5-Chloro-8-nitro-1-naphthoyl (NNap): A Selective Protective Group for Amines and Amino Acids

The synthesis of 5-chloro-8-nitro-1-naphthoyl chloride and its use as a protective group for amines is described. Protection is carried out with an auxiliary amine or under mild Schotten-Baumann conditions in high yield (>86%), while deprotection can be achieved easily under gentle reducing conditions due to the large steric tension between C-1 and C-8 naphthalene substituents. The reaction has been successfully tested in dipeptide synthesis and amino alcohols protection, and it has proved selective for the ε-amine group of lysine.

Potassium carboxylate (K•3). Single crystals suitable for X-ray diffraction measurements were obtained by slowly evaporating aqueous solutions of stoichiometric mixtures of the carboxylic acid 3 (50 mg) and the equimolar amount of potassium hydroxide in 3 mL of water in a clean and dry 10 mL glass vial. The mouth of the glass vial was covered with a cap having a small hole and kept it for slow evaporation at room temperature. At the end of the process, single crystals were obtained after 5-6 days. Amide 17. Single crystals suitable for X-ray diffraction measurements were obtained by slowly evaporating solutions of the amide 17 (50 mg) in 2 mL of methanol in a clean and dry 10 mL glass vial. The mouth of the glass vial was covered with a cap having a small hole and kept it for slow evaporation at room temperature. At the end of the process, single crystals were obtained after 5-6 days.

• X-Ray Crystallography
Suitable single crystals of K•3 and 17 were mounted on glass fibre for data collection on a Bruker Kappa APEX II diffractometer. Data were collected at 298(2) K using Cu Kα radiation (λ = 1.54178 Å) and ω scan technique and were corrected for Lorentz and polarization effects. The detector was placed at approximately 37.5 mm from the crystal. A series of narrow frames of data were collected with a scan width of 0.5º in ω and an exposure time of 10 s per frame. The data were integrated with SAINT [3] to a resolution of 0.78 Å using a narrow-frame algorithm. Data were corrected for absorption effects using the multi-scan method using SADABS. [4] Subsequent structure solution and refinement were carried out with SHELXT and SHELXL, respectively. [5,6] The structures were solved by direct methods combined with difference Fourier synthesis and refined by full-matrix least-squares procedures, with anisotropic thermal parameters in the last cycles of refinement for all non-hydrogen atoms. The refinement was based on F 2 for all reflections, weighted R factors (wR) and goodness-of-fit (GoF) values are based on F 2 , while conventional R factors (R) are based on F. The Fo 2 > 2σ(Fo 2 ) criterion was used only for calculating R factors and it is not relevant to the choice of reflections for the refinement. The R factors based on F 2 are about twice as large as those based on F. Scattering factors were taken from the International Tables for Crystallography. [7] Hydrogen atom positions were calculated by geometrical methods and refined as a riding model. The potassium salt of the carboxylic acid (3) crystallizes with one molecule of water; it should be noticed that hydrogen atoms of water molecules are not shown in the structural formula of the complex, as they were not located by Xray diffraction. The amide 17 crystallizes with one molecule of methanol. Mercury 4.2.0 program was used for analysis and molecular and crystal structure drawings preparation. [8] The CCDC numbers assigned for the crystal structures are: 2252216 (K•3, potassium salt of carboxylic acid 3) and 2252212. (amide 17).

Synthesis of the protective group
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5-bromo-1-naphthoic acid (2)
This compound was prepared according to a modified published literature. [10]. 1-naphthoic acid (20.0 g, 116.2 mmol) was placed in a 250 mL round bottom flask equipped with a reflux condenser, and CHCl3 (80.0 mL) and acetic acid (10.0 mL) were added. The suspension was refluxed until a homogeneous solution was obtained. Bromine (12.0 mL, 234 mmol) was then added dropwise and after 3 minutes the brominated compound 2 started to precipitate. The thick solution was further refluxed for 2 hours. Cooling down and filtering the precipitate yielded the brominated compound (2) (26.0 g, 90% yield) as a white powder. The spectroscopic properties of compound 2 were in agreement with the published data [10]. 1

5-bromo-8-nitro-1-naphthoic acid (3)
5-bromo-1-naphthoic acid, 2 (20.0 g, 79.7 mmol) was placed in a round bottom flask with acetic anhydride (200 mL), p-toluene sulfonic acid (4.0 g) and P2O5 (5.0 g). Fuming nitric acid (20.0 mL) was carefully added to this suspension, keeping the temperature below 30ºC. After 30 minutes the reaction was poured over water and ice. Once the acetic anhydride was hydrolysed (2 hours), the precipitate was filtered to yield, after drying, the nitro compound (3). This crude compound can be further purified by recrystallization from THF/EtOH, affording pure compound 3 in 86% yield (20.  5-bromo-8-nitro-1-naphthoic acid (1.0 g, 3.38 mmol) was placed in a round bottom flask and thionyl chloride (5.0 mL) and a trace of pyridine were added. The round bottom flask was equipped with a condenser and a paraffin bubbler. The reaction mixture was stirred and heated in a water bath at 40ºC until a homogeneous solution was obtained. Once the acid was completely dissolved, the reaction was finished and the thionyl chloride was removed under vacuum. The crude acid chloride was used without further purification in the next step. Due to its instability, the acid chloride 4 is not characterized.

General Procedure (A) for amine (5-11) protection using NNapCl
4 was prepared according to the above-described procedure by reacting 5-bromo-8-nitro-1-naphthoic acid 3 (500 mg; 1.69 mmol; 1 eq.) with thionyl chloride (5 ml; 69 mmol; 40 eq.). The crude acid chloride 4 was dissolved in methylene chloride (15 mL) and cooled down in an ice bath. Then, a solution of the amine (1.69 mmol; 1 eq.) dissolved in CH2Cl2 (3.0 mL) was added dropwise. After stirring for 5 minutes, a solution of Na2CO3 (8.45 mmol, 890 mg) in water (4 mL) was added and the mixture allowed to warm to room temperature, stirring for an additional 25 minutes. Water (20 mL) was then added, and the layers were separated in a separatory funnel. The organic layer was washed with HCl 2N and dried over anhydrous Na2SO4. The solvent was removed by vacuum and the crude residue was purified.

Procedure for deprotection
Removal of the protective group was explored using a variety of reducing conditions: Zn/AcOH, SnCl2/MeOH, and H2/Pd (C)/AcOEt. Next, the different procedures are described for amide 16. Reduction of amines (12, 15, 17 and 18) proceeded under similar conditions. Lactam F was purified by column chromatography on silica gel using 100% CH2Cl2 to a mixture CH2Cl2: EtOAc (9:1).

Zn/AcOH (B)
A solution of the amide 16 (300 mg, 0.83 mmol, 1 eq.) in acetic acid (3 mL) was added to an Erlenmeyer flask containing a preheated (60ºC) and well-stirred suspension of Zn (3.0 g, 46 mmol, 55 eq.) in acetic acid (6 mL). The reaction mixture was stirred for 10 minutes and ethyl acetate (20 mL) and water (20 mL) were then added. Zn was removed by filtration (or careful decantation), the two phases separated, and the organic layer acidified with 2N HCl. The acidic aqueous layer and the ethyl acetate layer (containing the lactam F) were separated, and the acidic extract basified with solid Na2CO3. The basic aqueous layer was finally extracted with ethyl acetate, the combined organic layers dried over anhydrous Na2SO4, filtered, and the ethyl acetate was evaporated under reduced pressure, to afford di-n-butylamine 9 in 95% yield (160 mg).

SnCl 2 /MeOH (C)
The amide 16 (140 mg, 0.39 mmol, 1 eq.) was dissolved in methanol (10 mL) and excess of SnCl2 was added (300 mg, 1.58 mmol, 4 eq.). The reaction mixture was heated in a water bath at 60ºC for 10 minutes. Next, 2.0 mL of an aqueous 2M Na2CO3 solution, solid Na2CO3 (500 mg, 4.72 mmol) and ethyl acetate (20 mL) were added, and the reaction mixture was stirred for additional 10 minutes, until a basic pH was obtained. The reaction mixture was filtered, the phases separated, and the combined organic layers acidified with 2N HCl. Lactam F could be isolated from the organic layer, after drying over anhydrous Na2SO4, filtering and evaporating the solvent under reduced pressure. The acidic extracts (containing the protonated amine) were basified with solid Na2CO3 and extracted with ethyl acetate, dried over anhydrous Na2SO4, filtered, and evaporated under reduced pressure to afford di-n-butylamine 9 in 80% yield (63 mg).

▪ H 2 /Pd (C)/ EtOAc (D)
The amide 16 (140 mg, 0.39 mmol, 1eq.) was dissolved in ethyl acetate (7.0 mL). Then 100 mg of Pd/C 5% was added and the mixture was reacted with hydrogen under pressure (3.5 atm) overnight at 20ºC. The catalyst was removed by filtration and the solvent was evaporated under reduced pressure, but lactam cyclization was incomplete according to 1 H NMR spectrum. The crude residue was then dissolved in acetic acid and heated for 2 minutes at 100ºC to complete the amine liberation. The reaction mixture was worked up with ethyl acetate and water as described previously to afford di-n-butylamine 9 in 90% yield (71 mg).

Capture of the intermediate amine
Amide 16 (0.83 mmol, 300 mg) in acetic acid (3 mL) was added to a suspension of Zn (46 mmol, 3.0 g) in acetic acid (6 mL). The reaction mixture was stirred for 10 minutes. Then, acetic anhydride (10 mL) was added, and the reaction mixture was stirred at room temperature until acetylation was complete. Ethyl acetate (20 mL) and 4% (w/v) aqueous solution of Na2CO3 (20 mL) were then added and Zn was removed by filtration (or careful decantation). The two phases were separated, and the organic layer dried over anhydrous Na2SO4 and filtered. The solvent was evaporated under reduced pressure and the crude residue was purified by silica gel column chromatography using CH2Cl2: EtOAc (95:5) to afford 255 mg (82%) of the desired compound Ac16.

tert-butyl L-leucyl-L-leucinate (H-Leu-Leu-OtBu)
The protected dipeptide 20 (106 mg, 0.20 mmol) was dissolved in acetic acid (1.0 mL) and added to a preheated and wellstirred suspension of Zn (1.0 g) in AcOH (3.0 mL). Following the above-described procedure for deprotection (B), 45 mg of the dipeptide H-Leu-Leu-OtBu was obtained as a colourless oil in 70% yield. The spectroscopic properties agreed with those published [11].  H-Lys-OMe•2HCl (2.20 g, 10.2 mmol) was suspended in a dichloromethane-tert-butanol (5:1) solvent mixture (60 mL) containing i-Pr2Et (7.0 mL, 40.1 mmol) and stirred at room temperature until total dissolution. Next, a solution of the acid chloride 4 (920 mg, 3.4 mmol) in CH2Cl2 (10 mL) was added. The reaction mixture was stirred for 10 minutes until it was completed. The reaction mixture was worked-up, extracting with ethyl acetate (50 mL) and aqueous sodium carbonate 4% (w/v). The combined organic layers were dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The crude residue (1.3 g) was purified by crystallization to yield 1.1 g of the desired compound 21 in 85% yield as a pale yellow crystalline solid. +7.9 (c 1, MeOH).

Modelling studies
Theorical studies were carried out using GAMESS interface for Chem3D 19.1 software [9] using the RHF/3-21G. No imaginary frequencies were observed, and the calculated total energy was -954413.1802 kcal/mol. The figure shows a possible geometry for the compound Ac16, considering the information from the NMR spectra.