Enzymatic Kinetic Resolution of tert-Butyl 2-(1-Hydroxyethyl)phenylcarbamate, A Key Intermediate to Chiral Organoselenanes and Organotelluranes

The enzymatic kinetic resolution of tert-butyl 2-(1-hydroxyethyl)phenylcarbamate via lipase-catalyzed transesterification reaction was studied. We investigated several reaction conditions and the carbamate was resolved by Candida antarctica lipase B (CAL-B), leading to the optically pure (R)- and (S)-enantiomers. The enzymatic process showed excellent enantioselectivity (E > 200). (R)- and (S)-tert-butyl 2-(1-hydroxyethyl)phenylcarbamate were easily transformed into the corresponding (R)- and (S)-1-(2-aminophenyl)ethanols.


Results and Discussion
As outlined in Scheme 2, chiral building blocks (R)-3 and (S)-3 could be synthesized from commercially available 1-(2-aminophenyl)ethanone (1). Initially, the amine protection leads to the N-Boc-protected arylketone 2, which by reduction of the ketone group affords (R,S)-3. Then, the latter could be submitted to an enzymatic kinetic resolution (EKR) and, at the end of the process, both enantiomers could be easily separated.

Enzymatic Kinetic Resolution of the (R,S)-tert-butyl 2-(1-Hydroxyethyl)phenylcarbamate
Among the different type of lipases that were used as biocatalysts in the transesterification reaction, CAL-B presented high values of both conversion and enantioselectivity (Entries 1-3). After 12 h CAL-B-catalyzed reaction showed 47% conversion, high enantiomeric excess (ee) for (S)-3 (88%) and (R)-4 (>99%), and an enantiomeric ratio (E) higher than 200 (Entry 1). After 24 h, the conversion increased to 50% and both (S)-3 and (R)-4 were obtained with ee > 99% (Entry 2). After 48 h, the conversion was higher than 50% and consequently the ee of (R)-4 dropped to 95%. Based on these results, 24 h was selected as the most appropriate time to interrupt the kinetic resolution process.
Pseudomonas cepacia lipases also presented interesting results, but slightly inferior to those of CAL-B. For P. cepacia immobilized on ceramics, we observed high ee for (R)-4 (>99%) and E > 200 after 12 h (Entry 4). But, even after 48 h the conversion did not reach 50% and consequently the maximum ee for (S)-3 was 95% (Entry 6). For P. cepacia immobilized on diatomite, the conversion was lower than 40% and 56% ee for (S)-3 (Entry 9).

Influence of Solvent in the Kinetic Resolution of (R,S)-3
The solvent influence on EKR of (R,S)-3 was also investigated ( Table 3). The results demonstrated that the reaction in hexane presented high conversion (50%) and excellent enantioselectivity. For those reactions in toluene (Entries 3 and 4) and methyl tert-butyl ether (Entries 5 and 6) slightly inferior results were observed, in comparison with hexane. On the other hand, THF, CHCl 3 , i-PrOH and i-BuOH (Entries 7-10) showed a dramatic influence on the conversion, which resulted in low values, <30%.
2.2.3. Influence of Temperature in the Kinetic Resolution of (R,S)-3.
In order to obtain the compounds (S)-3 and (R)-3 and to assign the absolute configuration, a reaction on a preparative scale (5 mmol) was carried out. After quenching the reaction, the compounds (S)-3 and (R)-4 were separated by flash gel column chromatography. Then, the ester (R)-4 was submitted to a hydrolysis reaction to give the alcohol (R)-3 (Scheme 4). In this way, both enantiomers of 3 were obtained in high enantiomeric purity (ee > 99%) and yields (>45%).

Scheme 4. Synthesis of (R)-and (S)-3.
The absolute configuration of the compound 3 was indirectly attributed after deprotection of the amino group of (−)-(S)-3 [18]. Then, the optical rotation of the resulting amino-alcohol 5 was measured, and by comparison with literature data [18] its absolute configuration was attributed to (S)-5 (Table 6). Consequently, the configuration of the NH-Boc protected precursor was also attributed to (S)-3.

Experimental Section
Commercially available materials were used without further purification. Lipase from Candida antarctica (fraction B, CAL-B) immobilized, and commercially available as Novozym® 435 was kindly donated by Novozymes Latin America Ltda. All solvents were HPLC or ACS grade. Solvents used for moisture sensitive operations were distilled from drying reagents under a nitrogen atmosphere: THF was distilled from Na/benzophenone. Analytical thin-layer chromatography (TLC) was performed using aluminum-backed silica plates coated with a 0.25 mm thickness of silica gel 60 F 254 (Merck), visualized with an ultraviolet light (λ = 254 nm), followed by exposure to p-anisaldehyde solution or vanillin solution and heating. Standard chromatographic purification methods were followed using 35-70 mm (240-400 mesh) silica gel purchased from Acros Organics ® .
Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AC 200 spectrometer at operating frequencies of 200 ( 1 H-NMR) and 50 MHz ( 13 C-NMR). The 1 H-NMR chemical shifts are reported in ppm relative to TMS peak. The data are reported as follows: chemical shift (δ), multiplicity (s = singlet, d = doublet, t = triplet, qd = quadruplet, dd = double dublet, td = triple dublet, m = multiplet), and coupling constant (J) in Hertz and integrated intensity. The 13 C-NMR chemical shifts are reported in ppm relative to CDCl 3 signal.
High-resolution mass spectra (HRMS) were acquired using a Bruker Daltonics MicroTOF instrument, operating in the electrospray ionization (ESI) mode.
Infrared spectra were recorded from KBr discs or from a thin film between NaCl plates on FTIR spectrometer (Bomem Michelson model 101). Absorption maxima (ν max ) are reported in wavenumbers (cm −1 ).
Optical rotations were measured on a Perkin Elmer-343 digital polarimeter in a 1 mL cuvette with a 1 dm pathlength. All values are reported in the following format: [α] D (temperature of measurement) = specific rotation (concentration of the solution reported in units of 10 mg sample per 1 mL solvent used).
(2-Acetylphenyl)carbamate (2) and the di-Boc derivative were isolated in 45% and 31% yields, respectively. The di-Boc compound (1.00 g) was dissolved in CH 2 Cl 2 (100 mL) and Amberlyst 15 resin (1.00 g) was added. The mixture was stirred for 24 h in an orbital shaker. Then, the solvent was removed and the residue filtered through a silica gel column with hexane/EtOAc 9:1. The compound 2 was obtained in 67% yield. 1

General Procedure to Remove Boc-Protecting Group (Adapted from Reference [19])
To a mixture of AcOEt:HCl 3 mol L −1 1:1 (5 mL), N-Boc protected compound (1 mmol) was added. The mixture was stirred at room temperature for 1 h. After that, the solvent was removed under vacuum. The residue was dissolved in CH 2 Cl 2 (10 mL) and washed with saturated NaHCO 3 solution (3 × 3 mL). Then, the organic layer was dried over MgSO 4 , filtered and concentrated to dryness under vacuum. The product was obtained in quantitative yield without further purification.

Conclusions
In summary, we have described an efficient methodology to obtain (R)-and (S)-tert-butyl 2-(1hydroxyethyl)phenylcarbamates in enantiopure form (ee > 99%), using a kinetic resolution process mediated by lipase as a biocatalyst. Both enantiomers can be employed in the preparation of organochalcogenanes for further application in biological studies.