3-Chlorothieno[2,3-b]pyridine-2-carboxylic Acid: Use Protocol

3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid is a halogenated bicyclic heteroaromatic carboxylic acid that serves as a versatile synthetic building block for the preparation of thienopyridine-based pharmaceutical intermediates and functionalised heterocyclic scaffolds.

Chemical Name

3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid

SKU

C195942

Molecular Formula

C8H4ClNO2S

Molecular Weight

213.64 g/mol

Core Scaffold

Thieno[2,3-b]pyridine fused bicyclic system

Key Functional Groups

Chloro substituent at C-3; carboxylic acid at C-2

Primary Application

Heterocyclic synthesis, pharmaceutical intermediate preparation

Overview and Chemical Properties

3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid (Aladdin SKU C195942; molecular formula C₈H₄ClNO₂S; MW 213.64 g/mol) is a fused bicyclic heteroaromatic compound in which a thiophene ring is annulated to a pyridine ring in the [2,3-b] fusion pattern. A chlorine atom occupies the C-3 position of the thiophene ring, and a carboxylic acid group is located at C-2, creating a multifunctional handle for downstream chemical elaboration.

The compound belongs to the thienopyridine class of heterocycles, which are well recognised as privileged scaffolds in medicinal chemistry owing to their drug-like physicochemical profiles and capacity for hydrogen-bond donation and acceptance. The coplanar bicyclic aromatic system confers moderate lipophilicity while the carboxylic acid and C–Cl bond provide orthogonal reactive sites for amide coupling and transition-metal-catalysed cross-coupling reactions respectively.

Appearance: Solid (exact colour should be confirmed on certificate of analysis).
Recommended storage: Store in a tightly sealed container at room temperature (15–25 °C) away from moisture and light unless otherwise stated on the product label.

Safety and Handling

Handle 3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid in accordance with standard laboratory safety practices for fine chemical reagents. Consult the Safety Data Sheet (SDS) supplied by Aladdin Scientific before use.

• Personal Protective Equipment (PPE): Wear appropriate gloves (nitrile recommended), safety glasses or goggles, and a laboratory coat. Work in a well-ventilated area or fume hood when weighing or dissolving the compound.
• Skin and eye contact: In case of skin or eye contact, rinse immediately with copious water for at least 15 minutes and seek medical attention as appropriate.
• Inhalation: Avoid generating dust. If inhalation of dust or aerosol occurs, move to fresh air.
• Disposal: Dispose of waste material in accordance with local, regional and national regulations governing chemical waste.

Note: Specific GHS hazard classifications and first-aid measures are detailed on the product SDS. Always refer to the current SDS before use.

Solubility and Solution Preparation

The carboxylic acid group renders 3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid sparingly soluble in water at neutral pH. Solubility is significantly enhanced in organic solvents and under basic aqueous conditions.

Recommended solvents:

• DMSO: Good solubility; prepare stock solutions at 10–50 mM for biochemical assays. Warm gently if needed and vortex to ensure complete dissolution.
• DMF or NMP: Suitable for synthetic applications requiring higher concentrations.
• Dilute aqueous base (e.g., 0.1 M NaOH or K₂CO₃ solution): Deprotonates the carboxylate, greatly increasing aqueous solubility; useful for preparing sodium or potassium carboxylate salts in situ.
• Methanol or ethanol: Moderate solubility; suitable for recrystallisation or LC-MS sample preparation.

Protocol for stock solution preparation (DMSO, 10 mM, 1 mL):

1. Weigh 2.14 mg of 3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid (MW 213.64 g/mol) into a clean 1.5 mL microcentrifuge tube.
2. Add 1.00 mL of anhydrous DMSO.
3. Vortex for 30 seconds; if necessary, sonicate in a water bath for up to 5 minutes.
4. Confirm complete dissolution by visual inspection; the solution should be clear.
5. Aliquot into single-use volumes, seal under inert gas (N₂ or Ar) if air-sensitive downstream use is anticipated, and store at −20 °C.

Exact solubility values have not been confirmed in the source material; the above guidance is based on general physicochemical principles for halogenated heteroaromatic carboxylic acids and should be experimentally verified.

Synthetic Applications and Reaction Protocols

3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid offers two principal reactive handles for chemical synthesis: (1) the carboxylic acid at C-2, amenable to standard amide-bond-forming reactions; and (2) the C-3 chloro substituent, suitable for nucleophilic aromatic substitution (SNAr) or transition-metal-catalysed cross-coupling reactions.

A. Amide Coupling via the Carboxylic Acid

Standard peptide/amide coupling conditions are applicable:

1. Dissolve the carboxylic acid (1.0 equiv.) and amine coupling partner (1.0–1.2 equiv.) in anhydrous DMF or DCM under N₂.
2. Add a coupling reagent such as HATU or EDC·HCl (1.1–1.2 equiv.) together with a base (e.g., DIPEA or Et₃N, 2.0–3.0 equiv.).
3. Stir at room temperature for 2–16 hours; monitor by TLC or LC-MS.
4. Work up by dilution with ethyl acetate, wash with aqueous citric acid, saturated NaHCO₃, and brine. Dry over MgSO₄, filter, and concentrate.
5. Purify by column chromatography or recrystallisation.

B. Nucleophilic Aromatic Substitution at C-3

The electron-deficient bicyclic ring system activates the C-3 chloro group toward SNAr with nitrogen or oxygen nucleophiles:

1. Combine the substrate (1.0 equiv.) with the nucleophile (1.5–3.0 equiv.) in DMSO or NMP.
2. Add an inorganic base (K₂CO₃ or Cs₂CO₃, 2.0 equiv.) if required to deprotonate the nucleophile.
3. Heat at 80–120 °C for 4–24 hours under an inert atmosphere; monitor by LC-MS.
4. Cool, dilute with water, extract with ethyl acetate, and purify.

C. Palladium-Catalysed Cross-Coupling at C-3

The aryl chloride can participate in Suzuki–Miyaura or Buchwald–Hartwig reactions:

1. Combine substrate (1.0 equiv.), boronic acid or amine partner (1.2–1.5 equiv.), Pd catalyst (e.g., Pd(PPh₃)₄ or Pd₂(dba)₃/ligand, 2–5 mol%), and base (K₂CO₃ or Cs₂CO₃, 2.0–3.0 equiv.) in dioxane/water (4:1) or toluene.
2. Degas the mixture by sparging with N₂ or Ar for 10 minutes.
3. Heat at 80–100 °C for 12–24 hours under inert atmosphere.
4. Cool, filter through Celite, concentrate, and purify by column chromatography.

Specific reaction conditions, yields, and selectivity data for this exact compound are not available in the source material and should be optimised empirically. The protocols above represent standard best-practice starting points for analogous heteroaryl chloride carboxylic acids.

Quality Control and Analytical Characterisation

Upon receipt of 3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid (C195942), the following analytical methods are recommended to confirm identity and purity before use in critical experiments.

• HPLC purity: Reversed-phase HPLC (C18 column; acetonitrile/water with 0.1% TFA gradient; UV detection at 254 nm or 280 nm) is the primary method for assessing purity. Target purity ≥95% for synthetic applications; ≥98% for biological assays.
• LC-MS: Confirm the molecular ion [M+H]⁺ at m/z 214.6 (monoisotopic mass for C₈H₄ClNO₂S; note the characteristic chlorine isotope pattern, M and M+2 in approximately 3:1 ratio).
• ¹H NMR (400 MHz, DMSO-d₆): Expect signals consistent with the pyridine ring protons and the acidic carboxylic OH; absence of extraneous peaks confirms purity. Exact chemical shifts should be verified against the certificate of analysis (CoA).
• Melting point: Determine and compare against the value stated on the CoA as a rapid identity check.
• Karl Fischer titration: Recommended if moisture content is critical for the intended application.

All analytical data should be archived alongside the batch CoA for regulatory traceability.

Frequently asked questions

What solvents are recommended for dissolving 3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid?

DMSO and DMF are the preferred solvents for most laboratory applications, providing good solubility at working concentrations. Dilute aqueous base (e.g., 0.1 M NaOH) can be used to prepare aqueous carboxylate solutions; methanol or ethanol are suitable for recrystallisation. Water alone at neutral pH is not recommended due to limited solubility.

What are the main reactive sites of 3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid and how can they be exploited in synthesis?

The compound has two principal reactive sites: the C-2 carboxylic acid group and the C-3 chloro substituent. The carboxylic acid can be converted to amides, esters, or acyl halides using standard functional-group transformations. The C-3 chloro group is activated toward nucleophilic aromatic substitution by the electron-deficient bicyclic system, and can also undergo palladium-catalysed Suzuki–Miyaura or Buchwald–Hartwig cross-coupling reactions, making this compound a highly versatile building block.

How should 3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid (C195942) be stored to maintain stability?

Store the solid reagent in a tightly sealed container at room temperature (15–25 °C), protected from moisture and light. DMSO stock solutions should be aliquoted and stored at −20 °C under an inert atmosphere where possible to minimise freeze–thaw cycles and oxidative degradation. Always consult the product label and current SDS for the most up-to-date storage recommendations.

How is the molecular weight of 3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid used to calculate the amount needed for a 10 mM solution?

Using the molecular weight of 213.64 g/mol, to prepare 1 mL of a 10 mM solution you need 0.01 mmol, which equals 0.01 × 213.64 mg = 2.136 mg. Weigh this amount accurately, dissolve in 1 mL of the chosen solvent (e.g., DMSO), and vortex to ensure complete dissolution. Scale proportionally for other volumes or concentrations.

References

1. Joule, J. A.; Mills, K. Heterocyclic Chemistry, 5th ed.; Wiley-Blackwell: Oxford, 2010. General reference for thienopyridine synthesis and reactivity.
2. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed.; Oxford University Press: Oxford, 2012. General reference for amide coupling and cross-coupling reaction protocols.
3. Aladdin Scientific. Product Information Sheet and Certificate of Analysis for 3-Chlorothieno[2,3-b]pyridine-2-carboxylic acid, SKU C195942.

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