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General procedure for the synthesis of rac-4a-d:
Esters rac-3a-d (0.1 mmol) (Sternativo et al. 2012) were deprotected by treatment with TFA
(1 ml) in DCM (2 ml). The reaction mixtures were stirred overnight at room temperature. After
removal of the solvent under reduced pressure, methanol was added and evaporated in vacuum..
This procedure was repeated three times to remove residual TFA. TFA salts rac-4a-d were all
obtained in quantitative yields. Spectral data of TFA salts 4a-d are reported below.
rac-(3aR,8bS)-3a-Carboxy-7-chloro-1,2,3,3a,4,8b-hexahydroindeno[1,2-b]pyrrolium
trifluoroacetate (4a)
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H NMR (400 MHz, CD3OD, 25 °C, TMS): 7.59 (s, 1H, CH), 7.46 (br d, 1H, J = 8.3 Hz,
CH), 7.35 (d, 1H, J = 8.3 Hz, CH), 5.55 (s, 1H, CHN), 3.66 (d, 1H; J = 17.4 Hz, CH2), 3.50 (td, 1H;
J = 6.8, 11.8 Hz, CH2N), 3.34-3.22 (m, 2H; CH2N and CH2), 2.73 (td, 1H; J = 6.8, 13.7 Hz, CH2),
2.19 (td, 1H; J = 6.8, 13.7 Hz, CH2); 13C NMR (100 MHz, CD3OD, 25 °C):  = 175.0, 141.2, 137.2,
133.3, 130.3, 126.5, 125.7, 70.4, 58.9, 45.6, 41.1, 35.8; 19F NMR (376 MHz, CD3OD, 25 °C): δ = 77.0. 19F NMR (376 MHz, CD3OD, 25 °C): δ = -77.4 (s).
rac-(3aR,8bS)-3a-Carboxy-6,7-dimethoxy-1,2,3,3a,4,8b-hexahydroindeno [1,2-b]pyrrolium
trifluoroacetate (4b)
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H NMR (400 MHz, CD3OD, 25 °C, TMS): 7.08 (s, 1H, CH), 6.9 (s, 1H, CH), 5.46 (s,
1H; CHN), 3.86 (s, 3H, OMe), 3.84 (s, 3H, OMe), 3.61 (d, 1H; J = 16.9 Hz, CH2), 3.51-3.44 (m,
1H; CH2N), 3.30-3.21 (m, 1H; CH2N), 3.20 (d, 1H; J = 16.9 Hz, CH2), 2.69 (td, 1H; J = 6.9, 13.7
Hz, CH2), 2.18 (td, 1H; J = 6.9, 13.7 Hz, CH2); 13C NMR (100 MHz, CD3OD, 25 °C):  = 175.2,
161.1 (q, J = Hz), 151.7, 149.6, 134.9, 126.3, 107.9, 107.3, 71.3, 58.9, 55.2, 55.1, 45.3, 41.9, 36.0;
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F NMR (376 MHz, CD3OD, 25 °C): δ = -77.62 (s).
rac-(3aS,4S,8bS)-3aCarboxy-4-phenyl-1,2,3,3a,4,8b-hexahydroindeno[1,2-b]pyrrolium
trifluoroacetate (4c)
1
H NMR (400 MHz, CDCl3, 25°C, TMS): 7.75-7.62 (m, 1H), 7.52-7.41 (m, 2H, CH),
7.28-7.20 (m, 3H, CH), 7.15-7.10 (m, 1H, CH), 7.08-7.00 (m, 2H, CH), 5.86 (s, 1H; CHN), 4.80
(s, 1H; CHPh), 3.45-3.32 (m, 2H; CH2N), 2.88 (dt, 1H; J = 6.6, 13.9 Hz, CH2), 2.391.94 (dt, 1H; J
= 7.4, 13.9 Hz, CH2); 13C NMR (100 MHz, CDCl3, 25°C):  = 174.6, 147.1, 141.9, 137.4, 132.1,
130.3 (2 C), 130.1, 129.5 (2 C), 128.7, 127.7, 127.0, 70.6, 67.8, 61.5, 46.6, 37.4;
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F NMR (376
MHz, CD3OD, 25 °C): δ = -77.33 (s).
rac-(3aR,8bS)-3a-Carboxy-1,2,3,3a,4,8b-hexahydroindeno[1,2-b]pyrrolium trifluoroacetate (4d)
1
H NMR (400 MHz, CD3OD, 25 °C, TMS): 7.62-7.25 (m, 4H, CH), 5.51 (s, 1H, CHN),
5.39, 3.67 (d, 1H; J = 17.3 Hz, CH2), 3.48 (td, 1H; J = 6.9, 11.5 Hz, CH2N), 3.33-3.22 (m, 2H;
CH2N and CH2), 2.76 (td, 1H; J = 6.9, 13.7 Hz, CH2), 2.16 (td, 1H; J = 6.9, 13.7 Hz, CH2);
13
C
NMR (100 MHz, CD3OD, 25 °C):  = 175.1, 142.4, 135.2, 130.2, 127.7, 125.4, 125.0, 70.9, 58.2,
45.6, 41.7, 35.9; 19F NMR (376 MHz, CD3OD, 25 °C): δ = -77.0 (s).
Experimental section
1
H, 13C and 19F NMR were recorded in CD3OD at 400, 100 and 376 MHz, respectively, on a
Bruker Avance-DRX 400 instrument. Chemical shifts (δ) are reported in ppm relative to solvent
signals (CH3OH, 3.31 ppm for 1H NMR, CD3OD, 49.0 ppm for
13
C NMR).
19
F NMR chemical
shifts were referenced to external CFCl3 (0.0 ppm). Coupling constants are given in Hz. Due to the
fast exchange, resonance signals of NH and OH protons were not detected in CD3OD spectra. The
following abbreviations are used to indicate the multiplicity: s, singlet; d, doublet; t, triplet; q,
quartet; m, multiplet. Commercial grade solvents and reagents were used without further
purification.