Regioselective Nucleophilic Aromatic Substitution: Theoretical and Experimental Insights into 4-Aminoquinazoline Synthesis as a Privileged Structure in Medicinal Chemistry

Abstract

The 4-aminoquinazoline scaffold is a privileged structure in medicinal chemistry. Regioselective nucleophilic aromatic substitution (S_NAr) for replacing the chlorine atom at the 4-position of 2,4-dichloroquinazoline precursors is well documented in the scientific literature and has proven useful in synthesizing 2-chloro-4-aminoquinazolines and/or 2,4-diaminoquinazolines for various therapeutic applications. While numerous reports describe reaction conditions involving different nucleophiles, solvents, temperatures, and reaction times, discussions on the regioselectivity of the S_NAr step remain scarce. In this study, we combined DFT calculations with 2D-NMR analysis to characterize the structure and understand the electronic factors underlying the regioselective S_NAr of 2,4-dichloroquinazolines for the synthesis of bioactive 4-aminoquinazolines. DFT calculations revealed that the carbon atom at the 4-position of 2,4-dichloroquinazoline has a higher LUMO coefficient, making it more susceptible to nucleophilic attack. This observation aligns with the calculated lower activation energy for nucleophilic attack at this position, supporting the regioselectivity of the reaction. To provide guidance for the structural confirmation of 4-amino-substituted product formation when multiple regioisomers are possible, we employed 2D-NMR methods to verify the 4-position substitution pattern in synthesized bioactive 2-chloro-4-aminoquinazolines. These findings are valuable for future research, as many synthetic reports assume regioselective outcomes without sufficient experimental verification.

Keywords: NMR spectroscopy; aminoquinazoline; density functional calculations; medicinal chemistry; nucleophilic substitution; regioselectivity.

Scheme 1

(a) General scheme for regioselective nucleophilic aromatic substitution (S_NAr) reaction from 2,4-dichloro-quinazoline precursors and primary or secondary amine nucleophiles; (b) intermediates involved in the regioselective S_NAr reaction from 2,4-dichloro-quinazolines (1) and primary or secondary amine nucleophiles (2), providing the corresponding 2-chloro-4-aminoquinazoline derivatives (3).

Figure 1

Bioactive 4-aminoquinazolines (6–18) described as drugs or as drug candidates presenting vasodilator, smooth muscle relaxant, antipsychotic, anti-Alzheimer, anti-inflammatory, antitumor, antiviral, antibiotic, and antiparasitic pharmacological properties. The 4-aminoquinazoline scaffold is highlighted in red, and the second amino substituent of the 2,4-diaminoquinazolines is indicated in green. Aβ = amyloid beta peptide; ChE = cholinesterase; H4R = histamine H4 receptor; 5-HT2A = serotonin 5-HT2A subtype receptor; PAK4i = p21 activated kinase 4 inhibitor; PDE5i = phosphodiesterase 5 inhibitor; Tcf-4 = transcription factor 4; HIV = human immunodeficiency virus; NNRTi = non-nucleoside reverse transcriptase inhibitor; RSV = respiratory syncytial virus.

Figure 2

Potential energy surface analysis between (2a) and (1a) by varying the distance between the aniline N atom in relation to the C2 (black curve) and C4 (blue curve) atoms of the quinazoline moiety from 2 to 8 Å, with steps of 1 Å.

Scheme 2

Activation energy determination for the nucleophilic aromatic substitution reaction at C4 (A) and C2 (B) of the 2,4-dichloro-quinazoline (1a) at the ωB97X-D/6-31G(d) level of theory using the C-PCM solvation model for polar solvents, available on Spartan’20. ^‡ Transition state.

Scheme 3

Regioselective S_NAr in the synthesis of 2-chloro-4-anilinoquinazolines 21 and 22 [5].

Figure 3

¹H NMR (800 MHz; 25 °C; DMSO-d₆) spectrum and signal assignment for bioactive 2-chloro-4-anilinoquinazoline 21 (LASSBio-1812).

Figure 4

2D-NOESY predicted (A) and observed (B) (800 MHz; 25 °C; DMSO-d₆) correlations for bioactive 2-chloro-4-anilinoquinazoline 21 (LASSBio-1812). The observed NOE correlations are indicated in red, with the spatial proximity between the NH and H5 signals confirmed as correlation B, highlighted in blue.

Figure 5

Chemical structure (A) and 2D-HSQC NMR spectrum (B) of 2-chloro-4-anilinoquinazoline 21 (LASSBio-1812) (500 MHz spectrometer; 25 °C; DMSO-d₆). Hydrogen and carbon signals are assigned according to the chemical structure atom numbers.

Figure 6

Chemical structure (A) and 2D-HMBC NMR spectrum (B) of 2-chloro-4-anilinoquinazoline 21 (LASSBio-1812) (500 MHz spectrometer; 25 °C; DMSO-d₆). The annotated cross-peaks indicate the ³J_CH coupling between C4a and the NH hydrogen and the ³J_CH and ²J_CH coupling of C4 with hydrogens H5 and NH, respectively, as highlighted in blue in the chemical structure.

Figure 7

¹H NMR (800 MHz; 25 °C; DMSO-d₆) spectrum and signal assignment for bioactive 2-chloro-4-anilinoquinazoline 22 (LASSBio-1821).

Figure 8

2D-NOESY predicted (A) and observed (B) (800 MHz; 25 °C; DMSO-d₆; chemical shift ranges from 6.5 ppm to 11.0 ppm) correlations for bioactive 2-chloro-4-anilinoquinazoline 22 (LASSBio-1821). The observed NOE correlations are indicated in red, with the spatial proximity between the NH and H5 signals confirmed as correlation B, highlighted in blue.