Reaction Steps
To a stirred solution of (1R,3S)-(+)-camphoric acid (5 g, 25 mmol) in 30 mL of methanol, anhydrous HCl was bubbled for 2 h at room temperature. Methanol was evaporated and the residue was mixed with 5% sodium bicarbonate solution until the effervescence ceased and then 5% sodium hydroxide was added. The diester by-product was removed by extraction with diisopropyl ether. The aqueous layer was acidified with 10% HCl and extracted with diisopropyl ether. The combined ether extracts were dried over anhydrous sodium sulfate and evaporated to obtain (1R,3S)-3-(methoxycarbonyl)-1,2,2-trimethylcyclopentanecarboxylic acid (4.38 g) in 83% yield. 1H NMR (400 MHz, CDCl3) δ ppm 0.84 (s, 3H), 1.25 (s, 6H), 1.53-1.55 (m, 1H), 1.80-1.84 (m, 1H), 2.19-2.22 (m, 1H), 2.25-2.57 (m, 1H), 239-2.84 (m, 1H); 3.70 (s, 3H); m/z (M+1): 214. Step II: To a solution of (1R,3S)-3-(methoxycarbonyl)-1,2,2-trimethyl cyclopentanecarboxylic acid (4.1 g, 19.2 mmol) in 18 mL of DCM and oxalyl chloride (2.1 mL, 24.9 mmol), 2 drops of DMF were added. The solution was stirred for 5 hours at −15° C. All the volatiles were removed by passing nitrogen gas. The residue was dissolved in THF and the solution added dropwise to 80 mL of anhydrous acetonitrile saturated with NH3 gas maintained at −30° C. The reaction mixture was stirred for another 15 minutes and the volatiles were removed under reduced pressure. The residue was taken in hot ethyl acetate and the solution was filtered; the crude product obtained after evaporation of the solvent was purified by column chromatography to furnish methyl (1S,3R)-3-carbamoyl-2,2,3-trimethyl cyclopentanecarboxylate (3.45 g) in 84% yield. 1H NMR (400 MHz, CDCl3) δ ppm 0.85 (s, 3H), 1.22 (s, 3H), 1.30 (s, 3H), 1.45-1.48 (m, 1H), 1.80-1.91 (m, 1H), 2.20-2.27 (m, 1H), 2.35-2.43 (m, 1H), 2.79-2.84 (m, 1H), 3.69 (s, 3H), 5.60 (d, J=37.96, 2H); m/z (M+1): 213. Step III: To a stirred solution of methyl (1S,3R)-3-carbamoyl-2,2,3-trimethyl cyclopentanecarboxylate (2.6 g; 12.2 mmol) in 12 mL of t-butanol, 0.2 mL of stannic chloride was added followed by lead tetraacetate (7.02 g, 15.86 mmol). The reaction mixture was heated under reflux for 24 hours. The solvent was evaporated under reduced pressure; the residue was taken up in diethyl ether, washed with 10% K2CO3 solution. The ether extract was concentrated and the crude product was purified by column chromatography to obtain methyl (1S,3R)-3-[(t-butoxycarbonyl)amino]-2,2,3-trimethylcyclopentanecarboxylate (2.58 g) in 74% yield. 1H NMR (400 MHz, CDCl3) δ ppm 0.86 (s, 3H), 1.18 (s, 3H), 1.35 (s, 3H), 1.43 (s, 9H), 1.8-1.9 (m, 1H), 1.98-2.05 (m, 3H), 2.72-2.76 (m, 1H), 3.71 (s, 3H), 4.73 (bs, 1H); m/z (M+1): 285.
