Comparison of racemic epi-inosose and (-)-epi-inosose

The conversion of myo-inositol to epi-inositol can be achieved by the hydride reduction of an inter­mediate epi-inosose derived from myo-inositol. (-)-epi-Inosose, (I), crystallized in the monoclinic space group P2₁, with two independent mol­ecules in the asymmetric unit [Hosomi et al. (2000). Acta Cryst. C56, e584-e585]. On the other hand, (2RS,3SR,5SR,6SR)-epi-inosose, C₆H₁₀O₆, (II), crystallized in the ortho­rhom­bic space group Pca2₁. Inter­estingly, the conformation of the mol­ecules in the two structures is nearly the same, the only difference being the orientation of the C-3 and C-4 hy­droxy H atoms. As a result, the mol­ecular organization achieved mainly through strong O-H...O hydrogen bonding in the racemic and homochiral lattices is similar. The compound also follows Wallach's rule, in that the racemic crystals are denser than the optically active form.

S1. Comment

epi-Inositol is known to affect regulation of the myo-inositol biosynthetic pathway (Shaldubina et al., 2002) and has been evaluated as a potential antidepressant drug that could interact with the Li⁺ ion and myo-inositol receptors in the brain (Einat et al., 1998; Belmaker et al., 1998; Williams et al., 2002). We have earlier reported the synthesis of epi-inositol by the reduction of racemic epi-inosose (Patil et al., 2011).

A Cambridge Structural Database (CSD, version 5.31; Allen, 2002) search yielded the structure of the optically active (-)-epi-inosose (CSD refcode: XEGVUA) prepared enantioselectively by a bioconversion from myo-inositol (Hosomi et al., 2000). We were thus presented with an opportunity for the comparison of the molecular assembly in the crystals of these homochiral, (I), and racemic, (II), inososes. Single-crystal X-ray intensity measurements for crystals of (II) were recorded at ambient temperature (297 K) as reported for (I). Crystals of the racemic ketone are orthorhombic, belonging to the space group Pca2₁ (Fig. 1), while the homochiral ketone crystallizes in the non-centrosymmetric space group P2₁, with two independent molecules (A and B) in the asymmetric unit. The atom numbering for the racemic form is consistent with that reported for the optically active compound to enable easier comparison of the crystal structures.

The overlap of molecules in the asymmetric unit of (I) and the corresponding enantiomer in (II) reveals orientational differences in the hydroxy hydrogen atoms at C3 and C4 (Fig. 2). The conformation of the C3 hydroxy hydrogen of (II) (red, Fig. 2b) matches that of molecule B (green, Fig. 2b) in the asymmetric unit of crystals of (II), whereas the conformation of the C4 hydroxy group (red, Fig. 2a) matches that of molecule A (blue, Fig. 2a). There is a close correspondence in the unit-cell parameters of the two structures: the a axes lengths are nearly the same and interchange of the b and c axes of the orthorhombic racemic form results in edge lengths that are nearly identical to those of the homochiral crystal lattice [a = 11.197 (2), b = 8.932 (2), c = 6.976 (2) Å, β = 90.21 (2)° for (I)]. In accordance with Wallach's rule (Wallach, 1895; Brock et al., 1991), the racemic crystal is 1.7% denser than the enantiomer crystal, and its melting point is 492–495 K. The melting point of the crystals of (I) is not available for comparison. The unit cell of racemic epi-inosose consists of four molecules, i.e. two pairs of enantiomers, whereas that of (-)-epi-inosose contains two pairs of the two symmetry-independent molecules of the asymmetric unit. The presence of five hydroxy groups and a ketone carbonyl results in extensive hydrogen-bonding interactions in the crystal.

In the crystals of (II), each enantiomer forms a homochiral O6—H6···O4 hydrogen-bonded chain along the c axis with adjacent heterochiral molecular chains along the a axis linked by short and linear O3—H3···O2, O4—H4···O6, O5—H5···O3 and C3—H8···O4 interactions (Fig. 3a, Table 1). In the case of (I), each of the two molecules in the asymmetric unit forms a similar O6—H5···O4 hydrogen-bonded chain along the b axis. Interestingly, the ketone carbonyl oxygen (O7) of only one of the molecules (molecule B) is involved in O—H···O hydrogen bonding (O9—H12···O7), because of the conformational differences in the hydroxy groups of the two molecules in the asymmetric unit. The adjacent molecular chains along the a axis are linked by a large number of hydrogen-bonding interactions (Fig. 3b).

A view of these molecular chains down the c axis in (II) and b axis in (I) shows a corrugated sheet-like assembly (Fig. 4). Adjacent sheets are linked by bifurcated hydrogen-bonding interactions involving the ketone carbonyl oxygen O1 (O2—H2···O1 and C2—H7···O1) and O2—H2···O6 and C6—H11···O2 contacts in the racemic crystal (Fig. 4a, Table 1). In the crystals of the optically active form, the neighbouring sheets are linked by O2—H1···O9, O3—H2···O10, O10—H13···O11, O11—H14···O6, C2—H6···O1 and C6—H10···O2 contacts (Fig. 4b). Thus, the overall molecular organization in the crystals of the racemic and enantiopure compound is remarkably similar. This is primarily due to the fact that the second molecule in the asymmetric unit of (I) plays the role of the second enantiomer in the crystal packing. While the thermodynamic stability of the two crystals cannot be experimentally evaluated owing to the absence of adequate thermal data, estimation of lattice energies for (I) and (II) using the Oprop module of the OPiX program suite (Gavezzotti, 2003) yielded values of -204.75 (I) and -253.5 (II) kJ mol^-1, consistent with the crystal densities.

Related literature

For related literature, see: Belmaker et al. (1998); Brock et al. (1991); Einat et al. (1998); Gavezzotti (2003); Hosomi et al. (2000); Patil et al. (2011); Shaldubina et al. (2002); Sheldrick (2008); Wallach (1895); Williams et al. (2002).

S2. Experimental

Racemic epi-inosose, (II), was synthesized as reported previously (Patil et al., 2011). Prism-shaped crystals (m.p. 492–495 K) were obtained by slow evaporation from a solution in hot water.

S3. Refinement

All inositol ring H atoms were placed in geometrically idealized positions with C—H = 0.98 Å. They were constrained to ride on their parent atoms, with U_iso(H) = 1.2U_eq(C). The O-bound H atoms were located in difference Fourier maps and refined isotropically. The refined O—H distances were in the range 0.79 (2)–0.92 (2) Å. The data were merged using MERG3 in SHELXL97 (Sheldrick, 2008), according to the standard procedure for X-ray Mo Kα measurements of chemical compounds without heavy atoms.

Computing details

Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures

	Fig. 1. The molecular structure of racemic epi-inosose, (II) [the (2R,3S,5S,6S)-enantiomer], showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. The overlap of the molecules in the crystals of (-)-epi-inosose, (I), and racemic epi-inosose, (II), showing the difference in orientation of the hydroxy groups. In (a), one of the two independent molecules in the asymmetric unit of (I) (blue in the electronic version of the paper) and the corresponding enantiomer in (II) (red) is shown, while in (b) the second independent molecule in the asymmetric unit of (I) (green) and the corresponding enantiomer in (II) (red) is shown.
	Fig. 3. Molecular chains linked through hydrogen-bonding interactions (dotted lines) in the crystal structures of (a) (II) and (b) (I). The different colours represent the enantiomers of (II) in (a) (green and yellow) and the independent molecules in the asymmetric unit of (I) in (b) (green and blue). Grey molecules are present in the asymmetric unit. H atoms not involved in hydrogen bonding have been omitted. [Symmetry codes: (i) -x, -y, z-1/2; (ii) x-1/2, -y, z; (iii) -x+1/2, y, z+1/2; (iv) x+1/2, -y, z; (v) -x+1/2, y, z-1/2; (vi) -x+2, y-1/2, -z; (vii) -x+2, y+1/2, -z; (viii) -x+1, y+1/2, -z; (ix) -x+1, y-1/2, -z; (x) x, y-1, z.]
	Fig. 4. A view of the molecular packing down (a) the c axis in crystals of (II) and (b) the b axis in crystals of (I). Dotted lines represent hydrogen-bonding interactions, some of which (shown in Fig. 3) have been omitted for clarity. [Symmetry codes: (xi) -x, -y+1, z+1/2; (xii) x+1/2, -y+1, z; (xiii) x+2, y+1/2, -z+1; (xiv) -x+2, y-1/2, -z+1; (xv) -x+1, y+1/2, -z+1; (xvi) -x+1, y-1/2, -z+1.]

(2RS,3SR,5SR,6SR)-epi-inosose

Crystal data

C6H10O6	F(000) = 376
Mr = 178.14	Dx = 1.725 Mg m^−3
Orthorhombic, Pca21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2c -2ac	Cell parameters from 2280 reflections
a = 11.1825 (18) Å	θ = 2.9–27.8°
b = 6.9752 (12) Å	µ = 0.16 mm^−1
c = 8.7930 (15) Å	T = 297 K
V = 685.9 (2) Å^3	Prism, colourless
Z = 4	0.29 × 0.29 × 0.17 mm

Data collection

Bruker SMART APEX CCD area-detector diffractometer	653 independent reflections
Radiation source: fine-focus sealed tube	647 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.016
ϕ and ω scans	θmax = 25.0°, θmin = 2.9°
Absorption correction: multi-scan (SADABS; Bruker, 2003)	h = −10→13
Tmin = 0.956, Tmax = 0.974	k = −8→8
3225 measured reflections	l = −10→10

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.026	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.068	H atoms treated by a mixture of independent and constrained refinement
S = 1.15	w = 1/[σ^2(Fo^2) + (0.0491P)^2 + 0.0637P] where P = (Fo^2 + 2Fc^2)/3
653 reflections	(Δ/σ)max = 0.002
129 parameters	Δρmax = 0.25 e Å^−3
1 restraint	Δρmin = −0.13 e Å^−3

Special details

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	−0.03349 (15)	0.3927 (3)	−0.0752 (2)	0.0405 (5)
O2	0.19141 (13)	0.4974 (2)	−0.03356 (19)	0.0276 (4)
O3	0.31840 (13)	0.2909 (2)	0.1949 (2)	0.0285 (4)
O4	0.19401 (13)	−0.0091 (2)	0.3489 (2)	0.0262 (4)
O5	0.01515 (14)	−0.0554 (2)	0.1209 (2)	0.0286 (4)
O6	−0.17930 (12)	0.2105 (2)	0.1184 (2)	0.0260 (4)
C1	0.00672 (18)	0.3622 (3)	0.0495 (3)	0.0226 (5)
C2	0.13256 (17)	0.4188 (3)	0.0947 (2)	0.0222 (5)
H7	0.1287	0.5159	0.1752	0.027*
C3	0.19932 (18)	0.2409 (3)	0.1551 (3)	0.0222 (5)
H8	0.2032	0.1469	0.0725	0.027*
C4	0.13021 (18)	0.1496 (3)	0.2869 (2)	0.0215 (4)
H9	0.1212	0.2461	0.3670	0.026*
C5	0.0049 (2)	0.0883 (3)	0.2355 (2)	0.0217 (5)
H10	−0.0385	0.0353	0.3227	0.026*
C6	−0.06406 (18)	0.2616 (3)	0.1732 (3)	0.0227 (4)
H11	−0.0750	0.3526	0.2570	0.027*
H6	−0.166 (3)	0.147 (4)	0.046 (4)	0.040 (9)*
H5	−0.044 (3)	−0.142 (4)	0.139 (3)	0.031 (7)*
H2	0.246 (4)	0.573 (4)	0.000 (5)	0.058 (9)*
H4	0.215 (3)	−0.081 (4)	0.286 (4)	0.040 (9)*
H3	0.316 (3)	0.365 (4)	0.275 (5)	0.061 (11)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0343 (8)	0.0523 (10)	0.0348 (10)	−0.0097 (7)	−0.0117 (8)	0.0174 (10)
O2	0.0275 (8)	0.0312 (8)	0.0241 (8)	−0.0091 (6)	−0.0006 (6)	0.0040 (7)
O3	0.0182 (7)	0.0361 (9)	0.0311 (9)	0.0002 (6)	−0.0011 (7)	−0.0015 (8)
O4	0.0284 (8)	0.0282 (8)	0.0221 (8)	0.0059 (6)	−0.0001 (6)	0.0037 (8)
O5	0.0314 (8)	0.0255 (8)	0.0290 (9)	−0.0023 (7)	−0.0005 (8)	−0.0047 (7)
O6	0.0187 (8)	0.0320 (8)	0.0273 (9)	−0.0007 (6)	−0.0010 (7)	−0.0028 (8)
C1	0.0239 (10)	0.0206 (9)	0.0234 (11)	0.0027 (7)	−0.0029 (9)	0.0023 (9)
C2	0.0246 (10)	0.0227 (10)	0.0193 (10)	−0.0021 (7)	−0.0003 (8)	−0.0007 (8)
C3	0.0188 (8)	0.0263 (10)	0.0215 (12)	−0.0004 (8)	−0.0008 (8)	−0.0030 (8)
C4	0.0234 (10)	0.0242 (9)	0.0169 (10)	0.0035 (8)	0.0006 (8)	−0.0001 (9)
C5	0.0227 (10)	0.0248 (10)	0.0175 (11)	−0.0005 (8)	0.0024 (8)	0.0006 (8)
C6	0.0196 (9)	0.0258 (9)	0.0225 (11)	−0.0016 (8)	−0.0010 (8)	−0.0017 (9)

Geometric parameters (Å, º)

O1—C1	1.204 (3)	C1—C2	1.515 (3)
O2—C2	1.416 (3)	C1—C6	1.517 (3)
O2—H2	0.85 (4)	C2—C3	1.543 (3)
O3—C3	1.420 (2)	C2—H7	0.9800
O3—H3	0.87 (4)	C3—C4	1.531 (3)
O4—C4	1.425 (2)	C3—H8	0.9800
O4—H4	0.78 (4)	C4—C5	1.533 (3)
O5—C5	1.426 (3)	C4—H9	0.9800
O5—H5	0.91 (3)	C5—C6	1.535 (3)
O6—C6	1.421 (3)	C5—H10	0.9800
O6—H6	0.79 (3)	C6—H11	0.9800
C2—O2—H2	107 (3)	C2—C3—H8	107.8
C3—O3—H3	108 (2)	O4—C4—C5	110.74 (16)
C4—O4—H4	112 (2)	O4—C4—C3	111.10 (17)
C5—O5—H5	106.4 (17)	C5—C4—C3	110.75 (17)
C6—O6—H6	104 (2)	O4—C4—H9	108.0
O1—C1—C2	122.7 (2)	C5—C4—H9	108.0
O1—C1—C6	122.65 (19)	C3—C4—H9	108.0
C2—C1—C6	114.65 (18)	O5—C5—C4	109.33 (16)
O2—C2—C1	108.87 (17)	O5—C5—C6	109.98 (18)
O2—C2—C3	111.13 (16)	C4—C5—C6	110.20 (16)
C1—C2—C3	109.29 (16)	O5—C5—H10	109.1
O2—C2—H7	109.2	C4—C5—H10	109.1
C1—C2—H7	109.2	C6—C5—H10	109.1
C3—C2—H7	109.2	O6—C6—C1	110.24 (18)
O3—C3—C4	112.90 (19)	O6—C6—C5	112.30 (17)
O3—C3—C2	109.93 (18)	C1—C6—C5	110.97 (16)
C4—C3—C2	110.54 (16)	O6—C6—H11	107.7
O3—C3—H8	107.8	C1—C6—H11	107.7
C4—C3—H8	107.8	C5—C6—H11	107.7
O1—C1—C2—O2	−3.6 (3)	O4—C4—C5—O5	−60.6 (2)
C6—C1—C2—O2	175.53 (16)	C3—C4—C5—O5	63.1 (2)
O1—C1—C2—C3	−125.2 (2)	O4—C4—C5—C6	178.37 (18)
C6—C1—C2—C3	54.0 (2)	C3—C4—C5—C6	−57.9 (2)
O2—C2—C3—O3	58.6 (2)	O1—C1—C6—O6	0.6 (3)
C1—C2—C3—O3	178.73 (18)	C2—C1—C6—O6	−178.56 (16)
O2—C2—C3—C4	−176.14 (16)	O1—C1—C6—C5	125.7 (2)
C1—C2—C3—C4	−56.0 (2)	C2—C1—C6—C5	−53.5 (2)
O3—C3—C4—O4	−53.4 (2)	O5—C5—C6—O6	57.1 (2)
C2—C3—C4—O4	−176.96 (16)	C4—C5—C6—O6	177.66 (18)
O3—C3—C4—C5	−176.85 (17)	O5—C5—C6—C1	−66.8 (2)
C2—C3—C4—C5	59.5 (2)	C4—C5—C6—C1	53.8 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O1^i	0.85 (4)	2.57 (4)	3.191 (2)	131 (3)
O2—H2···O6^i	0.85 (4)	2.02 (4)	2.833 (2)	159 (4)
O3—H3···O2^ii	0.87 (4)	1.93 (4)	2.791 (3)	172 (4)
O4—H4···O6^iii	0.78 (4)	2.10 (4)	2.844 (2)	161 (3)
O5—H5···O3^iv	0.91 (3)	1.92 (3)	2.822 (2)	171 (2)
O6—H6···O4^v	0.79 (3)	2.01 (4)	2.760 (2)	160 (3)
C2—H7···O1^vi	0.98	2.52	3.374 (3)	145
C3—H8···O4^vii	0.98	2.52	3.422 (3)	152
C6—H11···O2^vi	0.98	2.49	3.392 (3)	154

Symmetry codes: (i) x+1/2, −y+1, z; (ii) −x+1/2, y, z+1/2; (iii) x+1/2, −y, z; (iv) x−1/2, −y, z; (v) −x, −y, z−1/2; (vi) −x, −y+1, z+1/2; (vii) −x+1/2, y, z−1/2.