1,8-Naphthyridine-Based Aldose Reductase Inhibitors: Scaffold Construction, In Vitro Inhibitory Activity, and Directions for Structural Optimization

1 Introduction

Aldose reductase (ALR2) is a reduced nicotinamide adenine dinucleotide phosphate-dependent enzyme in the polyol pathway. The human enzyme is encoded by the aldo-keto reductase family 1 member B1 gene (AKR1B1). The molecular design of aldose reductase inhibitors (ARIs) generally requires coordination among polar binding in the catalytic region, positioning of the aromatic or heteroaromatic scaffold, linker geometry, and spatial matching of distal substituents. Early studies focused primarily on carboxylic acid derivatives, hydantoins, and other cyclic imide structures. These compounds generally bind to the catalytic region of the enzyme through polar groups and contact adjacent hydrophobic regions through aromatic or heteroaromatic moieties [2].

To expand the chemical space beyond traditional carboxylic acid and hydantoin structures, researchers used 1,8-naphthyridine as the core scaffold in the study Synthesis of Naphthyridine Derivatives and Their Aldose Reductase Inhibitory Activity. An acetamido group was introduced at the 2-position, while morpholine, piperidine, or pyrrolidine was connected to the 7-position through a methylene group. Three structurally related derivatives were synthesized, and their in vitro inhibitory activities were measured in a crude enzyme system consisting of the supernatant of rat lens homogenates [1]. The half-maximal inhibitory concentrations (IC₅₀ values) of all three compounds were in the micromolar range, with the morpholine derivative exhibiting an IC₅₀ of 3.5 μmol/L.

These results indicate that the overall structure comprising a 2-acetamido-1,8-naphthyridine core, a methylene group at the 7-position, and a terminal cyclic amine possesses preliminary inhibitory activity in the above crude enzyme system and may serve as a candidate heteroaromatic scaffold for subsequent structure–activity relationship studies. The observed activity also suggests that polar anchoring in the catalytic region, linker length and orientation, the properties of the terminal heterocycle, and occupation of hydrophobic space require further optimization. This article discusses the construction of the 1,8-naphthyridine core, the structural origins of its preliminary inhibitory activity, and directions for further pharmacophore refinement.

Keywords: 1,8-naphthyridine; aldose reductase; ALR2; AKR1B1; aldose reductase inhibitor; structure–activity relationship; pharmacophore; benzylic bromination

2 Catalytic Reaction of ALR2 and the Basis of Inhibitor Recognition

2.1 Reaction of ALR2 in the Polyol Pathway

ALR2 belongs to the aldo-keto reductase family and uses reduced nicotinamide adenine dinucleotide phosphate (NADPH) to reduce various aldehydes, ketones, and sugar carbonyl groups to their corresponding alcohols. Its representative reaction in the polyol pathway is [3]:

D-Glucose + NADPH + H⁺

—ALR2→ D-Sorbitol + NADP⁺

Under normal glucose metabolic conditions, the polyol pathway accounts for only a small proportion of glucose metabolism. Under persistent hyperglycemic conditions, flux through this pathway may increase, accompanied by NADPH consumption, increased sorbitol formation, and changes in the cellular redox state [3].

ALR2 also participates in the metabolism of various endogenous and exogenous carbonyl compounds. Therefore, in addition to enzyme inhibitory potency, inhibitor studies must consider the tissue environment, substrate type, and selectivity among related aldo-keto reductases.

2.2 Key Recognition Regions in the ALR2 Active Site

Structural and site-directed mutagenesis studies of human ALR2 have shown that tyrosine 48 (Tyr48) participates in catalytic proton transfer, histidine 110 (His110) contributes to substrate positioning and stereoselectivity, and lysine 77 (Lys77) helps maintain the hydrogen-bonding network in the catalytic region [4]. The nicotinamide moiety of NADP⁺ or NADPH is located near the catalytic region and also contributes to the local polar environment required for inhibitor binding [5].

Based on resolved inhibitor–enzyme complex structures, the common molecular recognition elements of ALR2 inhibitors can be summarized as follows:

① Polar anchoring region

Carboxylates, cyclic imides, carbonyl groups, sulfonyl groups, or other polar groups form hydrogen-bonding or electrostatic interactions with Tyr48, His110, NADP⁺, and associated bound water molecules.

② Aromatic or heteroaromatic scaffold

Hydrophobic interactions, van der Waals interactions, and aromatic interactions maintain the basic orientation of the molecule within the active site.

③ Linker

The linker regulates the distance, orientation, and conformational freedom between the polar region and the distal substituent.

④ Distal occupancy group

This group extends toward hydrophobic regions or specific spatial regions adjacent to the active site and affects binding affinity and enzyme subtype selectivity.

The crystal structure of the Fidarestat–human ALR2 complex shows that the compound is positioned in the active site through both hydrophilic and hydrophobic interactions. Its carbamoyl group makes an important contribution to affinity and to selectivity between ALR2 and aldehyde reductase [6]. This finding indicates that highly potent inhibition does not depend on a single functional group but instead relies on the coordinated three-dimensional arrangement of multiple structural regions.

3 Molecular Design of 1,8-Naphthyridine Derivatives

3.1 Structural Characteristics of the 1,8-Naphthyridine Core

1,8-Naphthyridine consists of two fused nitrogen-containing six-membered aromatic rings and has a planar, rigid, and highly conjugated heteroaromatic structure. Its two ring nitrogen atoms alter the electronic distribution and local polarity of the aromatic system, while the two ends of the core can be functionalized differentially.

In the design of ALR2 inhibitors, this core has the following functions:

① It provides a fused aromatic structure with a certain hydrophobic surface area;

② It constrains the relative orientations of substituents at the 2- and 7-positions;

③ Its ring nitrogen atoms provide potential hydrogen-bond acceptors;

④ It provides independently modifiable positions for a polar group and a distal side chain.

In this series of compounds, 1,8-naphthyridine primarily serves as a scaffold-positioning element. It does not itself constitute a complete ALR2 pharmacophore and must be combined with an appropriate polar group, linker, and distal substituent to form an effective binding structure.

3.2 Four Structural Regions of the Target Compounds

Compounds 5–7 designed in the original study share the same 2-acetamido-1,8-naphthyridine core but differ in the terminal heterocycle at the 7-position [1]. Their structures can be divided into four regions.

① 1,8-Naphthyridine core

Forms a rigid heteroaromatic scaffold and fixes the orientations of the two substituents.

② 2-Acetamido group

Provides an amide carbonyl, an amide N—H, and local polarity.

③ 7-Methylene linker

Separates the naphthyridine core from the terminal heterocycle and allows limited conformational adjustment of the terminal ring.

④ Terminal saturated nitrogen-containing heterocycle

Morpholine, piperidine, and pyrrolidine were used to compare the effects of heteroatom composition, ring size, basicity, and spatial conformation on activity.

The differences among the three target compounds are concentrated in the terminal heterocycles, allowing preliminary comparison of the side chains while maintaining the same core and linking mode.

4 Synthetic Route to the 1,8-Naphthyridine Derivatives

In the original study, 2,6-diaminopyridine was used as the starting material, and target compounds 5–7 were obtained through four types of reactions [1]. The synthetic route can be summarized as follows:

2,6-Diaminopyridine

→ 2-Amino-7-methyl-1,8-naphthyridine

→ 2-Acetamido-7-methyl-1,8-naphthyridine

→ 2-Acetamido-7-(bromomethyl)-1,8-naphthyridine

→ Compounds 5–7

4.1 Construction of the Fused Ring System

2,6-Diaminopyridine and 4,4-dimethoxy-2-butanone were reacted in phosphoric acid at 100 °C for 6 h to form 2-amino-7-methyl-1,8-naphthyridine in a reported yield of 62% [1].

This reaction constructs the fused 1,8-naphthyridine ring system while retaining the amino group at the 2-position and the methyl group at the 7-position, thereby providing a structural basis for subsequent modification at both positions.

4.2 Acetylation of the Amino Group

2-Amino-7-methyl-1,8-naphthyridine was reacted in acetic anhydride at 120 °C for 1 h to afford 2-acetamido-7-methyl-1,8-naphthyridine in a reported yield of 51% [1].

Acetylation reduces the nucleophilicity of the exocyclic amino group and facilitates selective functionalization of the methyl group at the 7-position in the subsequent step. The acetamido group is retained in the target compounds and forms the polar region at the 2-position.

4.3 Benzylic Bromination of the 7-Methyl Group

2-Acetamido-7-methyl-1,8-naphthyridine was reacted with N-bromosuccinimide (NBS) under reflux in chloroform for 4 h to afford 2-acetamido-7-(bromomethyl)-1,8-naphthyridine in a reported yield of 24% [1].

This step converts the relatively unreactive methyl group into a bromomethyl group capable of undergoing nucleophilic substitution. The resulting bromomethyl intermediate can react with different amine nucleophiles and serves as a general intermediate for expanding the structural diversity of the side chain at the 7-position.

The reported yield of this step was lower than those of the other steps. In subsequent derivative synthesis, the bromination process may be optimized by adjusting the amount of NBS, reaction concentration, temperature, radical initiation conditions, and purification method.

4.4 Introduction of Terminal Heterocycles by Nucleophilic Substitution

The bromomethyl intermediate was reacted separately with morpholine, piperidine, and pyrrolidine, using N,N-diisopropylethylamine (DIPEA) as the base in 1,4-dioxane at room temperature for 48 h, to afford compounds 5–7 [1].

General reaction:

2-Acetamido-7-[(cyclic amino)methyl]-1,8-naphthyridine + cyclic amine

—DIPEA, 1,4-dioxane, room temperature, 48 h→ 2-acetamido-7-(cyclic aminomethyl)-1,8-naphthyridine

Compound	Terminal group at the 7-position	Standardized structural name	Reported yield
5	Morpholine	N-[7-(Morpholin-4-ylmethyl)-1,8-naphthyridin-2-yl]acetamide	65%
6	Piperidine	N-[7-(Piperidin-1-ylmethyl)-1,8-naphthyridin-2-yl]acetamide	61%
7	Pyrrolidine	N-[7-(Pyrrolidin-1-ylmethyl)-1,8-naphthyridin-2-yl]acetamide	49%

The final step uses a common intermediate and different cyclic amines, allowing the terminal side chain to be varied independently and making the route suitable for constructing a structurally controlled comparison series.

5 In Vitro ALR2 Inhibitory Activity

5.1 Enzyme Source and Assay Principle

In the original study, lens homogenates were prepared from male Sprague–Dawley rats weighing approximately 250 g. After low-temperature centrifugation, the supernatant was collected as the crude aldose reductase enzyme source [1].

The enzyme assay was based on the characteristic absorbance of NADPH at 340 nm, whereas its oxidation product, NADP⁺, shows markedly lower absorbance at this wavelength. By monitoring the change in absorbance of the reaction system at 340 nm over time, the rate of NADPH consumption and the inhibitory effect of the compounds on the enzymatic reaction could be evaluated.

The inhibition rate of each compound was measured at multiple concentrations, and the half-maximal inhibitory concentration (IC₅₀) was calculated. Fidarestat was used as the reference compound in the same experimental system.

5.2 Activity Results

Compound	Terminal heterocycle	IC₅₀/μmol·L⁻¹
5	Morpholine	3.5
6	Piperidine	8.8
7	Pyrrolidine	7.8
Fidarestat	Spirocyclic imide reference compound	0.088

Compounds 5–7 all exhibited micromolar inhibitory activity in the rat lens crude enzyme system, indicating that the overall structure comprising a 2-acetamido-1,8-naphthyridine core, a methylene group at the 7-position, and a terminal cyclic amine possesses preliminary inhibitory activity [1].

The IC₅₀ values of the three compounds were within the same order of magnitude, indicating that the overall structure comprising the shared core, the 2-acetamido group, the one-carbon linker, and the terminal cyclic amine is compatible with micromolar inhibitory activity. The IC₅₀ of morpholine derivative 5 was lower than those of piperidine derivative 6 and pyrrolidine derivative 7, indicating that an oxygen-containing six-membered heterocycle merits further investigation in this structural series.

Compared with Fidarestat, the IC₅₀ values of compounds 5–7 were approximately 40- to 100-fold higher. This difference indicates that the 1,8-naphthyridine series already possesses basic inhibitory activity, but its polar interactions in the catalytic region and occupation of adjacent space still require further optimization.

6 Structural Origins of the Preliminary Activity

6.1 The Rigid Heteroaromatic Ring Maintains the Basic Molecular Orientation

1,8-Naphthyridine is the common structural element of compounds 5–7. Its fused aromatic ring system has relatively low conformational freedom and fixes the relative orientations of the 2-acetamido group and the side chain at the 7-position.

The core also provides a certain aromatic surface area and may form van der Waals interactions and hydrophobic contacts with hydrophobic or aromatic residues in the ALR2 active site. The two ring nitrogen atoms increase the electronic asymmetry of the heteroaromatic ring and provide potential hydrogen-bond acceptors.

Therefore, 1,8-naphthyridine serves both spatial positioning and local polarity-regulating functions in this series.

6.2 The Acetamido Group and Ring Nitrogen Atoms Form a Basic Polar Region

The 2-acetamido group contains a carbonyl oxygen and an amide N—H, which can act as a hydrogen-bond acceptor and a hydrogen-bond donor, respectively. The ring nitrogen atoms of 1,8-naphthyridine may also participate in direct or water-mediated hydrogen-bonding interactions.

These polar atoms give the molecule the potential to interact with the catalytic region of ALR2, although their actual contributions depend on spatial distance, hydrogen-bond orientation, desolvation energy, and binding conformation. Subsequent modification of the group at the 2-position may identify a more suitable ionization state and hydrogen-bonding arrangement for this region.

6.3 The One-Carbon Linker Balances Positioning and Local Adjustment

The methylene group at the 7-position separates the naphthyridine core from the terminal ring. The one-carbon linker does not introduce an excessive number of rotatable bonds while allowing the terminal heterocycle a certain degree of conformational adjustment within the binding region.

This linking mode supports micromolar activity, indicating that the basic distance between the core and the terminal ring lies within an acceptable range. Linkers of different lengths and rigidities should still be compared to determine the geometry favorable for simultaneous binding of the two structural regions.

6.4 The Morpholine Side Chain Simultaneously Alters Multiple Molecular Properties

The morpholine ring in compound 5 contains one more ether oxygen than piperidine and pyrrolidine, thereby providing an additional hydrogen-bond acceptor. Introduction of the ether oxygen also affects:

① The basicity of the terminal tertiary amine;

② The proportion of the protonated form under the assay conditions;

③ The water solubility and solvation state of the side chain;

④ The conformation of the six-membered ring and the spatial distribution of its heteroatoms;

⑤ The desolvation penalty associated with entry into the binding region.

The relatively lower IC₅₀ of compound 5 may result from the combined effects of these factors. Further design of matched compounds with the same ring size and similar basicity but different heteroatom compositions would help distinguish the specific contributions of hydrogen bonding, protonation, and conformation.

7 Limitations of the Current Activity Data and Requirements for Subsequent Validation

7.1 Scope of the Conclusions Supported by the Current Data

The three compounds in the original study share the same 2-acetamido-1,8-naphthyridine core and the same methylene linkage at the 7-position, differing only in whether the terminal heterocycle is morpholine, piperidine, or pyrrolidine. All three compounds exhibited micromolar inhibitory activity in the crude enzyme system consisting of the supernatant of rat lens homogenates, indicating that the overall structure comprising the 1,8-naphthyridine core, the 2-acetamido group, the methylene group at the 7-position, and the terminal cyclic amine possesses preliminary inhibitory activity in this system.

However, the current data cannot determine the following:

① The independent contribution of the 2-acetamido group, the 1,8-naphthyridine core, and the linker at the 7-position to activity;

② Whether the compounds retain inhibitory activity against recombinant human AKR1B1 and their possible binding modes;

③ Whether the numerically lower IC₅₀ of the morpholine derivative results from the ether oxygen, basicity, ring conformation, protonation state, or differences in solvation;

④ Whether the compounds possess sufficient selectivity over other aldo-keto reductases and NADPH-dependent enzymes;

⑤ Whether micromolar enzyme inhibition can be translated into inhibition of the intracellular polyol pathway.

Therefore, at the current stage, this series may be regarded as a 1,8-naphthyridine-based candidate scaffold with preliminary activity. However, the specific binding mode or complete pharmacophore should not be determined directly from only three compounds.

7.2 Subsequent Evaluation Should Progress from Crude Enzyme Screening to Tiered Validation

Subsequent studies may proceed according to the following levels:

① Re-evaluate the IC₅₀ values using recombinant human AKR1B1 to confirm direct inhibition of the human target enzyme;

② Determine the inhibition constant Kᵢ and the inhibition mechanism to characterize inhibitory potency and mechanism more accurately;

③ Compare selectivity toward related aldo-keto reductase subtypes and exclude broad nonspecific inhibition;

④ Determine solubility, acid dissociation constant, lipophilicity, chemical stability, and membrane permeability to assess whether changes in activity are influenced by physicochemical properties;

⑤ Verify the actual binding mode through site-directed mutagenesis, complex crystal structures, or other experimentally supported structural approaches, while using molecular docking only to propose binding hypotheses requiring experimental validation;

⑥ Measure sorbitol accumulation and related polyol pathway indicators in high-glucose cell or tissue models to evaluate intracellular efficacy.

This evaluation sequence can progressively connect crude enzyme inhibition, activity against the human target, enzyme selectivity, binding mechanism, and intracellular effects, thereby providing a reliable basis for subsequent structure–activity relationship analysis.

8 Directions for Structural Optimization of 1,8-Naphthyridine-Based ALR2 Inhibitors

The three existing compounds only compare terminal heterocycles and are insufficient to determine the roles of the other structural regions. In subsequent studies, the molecules may be divided into four parts—the polar region at the 2-position, the linker at the 7-position, the 1,8-naphthyridine core, and the terminal side chain. Single-variable structural comparisons should first be established, followed by combinations of modifications showing favorable performance.

8.1 Optimization of the Polar Region at the 2-Position

The 2-acetamido group may participate in polar interactions in the catalytic region and may also affect the electronic distribution, solubility, and conformation of the molecule. Because this group was retained in all three compounds in the original study, its specific contribution has not been independently evaluated.

The following structural comparisons may be established:

① Deacetylate the acetamido group to generate the free amino group and evaluate the contributions of the acetyl group and the amide carbonyl;

② Retain the amide structure while varying the size, electronic properties, and substitution pattern of the acyl group;

③ Replace the acetamido group with a urea, carbamate, or sulfonamide to compare the number and orientation of hydrogen-bond donors and acceptors;

④ Introduce suitable weakly acidic carbonyl-containing structures to explore the influence of an ionizable polar region on binding to the catalytic region;

⑤ Establish control structures lacking a specific hydrogen-bond donor or acceptor to distinguish hydrogen-bonding effects from steric effects.

The polar regions of ALR2 inhibitors generally need to form appropriately oriented interactions near Tyr48, His110, and the NADP⁺ cofactor [4–6]. Therefore, optimization at the 2-position should not only compare IC₅₀ values but should also incorporate Kᵢ, pK_a, and solubility data to comprehensively assess the relationships among activity changes, inhibitory potency, ionization state, and solubility.

8.2 Optimization of the Linking Distance and Orientation at the 7-Position

The original study used one methylene group to connect the 1,8-naphthyridine core to the terminal cyclic amine:

1,8-Naphthyridine—CH₂—N (cyclic amine)

This linking mode is compatible with micromolar inhibitory activity but has not been systematically compared in terms of linking distance, flexibility, or substitution orientation.

The following linking modes may be investigated:

① Directly connect the core to the terminal nitrogen-containing group;

② Use saturated linkers containing one, two, or three carbon atoms;

③ Introduce an oxygen or nitrogen atom into the linker to regulate polarity, basicity, and conformation;

④ Use an amide or another directional linking structure;

⑤ Use an alkenyl or another conformationally restricted linker to reduce the number of rotatable bonds.

When comparing linkers, the polar group at the 2-position and the terminal heterocycle should be kept unchanged whenever possible. For amide linkages, orientations such as —CO—NH— and —NH—CO— should be evaluated separately because they have different hydrogen-bonding arrangements and dipole orientations.

8.3 Modulation of the Electronic and Spatial Properties of the 1,8-Naphthyridine Core

The 1,8-naphthyridine core provides a rigid heteroaromatic scaffold and two potential hydrogen-bond acceptors. However, current studies have not established whether the two ring nitrogen atoms directly participate in binding or which additional positions on the core can accommodate substituents.

While maintaining the principal substitution orientations at the 2- and 7-positions, the following modifications may be investigated:

① Introduce small halogen substituents, such as fluorine or chlorine, to modulate electronic properties and local spatial occupancy;

② Introduce a methyl group to compare local hydrophobicity and steric filling effects;

③ Introduce a methoxy, hydroxy, or another small polar group to alter electronic distribution and hydrogen-bonding properties;

④ Use quinoline or other naphthyridine isomers as scaffold controls to compare the effects of the number and arrangement of ring nitrogen atoms.

Changes to the core may also affect lipophilicity, solvation state, metabolic stability, and the overall binding conformation. Substitution positions may be initially guided by structural models, but their effects must ultimately be confirmed through actual synthesis, enzyme activity data, and physicochemical measurements.

8.4 Optimization of the Terminal Heterocycle and Distal Occupancy Structure

The original study compared only three protonatable saturated nitrogen-containing heterocycles: morpholine, piperidine, and pyrrolidine. The IC₅₀ of the morpholine derivative was numerically lower, but the three heterocycles differ simultaneously in heteroatom composition, ring size, basicity, conformation, and solvation. Therefore, the activity difference cannot be attributed solely to the ether oxygen in morpholine.

The following comparisons may be established:

① Compare morpholine, thiomorpholine, piperazine, and other structures with similar ring sizes but different heteroatoms;

② Compare neutral heterocycles, weakly basic heterocycles, and cyclic tertiary amines with different pK_a values;

③ Introduce a hydroxy, amide, or urea group into the side chain to evaluate the effect of an additional polar site;

④ Introduce a small phenyl or heteroaryl group to explore whether the side chain can extend into an adjacent hydrophobic binding region;

⑤ Introduce small substituents stepwise onto the distal aromatic structure to compare the effects of halogens, hydroxy groups, or other small groups on hydrophobic contacts, polarity, and molecular volume.

Expansion of the distal structure should simultaneously consider enzyme inhibitory activity, solubility, protonation state, molecular volume, and enzyme selectivity. Increasing apparent activity solely by increasing hydrophobicity should be avoided.

8.5 Stepwise Establishment of Structure–Activity Relationships

Studies of other heteroaromatic ALR2 inhibitors have shown that coordinated optimization of the polar region, aromatic scaffold, and distal substituents can significantly improve inhibitory activity [7,8]. However, different scaffolds have different binding orientations and electronic properties, and their specific substitution patterns cannot be directly transferred to the 1,8-naphthyridine series.

This series may be advanced according to the following sequence:

① Independently modify the polar region at the 2-position, the linker at the 7-position, substituents on the core, and the terminal side chain to establish single-variable comparison series;

② Combine activity against human AKR1B1 with key physicochemical properties to identify structures showing clear structure–activity trends;

③ Combine favorable modifications from different regions and determine whether their effects are synergistic, additive, or mutually offsetting;

④ Conduct tiered enzymological, selectivity, and binding-mode validation on representative compounds;

⑤ Further narrow the structural range according to the experimental results and establish reproducible structure–activity relationships for 1,8-naphthyridine-based ALR2 inhibitors.

This strategy enables separate comparison of polar anchoring, core positioning, linker geometry, and distal occupancy, followed by evaluation of their combined effects, thereby avoiding the inference of a specific pharmacophore from only a small number of compounds.

9 Classification and Research Applications of Representative Chemicals Related to Scaffold Construction, Enzyme Activity Evaluation, and Pharmacophore Optimization of 1,8-Naphthyridine-Based Aldose Reductase Inhibitors

Note: The following products are organized into four categories: the 1,8-naphthyridine core and the original synthetic route; reaction conditions, separation, and purification; aldose reductase activity and polyol pathway research; and side-chain and structural expansion and inhibitor controls. Product features and applications are described according to the experimental roles of each product within the context of this article. Except for the starting materials, reagents, and control compounds explicitly used in the original study, some products are extension, analytical, or control products listed on the basis of the research directions discussed in this article and were not necessarily used in the original study. Different products vary in structure, purity, grade, and experimental use and therefore should not be considered directly interchangeable under the same reaction or testing conditions.

Table 1. Products Related to the 1,8-Naphthyridine Core, Starting Materials Used in the Original Synthesis, and Side-Chain Introduction

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
1,8-Naphthyridine core	254-60-4	N133764	1,8-Diazanaphthalene	≥98% (GC)	Used to study the aromaticity of the 1,8-naphthyridine core, the electronic effects of the ring nitrogen atoms, basic binding characteristics, and structural differences before and after substitution.
Starting material for naphthyridine fused-ring construction	141-86-6	D106446	2,6-Diaminopyridine (DAP)	≥98%	Used in an acid-promoted fused-ring reaction with 4,4-dimethoxy-2-butanone to construct 2-amino-7-methyl-1,8-naphthyridine.
Carbonyl component for naphthyridine fused-ring construction	5436-21-5	D498461	4,4-Dimethoxy-2-butanone	≥92%	Used to provide the carbonyl and acetal-containing carbon-chain fragments required for the fused-ring reaction, forming the 7-methyl-substituted 1,8-naphthyridine ring system.
Acidic medium for promoting fused-ring formation	7664-38-2	P112028	Phosphoric acid	≥99%, crystalline	Used in the condensation, dehydration, and fused-ring formation of 2,6-diaminopyridine with the carbonyl acetal component.
Key 2-aminonaphthyridine intermediate	1568-93-0	M698289	7-Methyl-1,8-naphthyridin-2-amine	≥97%	Used for acetylation of the amino group at the 2-position, benzylic functionalization of the methyl group at the 7-position, and construction of 1,8-naphthyridine derivatives.
Reagent for acetylation of the amino group at the 2-position	108-24-7	A1506320	Acetic anhydride (controlled precursor chemical)	European Pharmacopoeia (Ph. Eur.), puriss. p.a., ISO, ACS, ≥99% (GC)	Used to acetylate 7-methyl-1,8-naphthyridin-2-amine and prepare the 2-acetamido-substituted intermediate.
Reagent for benzylic bromination at the 7-position	128-08-5	B105059	N-Bromosuccinimide (NBS)	Chemically pure (CP), ≥98% (T)	Used for benzylic bromination of the methyl group at the 7-position to prepare a bromomethyl intermediate capable of undergoing nucleophilic substitution with cyclic amines.
Reagent for introducing a morpholine side chain	110-91-8	M109058	Morpholine	Distillation grade, ≥99.5%	Used to introduce a morpholinomethyl side chain and investigate the effects of the ether oxygen, tertiary-amine basicity, solvation, and six-membered-ring conformation on inhibitory activity.
Reagent for introducing a piperidine side chain	110-89-4	P1506346	Piperidine (controlled precursor chemical)	Biotechnology grade, ≥99.5%	Used to introduce a piperidinylmethyl side chain and study the basicity, hydrophobicity, and spatial conformation of the six-membered nitrogen-containing ring.
Reagent for introducing a pyrrolidine side chain	123-75-1	P108562	Pyrrolidine	≥99%	Used to introduce a pyrrolidinylmethyl side chain and compare the volume, conformation, and side-chain positioning characteristics of the five-membered nitrogen-containing ring.
Acid scavenger for nucleophilic substitution	7087-68-5	D109322	N,N-Diisopropylethylamine	Distillation grade, ≥99.5%	Used as an acid scavenger and for adjustment of reaction conditions during nucleophilic substitution between the bromomethyl intermediate and cyclic amines.

Table 2. Products Related to Benzylic Functionalization Conditions, Reaction Solvents, Separation, and Purification

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Initiator for expanding benzylic radical bromination conditions	78-67-1	A104256	2,2′-Azobisisobutyronitrile (AIBN)	Recrystallized, ≥99%	Used to screen radical initiation conditions for N-bromosuccinimide-mediated benzylic bromination and to study functionalization of the methyl group at the 7-position.
Reaction solvent for benzylic bromination	67-66-3	C1506275	Chloroform (controlled precursor chemical)	AR, ≥99.0%	Used for the heated reflux reaction of 2-acetamido-7-methyl-1,8-naphthyridine with N-bromosuccinimide.
Reaction solvent for nucleophilic substitution with cyclic amines	123-91-1	D431640	1,4-Dioxane	Anhydrous, ≥99.8%	Used for nucleophilic substitution of the bromomethyl naphthyridine intermediate with morpholine, piperidine, or pyrrolidine.
Extraction and chromatographic elution solvent for fused-ring products	75-09-2	D1519839	Dichloromethane	Column chromatography elution grade	Used for extraction and post-reaction processing of naphthyridine fused-ring products and for chromatographic elution of intermediates and target compounds.
Washing and crystallization-assisting solvent for acetylated products	60-29-7	D1506342	Diethyl ether (controlled precursor chemical)	For HPLC, ≥99%, stabilized with ethanol	Used for washing, precipitation, crystallization treatment, and removal of nonpolar impurities from acetylated intermediates.
Solvent for candidate-compound dissolution and enzyme-assay solvent controls	64-17-5	E111991	Ethanol	Moligand™, molecular biology grade, ≥99.8%	Used to prepare candidate-inhibitor solutions, establish solvent controls in enzyme activity assays, and process biological activity samples.
Silica-based adsorption and analytical separation material	7631-86-9	S419438	Silicon dioxide	≥99.95% metals basis, particle size: 2 μm	Used for silica-based adsorption of naphthyridine intermediates and target compounds, development of normal-phase chromatographic methods, and analytical-scale separation studies.

Table 3. Products Related to Aldose Reductase Activity, Cofactors, and Polyol Pathway Research

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Basic component of the enzyme activity buffer	7558-79-4	D433613	Disodium hydrogen phosphate	Anhydrous, European Pharmacopoeia (Ph. Eur.), superior-grade reagent, suitable for analysis, ACS	Used to prepare phosphate buffer systems and regulate the pH and ionic environment of aldose reductase reaction systems.
Acidic component of the enzyme activity buffer	7558-80-7	S433620	Monobasic sodium phosphate	Anhydrous, USP	Used with disodium hydrogen phosphate to prepare phosphate buffers for enzyme activity assays, cofactor stability studies, and condition screening.
Starting substrate of the polyol pathway	50-99-7	D755713	D-(+)-Glucose	Anhydrous, UltraBio™, ≥99.5% (HPLC), sum of enantiomers	Used in polyol pathway substrate studies, high-glucose experimental models, and analysis of the conversion of glucose to sorbitol.
Polyol pathway product control	50-70-4	S104833	D-Sorbitol	AR, ≥98%	Used for qualitative and quantitative analysis of polyol pathway products, establishment of calibration curves, and evaluation of aldose reductase inhibition.
Oxidized cofactor and reaction-product control	53-59-8	N303921	β-Nicotinamide adenine dinucleotide phosphate	Moligand™, ≥90%	Used as an oxidized cofactor control and for cofactor-conversion analysis and spectroscopic and methodological studies of enzyme activity assay systems.
Reduced cofactor for aldose reductase	2646-71-1	N1510346	Reduced coenzyme II tetrasodium salt hydrate (β-NADPH tetrasodium salt hydrate)	≥99%	Used in aldose reductase-catalyzed reactions, monitoring absorbance changes at 340 nm, and evaluating the activity of candidate inhibitors.

Table 4. Products for Side-Chain Expansion, Pharmacophore Scaffolds, and Aldose Reductase Inhibitor Controls

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Diaza six-membered heterocyclic starting material for side-chain expansion	110-85-0	P755827	Piperazine	Anhydrous, UltraBio™, ≥99% (T)	Used to replace monoaza cyclic-amine side chains and investigate the effects of a second nitrogen atom, protonation state, and subsequent nitrogen substitution on activity.
N-Methylpiperazine starting material for side-chain expansion	109-01-3	M108683	1-Methylpiperazine	≥99%	Used to introduce a methylpiperazine side chain and compare the basicity, substitution state, and spatial volume of diaza heterocycles.
Sulfur-containing six-membered heterocyclic starting material for side-chain expansion	123-90-0	T111124	Thiomorpholine	≥98%	Used to investigate replacement of the oxygen atom in the morpholine side chain with sulfur and compare heteroatom polarizability, hydrophobicity, and ring conformation.
Hydroxy-containing five-membered heterocyclic starting material for side-chain expansion	40499-83-0	S161346	DL-3-Pyrrolidinol	≥97%	Used to construct hydroxy-containing pyrrolidine side chains and investigate additional hydrogen-bonding sites, side-chain polarity, and conformational effects.
Hydantoin-type cyclic imide pharmacophore scaffold	461-72-3	H100923	Hydantoin	≥98%	Used to study hydantoin-type cyclic dicarbonyl structures, hydrogen-bonding arrangements, weakly acidic characteristics, and spirohydantoin derivatives.
Sulfur-containing cyclic dicarbonyl pharmacophore scaffold	2295-31-0	T109725	2,4-Thiazolidinedione	≥99%	Used to study sulfur-containing cyclic dicarbonyl structures, functionalization at the 5-position, weakly acidic pharmacophores, and polar anchoring modes.
Carboxylic acid-type aldose reductase inhibitor control	110703-94-1	Z614985	Zopolrestat	Moligand™, ≥98%	Used to compare carboxylate-type polar anchoring, occupancy of heteroaromatic regions, enzyme inhibitory activity, and structure–activity relationships.
Carboxylic acid-type aldose reductase inhibitor control	82964-04-3	T304733	Tolrestat (AY-27773)	Moligand™, ≥98%	Used to compare the aldose reductase inhibitory activity associated with the carboxylic acid group, the hydrophobic naphthalene scaffold, and the sulfur-containing linker structure.
Spirohydantoin-type aldose reductase inhibitor control	68367-52-2	S613657	Sorbinil	Moligand™, ≥98%	Used to study spirohydantoin dicarbonyl anchoring, rigid scaffolds, stereostructure, and aldose reductase inhibitory activity.
Carboxylic acid–sulfur-containing heterocycle-type aldose reductase inhibitor control	82159-09-9	E129897	Epalrestat	Moligand™, ≥97%	Used to compare the combined structure of a carboxylic acid and sulfur-containing heterocycle, inhibition of the polyol pathway, and different pharmacophore types.
Positive control used in the original study (spirohydantoin type)	136087-85-9	F651787	Fidarestat	≥99%	Used as the activity reference in the original study and for research on the spirohydantoin scaffold, the role of the carbamoyl group, and hydrophilic–hydrophobic binding modes.
Spirocyclic imide-type aldose reductase inhibitor control	147254-64-6	R651726	Ranirestat	≥96%	Used to compare the spirocyclic imide scaffold, stereochemistry, aromatic side chain, and aldose reductase inhibitory activity.
Carboxylic acid-type aldose reductase inhibitor control	72702-95-5	S343067	Ponalrestat	≥95%	Used to study carboxylate anchoring, the phthalazinone scaffold, halogenated aromatic side chains, and enzyme inhibitory activity.

Note: The products listed above are representative Aladdin research products. Additional product specifications, grades, and certificate of analysis information can be retrieved from the Aladdin website using the product name, CAS number, or catalog number.

References

[1] DU R, ZHU C J. Synthesis of naphthyridine derivatives and their aldose reductase inhibitory activity[EB/OL]. Renren Wenku, Document No. 209769538.

[2] KADOR P F, KINOSHITA J H, SHARPLESS N E. Aldose reductase inhibitors: a potential new class of agents for the pharmacological control of certain diabetic complications[J]. Journal of Medicinal Chemistry, 1985, 28(7): 841-849. DOI: 10.1021/jm00145a001.

[3] SINGH M, KAPOOR A, BHATNAGAR A. Physiological and pathological roles of aldose reductase[J]. Metabolites, 2021, 11(10): 655. DOI: 10.3390/metabo11100655.

[4] BOHREN K M, GRIMSHAW C E, LAI C J, et al. Tyrosine-48 is the proton donor and histidine-110 directs substrate stereochemical selectivity in the reduction reaction of human aldose reductase: enzyme kinetics and crystal structure of the Y48H mutant enzyme[J]. Biochemistry, 1994, 33(8): 2021-2032. DOI: 10.1021/bi00174a007.

[5] EHRIG T, BOHREN K M, PRENDERGAST F G, GABBAY K H. Mechanism of aldose reductase inhibition: binding of NADP⁺/NADPH and alrestatin-like inhibitors[J]. Biochemistry, 1994, 33(23): 7157-7165. DOI: 10.1021/bi00189a019.

[6] OKA M, MATSUMOTO Y, SUGIYAMA S, TSURUTA N, MATSUSHIMA M. A potent aldose reductase inhibitor, (2S,4S)-6-fluoro-2′,5′-dioxospiro[chroman-4,4′-imidazolidine]-2-carboxamide (Fidarestat): its absolute configuration and interactions with the aldose reductase by X-ray crystallography[J]. Journal of Medicinal Chemistry, 2000, 43(12): 2479-2483. DOI: 10.1021/jm990502r.

[7] MYLARI B L, ARMENTO S J, BEEBE D A, et al. A novel series of non-carboxylic acid, non-hydantoin inhibitors of aldose reductase with potent oral activity in diabetic rat models: 6-(5-chloro-3-methylbenzofuran-2-sulfonyl)-2H-pyridazin-3-one and congeners[J]. Journal of Medicinal Chemistry, 2005, 48(20): 6326-6339. DOI: 10.1021/jm050462t.

[8] MA B, FAN Z Y, WANG X, LI C Y, ZHU C J. Design, synthesis, and structure–activity relationship study of aldose reductase inhibitors based on a quinoxalinone scaffold[J]. Transactions of Beijing Institute of Technology (Natural Science Edition), 2021, 41(4): 445-450. DOI: 10.15918/j.tbit1001-0645.2020.006.

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