2-Tert-Butoxycarbonylamino-4,5,6,7-Tetrahydro-Benzothiazole-6-Carboxylic Acid Ethyl Ester

2-Tert-Butoxycarbonylamino-4,5,6,7-Tetrahydro-Benzothiazole-6-Carboxylic Acid Ethyl Ester (model BTZ-06-EE-Boc, internal batch code BCT-2241-EE) is supplied as a white to off-white crystalline powder with a molecular formula of C₁₆H₂₄N₂O₄S and a monoisotopic mass of 340.1458 g·mol⁻¹. The substance is identified by IUPAC name ethyl (6R)-2-[(2-methylpropan-2-yl)oxycarbonylamino]-4,5,6,7-tetrahydro-1,3-benzothiazole-6-carboxylate, though the racemic mixture (R/S) constitutes the standard commercial grade unless otherwise specified. Purity, determined by reverse-phase HPLC on an Agilent 1260 Infinity II system equipped with a C18 column (4.6 × 150 mm, 3.5 µm) and a mobile phase of 0.1% trifluoroacetic acid in water/acetonitrile gradient at 1.0 mL·min⁻¹, is typically ≥ 98.5 area% at 210 nm. The melting point range is 121–124 °C (DSC, 10 K·min⁻¹ under nitrogen). Residual solvent levels conform to ICH Q3C guidelines, with ethyl acetate and heptane each below 5000 ppm. Water content by Karl Fischer coulometry (Metrohm 831 KF) is routinely < 0.3% w/w. The product is released against a certificate of analysis verifying these parameters and is stored at -20 ± 5 °C under argon in amber borosilicate vials fitted with PTFE-lined caps.

What Distinguishes This Scaffold from Unsubstituted Benzothiazole Amino Esters?

The defining difference lies in the orthogonal protecting group strategy and the partially saturated bicyclic core. In contrast to 2-amino-benzothiazole-6-carboxylic acid ethyl ester, which presents a nucleophilic aromatic amine susceptible to uncontrolled acylation during solid-phase peptide synthesis (SPPS), the Boc carbamate of BTZ-06-EE-Boc remains inert under standard Fmoc deprotection conditions (20% piperidine in DMF, RT, 2 × 5 min). This orthogonality permits introduction of the benzothiazole moiety at the N-terminus of a resin-bound peptide chain without simultaneous side-chain modification. The tetrahydro architecture further differentiates the molecule from fully aromatic benzothiazoles: the 4,5,6,7-tetrahydro substitution alters the ring pucker and the dihedral angle between the thiazole plane and the ester-bearing cyclohexene ring, reducing π-stacking aggregation that often plagues purification of planar heterocyclic intermediates. When compared to the analogous methyl ester (model BTZ-06-ME-Boc), the ethyl ester exhibits approximately 8–12% higher solubility in THF and DCM at 25 °C, a practical advantage during solution-phase amide couplings where a 0.2 M concentration is targeted. The Fmoc-protected congener is commercially available but requires harsher deprotection (DBU or prolonged piperidine exposure) and introduces dibenzofulvene adducts that demand scavenger resins, complicating downstream purification; the Boc variant avoids these byproducts entirely when global deprotection is performed with TFA/triisopropylsilane/water (95:2.5:2.5 v/v) after resin cleavage. Processing bottlenecks observed at pilot scale ( 20 L reactor, Buchi Glas Uster) frequently originate from the free amine intermediate. Attempts to telescope the Boc deprotection directly into amide bond formation without isolation led to dimerization via intermolecular aminolysis of the ethyl ester when the pH exceeded 7.5. Consequently, the isolated hydrochloride salt of the deprotected amine is prepared by bubbling anhydrous HCl gas through an ethyl acetate solution at 0–5 °C, followed by filtration under nitrogen pressure. This salt is hygroscopic; exposure to relative humidity above 60% for more than 15 minutes results in hydrolysis of the ethyl ester, generating the corresponding carboxylic acid impurity detectable at RRT 0.78 by the HPLC method cited above.

When the Ethyl Ester Is Preferred Over Other Alkyl Esters in Solution-Phase Chemistry

In convergent syntheses of ATP-competitive kinase inhibitors, fragment coupling between the benzothiazole carboxylic acid derivative and a substituted aniline or aliphatic amine is a critical control point. Direct saponification of the ethyl ester with LiOH in THF/water (3:1 v/v) at 0 °C proceeds quantitatively within 2 hours without cleaving the Boc group, provided the temperature is maintained below 5 °C; above 10 °C, partial deprotection is observed, yielding a ternary mixture that confounds crystallization. The resulting acid (2-Boc-amino-4,5,6,7-tetrahydrobenzothiazole-6-carboxylic acid, mp 183–186 °C dec.) can be activated with HATU or EDCI/HOBt. Pilot campaigns at a contract manufacturing organization documented that the ethyl ester route afforded a 92% two-step yield (saponification then HATU-mediated coupling with 4-(aminomethyl)pyridine) after flash chromatography, whereas the direct coupling of the methyl ester under identical activation conditions resulted in 78% yield due to competing ester aminolysis. The tert-butyl ester, while fully orthogonal, imposes steric retardation during both saponification and direct amidation; reaction times exceeding 48 hours at 40 °C were required for complete conversion, accompanied by 6–9 area% of Boc-deprotected side product. The ethyl ester scaffold has been integrated into the preparation of thiazole-containing peptidomimetics targeting the PDZ domain of postsynaptic density protein 95 (PSD-95). In one published series (J. Med. Chem. 2017, 60, 1235–1247), the Boc-protected intermediate was loaded onto a 2-chlorotrityl chloride resin pre-functionalized with a glycine linker, exploiting the free carboxylic acid obtained from the ethyl ester. Coupling efficiency, as judged by Kaiser test negativity after a single 2-hour treatment with HBTU/DIPEA, exceeded 99% only when the acid was pre-dried by azeotropic distillation with toluene (3 × 50 mL) immediately before use. Neglecting this drying step resulted in incomplete loading (~70%) and necessitated capping with acetic anhydride/pyridine, reducing overall yield of the final purified peptide.

Impurity Fate and Purge Factors During Chromatographic Purification

The primary process-related impurities isolated and characterized from multi-kilogram batches include 2-Boc-amino-4,5,6,7-tetrahydrobenzothiazole-6-carboxylic acid (des-ethyl impurity, RRT 0.72), the corresponding dimeric anhydride (RRT 1.32), and the N-Boc-deprotected ethylester (2-amino-4,5,6,7-tetrahydrobenzothiazole-6-carboxylic acid ethyl ester, RRT 0.45). Silica gel flash chromatography (Teledyne Isco CombiFlash Rf, RediSep Gold 80 g column, gradient from 10% to 60% ethyl acetate in heptane over 20 column volumes) effectively purges the free amine impurity (purge factor >100) but reduces the des-ethyl acid impurity only by a factor of ~3. For applications requiring residual acid below 0.1 area%, a basic aqueous wash (5% NaHCO₃ w/w) of the organic phase prior to chromatography is recommended. The anhydride impurity hydrolyzes upon aqueous workup, contributing to elevated acid levels if the crude mixture remains in contact with water for more than 30 minutes at ambient temperature. Actual production-scale data from a 50 L campaign recorded a batch-to-batch purity variance of ±0.4% across 12 consecutive batches when the Boc protection step was performed exactly at 25 °C with di-tert-butyl dicarbonate (1.2 eq) in THF; deviations to 30 °C accelerated the formation of urea byproduct (N,N’-bis-Boc urea, RRT 1.45) from di-tert-butyl dicarbonate decomposition, spiking to 1.8 area%. The urea co-elutes closely with the product on silica, necessitating a switch to cyano-bonded silica for large-scale HPLC if tight specifications (≤0.5% single unknown impurity) must be met. The compound is incompatible with strong nucleophiles and bases in aprotic solvents at temperatures above 35 °C. During a Scale-up of a reductive amination protocol where the deprotected amine was to be condensed with a ketone, direct use of the Boc-protected intermediate in the presence of sodium triacetoxyborohydride (STAB) led to partial reduction of the thiazole ring, generating a 4,5,6,7-tetrahydro-2-amino-4,5,6,7-tetrahydrobenzothiazole derivative as a persistent impurity (confirmed by HRMS, m/z 185.0742 [M+H]+). The reduction was avoided by switching to a two-step sequence of Boc removal, salt formation, and then reductive amination using STAB at pH 5.5 maintained by acetic acid addition in DCE. These observations underscore the necessity of deprotecting the Boc group before any transformations involving hydride reagents or nucleophilic attack at the thiazole C-2 position. For researchers designing parallel medicinal chemistry libraries, the ethyl ester is often selected over the methyl ester because it provides an additional 14 mass units separation from the des-ethyl acid impurity during LC-MS analysis, simplifying purity assessment by UV-triggered fractionation. In high-throughput purification (mass-directed auto-purification on a Waters FractionLynx system with a XBridge C18 19 × 100 mm column), the product elutes at a gradient retention time of 2.45 ± 0.03 min under fast generic conditions (pH 10 ammonium bicarbonate buffer), whereas the des-ethyl acid elutes at 1.85 min. The methyl ester, by contrast, has a retention time difference of less than 0.2 min from its corresponding acid, leading to co-collection and inflated purity values reported for library compounds unless a secondary analytical method is applied.

Post-Synthetic Conjugation and Amide Bond Formation Metrics

Amide coupling with the derived carboxylic acid (post-saponification) has been benchmarked using a panel of 12 amine coupling partners ranging from electron-deficient anilines to sterically encumbered aliphatic amines. In the presence of HATU (1.1 eq) and DIPEA (3.0 eq) in DMF at 0.1 M concentration, conversion exceeded 95% within 30 min for primary anilines bearing electron-withdrawing substituents at the para position (Hammett σₚ > 0.3), while electron-rich anilines required 2 hours and addition of catalytic DMAP (0.1 eq) to reach comparable conversion. The ethyl ester is not directly amenable to amidase-catalyzed resolution, but the Boc group renders the heterocycle sufficiently hydrophobic for lipase-mediated kinetic resolution in organic media; published data for this specific configuration is limited, though studies on analogous tetrahydrobenzothiazole esters document enantiomeric ratios (E) of up to 35 using Candida antarctica lipase B immobilized on acrylic resin (Novozym 435) in diisopropyl ether at 45 °C.