Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270112032933/uk3047sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270112032933/uk3047Isup2.hkl |
CCDC reference: 908129
Compound (I) was prepared according to the method of Shiraiwa et al. (1998). (RS)-H3msa (5.0 g, 33 mmol) and (S)-pea (4.0 g, 33 mmol) were dissolved in propan-1-ol (27 ml). After allowing the mixture to stand in a freezer for 1 week, the crude product of (I) (4.6 g) was collected by filtration. This product was dissolved in propan-1-ol at 353 K to give a colourless solution. The solution was cooled slowly to room temperature and colourless plate-shaped crystals of (I) appeared after several hours.
H atoms bound to C atoms were placed at calculated positions [C—H = 0.98 (CH3), 0.99 (CH2) and 1.00 Å (CH)] and refined as riding, with Uiso(H) = 1.2Ueq(C) for CH2 and CH groups, and 1.5Ueq(C) for methyl groups (rotating group model). H atoms bound to O and S atoms were located in a difference Fourier map and were refined with constrained displacement parameters [Uiso(H) = 1.2Ueq(O,S)]. H atoms bound to N atoms were located in a difference Fourier map and refined with distance restraints and constrained displacement parameters [N—H = 0.89 (2) Å and Uiso(H) = 1.5Ueq(N)].
Data collection: RAPID-AUTO (Rigaku, 2000); cell refinement: RAPID-AUTO (Rigaku, 2000); data reduction: RAPID-AUTO (Rigaku, 2000); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Yadokari-XG 2009 (Kabuto et al., 2009); software used to prepare material for publication: Yadokari-XG 2009 (Kabuto et al., 2009).
C8H12N+·C4H5O4S− | F(000) = 288 |
Mr = 271.33 | Dx = 1.301 Mg m−3 |
Monoclinic, P21 | Mo Kα radiation, λ = 0.71075 Å |
Hall symbol: P 2yb | Cell parameters from 4466 reflections |
a = 9.0547 (7) Å | θ = 3.1–27.5° |
b = 8.2304 (5) Å | µ = 0.24 mm−1 |
c = 9.3016 (7) Å | T = 200 K |
β = 92.760 (2)° | Platelet, colourless |
V = 692.39 (9) Å3 | 0.30 × 0.15 × 0.05 mm |
Z = 2 |
Rigaku R-AXIS RAPID diffractometer | 3100 independent reflections |
Radiation source: rotating-anode X-ray tube | 2723 reflections with I > 2σ(I) |
Detector resolution: 10.000 pixels mm-1 | Rint = 0.034 |
ω scans | θmax = 27.5°, θmin = 3.1° |
Absorption correction: multi-scan (ABSCOR; Rigaku, 1995) | h = −11→11 |
Tmin = 0.788, Tmax = 0.988 | k = −10→10 |
6800 measured reflections | l = −12→12 |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.041 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.107 | w = 1/[σ2(Fo2) + (0.0561P)2 + 0.051P] where P = (Fo2 + 2Fc2)/3 |
S = 1.12 | (Δ/σ)max < 0.001 |
3100 reflections | Δρmax = 0.40 e Å−3 |
179 parameters | Δρmin = −0.26 e Å−3 |
4 restraints | Absolute structure: Flack (1983), 1408 Friedel pairs |
0 constraints | Absolute structure parameter: 0.07 (8) |
Primary atom site location: structure-invariant direct methods |
C8H12N+·C4H5O4S− | V = 692.39 (9) Å3 |
Mr = 271.33 | Z = 2 |
Monoclinic, P21 | Mo Kα radiation |
a = 9.0547 (7) Å | µ = 0.24 mm−1 |
b = 8.2304 (5) Å | T = 200 K |
c = 9.3016 (7) Å | 0.30 × 0.15 × 0.05 mm |
β = 92.760 (2)° |
Rigaku R-AXIS RAPID diffractometer | 3100 independent reflections |
Absorption correction: multi-scan (ABSCOR; Rigaku, 1995) | 2723 reflections with I > 2σ(I) |
Tmin = 0.788, Tmax = 0.988 | Rint = 0.034 |
6800 measured reflections |
R[F2 > 2σ(F2)] = 0.041 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.107 | Δρmax = 0.40 e Å−3 |
S = 1.12 | Δρmin = −0.26 e Å−3 |
3100 reflections | Absolute structure: Flack (1983), 1408 Friedel pairs |
179 parameters | Absolute structure parameter: 0.07 (8) |
4 restraints |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R– factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | ||
S1 | 0.42305 (6) | 0.38313 (8) | 0.79560 (6) | 0.04537 (17) | |
H1 | 0.498 (3) | 0.440 (3) | 0.700 (3) | 0.054* | |
O1 | 0.10777 (16) | 0.1497 (2) | 0.46721 (18) | 0.0386 (4) | |
H1A | 0.041 (3) | 0.076 (4) | 0.418 (3) | 0.046* | |
O2 | 0.28726 (19) | 0.0139 (3) | 0.3674 (3) | 0.0688 (7) | |
O3 | 0.06554 (15) | 0.46249 (17) | 0.66908 (16) | 0.0358 (3) | |
O4 | 0.23275 (17) | 0.5239 (2) | 0.50889 (17) | 0.0421 (4) | |
N1 | 0.1426 (2) | 0.7201 (2) | 0.2819 (2) | 0.0361 (4) | |
H1B | 0.154 (3) | 0.658 (3) | 0.367 (2) | 0.054* | |
H1C | 0.056 (2) | 0.777 (3) | 0.284 (3) | 0.054* | |
H1D | 0.220 (2) | 0.786 (3) | 0.287 (3) | 0.054* | |
C1 | 0.2845 (2) | 0.3014 (3) | 0.6675 (2) | 0.0320 (4) | |
H1E | 0.2202 | 0.2255 | 0.7206 | 0.038* | |
C2 | 0.3547 (2) | 0.2056 (3) | 0.5492 (3) | 0.0374 (5) | |
H2 | 0.4108 | 0.2815 | 0.4898 | 0.045* | |
H2A | 0.4261 | 0.1273 | 0.5940 | 0.045* | |
C3 | 0.2457 (2) | 0.1138 (3) | 0.4524 (2) | 0.0363 (5) | |
C4 | 0.1875 (2) | 0.4410 (2) | 0.6086 (2) | 0.0310 (4) | |
C5 | 0.2609 (3) | 0.5042 (4) | 0.1466 (4) | 0.0635 (8) | |
H5 | 0.2557 | 0.4299 | 0.2286 | 0.095* | |
H5A | 0.3537 | 0.5654 | 0.1548 | 0.095* | |
H5B | 0.2568 | 0.4416 | 0.0569 | 0.095* | |
C6 | 0.1313 (3) | 0.6215 (3) | 0.1460 (2) | 0.0422 (5) | |
H6 | 0.1394 | 0.6971 | 0.0626 | 0.051* | |
C7 | −0.0189 (2) | 0.5413 (2) | 0.1337 (2) | 0.0339 (4) | |
C8 | −0.1179 (3) | 0.5811 (3) | 0.0209 (3) | 0.0444 (5) | |
H8 | −0.0892 | 0.6555 | −0.0507 | 0.053* | |
C9 | −0.2589 (3) | 0.5132 (4) | 0.0114 (3) | 0.0526 (6) | |
H9 | −0.3263 | 0.5426 | −0.0657 | 0.063* | |
C10 | −0.3007 (2) | 0.4033 (3) | 0.1138 (3) | 0.0470 (6) | |
H10 | −0.3970 | 0.3574 | 0.1077 | 0.056* | |
C11 | −0.2021 (2) | 0.3603 (3) | 0.2254 (3) | 0.0420 (5) | |
H11 | −0.2308 | 0.2844 | 0.2957 | 0.050* | |
C12 | −0.0616 (2) | 0.4273 (2) | 0.2353 (2) | 0.0354 (5) | |
H12 | 0.0060 | 0.3956 | 0.3114 | 0.043* |
U11 | U22 | U33 | U12 | U13 | U23 | |
S1 | 0.0360 (3) | 0.0627 (4) | 0.0367 (3) | −0.0065 (3) | −0.0065 (2) | −0.0025 (3) |
O1 | 0.0312 (7) | 0.0395 (8) | 0.0447 (9) | 0.0006 (6) | −0.0028 (6) | −0.0084 (7) |
O2 | 0.0384 (9) | 0.0769 (14) | 0.0922 (16) | −0.0091 (10) | 0.0151 (9) | −0.0504 (13) |
O3 | 0.0322 (7) | 0.0350 (7) | 0.0402 (8) | 0.0034 (6) | 0.0029 (6) | 0.0012 (7) |
O4 | 0.0401 (8) | 0.0445 (8) | 0.0420 (9) | 0.0004 (7) | 0.0040 (7) | 0.0079 (7) |
N1 | 0.0380 (10) | 0.0339 (9) | 0.0367 (10) | −0.0040 (8) | 0.0042 (8) | −0.0038 (8) |
C1 | 0.0273 (9) | 0.0361 (10) | 0.0322 (10) | −0.0032 (8) | −0.0010 (8) | −0.0001 (9) |
C2 | 0.0267 (9) | 0.0402 (11) | 0.0451 (12) | 0.0009 (8) | −0.0003 (9) | −0.0031 (10) |
C3 | 0.0324 (10) | 0.0357 (11) | 0.0410 (12) | −0.0029 (8) | 0.0055 (9) | −0.0045 (9) |
C4 | 0.0304 (9) | 0.0319 (9) | 0.0302 (10) | −0.0035 (8) | −0.0039 (8) | −0.0038 (8) |
C5 | 0.0436 (13) | 0.0740 (19) | 0.0746 (19) | −0.0067 (14) | 0.0206 (13) | −0.0354 (17) |
C6 | 0.0494 (13) | 0.0451 (13) | 0.0331 (11) | −0.0123 (11) | 0.0117 (9) | −0.0058 (10) |
C7 | 0.0421 (11) | 0.0303 (10) | 0.0294 (10) | 0.0003 (9) | 0.0036 (8) | −0.0044 (8) |
C8 | 0.0616 (14) | 0.0388 (12) | 0.0322 (11) | −0.0003 (11) | −0.0033 (10) | −0.0002 (9) |
C9 | 0.0566 (15) | 0.0544 (14) | 0.0449 (13) | 0.0085 (12) | −0.0177 (11) | −0.0060 (13) |
C10 | 0.0379 (11) | 0.0470 (14) | 0.0559 (13) | −0.0016 (11) | −0.0021 (9) | −0.0098 (13) |
C11 | 0.0437 (11) | 0.0369 (11) | 0.0457 (12) | −0.0023 (10) | 0.0072 (9) | −0.0007 (11) |
C12 | 0.0382 (10) | 0.0341 (11) | 0.0339 (10) | 0.0032 (8) | 0.0008 (8) | 0.0014 (8) |
S1—C1 | 1.817 (2) | C5—C6 | 1.519 (4) |
S1—H1 | 1.24 (3) | C5—H5 | 0.9800 |
O1—C3 | 1.297 (3) | C5—H5A | 0.9800 |
O1—H1A | 0.96 (3) | C5—H5B | 0.9800 |
O2—C3 | 1.213 (3) | C6—C7 | 1.511 (3) |
O3—C4 | 1.276 (2) | C6—H6 | 1.0000 |
O4—C4 | 1.237 (3) | C7—C8 | 1.386 (3) |
N1—C6 | 1.501 (3) | C7—C12 | 1.399 (3) |
N1—H1B | 0.943 (17) | C8—C9 | 1.392 (4) |
N1—H1C | 0.917 (17) | C8—H8 | 0.9500 |
N1—H1D | 0.886 (17) | C9—C10 | 1.380 (4) |
C1—C2 | 1.518 (3) | C9—H9 | 0.9500 |
C1—C4 | 1.531 (3) | C10—C11 | 1.382 (3) |
C1—H1E | 1.0000 | C10—H10 | 0.9500 |
C2—C3 | 1.507 (3) | C11—C12 | 1.385 (3) |
C2—H2 | 0.9900 | C11—H11 | 0.9500 |
C2—H2A | 0.9900 | C12—H12 | 0.9500 |
C1—S1—H1 | 93.1 (13) | H5—C5—H5A | 109.5 |
C3—O1—H1A | 113.0 (16) | C6—C5—H5B | 109.5 |
C6—N1—H1B | 114.3 (18) | H5—C5—H5B | 109.5 |
C6—N1—H1C | 105.7 (18) | H5A—C5—H5B | 109.5 |
H1B—N1—H1C | 109 (2) | N1—C6—C7 | 108.94 (18) |
C6—N1—H1D | 113.3 (19) | N1—C6—C5 | 108.6 (2) |
H1B—N1—H1D | 103 (2) | C7—C6—C5 | 114.5 (2) |
H1C—N1—H1D | 111 (3) | N1—C6—H6 | 108.2 |
C2—C1—C4 | 112.52 (18) | C7—C6—H6 | 108.2 |
C2—C1—S1 | 111.56 (14) | C5—C6—H6 | 108.2 |
C4—C1—S1 | 108.82 (14) | C8—C7—C12 | 118.7 (2) |
C2—C1—H1E | 107.9 | C8—C7—C6 | 120.0 (2) |
C4—C1—H1E | 107.9 | C12—C7—C6 | 121.3 (2) |
S1—C1—H1E | 107.9 | C7—C8—C9 | 120.7 (2) |
C3—C2—C1 | 114.12 (17) | C7—C8—H8 | 119.6 |
C3—C2—H2 | 108.7 | C9—C8—H8 | 119.6 |
C1—C2—H2 | 108.7 | C10—C9—C8 | 120.0 (2) |
C3—C2—H2A | 108.7 | C10—C9—H9 | 120.0 |
C1—C2—H2A | 108.7 | C8—C9—H9 | 120.0 |
H2—C2—H2A | 107.6 | C9—C10—C11 | 119.9 (2) |
O2—C3—O1 | 123.7 (2) | C9—C10—H10 | 120.1 |
O2—C3—C2 | 121.0 (2) | C11—C10—H10 | 120.1 |
O1—C3—C2 | 115.36 (19) | C10—C11—C12 | 120.4 (2) |
O4—C4—O3 | 125.38 (19) | C10—C11—H11 | 119.8 |
O4—C4—C1 | 118.54 (18) | C12—C11—H11 | 119.8 |
O3—C4—C1 | 116.08 (18) | C11—C12—C7 | 120.3 (2) |
C6—C5—H5 | 109.5 | C11—C12—H12 | 119.8 |
C6—C5—H5A | 109.5 | C7—C12—H12 | 119.8 |
C4—C1—C2—C3 | −66.1 (2) | N1—C6—C7—C12 | −63.2 (3) |
S1—C1—C2—C3 | 171.31 (16) | C5—C6—C7—C12 | 58.6 (3) |
C1—C2—C3—O2 | −169.0 (2) | C12—C7—C8—C9 | 2.1 (3) |
C1—C2—C3—O1 | 11.0 (3) | C6—C7—C8—C9 | −177.3 (2) |
C2—C1—C4—O4 | −41.8 (2) | C7—C8—C9—C10 | −0.9 (4) |
S1—C1—C4—O4 | 82.4 (2) | C8—C9—C10—C11 | −0.3 (4) |
C2—C1—C4—O3 | 138.90 (18) | C9—C10—C11—C12 | 0.2 (4) |
S1—C1—C4—O3 | −96.94 (18) | C10—C11—C12—C7 | 1.1 (3) |
N1—C6—C7—C8 | 116.3 (2) | C8—C7—C12—C11 | −2.2 (3) |
C5—C6—C7—C8 | −121.9 (3) | C6—C7—C12—C11 | 177.2 (2) |
D—H···A | D—H | H···A | D···A | D—H···A |
S1—H1···O2i | 1.24 (3) | 2.16 (3) | 3.2741 (19) | 148.0 (19) |
O1—H1A···O3ii | 0.96 (3) | 1.54 (3) | 2.501 (2) | 177 (3) |
N1—H1B···O4 | 0.94 (2) | 1.84 (2) | 2.751 (3) | 162 (3) |
N1—H1C···O3iii | 0.92 (2) | 1.94 (2) | 2.797 (2) | 154 (3) |
N1—H1D···O2iv | 0.89 (2) | 2.10 (2) | 2.845 (3) | 141 (3) |
C1—H1E···Cgii | 1.00 | 2.62 | 3.56 | 156 |
C6—H6···Cgv | 1.00 | 2.85 | 3.83 | 166 |
Symmetry codes: (i) −x+1, y+1/2, −z+1; (ii) −x, y−1/2, −z+1; (iii) −x, y+1/2, −z+1; (iv) x, y+1, z; (v) −x, y+1/2, −z. |
Experimental details
Crystal data | |
Chemical formula | C8H12N+·C4H5O4S− |
Mr | 271.33 |
Crystal system, space group | Monoclinic, P21 |
Temperature (K) | 200 |
a, b, c (Å) | 9.0547 (7), 8.2304 (5), 9.3016 (7) |
β (°) | 92.760 (2) |
V (Å3) | 692.39 (9) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 0.24 |
Crystal size (mm) | 0.30 × 0.15 × 0.05 |
Data collection | |
Diffractometer | Rigaku R-AXIS RAPID diffractometer |
Absorption correction | Multi-scan (ABSCOR; Rigaku, 1995) |
Tmin, Tmax | 0.788, 0.988 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 6800, 3100, 2723 |
Rint | 0.034 |
(sin θ/λ)max (Å−1) | 0.649 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.041, 0.107, 1.12 |
No. of reflections | 3100 |
No. of parameters | 179 |
No. of restraints | 4 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.40, −0.26 |
Absolute structure | Flack (1983), 1408 Friedel pairs |
Absolute structure parameter | 0.07 (8) |
Computer programs: RAPID-AUTO (Rigaku, 2000), SIR92 (Altomare et al., 1994), SHELXL97 (Sheldrick, 2008), Yadokari-XG 2009 (Kabuto et al., 2009).
D—H···A | D—H | H···A | D···A | D—H···A |
S1—H1···O2i | 1.24 (3) | 2.16 (3) | 3.2741 (19) | 148.0 (19) |
O1—H1A···O3ii | 0.96 (3) | 1.54 (3) | 2.501 (2) | 177 (3) |
N1—H1B···O4 | 0.943 (17) | 1.838 (19) | 2.751 (3) | 162 (3) |
N1—H1C···O3iii | 0.917 (17) | 1.94 (2) | 2.797 (2) | 154 (3) |
N1—H1D···O2iv | 0.886 (17) | 2.10 (2) | 2.845 (3) | 141 (3) |
C1—H1E···Cgii | 1.00 | 2.62 | 3.56 | 156 |
C6—H6···Cgv | 1.00 | 2.85 | 3.83 | 166 |
Symmetry codes: (i) −x+1, y+1/2, −z+1; (ii) −x, y−1/2, −z+1; (iii) −x, y+1/2, −z+1; (iv) x, y+1, z; (v) −x, y+1/2, −z. |
Thiomalic acid (2-mercaptosuccinic acid, H3msa) is one of the simplest chiral thiol-containing dicarboxylic acids and has been widely employed as a raw material for sulfur-containing organic materials. Because commercially available H3msa is a racemic mixture of the S and R enantiomers, the preparation of enantiopure H3msa has been intensively investigated, prompted by the finding of efficient antirheumatic activity in a gold(I) adduct of the thiomalate ion, viz. {Na2[Au(msa)].1.75H2O}n, by Nomiya et al. (1995). For example, LeBlanc et al. (1997) reported the asymmetric synthesis of pure (R)-thiomalic acid from L-aspartic acid in three steps, while Shiraiwa et al. (1998) reported the optical resolution of the racemic H3msa with the use of (S)-pea (pea = 1-phenylethanamine), which led to the preferential crystallization of the title compound, (S)-Hpea (R)-H2msa, (I). The latter method is undoubtedly superior to the former, but the resulting salt, (I), has not been crystallographically characterized.
As part of our studies on the rational construction of coordination systems based on chiral thiol-containing multidentate ligands (Konno, 2004; Igashira-Kamiyama & Konno, 2011), we started to investigate the coordination system derived from H3msa. In the course of this investigation, we obtained an optically active single crystal of (I) from the reaction of racemic H3msa and (S)-pea, and its structure was determined by X-ray crystallography.
The asymmetric unit of (I) contains an (S)-Hpea+ cation and an (R)-H2msa- anion, the absolute configurations of which are consistent with the previous prediction made by the optical rotation measurement (Shiraiwa et al., 1998). In (I), the amine group of pea is protonated to form a Hpea+ cation, while one of the two carboxy groups of thiomalic acid (C3, O1 and O2) is protonated and the other (C4, O3 and O4) is deprotonated to form a H2msa- anion (Fig. 1). Reflecting the protonation of the O1 atom, the C3—O1 bond length [1.297 (3) Å] is obviously longer than that of C3—O2 [1.213 (3) Å]. On the other hand, the difference between the C4—O3 [1.276 (2) Å] and C4—O4 [1.237 (3) Å] bond lengths is smaller, which is consistent with the deprotonation form of the COO- group. The other bond lengths and angles of the cation and the anion are in the range normally observed for related compounds.
In the crystal, the protonated carboxy group of each (R)-H2msa- anion acts as a hydrogen-bond donor, forming an intermolecular O—H···O hydrogen bond with a deprotonated carboxylate group of a neighbouring anion [O1···O3ii = 2.501 (2) Å; symmetry code: (ii) -x, y-1/2, -z+1]. In addition, its protonated carboxy group also acts as a hydrogen-bond acceptor, forming an intermolecular S—H···O hydrogen bond with a thiol group of another neighbouring anion [S1···O2i = 3.2741 (19) Å; symmetry code: (i) -x+1, y+1/2, -z+1]. Based on these two kinds of hydrogen bonds, (R)-H2msa- anions construct a two-dimensional grid network having rectangular cavities surrounded by four anions parallel to the ab plane (Fig. 2). It is noted that each rectangular cavity accommodates an ammonium group of an (S)-Hpea+ cation through three N—H···O hydrogen bonds [N1···O4 = 2.751 (3) Å, N1···O3iii = 2.797 (2) Å and N1···O2iv = 2.845 (3) Å; symmetry codes: (iii) -x, y+1/2, -z+1; (iv) x, y+1, z]. Besides these hydrogen-bonding interactions, two kinds of C—H···π interactions exist in the crystal; one is a contact between a methine group of an (R)-H2msa- anion and a phenyl group of an (S)-Hpea+ cation (H1E···Cgii = 2.62 Å; Cg is the centroid of the C6–C12 ring), and the other is between a methine group of an (S)-Hpea+ cation and a phenyl group of a neighbouring cation [H6···Cgv = 2.85 Å; symmetry code: (v) -x, y+1/2, -z] (Fig. 3). The latter interaction connects the two-dimensional grids along the c axis, giving a dense three-dimensional structure in (I).
From these structural features, it is likely that the cationic (S)-Hpea+ selects the R isomer of the anionic H2msa- such that each (S)-Hpea+ ammonium group forms multiple hydrogen bonds with three H2msa- carboxy groups and that each (S)-Hpea+ phenyl group forms a C—H···π interaction with a H2msa- methine group, leading to the excellent optical resolution of the racemic H3msa with the use of (S)-pea.