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DOI: 10.1055/s-2006-958408
Pauson-Khand Reaction of Amino Acid Derived 3-Pyrrolines: New Enantiopure Scaffolds for Diversity-Oriented Synthesis
Publication History
Publication Date:
20 December 2006 (online)
Abstract
Amino acid derived 3-pyrrolines can be readily prepared in a single step in enantiomerically pure form by cycloalkylation of the corresponding amino esters with (Z)-1,4-dichloro-2-butene. Enantiopure azabicyclo[3.3.0]octenones can be then easily obtained through an unprecedented Pauson-Khand reaction followed by diastereomer separation.
Key words
Pauson-Khand reaction - pyrrolines - amino acids - cyclization - alkynes
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References and Notes
The preparation of 2a-f can also be performed, albeit in somewhat decreased yield, by treating cis-1,4-dichloro-2-butene with 1.1 equiv of amino ester hydrochloride and 3.5 equiv of diisopropylethylamine in CH2Cl2 at r.t. for 24 h. In this way, 2d is obtained in 57% yield (vs. 65% yield with the standard procedure).
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Representative Procedure for the Synthesis of 2a-f.
A mixture of cis-1,4-dichloro-2-butene (1 mmol) and amino ester 1a-f (3.5 mmol) in CH2Cl2 (1 mL) was stirred at r.t. for 24 h. Then, CH2Cl2 was added and the precipitated was filtered. The filtrate was washed with H2O and sat. solution of NaCl, dried over Na2SO4, filtered and concentrated. The crude was purified by flash chromatography on silica gel with EtOAc and n-hexane, gradient 1:2 to 2:1.
Spectral Data for Selected Compounds.
Compound 2a: yellow oil. 1H NMR (400 MHz, CDCl3): δ = 5.76 (s, 2 H), 4.17 (q, J = 7 Hz, 2 H), 3.63 (m, 4 H), 3.46 (q, J = 7 Hz, 1 H), 1.36 (d, J = 7 Hz, 3 H), 1.26 (t, J = 7 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 173.86, 127.13, 60.35, 60.28, 56.55, 17.22, 14.26 ppm. MS (EI+): m/z = 170.1 [M + H+].
Compound 2b: yellow oil. 1H NMR (400 MHz, CDCl3): δ = 5.75 (s, 2 H), 4.10 (q, J = 7 Hz, 2 H), 3.70 (m, 2 H), 3.51 (m, 2 H), 2.98 (d, J = 7 Hz, 1 H), 1.95 (m, 1 H), 1.20 (t, J = 7 Hz, 3 H), 0.96 (d, J = 7 Hz, 3 H), 0.86 (d, J = 7 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 172.67, 127.08, 70.72, 59.70, 55.79, 29.16, 19.94, 19.07, 14.45 ppm. MS (EI+): m/z = 198.1 [M + H+].
Compound 2e: colorless oil. 1H NMR (400 MHz, CDCl3): δ = 7.48 (m, 2 H), 7.34 (m, 3 H), 5.75 (s, 2 H), 4.32 (s, 1 H), 3.69 (s, 3 H), 3.54 (m, 2 H), 3.52 (m, 2 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 172.23, 137.31, 128.65, 128.52, 128.33, 127.20, 72.56, 58.09, 52.09 ppm. MS (EI+): m/z = 218.1 [M + H]+.
General Procedure for the Pauson-Khand Reaction of Amino Acid Derived 3-Pyrrolines.
A mixture of alkyne (0.60 mmol) and Co2(CO)8 (0.65 mmol) in 1,2-dichloroethane (2 mL) was stirred at r.t. for 2 h. A solution of 2a-f (0.50 mmol) in 1,2-dichloroethane (1 mL) and DMSO (1.75 mmol) was added and the mixture was heated at 83 °C for 24 h. The reaction mixture was cooled at r.t., filtrated through Celite® and washed with CH2Cl2. The filtrate was concentrated and purified by flash chromatog-raphy on silica gel, gradient with neat n-hexane to n-hexane-EtOAc (1:2). Compounds 3 were obtained as mixture of two diastereomers that were purified by HPLC: Chiralcel® OD-H (1 cm × 25 cm), n-hexane-2-PrOH, 5 mL/min.
Spectral Data for Selected Compounds.
Compound 3cp: white solid.
Diastereomer 1: (2S)-methyl 4-methyl-2-[(3aR,6aS)-4-oxo-5-phenyl-1,3a,4,6a-tetrahydrocyclo penta[c]pyrrol-2 (1H)-yl]pentanoate.1H NMR (400 MHz, CDCl3): δ = 7.67 (m, 2 H), 7.65 (d, J = 3.0 Hz, 1 H), 7.40-7.31 (m, 3 H), 3.68 (s, 3 H), 3.38 (m, 2 H), 3.16 (d, J = 9.0 Hz, 1 H), 2.89 (m, 1 H), 2.84 (m, 2 H), 2.77 (m, 1 H), 1.58-1.42 (m, 3 H), 0.85 (d, J = 6.5 Hz, 3 H) 0.80 (d, J = 6.5 Hz, 3 H) ppm. HPLC [Chiralcel® OD-H (0.46 cm × 25 cm), n-hexane-2-PrOH (99:1), 0.5 mL/min, 254 nm]: r.t. 11.7 min. HRMS (ES+): m/z calcd for C20H25NO3 + H+: 328.1913; found: 328.1923. [α]D
20 +46.1 (c 1, CHCl3).
Diastereomer 2: (2S)-methyl 4-methyl-2-[(3aS,6aR)-4-oxo-5-phenyl-1,3a,4,6a-tetrahydrocyclopenta[c]pyrrol-2 (1H)-yl]pentanoate. 1H NMR (400 MHz, CDCl3): δ = 7.70-7.65 (m, 3 H), 7.40-7.30 (m, 3 H), 3.68 (s, 3 H), 3.37 (m, 2 H), 3.18 (d, J = 9.0 Hz, 1 H), 2.92 (m, 2 H), 2.82 (m, 2 H), 1.59-1.44 (m, 3 H), 0.86 (d, J = 6.5 Hz, 3 H), 0.80 (d, J = 6.5 Hz, 3 H) ppm. HPLC [Chiralcel® OD-H (0.46 cm × 25 cm), n-hexane-2-PrOH (99:1), 0.5 mL/min, 254 nm]: r.t. 15.8 min. HRMS (ES+): m/z calcd for C20H25NO3 + H+: 328.1913; found: 328.1922. [α]D
20 -79.2 (c 1, CHCl3).
Compound 3ab: yellow oil.
Diastereomer 1: (2S)-ethy l2-[(3aR,6aS)-5-butyl-4-oxo-1,3a,4,6a-tetrahydrocyclopenta[c]pyrrol-2 (1H)-yl]propanoate.1H NMR (400 MHz, CDCl3): δ = 7.12 (m, 1 H), 4.14 (q, J = 7 Hz, 2 H), 3.26 (m, 1 H), 3.22 (m, 1 H), 3.06 (d, J = 9 Hz, 1 H), 2.76 (m, 2 H), 2.66 (m, 2 H), 2.16 (m, 2 H), 1.45 (m, 2 H), 1.32 (m, 2 H), 1.26 (d, J = 7 Hz, 3 H), 1.25 (t, J = 7 Hz, 3 H), 0.89 (t, J = 7 Hz, 3 H) ppm. HPLC [Chiralcel® OD-H (0.46 cm × 25 cm), n-hexane-2-PrOH (99.5:0.5), 0.5 mL/min, 254 nm]: r.t. 14.46 min. HRMS (ES+): m/z calcd for C16H26NO3 + H+: 280.1913; found: 280.1906. [α]D
20 +70.5 (c 1, CHCl3).
Diastereomer 2: (2S)-ethyl 2-[(3aS,6aR)-5-butyl-4-oxo-1,3a,4,6a-tetrahydrocyclopenta[c]pyrrol-2 (1H)-yl]propanoate. 1H NMR (400 MHz, CDCl3): δ = 7.13 (m, 1 H), 4.14 (q, J = 7 Hz, 2 H), 3.26 (m, 2 H), 3.04 (m, 1 H), 2.78 (m, 2 H), 2.72 (m, 2 H), 2.16 (m, 2 H), 1.45 (m, 2 H), 1.33 (m, 2 H), 1.27 (d, J = 7 Hz, 3 H), 1.25 (t, J = 7 Hz, 3 H), 0.89 (t, J = 7 Hz, 3 H) ppm. HPLC [Chiralcel® OD-H (0.46 cm × 25 cm), n-hexane-2-PrOH (99.5:0.5), 0.5 mL/min, 254 nm]: r.t. 15.80 min. HRMS (ES+): m/z calcd for C16H26NO3 + H+: 280.1913; found: 280.1906. [α]D
20 -83.6 (c 1, CHCl3).