2-((Tert-Butoxycarbonyl)Amino)-4,5,6,7-Tetrahydrobenzo[D]Thiazole-6-Carboxylic Acid

The compound 2-((tert-butoxycarbonyl)amino)-4,5,6,7-tetrahydrobenzo[d]thiazole-6-carboxylic acid (C₁₃H₁₈N₂O₄S, exact mass 298.10 Da) functions as a conformationally constrained, Boc-protected amino acid surrogate in modular target synthesis. Its core structure consists of a thiazole ring fused to a fully saturated cyclohexane, with the carboxylic acid group positioned at the 6-carbon of the carbocycle. This scaffold introduces a non-planar, sp³-rich topology while retaining the hydrogen-bond-accepting properties of the thiazole nitrogen and the protected amine at position 2. As a racemic building block, it presents both opportunities and challenges in stereochemical control during peptide coupling and fragment-based library construction. The Boc carbamate imparts orthogonality in Fmoc-based solid-phase peptide synthesis (SPPS), allowing selective deprotection under acidic conditions without cleavage of the backbone-protecting group. Its rigid bicyclic architecture reduces the entropic penalty of binding in protein–ligand interactions relative to flexible aliphatic amino acids, and the saturation of the benzo ring mitigates the cytochrome P450-mediated oxidation typically observed with flat, aromatic heterocycles.

Why Does the Tetrahydrobenzo[d]thiazole Core Enhance Metabolic Stability?

A persistent liability of 2-aminothiazole and benzothiazole-containing pharmacophores is rapid oxidative metabolism at electron-rich positions on the aromatic system, frequently documented in human liver microsome (HLM) assays (test method: standard NADPH-fortified HLM incubation at 1 µM substrate, 37 °C). Substitution of the planar benzothiazole with the 4,5,6,7-tetrahydro analogue introduces conformational flexibility and increases the fraction of sp³ carbon, factors that disrupt π–π stacking with heme iron and reduce affinity for CYP3A4 and CYP2D6 isoforms. In comparative intrinsic clearance measurements performed using pooled HLM, the tetrahydrobenzo[d]thiazole-6-carboxylic acid scaffold exhibited a greater than **2.5-fold** prolongation of in vitro half-life compared to the aromatic benzothiazole-6-carboxylic acid equivalent under identical incubation conditions. This improvement mirrors observations in saturated heterocycle series where the absence of extended conjugation lowers the oxidation potential measured via cyclic voltammetry (shift of > +**0.3 V** vs. Ag/AgCl). Additionally, the cyclohexane ring adopts multiple chair conformations at ambient temperature, which can modulate solvent exposure of the thiazole sulfur—a potential site for S-oxidation—though published data for this specific molecular configuration is limited. In permeability assessments using parallel artificial membrane permeability assay (PAMPA) at pH 7.4, the effective permeability log Pe remained above **−5.8 cm/s**, indicating acceptable passive diffusion despite the addition of a saturated ring. When integrated into automated microwave-assisted synthesis, the Boc-protected acid requires activation with a combination of HATU (**3.9 equivalents** relative to resin loading) and DIPEA (**6.0 equivalents**) in anhydrous DMF at a concentration of **0.4 M**. A double-coupling protocol on a Liberty Blue™ synthesizer (CEM Corporation), with each cycle lasting **5 minutes** at **75 °C** and a pre-activation period of **45 seconds**, consistently delivers crude peptide purities exceeding **87%** by UPLC at **220 nm** for sequences containing a sterically hindered amine incoming nucleophile. Residual HATU-related tetramethylguanidinium byproducts are removed by a DMF wash (3 × 2 mL) followed by a **20%** piperidine/DMF deprotection step for Fmoc removal, leaving the Boc group intact. When the target peptide contains an N-terminal secondary amine, diketopiperazine formation is suppressed by immediate coupling after deprotection without a prolonged post-piperidine hold step—delays beyond **30 minutes** at **25 °C** increase the side-product yield to above **8%**.

Analytical and Storage Specifications

Table 1 consolidates the release criteria and analytical test methods applied to each manufactured batch. All values are verified against primary reference standards traceable to the manufacturer’s internal master lot, with acceptance limits derived from a minimum of **15** consecutive production campaigns processed in a GMP-compliant facility (ISO 9001:2015).

Parameter	Specification	Test Method
Appearance	White to off-white crystalline powder	Visual inspection against USP Reference Standard <631>
Purity (HPLC area %)	≥ 98.0%	RP-HPLC, C18 column (150 × 4.6 mm, 3 µm), gradient: 5–95% acetonitrile in water + 0.1% TFA over 20 min, detection at 210 nm, flow rate 1.0 mL/min
Moisture content	≤ 0.5% (w/w)	Karl Fischer coulometric titration (Metrohm 831 KF coulometer), sample dissolved in anhydrous methanol
Residual solvents (DCM)	≤ 600 ppm	Headspace GC-FID, Restek Rtx-624 column, oven 40 °C (5 min) to 240 °C at 20 °C/min
Storage temperature	−20 °C ± 5 °C, desiccated	Continuous monitoring with data-logging thermocouple; protect from moisture with silica gel desiccant pack
Solubility (DMF)	> 50 mg/mL	Gravimetric determination after 30 min sonication at 25 °C, filtered through 0.22 µm PVDF

A 50% (v/v) trifluoroacetic acid solution in dichloromethane containing **2.5%** triisopropylsilane as scavenger removes the Boc group within **2 hours** at room temperature. Deprotection progress is monitored by withdrawing aliquots, evaporating the volatile acids under a nitrogen stream, resuspending in deuterated DMSO, and integrating the tert-butyl singlet at **1.38 ppm** in ¹H NMR (Bruker Avance III HD **400 MHz**, 64 scans). Complete disappearance of this resonance confirms quantitative release of the free amine. When residual TFA salts interfere with subsequent coupling steps, a short DIEA wash cycle (5% DIEA in DCM, 3 × **1 min**) on the resin restores the free amine, reducing salt-related suppression of acylation kinetics.

When Orthogonal Boc/Fmoc Protection is Required in Macrocycle Engineering

Macrocyclic peptides often demand orthogonal protection schemes to permit sequential ring-closing metathesis or lactamization while preserving side-chain functional groups. The Boc-protected tetrahydrobenzo[d]thiazole acid serves a particular niche here: its acid-labile Boc group is cleaved with TFA before global cleavage of the peptide from the resin, exposing a nucleophilic amine that can participate in on-resin cyclization or be capped with a reporter tag, while Fmoc groups remain intact until the final elongation step. By contrast, the commercially available Fmoc-2-((tert-butoxycarbonyl)amino)-4,5,6,7-tetrahydrobenzo[d]thiazole-6-carboxylic acid—where the Boc group remains as a permanent protecting group until final TFA cleavage—forces a different sequence of deprotection if the thiazole amine must be derivatized early. In scaffold-hopping campaigns where the tetrahydrobenzothiazole replaces a phenylalanine or a tetrahydroisoquinoline-3-carboxylic acid residue, the 6-carboxy substituent provides a carboxylate handle for amide bond formation with a spatial orientation that differs from the 2-aminothiazole-4-carboxylic acid series: the sp³ carbon at position 6 creates a projected vector that deviates by approximately **30°** from the plane of the thiazole ring, impacting macrocycle ring strain and conformational pre-organization.

Building Block	Chiral Center	Acid-Labile Group	Ring Saturation	Solubility in DMF (mg/mL, 25 °C)	Relative HLm t½ (vs aromatic analogue)
Boc-2-amino-4,5,6,7-tetrahydrobenzo[d]thiazole-6-COOH (this compound)	Racemic C6	Boc	Full (cyclohexane)	> 50	~2.5× increase
Boc-2-aminothiazole-4-COOH	None	Boc	None (aromatic)	> 100	Baseline (1.0×)
Fmoc-2-amino-4,5,6,7-tetrahydrobenzo[d]thiazole-6-COOH	Racemic C6	Fmoc	Full (cyclohexane)	~45	~2.5× increase
Fmoc-2-aminothiazole-4-COOH	None	Fmoc	None (aromatic)	> 100	Baseline (1.0×)

The racemic nature of the 6-carboxy derivative introduces a composition of two enantiomers that, in the absence of chiral resolution, complicates the interpretation of biological activity and X-ray crystallography electron density. Diastereomeric peptide sequences synthesized from the racemic building block exhibit split peaks on RP-HPLC when coupled to a chiral amine downstream, with baseline resolution (Rs > **1.5**) achievable on a Chiralpak IA-3 column using a heptane/ethanol/diethylamine mobile phase. During off-resin chiral separation of the final peptide, the late-eluting diastereomer often shows an **80–90%** reduction in IC₅₀ in biochemical assays, illustrating the necessity for single-enantiomer supply or on-resin diastereomer separation. Coupling efficiency in manual solid-phase synthesis with DIC/HOBt activation (each **8 equivalents**, 0–5 °C pre-activation in DMF) averages **78%** after a single **4 h** coupling at room temperature, as quantified by the Fmoc-release UV absorbance at **301 nm** (ε = 7800 M⁻¹cm⁻¹). For difficult sequences bearing a β-branched amino acid at the N-terminus of the incoming resin-bound amine, the double incorporation rate with PyBroP (**5 equivalents**) and DIEA (**10 equivalents**) in DCM/DMF (**1:1**) increases the single-step conversion to > **91%**, monitored by LC-MS (ESI positive mode, [M+H]⁺ for the Fmoc-deprotected intermediate).

Addressing Batch-to-Batch Variability in Solid-Phase Peptide Synthesis

Process robustness across multiple synthesis campaigns requires monitoring of residual ammonium salts remaining from incomplete scavenging of the Boc deprotection cocktail, as these can form ion pairs with the free carboxylic acid of the building block and impede activation. When switching to a new lot of 2-((tert-butoxycarbonyl)amino)-4,5,6,7-tetrahydrobenzo[d]thiazole-6-carboxylic acid, a test coupling on a model resin-bound amine (Leu-Wang, loading **0.6 mmol/g**) is recommended, followed by cleaved peptide analysis. Drift in crude purity exceeding **±3%** from the validated mean of **87%** triggers a re-examination of moisture content, as water uptake above **0.8%** (determined by Karl Fischer) leads to hydrolysis of the activated ester before nucleophilic attack. Exposure of the lyophilized powder to ambient relative humidity > **60%** for more than **4 hours** during weighing increases moisture above this threshold; therefore, aliquoting under dry nitrogen in a glovebox (O₂ < **10 ppm**, H₂O < **5 ppm**) is enforced for large library syntheses exceeding **96-well** format. Incompatibilities include prolonged contact with amine bases without immediate coupling—the free-acid form is stable in DIPEA/DMF solutions for less than **45 minutes** at **25 °C**, after which HPLC shows a **4–6%** decomposition to the corresponding diketopiperazine dimer.