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12 All compounds were purified by the appropriate techniques and fully characterized by spectroscopic means.
Typical Experimental Procedure - Synthesis of Methyl-(1
S
,3
R
,3a
R
,7a
S
)-4-ethoxy-6-hydroxymethyl-7-oxo-1-phenyl-octahydro-pyrano[3,4-
c
]pyrrole-3-carboxylate (
9a).
To a solution of methyl-N-benzylideneglycinate (7a, 0.23 g, 1.3 mmol, 1.5 equiv) in 20 mL of dry MeCN in a two-necked, round-bottomed flask equipped with a magnetic stirring bar and a reflux condenser were added enone 5 (0.15 g, 0.9 mmol), AgOAc (0.18 g, 1.05 mmol, 1.2 equiv), and 0.16 mL of DBU (1.05 mmol, 1.2 equiv). The mixture was stirred at r.t. during 4 h in absence of light (flask covered in aluminium foil). After filtration on celite and evaporation of the solvent under reduced pressure, the resulting brown oil was dissolved in 15 mL of CH2Cl2 and washed with 20 mL of NH4Cl solution. The organic layer was then dried over MgSO4, and the solvent was removed under reduced pressure. The resulting oil was purified by flash chromatography on silica gel (EtOAc-pentane 70:30, R
f
= 0.30) to provide 9a in 60% yield as a viscous colorless oil. 1H NMR and 13C NMR spectroscopy revealed two conformers.
Major conformer: 1H NMR (300 MHz, CDCl3): δ = 1.21 (t, J = 7.1 Hz, 3 H, ethyl-CH3), 2.86 (dd, J
3a,7a = 9.4 Hz, J
3a,3 = 9.0 Hz, 1 H, H-3a), 3.07 (dd, J
7a,3a = 9.4 Hz, J
7a,1 = 5.7 Hz, 1 H, H-7a), 3.51-3.98 (m, 5 H, H-5, CH2-O and ethyl-CH2), 3.77 (s, 3 H, ester-CH3), 4.04 (d, J
3,3a = 9.0 Hz, 1 H, H-3), 4.69 (d, J
1,7a = 5.7 Hz, 1 H, H-1), 5.12 (br s, 1 H, H-4α), 7.25-7.46 (m, 5 H, H-arom.) ppm. 13C NMR (CDCl3): δ = 14.7 (ethyl-CH3), 48.3 (C-3a), 52.5 (ester-CH3), 55.7 (C-7a), 62.1 (ethyl-CH2), 62.7 (C-3), 63.0 (C-1), 63.8 (CH2-O), 76.8 (C-6), 97.0 (C-4), 124.9-139.3 (C-Ar), 172.8 (ester C=O), 207.4 (C-7) ppm. IR = 3362 (OH), 3059 (arom. C-H), 2972, 2929, 2905 (CHn), 1736 (C=O ester), 1715 (C=O ketone), 1606 (arom. C=C), 1180, 1132, 1063 (C-O), 736 (arom. C-H), 702 (arom. C-H) cm-1.
Minor conformer: 1H NMR (300MHz, CDCl3): δ = 1.17 (t, J = 7.1 Hz, 3 H, ethyl-CH3), 3.12-3.15 (m, 1 H, H-3a, 3.25 (dd, J
7a,3a = 7.7 Hz, J
7a,1 = 7.2 Hz, 1 H, H-7a), 3.51-3.98 (m, 5 H, H-5, CH2-O and ethyl-CH2), 3.86 (s, 3 H, ester-CH3), 4.19 (d, J
3,3a = 9.4 Hz, 1 H, H-3), 4.41 (d, J
1,7a = 7.2 Hz, 1 H, H-1), 5.12 (br s, 1 H, H-4α), 7.25-7.46 (m, 5 H, H-arom.) ppm. 13C NMR (CDCl3): δ = 14.2 (ethyl-CH3), 45.9 (C-3a), 52.3 (ester-CH3), 52.4 (C-7a), 60.9 (C-1), 62.0 (ethyl-CH2), 63.7 (CH2-O), 65.9 (C-3), 75.5 (C-6), 96.1 (C-4), 124.9-139.3 (C-Ar), 172.2 (ester C=O), 207.4 (C-7).