with diverse pharmacological properties, commonly synthesized through the Hantzsch reaction,⁶¹ which serves as a privileged pharmacophore in organic chemistry.⁶² Additionally, the photophysical characteristics^63–66 of 1,4-dihydropyridine derivatives have been extensively investigated and documented in the literature. However, to the best of our knowledge, the incorporation of DHP moieties into the nucleoside chemistry has not been explored till date. In this article, we are reporting an environmentally friendly, efficient and solvent-free reaction to incorporate the 1,4-dihydropyridine moiety at the C-5 position of uridine for the first time via microwave irradiation. This research work includes the comparative studies of the conventional heating methodology and microwave irradiation procedures, where the later one provided an excellent solution as a non-conventional energy source, offering uniform internal heating and significantly reducing the reaction time, followed by an easier workup.⁶⁷ A comprehensive study of the photophysical properties of the synthesized nucleoside analogues is also included in this article.

2. Results and discussion

2.1. Chemistry

The primary goal of this research work was to incorporate 1,4-dihydropyridines into the uridine moiety. For this purpose, a one-pot, three-component, Hantzsch condensation reaction was attempted with 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine (3) under both conventional as well as microwave heating conditions (Scheme 1).

Scheme 1 Method A: conventional Heating; Method B: microwave Irradiation.

At the outset, thymidine 1 was acetylated with acetic anhydride to produce 3′,5′-di-O-acetylthymidine (2). Subsequent oxidation with potassium persulfate, 2,6-lutidine, and cupric sulfate in acetonitrile yielded 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine (3).⁶⁸ Initially, both microwave and conventional methods were used for synthesizing the target compound 5a starting from nucleoside 3. In Method A (conventional heating), 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine (3) was treated with ethyl acetoacetate (4a) and ammonium acetate in the presence of Ba(NO₃)₂ as a catalyst in CH₃CN at 50 °C. It yielded the desired compound 5a in 42% yield. Whereas, in Method B (microwave heating), under identical reaction conditions, 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine (3) produced the desired product 5a in 58% yield in 30 minutes (Scheme 1).

After establishing the superiority of microwave heating over conventional heating for the synthesis of our desired compound 5a, optimization of the reaction conditions was carried out (Table 1). Initially, performing the reaction in acetonitrile (CH₃CN) at 50 °C yielded the desired 1,4-dihydropyridine nucleoside 5a in 58% yield in 30 minutes (entry 1, Table 1). Next, various solvents, such as 1,4-dioxane, MeOH, ethanol, THF, AcOH, EtOAc, H₂O, DMF, isopropyl alcohol, and toluene (entries 2–11, Table 1), were screened. The higher yield of 72% was obtained in 1,4-dioxane, while other solvents resulted in inferior yields. Interestingly, significant improvement in the yield was achieved under solvent-free reaction conditions, with 86% yield at 50 °C in just 20 minutes (entry 12, Table 1). This outcome encouraged us to conduct the next reactions under solvent free reaction conditions. Hence, the following reactions were carried out at different temperatures, where the highest yield (93%) was obtained at 60 °C (entry 13, Table 1). Higher temperatures like 70 °C, 80 °C, 100 °C, and 120 °C resulted in relatively inferior yields of 88%, 68%, 32%, and 25%, respectively (entries 14–17, Table 1). When the reaction was performed in the absence of catalyst Ba(NO₃)₂ under solvent-free conditions at 60 °C for 30 minutes under microwave irradiation, the reaction remained incomplete, yielding the desired product 5a in 20% yield only (entry 18, Table 1). Among the conditions explored, microwave irradiation under solvent-free conditions at 60 °C was found to be the most favourable for synthesizing the 1,4-dihydropyridine nucleoside analogue 5a (entry 13, Table 1).

Table 1 Optimization of the reaction of 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine 3 with ethyl acetoacetate 4a and ammonium acetate in the presence of Ba(NO₃)₂ as a catalyst to obtain 1,4-dihydropyridine nucleoside analogue 5a^ad

S. no.	Solvent	Temperature (°C)	Time (min)	Reaction yield (percent yield)^b
a Reaction conditions: 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine 3 (0.34 g, 1 mmol), ethyl acetoacetate (0.39 g, 3 mmol), NH4OAc (92.3 mg, 1.2 mmol) and barium nitrate (26.2 mg, 0.1 mmol).b Isolated yield.c Without catalyst: reaction remained incomplete within 30 min and only 20 percent of the final product was obtained.d When we attempted to extend the optimized reaction conditions to synthesize unsymmetrical DHP using 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine 3 (0.34 g, 1 mmol), ethyl acetoacetate (0.13 g, 1 mmol), allyl acetoacetate (0.14 g, 1 mmol), NH4OAc (92.3 mg, 1.2 mmol) and barium nitrate (26.2 mg, 0.1 mmol), the reaction produced a un-separable diastereomeric mixture, as indicated by un-separable spots on TLC. Unfortunately, we were unable to separate these diastereomeric mixture product (see ESI, Fig. S43 and S44) using column chromatography.
1	Acetonitrile	50	30	58
2	1,4-Dioxane	50	30	72
3	MeOH	50	30	62
4	EtOH	50	30	54
5	THF	50	30	46
6	AcOH	50	30	38
7	EtOAc	50	30	40
8	H2O	50	30	44
9	DMF	50	30	35
10	i-PrOH	50	30	38
11	Toluene	50	30	65
12	Solvent-free	50	20	86
13	Solvent-free	60	20	93
14	Solvent-free	70	20	88
15	Solvent-free	80	20	68
16	Solvent-free	100	20	32
17	Solvent-free	120	20	25
18	Solvent-free	60	30	20^c

---

The optimized reaction condition was employed for the condensation of 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine 3 with eight differently substituted β-keto ester 4a–h or acetyl acetone 4i in the presence of NH₄OAc, yielding base-modified 1,4-dihydropyridine nucleoside analogues 5a–i in 86–96% yields (Scheme 2). Deacetylation of the diacetylated nucleosides 5a–i was performed using NaOMe in MeOH at 25 °C, resulting in the desired 1,4-dihydropyridine nucleoside analogues 6a–i in quantitative yields (Scheme 2).

Scheme 2 Synthesis of fluorescent 1,4-dihydropyridine nucleoside analogues.

The structures of all the synthesized compounds, including 3′,5′-di-O-acetyl-thymidine 2, 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine 3, base modified 1,4-dihydropyridine nucleoside analogues 5a–i and 6a–i, were unambiguously established by spectral data analysis (IR, ¹H, ¹³C-NMR, ¹H–¹H COSY NMR, ¹H–¹³C HETCOR NMR, NOESY NMR, and HRMS). The structures of the known compounds 2 and 3 were confirmed by comparing their melting point and spectral data with literature reports.^32,59,60

2.2. Gram scale synthesis of 3′,5′-di-O-acetyl-5-(diethyl 2′′,6′′-dimethyl 1′′,4′′-dihydropyridine-3′′,5′′-dicarboxylate)-2′-deoxyuridine (5a)

To showcase the applicability and broaden the applicative range of our methodology, we carried out the gram scale synthesis of 3′,5′-di-O-acetyl-5-(diethyl 2′′,6′′-dimethyl-1′′,4′′-dihydropyridine-3′′,5′′-dicarboxylate)-2′-deoxyuridine 5a, which highlights the practicality and effectiveness of the methodology for large-scale reactions. A reaction mixture comprising of 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine 3 (1.02 g, 3 mmol), ethyl acetoacetate 4a (1.17 g, 9 mmol), NH₄OAc (0.277 g, 3.6 mmol) and Ba(NO₃)₂ (78.5 mg, 0.3 mmol) was subjected to microwave irradiation under solvent-free conditions. The reaction proceeded smoothly resulting in 86% yield (1.45 g), highlighting the efficacy of this protocol for large-scale reactions.

3. Photophysical characterization

Over the last few decades, several modified nucleosides with fluorescent properties have been synthesized and studied for their application for studying the structure and dynamics of nucleic acid. The photophysical characterization of the thus synthesized 5-(1′′,4′′-dihydropyridine)-2′-deoxyuridine nucleoside analogues 6a–i, consisting of a highly extended conjugation system, was carried out in acetonitrile at 2.5 × 10⁻⁵ M concentration.

3.1. Absorption spectra

The absorption spectra of the synthesized 5-(1′′,4′′-dihydropyridine)-2′-deoxyuridine nucleoside analogues 6a–i was recorded in acetonitrile at 2.5 × 10⁻⁵ M concentration at 25 °C and all the compounds exhibited three absorption bands in the region 200–250 nm, 250–300 nm and 300–400 nm (Fig. 3). The presence of multiple peaks in the absorption spectra can be attributed to the extended conjugated system at the C-5 position of the nucleoside, corresponding to n → π* and π → π* transitions in the UV-vis region. Further, no significant effect on the absorption spectra was observed on changing the substituents at the C-2′′, C-3′′, C-5′′ or C-6′′ positions of the 1′′,4′′-dihydropyridine ring for compounds 6a–6h. However, a significant hypsochromic shift for compound 6i was observed.

Fig. 3 UV/vis absorption spectra of compounds 6a–i in acetonitrile at 2.5 × 10⁻⁵ M concentration.

3.2. Emission spectra

The emission spectra of the 5-(1′′,4′′-dihydropyridine)-2′-deoxyuridine nucleoside analogues 6a–i was also recorded in acetonitrile at the same concentration of 2.5 × 10⁻⁵ M with λ_ex = 235 nm as the excitation wavelength (Fig. 4). All the compounds exhibited a strong red-shifted band in the region 446–477 nm with two weak bands at around 310 and 350 nm. The highest fluorescence intensity was observed for compounds 6a and 6h. No significant substituent effect was observed on the emission spectra for compounds 6a–6h. However, a significant bathochromic shift was observed for compound 6i with a red-shifted band at 504 nm. Also, a weak blue-shifted band appeared at 310 nm for compound 6i.

Fig. 4 Emission spectra of compounds 6a–i in acetonitrile at 2.5 × 10⁻⁵ M concentration (λ_ex = 235 nm).

3.3. Quantum yield calculations and Stokes shift analysis

The quantum yields Φ_F for compounds 6a–i were calculated using the following equation:

Φ_F = Φ_st × S_u/S_st × A_st/A_u × n_Du²/n_Dst²

with quinine sulphate (Φ_st = 0.54 in 0.1 M H₂SO₄) as the reference standard. Wherein, Φ_F = Emission quantum yield for the synthesized compounds 6a–i, Φ_st = emission quantum yield of the standard, S_st = integrated emission band area of the standard S_u = integrated emission band area of the samples A_st = absorbance of the standard at the excitation wavelength A_u = absorbance of the samples at the excitation wavelength η_Dst = solvent refractive index of the standard η_Du = solvent refractive index of the sample.

The subscripts ‘u’ and ‘st’ are used to denote the unknown samples and the standard, respectively. The quantum yields thus calculated are tabulated in Table 2.

Table 2 Photophysical properties of 5-(1′′,4′′-dihydropyridine)-2′-deoxyuridine nucleoside analogues 6a–i in acetonitrile

Compound no.	Absorbance λabs (nm)	Emission λem (nm)	Stokes shift (nm)	Quantum yield (ΦF)	Molar extinction coefficient ε (M^−1 cm^−1)	Brightness ε × ΦF (M^−1 cm^−1)
6a	356	464	108	0.077	1.9 × 10^4	1.5 × 10^3
6b	352	446	94	0.080	2.1 × 10^4	1.7 × 10^3
6c	356	477	121	0.119	1.9 × 10^4	2.2 × 10^3
6d	354	472	118	0.091	1.7 × 10^4	1.5 × 10^3
6e	356	449	93	0.089	1.6 × 10^4	1.5 × 10^3
6f	334	474	140	0.078	1.8 × 10^4	1.4 × 10^3
6g	360	474	114	0.123	9.4 × 10^3	1.1 × 10^3
6h	354	449	95	0.074	1.8 × 10^4	1.3 × 10^3
6i	342	504	162	0.022	2.0 × 10^4	4.4 × 10^2

---

The quantum yield values (Φ_F) were found to be in the range 0.022–0.123 with the highest being 0.123 for compound 6g. High values of Stokes shift were observed ranging from 93–162 nm. The brightness of all modified nucleosides 6a–i was calculated and enlisted in Table 2.

3.4. Solvatochromism

Now, in order to study the effect of solvent polarity on the photophysical properties of the synthesized nucleoside analogues 6a–i, the solvatochromic characterization of one of the compounds 6g was performed in six different organic solvents; acetonitrile, CHCl₃, MeOH, DMF, DMSO and ethyl acetate.

Minor bathochromic/hypsochromic shifts in the absorption spectra were observed on changing the solvent from acetonitrile to CHCl₃, MeOH, DMF, DMSO or ethyl acetate (Fig. 5). A dual emission was observed in MeOH (310 nm and 456 nm), DMF (313 nm and 450 nm), DMSO (315 nm and 454 nm) and ethyl acetate (338 nm and 444 nm) while a single emission band was observed in acetonitrile (474 nm) and CHCl₃ (442 nm) (Fig. 6). The highest fluorescence intensity as well as highest quantum yield (0.579) was observed in DMSO. The highest Stokes shift value (114 nm) was observed in case of acetonitrile while the lowest value (70 nm) was recorded in CHCl₃. The emission wavelength, quantum yield, Stokes shift values as well as the molar absorption coefficient of compound 6g in different solvents is tabulated in Table 3.

Fig. 5 UV/vis absorption spectra of compound 6g in different organic solvents; acetonitrile, CHCl₃, MeOH, DMF, DMSO, and ethyl acetate at 2.5 × 10⁻⁵ M concentration.

Fig. 6 Emission spectra of compound 6g in different organic solvents; acetonitrile, CHCl₃, MeOH, DMF, DMSO, and ethyl acetate at 2.5 × 10⁻⁵ M concentration.

Table 3 Photophysical properties of 5-(1′′,4′′-dihydropyridine)-2′-deoxyuridine 6g in acetonitrile, CHCl₃, MeOH, DMF, DMSO, and ethyl acetate

Properties	Solvent
	Acetonitrile	CHCl3	MeOH	DMF	DMSO	Ethyl acetate
Absorbance λabs (nm)	360	372	370	368	375	369
Emission λem (nm)	474	442	456	450	454	444
Stokes shift (nm)	114	70	86	82	79	75
Quantum yield (ΦF)	0.123	0.331	0.441	0.468	0.579	0.197
Molar extinction coefficient ε (M^−1 cm^−1)	9.4 × 10^3	1.5 × 10^4	1.4 × 10^4	8.8 × 10^3	9.9 × 10^3	1.2 × 10^4
Brightness ε × ΦF (M^−1 cm^−1)	1.1 × 10^3	4.9 × 10^3	6.2 × 10^3	4.1 × 10^3	5.7 × 10^3	2.3 × 10^3

---

3.5. Comparative discussion

The synthesized fluorescent 1,4-dihydropyridine nucleoside analogues 6a–i demonstrated superior photophysical properties compared to previously reported C-5 modified pyrimidine nucleosides. Specifically, these compounds exhibited higher quantum yields (0.022–0.579) and Stokes shift values (70–162 nm), as detailed in Table 4. This indicated that these modified nucleosides not only surpassed the quantum yield values obtained in previous reports but also showed competitive Stokes shift values, emphasizing their potential in fluorescence applications.

Table 4 Comparison of various photophysical parameters of compound 6g with reported C-5 modified pyrimidine nucleosides^a

Ref.	Structure	Absorbance λabs (nm)	Emission λem (nm)	Stokes shift (nm)	Quantum yield (ΦF)	Extinction coefficient ε (M^−1 cm^−1)	Solvent
a Nr: not reported.
Ref. 31d		320	405	85	0.039	Nr	100 mM HCl
Ref. 31e & 34		(I) 322	(I) 423	(I) 101	(I) 0.12	(I) nr	(I) MeOH
		(II) 316	(II) 431	(II) 115	(II) 0.03	(II) nr	(II) H2O
Ref. 31a		(X = O) 296	(X = O) 400	(X = O) 104	(X = O) < 0.01	(X = O) nr	(X = O) H2O
		(X = S) 316	(X = S) 404	(X = S) 88	(X = S) < 0.01	(X = S) nr	(X = S) H2O
Ref. 31b		306	457	151	0.009	Nr	MeOH
Ref. 32		242	346	107	0.072	1.46 × 10^4	MeOH
This work (6g)		370	456	86	0.441	1.4 × 10^4	Acetonitrile
		375	454	79	0.579	9.9 × 10^3	DMSO

---

4. Conclusion

In this study, we have successfully developed an environmentally-friendly, economical and highly efficient methodology for synthesizing a small library of novel fluorescent 1,4-dihydropyridine nucleoside analogues, 5a–i and 6a–i, via a one-pot three-component Hantzsch condensation reaction under solvent-free conditions. This process involved reaction between 3′,5′-di-O-acetyl-5-formyl-2′-deoxyuridine 3, β-keto ester 4a–h or acetyl acetone 4i, and ammonium acetate, catalyzed by Ba(NO₃)₂ under microwave irradiation. This method offered high efficiency, producing the desired products in high yields up to 96%. Comparative studies demonstrated the superior performance of microwave irradiation over conventional heating. The protocol demonstrated practicability and broad applicability through successful gram-scale synthesis. The synthesized compounds displayed strong fluorescence at around 442–477 nm and exhibited significantly higher absorption and emission bands compared to thymidine, attributed to extended conjugation. Photophysical investigations revealed a noteworthy fluorescence intensity, excellent Stokes shift values (70–162 nm), and superior quantum yields (0.022–0.579), particularly notable in DMSO (0.579). These highly fluorescent nucleoside derivatives are suitable for their incorporation into oligonucleotides for carrying out hybridization studies with complementary DNA strands, facilitating the investigation of DNA local structure.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI materials.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

We appreciate the funding provided by the Institute of Imminence (IOE), University of Delhi which has contributed to further research and development. We appreciate the assistance provided by USIC and Department of Chemistry, University of Delhi for recording NMR, HRMS and IR data. Aditi Arora (SRF, File No. 09/0045(11270)/2021/EMR-I) and Sumit Kumar (SRF, File No. 09/045(1798)/2020-EMR-1) are grateful to CSIR, New Delhi for their fellowship.

References

1. L. P. Jordheim, D. Durantel, F. Zoulim and C. Dumontet, Nat. Rev. Drug Discovery, 2013, 12, 447.
2. K. L. Seley-Radtke and M. K. Yates, Antivir. Res., 2018, 154, 66.
3. J. Zhang, F. He, J. Chen, Y. Wang, Y. Yang, D. Hu and B. Song, J. Agric. Food Chem., 2021, 69, 5575.
4. C. K. Chu, L. Ma, S. Olegen, C. Pierra, J. Du, G. Gumina, E. Gullen, Y. C. Cheng and R. F. Schinazi, J. Med. Chem., 2000, 43, 3906.
5. D. C. Schultz, R. M. Johnson, K. Ayyanathan, J. Miller, K. Whig, B. Kamalia, M. Dittmar, S. Weston, H. L. Hammond, C. Dillen, J. Ardanuy, L. Taylor, J. S. Lee, M. Li, E. Lee, C. Shoffler, C. Petucci, S. Constant, M. Ferrer, C. A. Thaiss, M. B. Frieman and S. Cherry, Nature, 2022, 604, 134.
6. P. Sharma, V. Nurpeisov, H. B. Santiago, T. Beltran and R. Schinazi, Curr. Top. Med. Chem., 2004, 4, 895.
7. Y. Yoshida, M. Honma, Y. Kimura and H. Abe, ChemMedChem, 2020, 16, 743.
8. P. L. Bonate, L. Arthaud, W. R. Cantrell, K. Stephenson, J. A. Secrist and S. Weitman, Nat. Rev. Drug Discovery, 2006, 5, 855.
9. M. Guinan, C. Benckendorff, M. Smith and G. J. Miller, Molecules, 2020, 25, 2050.
10. G. Elgemeie, Curr. Pharm. Des., 2003, 9, 2627.
11. W. Parker, J. Secrist and W. Waud, Curr. Opin. Invest. Drugs, 2004, 5, 592.
12. V. K. Sharma, R. K. Sharma and S. K. Singh, MedChemComm, 2014, 5, 1454.
13. Y. Mehellou and E. D. Clercq, J. Med. Chem., 2010, 53, 521.
14. E. D. Clercq, J. Med. Chem., 2010, 53, 1438.
15. W. H. Prusoff, Biochim. Biophys. Acta, 1959, 32, 295.
16. (a) G. B. Elion, P. A. Furman, J. A. Fyfe, P. Miranda, L. Beauchamp and H. J. Schaeffer, Proc. Natl. Acad. Sci. U.S.A., 1977, 74, 5716; (b) K. Katarzyna, P. Aneta, K. Anna and N. Maria, Curr. Med. Chem., 2020, 27, 4118; (c) Y. P. Wei, L. Y. Yao, Y. Y. Wu, X. Liu, L. H. Peng, Y. L. Tian, J. H. Ding, K. H. Li and Q. G. He, Molecules, 2021, 26, 6566.
17. (a) T. Waga, H. Ohrui and H. Meguro, Nucleosides Nucleotides, 1996, 15, 287; (b) R. Yarchoan, P. Brouwers, A. R. Spitzer, J. Grafman, B. Safai, C. F. Perno, S. M. Larson, G. B. M. Fischl, A. Wichman, R. V. Thomas, A. Brunetti, P. J. Schmidt, C. E. Myers and S. Broder, Lancet, 1987, 329, 132; (c) R. Yarchoan, R. V. Thomas, J. P. Allain, N. Mcatee, R. Dubinsky, H. Mitsuya, T. J. Lawley, B. Safai, C. E. Myers, C. F. Perno, R. W. Klecker, R. J. Wills, M. A. Fischl, M. C. Mcneely, J. M. Pluda, M. Leuther, J. M. Collins and S. Broder, Lancet, 1988, 331, 76; (d) R. Yarchoan, H. Mitsuya, R. V. Thomas, J. M. Pluda, N. R. Hartman, C. F. Perno, K. S. Marczyk, J. P. Allain, D. G. Johns and S. Broder, Science, 1989, 245, 412; (e) R. Yarchoan, H. Mitsuya, C. E. Myers and S. N. Broder, N. Engl. J. Med., 1989, 321, 726; (f) M. Miller, J. Schneider, B. K. Sathyanarayana, M. V. Toth, G. R. Marshall, L. Clawson, L. Selk, S. B. H. Kent and A. Wlodawer, Science, 1989, 246, 1149.
18. (a) K. Z. Rana and M. N. Dudley, Clin. Pharmacokinet., 1997, 33, 276; (b) M. Hurst and S. Noble, Drugs, 1999, 58, 919; (c) J. A. S. Al-Hussaini, O. A. Hatem and Z. A. H. Alebady, Syst. Rev. Pharm., 2020, 11, 96; (d) B. Kumar, A. Sharma, S. K. Tiwari, D. Agrawal, A. K. Sharma and M. Khandelwal, Eur. J. Biomed. Pharm., 2021, 8, 148.
19. (a) B. Jarvis and D. Faulds, Drugs, 1999, 58, 101; (b) R. Quercia, C. F. Perno, J. Koteff, K. Moore, C. McCoig, M. S. Clair and D. Kuritzkes, J. Acquir. Immune Defic. Syndr., 2018, 78, 125; (c) R. Patel, L. Evitt, I. Mariolis, S. D. Giambenedetto, A. Monforte, J. Casado, A. C. Úbeda, L. Hocqueloux, C. Allavena, T. Barber, D. Jha, R. Kumar, R. D. Kamath, T. Vincent, J. V. Wyk and J. Koteff, Infect. Dis. Ther., 2021, 10, 2051; (d) R. Roediger, E. K. Smyth and D. Dieterich, Antivir. Ther., 2022, 27, 1.
20. (a) N. Katagiri, M. Takebayashi, H. Kokufuda, C. Kaneko, K. Kanehira and M. Torihara, J. Org. Chem., 1997, 62, 1580; (b) V. E. Kataev and B. F. Garifullin, Chem. Heterocycl. Compd., 2021, 57, 326.
21. B. Y. Michel, D. Dziuba, R. Benhida, A. P. Demchenko and A. Burger, Front. Chem., 2020, 8, 112.
22. F. Seela and V. R. Sirivolu, Org. Biomol. Chem., 2008, 6, 1674.
23. I. Schönrath, V. B. Tsvetkov, T. S. Zatsepin, A. V. Aralov and J. Müller, J. Biol. Inorg. Chem., 2019, 24, 693–702.
24. A. C. Jones and R. K. Neely, Rev. Geophys., 2015, 48, 244–279.
25. T. Gustavsson, N. Sarkar, Á. Bányász, D. Markovitsi and R. Improta, Photochem. Photobiol., 2007, 83, 595–599.
26. M. Wilhelmsson & Y. Tor, Fluorescent Analogues of Biomolecular Building Blocks: Design and Applications, John Wiley & Sons, 2016.
27. Y. Saito and R. H. E. Hudson, J. Photochem. Photobiol. C Photochem., 2018, 36, 48–73.
28. L. Zilbershtein-Shklanovsky, M. Weitman, D. T. Major and B. Fischer, J. Org. Chem., 2013, 78, 11999–12008.
29. K. Seio, T. Kanamori and Y. Masaki, Tetrahedron Lett., 2018, 59, 1977–1985.
30. J. Zayas, M. Annoual, J. K. Das, Q. Felty, W. G. Gonzalez, J. Mikssovska, N. Sharifai, A. Chiba and S. F. Wnuk, Bioconjugate Chem., 2015, 26, 1519–1532.
31. (a) N. J. Greco and Y. Tor, Tetrahedron, 2007, 63, 3515–3527; (b) T. Kanamori, H. Ohzeki, Y. Masaki, A. Ohkubo, M. Takahashi, K. Tsuda, T. Ito, M. Shirouzu, K. Kuwasako, Y. Muto, M. Sekine and K. Seio, ChemBioChem, 2015, 16, 167–176; (c) M. S. Noé, R. W. Sinkeldam and Y. Tor, J. Org. Chem., 2013, 78, 8123–8128; (d) P. Guo, X. Xu, X. Qiu, Y. Zhou, S. Yan, C. Wang, C. Lu, W. Ma, X. Weng, X. Zhang and X. Zhou, Org. Biomol. Chem., 2013, 11, 1610–1613; (e) A. A. Tanpure and S. G. Srivatsan, ChemBioChem, 2012, 13, 2392–2399.
32. S. Kumar, S. Kumar, J. Maity, B. Kumar, S. B. Mehta and A. K. Prasad, New J. Chem., 2021, 45, 16635–16647.
33. N. Amann and H. A. Wagenknecht, Synlett, 2002, 5, 0687–0691.
34. N. J. Greco and Y. Tor, J. Am. Chem. Soc., 2005, 127, 10784–10785.
35. C. Isabelle, B. Norberg, A. Olivier, C. Evrard, G. Evrard, P. Wigerninck, P. Herdewijn and F. Durant, J. Chem. Crystallogr., 1996, 26, 777–789.
36. A. V. Ardhapure, V. Gayakhe, S. Bhilare, A. R. Kapdi, S. S. Bag, Y. S. Sanghvi and K. C. Gunturu, New J. Chem., 2020, 44, 14744–14754.
37. A. J. Gutierrez, T. J. Terhorst, M. D. Matteucci and B. C. Froehler, J. Am. Chem. Soc., 1994, 116, 5540–5544.
38. Z. Wen, P. R. Tuttle, A. H. Howlader, A. Vasilyeva, L. Gonzalez, A. Tangar, R. Lei, E. E. Laverde, Y. Liu, J. Miksovska and S. F. Wnuk, J. Org. Chem., 2019, 84, 3624–3631.
39. H. Song, X. Li, Y. Long, G. Schatte and H. B. Kraatz, Dalton Trans., 2006, 4696–4701.
40. X. Fan, X. Zhang, L. Zhou, K. A. Keith, E. R. Kern and P. F. Torrence, Bioorg. Med. Chem. Lett., 2006, 16, 3224–3228.
41. T. Ehrenschwender and H. A. Wagenknecht, Synthesis, 2008, 22, 3657–3662.
42. X. Fan, Y. Wang, Y. He, X. Zhang and J. Wang, Tetrahedron Lett., 2010, 51, 3493–3496.
43. X. Zhang, X. Li, X. Fan, X. Wang, D. Li, G. Qu and J. Wang, Mol. Divers., 2009, 13, 57–61.
44. A. P. Silverman and E. T. Kool, Chem. Rev., 2006, 106, 3775.
45. A. V. Ivanov, A. R. Simonyan, E. F. Belanov and L. A. Aleksandrova, Russ. J. Bioorg. Chem., 2005, 31, 556–562.
46. M. L. Bryant, E. G. Bridges, L. Placidi, A. Faraj, A. G. Loi, C. Pierra, D. Dukhan, G. Gosselin, J. L. Imbach, B. Hernandez, A. Juodawlkis, B. Tennant, B. Korba, P. Cote, E. Cretton-Scott, R. F. Schinazi and J. P. Sommadossi, Nucleos Nucleot. Nucleic Acids, 2001, 20, 597.
47. X. Fan, X. Zhang, C. Bories, P. M. Loiseau and P. F. Torrence, Bioorg. Chem., 2007, 35, 121–136.
48. P. Wigerinck, R. Snoeck, P. Claes, E. De Clercq and P. Herdewijn, J. Med. Chem., 1991, 34, 1767–1772.
49. J. Krim, C. Grunewald, M. Taourirte and J. W. Engels, Bioorg. Med. Chem., 2012, 20, 480–486.
50. S. Kumar, S. Kumar, J. Maity, A. Arora and A. K. Prasad, ChemistrySelect, 2022, 7(45), e202203432.
51. A. K. Prasad, M. D. Sorensen, V. S. Parmar and J. Wengel, Tetrahedron Lett., 1995, 36, 6163–6166.
52. C. Beuck, I. Singh, A. Bhattacharya, W. Hecker, V. S. Parmar, O. Seitz and E. Weinhold, Angew. Chem., Int. Ed., 2003, 42, 3958–3960.
53. Poonam, A. K. Prasad, C. Mukherjee, G. Shakya, G. K. Meghwanshi, J. Wengel, R. K. Saxena and V. S. Parmar, Pure Appl. Chem., 2005, 77, 237–243.
54. A. K. Prasad, N. Kalra, Y. Yadav, R. Kumar, S. K. Sharma, S. Patkar, L. Lange, J. Wengel and V. S. Parmar, Chem. Commun., 2007, 2616–2617.
55. J. Maity, G. Shakya, S. Singh, V. T. Ravikumar, V. S. Parmar and A. K. Prasad, J. Org. Chem., 2008, 73, 5629–5632.
56. S. Srivastava, V. K. Maikhuri, R. Kumar, K. Bohra, H. Singla, J. Maity and A. K. Prasad, Carbohydr. Res., 2018, 470, 19–25.
57. S. Kumar, A. Arora, R. Kumar, N. N. Senapati and B. K. Singh, Carbohydr. Res., 2023, 108857.
58. H. Singla, S. Kumar, V. K. Maikhuri, Kavita and A. K. Prasad, ChemistrySelect, 2023, 8, e202203412.
59. A. Arora, S. Kumar, S. Kumar, A. Dua and B. K. Singh, Org. Biomol. Chem., 2024, 22, 4922–4939.
60. S. Kumar, A. Arora, S. Kumar, J. Maity, A. Dua and B. K. Singh, J. Mol. Struct., 2024, 1321, 139915.
61. A. Hantzsch and J. Liebigs, Ann. Chem., 1882, 215, 1–82.
62. (a) N. Edraki, A. R. Mehdipour, M. Khoshneviszadeh and R. Miri, Drug Discovery Today, 2009, 14, 1058–1066; (b) H. S. Sohal, Mater. Today: Proc., 2022, 48, 1163–1170; (c) M. G. Sharma, J. Pandya, D. M. Patel, R. M. Vala, V. Ramkumar, R. Subramanian, V. K. Gupta, R. L. Gardas, A. Dhanasekaran and H. M. Patel, Polycyclic Aromat. Compd., 2021, 41, 1495–1505; (d) S. G. Ouellet, A. M. Walji and D. W. Macmillan, Acc. Chem. Res., 2007, 40, 1327–1339; (e) V. Dean, C. M. Wandikayi, R. Manian, I. Nadeem, W. W. Michael, E. H. Susan and E. K. Edward, J. Med. Chem., 1995, 38, 2851–2859; (f) F. G. Mikhail, V. P. Dinesh and M. G. Eric, J. Org. Chem., 1996, 61, 924–928; (g) A. Tambe, G. Sadaphal, R. Dhawale and G. Shirole, Res. Chem. Intermed., 2022, 48, 1273–1286; (h) D. S. Malhi, M. Kaur and H. S. Sohal, ChemistrySelect, 2019, 4, 11321–11336; (i) A. S. Khedkar and B. P. Auti, Mini Rev. Med. Chem., 2014, 14, 282–290; (j) A. M. Vijesha, A. M. Isloor, S. K. Peethambar, K. N. Shivananda, T. Arulmoli and N. A. Isloor, Eur. J. Med. Chem., 2011, 46, 5549; (k) K. Sirisha, D. Bikshapathi, G. Achaiah and V. M. Reddy, Eur. J. Med. Chem., 2011, 46, 1564; (l) A. González, J. Casado, M. G. Gündüz, B. Santos, A. Velázquez-Campoy, C. Sarasa-Buisan, M. F. Fillat, M. Montes, E. Piazuelo and A. Lanas, Front. Microbiol., 2022, 25(13), 874709; (m) T. Narsinghani, L. K. Soni and S. Chourey, J. Drug Deliv. Therapeut., 2017, 7, 142–145; (n) A. Malani, A. Makwana, J. Monapara, I. Ahmad, H. Patel and N. Desai, J. Biochem. Mol. Toxicol., 2021, 35, e22903; (o) D. S. Malhi, H. S. Sohal, K. Singh, Z. M. Almarhoon, A. B. Bacha and M. I. Al-Zaben, ACS Omega, 2022, 7, 16055–16062; (p) A. K. Chillar, P. Arya, C. Mukherjee, P. Kumar, Y. Yadav, A. K. Sharma, V. Yadav, J. Gupta, R. Dabur, H. N. Jha, A. C. Watterson, V. S. Parmar, A. K. Prasad and G. L. Sharma, Bioorg. Med. Chem., 2006, 14, 973–981.
63. Y. He, H. Wang, C. Ge and H. Yan, J. Mol. Struct., 2023, 1281, 135167.
64. Q. Fan, P. Li, H. Yan and J. Photocgem, J. Photochem. Photobiol., A, 2018, 358, 51–60.
65. J. Ramchander, G. Raju, N. Rameshwar, T. S. Reddy and A. R. Reddy, Spectrochim. Acta, Part A, 2012, 85, 210–216.
66. L. Camargo da Luz, M. G. Gündz, R. Beal, G. M. Zanotto, E. R. Kuhn, P. A. Netz, C. Şafak, P. F. B. Gonçalves, F. da Silveira Santos, F. S. Rodembusch and J. Photocgem, J. Photochem. Photobiol., A, 2018, 358, 51–60.
67. (a) A. S. Grewal, K. Kumar, S. Redhu and S. Bhardwaj, Int. Res. J. Pharmaceut. Appl. Sci., 2013, 3, 278–285; (b) A. R. Yadav, S. K. Mohie and C. S. Magdum, Asian J. Res. Chem., 2020, 13, 275–278.
68. A. Amer, S. Senior and X. Fan, Nucleosides, Nucleotides Nucleic Acids, 2012, 31, 42–54.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra07295b

---