ELSEVIER

CARBOHYDRATE RESEARCH

Carbohydrate Research 265 ( 1994) 197-206

Conformations and structure studies of sugar lactones. Part III. The composition and conformation

of D-mannurono- y-lactone in solution, and the structural analysis of its p anomer in the solid

state * M. Ashraf Shalaby *, Frank R. Fronczek, David Vargas,

Ezzat S. Younathan Departments of Biochemistry and Chemistry, Louisiana State Universiv, Baton Rouge, LA 70803 USA

Received 3 March 1994; accepted 16 June 1994

Abstract

Complete analyses of the proton and carbon chemical-shift assignments of D-mannurono-y-lactone (1) have been achieved by 1D and 2D NMR spectral measurements. At equilibrium, the anomeric LY and p forms were present in the ratio of 34:66 in D,O and 72:28 in Me,SO-d,. The solution data indicated that the dienvelope conformation ‘E:E, to be the favored conformation of 1 in solution. The crystal structure of 1 was determined, and it showed that the crystalline form is the /3 anomer, a bicyclic structure, consisting of fused five-membered furanose and lactone rings, in agreement with an earlier deduction from chemical evidence. In contrast to the solution conformation, the furanose ring adopts a twist conformation lying between the $T and ‘E conformations with phase angle (P) and pseudorotation amplitude (7,) of - 44.23” and 37.9”, respectively, whereas the lactone ring adopts an envelope E5 conformation slightly distorted towards 6T5 with a phase angle (P) of - 22.3” and a pseudorotation amplitude (7,) of 18.4”. The molecules are linked in the crystal through a hydrogen-bonding network that involves all hydroxyl groups as well as the ring oxygen atoms.

Keywords: X-ray crystal structure; Lactone; NMR spectroscopy; Conformation

1. Introduction

D-Mannuronic acid, one of the three naturally occurring uranic acids, is of particular interest because it plays an important role in the structure of many marine plants. It consti-

* For Parts I and II, see Refs. [ 11 and [ 21, respectively. * Corresponding author (present address) : Center for Advanced Materials, Lawrence Berkeley Laboratory, 66-

226, One Cyclotron Road, University of California-Berkeley, Berkeley, CA 94720, USA.

OOOB-6215/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO8-6215 (94) 00234-7

198 MA. Shalaby et al. / Carbohydrate Research 265 (1994) 197-206

tutes the major component of alginic acid, the main polysaccharide of brown algae [l-3]. The compound was first prepared by Nelson and Cretcher [4]in the form of crystalline lactone by acid hydrolysis of alginic acid. The study of stereomodels of D-mannuronic lactone (1) reveals that it can form a bicyclic structure having two five-membered rings or two six-membered rings. The stereomeric relationship of the groups is such that a compound having both a five- and a six-membered ring is highly improbable. Thus the aldose function of a mannuronic y-lactone can exist only in the open-chain form or in the furanoid form, whereas the aldose function of mannuronic S-lactone can exist only in the open-chain or in the pyranoid form. Isbell and Frush [5] have shown that by oxidation with bromine, mannosaccharidic y-dilactone (2) [ 61 of well established structure, is formed directly from mannuronic lactone without change in the ring structure. Consequently, the latter substance, like mannosaccharic dilactone 2, has a bicyclic structure consisting of a furanoid structure for the sugar ring and a gamma structure for the lactone ring. The aim of the present work was to confirm this conclusion and report the 3D molecular structure of 1 as determined by X-ray crystallography, and compare its conformation in the crystal with that in solution as revealed by NMR spectroscopy.

2. Experimental

The crystalline sample of D-mannurono- y-lactone (mp 197-199°C) was obtained from the late Dr. H. S. Isbell’s collection of rare sugars at The American University, Washington D.C., and was utilized in this study as received without further purification.

iVMR spectroscopy.-NMR spectra were recorded with Bruker Instruments Model AM 400 and AM 500 spectrometers. lH NMR spectra were measured at 400 and 500 MHz and 13C spectra at 100 and 125 MHz in D20 and Me,SO-d, solutions, using the standard pulse sequences and procedures. 2D COSY lH NMR spectra were recorded using 1024 (Qx256 ( tl) data points matrix with zero filling in the t, domain. 2D ‘H-13C chemical shift correlated spectra (HETCOR) in D,O were acquired by use of 2048 (t2) X 128 (tl) point data sets and 32 scans per acquisition with broad-band ‘H decoupling. 2D lH-13C inverse correlation spectra in Me,SO-d, were measured using the BIRD sequence, 2048 ( t,) X 128 ( tl> data point sets, zero-filled to 2048 points in the t, domain using broad-band GARP 13C resonance decoupling.

X-ray crystallography.-A colorless octahedral crystal with dimensions 0.50 X 0.50 X 0.50 mm was used for the structure determination. Intensity data were col- lected on an Enraf-Nonius CAD4 diffractometer with graphite-monochromated MoKa radiation (A = 0.71073) at 21°C. Unit-cell parameters were determined by least-squares refinement using 25 accurately centered reflections (23” < 213< 26”). The crystal data are

MA. Shalaby et al, / Carbohydrate Research 265 (1994) 197-206 199

Table 1 Crystallographic data for n-mannurono-y-la&one ( 1)

Molecular formula Molecular weight Melting point (“C) Crystal dimensions (mm) Space group a (A> Volume ( A3) 2 (molecules/cell) F (000) P (cm-l) Radiation (graphite monochromator) Calculated density (g cm - ‘) Unique reflections Observed data [I> lo(I)] S (141 variables) Final residual factors R

RW

C&O, 176.1 197-199 0.50 x 0.50 x 0.50 Cubic P213 12.8170(7) 2105.5( 1) 12 1104 1.44 M0Ka

1.667 1155 923 1.093

0.030 0.031

summarized in Table 1. A total of 4497 reflections was measured by w - 28 scans (one quadrant having 2” < 20 < 40’ and an octant having 40” < 20 < 60”) with variable scan rate (0.61-3.30 deg min-1 ). Redundant data were averaged to yield 1155 unique data ( Z?int = 0.021) ; 923 reflections with I > 1 u(Z) were used in the refinement. Crystal stability was monitored by recording three standard reflections every 10 000 s; a linear decay cor- rection of 2.1% was applied. The effect of absorption for this compound was very small and was neglected in our calculations. Systematic absences uniquely specified that the crystal belongs to cubic space group P213.

The structure was solved by direct methods using the program MULTAN-80 [ 71 which revealed the positions of all nonhydrogen atoms. It was refined by a full-matrix least-squares method based upon F, using data for which 1> lcr(l), weights w = 4Fz[ a’(1) + (0.02 Fz) 2] - ’ using the MolEN programs [ 81. Scattering factors [ 91 and anomalous coefficients [lo] were taken from the International Tables for X-ray Crystallography. Nonhydrogen atoms were refined anisotropically. The H-atom coordinates were located by AF, and hydrogen atoms were refined isotropically. Atomic coordinates and equivalent isotropic thermal parameters are given in Table 2 2. Final R = 0.030 for 923 observed data (0.052 for all 1155 data), R, = 0.031, and S= 1.093 for 141 variables. In the final cyclic of refinement, the maximum shift was 0.01~~ and the final difference-electron-density synthesis showed maximum and minimum electron densities of 0.20 and -0.06 eA-3, respectively. All calculations were performed on a VAX computer. The crystal structure is represented by an ORTEP [ 111 drawing, which also shows the atom numbering in the molecule.

’ Lists of observed and calculated structure-amplitudes, anisotropic thermal parameters, and torsion angles for 1 have been deposited with the Cambridge Crystallographic Data Centre and may be obtained on request from the Director, Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 lEZ, UK.

200 MA. Shalaby et al. / Carbohydrate Research 265 (1994) 197-206

Table 2 Atomic coordinates and isotropic a thermal parameters for D-mannurono-y-lactone ( 1)

o-1 o-2 o-3 o-4 o-5 O-6 C-l c-z c-3 c-4 c-s c-4 H-IOH H-20H H-50H H-l H-Z H-3 H-4 H-5

0.2812(l) 0.45991( 8) 0.29683( 9) 0.2026( 1) 0.0471919) 0.1841( 1) 0.2957(2) 0.3721( 1) 0.3026( 1) 0.1943( 1) 01248( 1) 0.2011(l) 0.256( 2) 0.436( 1) 0.078( 1) 0.318( 1) 0.394( 1) 0.327( 1) 0.170(l) 0.093( 1)

0.0149( 1) 0.1281(l) 0.18183(9) 0.1544( 1) 0.1163(l) 0.0846( 1) 0.0950( 2) 0.1724( 1) 0.2352( 1) 0.2276( 1) 0.1864( 1) 0.1421( 1)

-0.037(2) O.OSO( 1) 0.075 ( 1) 0.076(l) 0.212(l) 0.304( 1) 0.295( 1) 0.243(2)

0.5290( 1) 0.5060( 1) 0.38155 (9) 0.6144( 1) 0.4752( 1) 0.2935( 1) 0.6015(l) 0.5547( 1) 0.4815( 1) 0.5308( 2) 0.4436( 1) 0.3647( 1) 0.555(Z) 0.465(2) 0.525( 2) 0.674( 1) 0.612( 1) 0.472( 1) 0.557( 1) 0.409( 2)

3.68( 3) 2.62( 2) 2.42( 2) 3.05(3) 3.02( 2) 4.01(3) 2.96(4) 2.39(3) 2.22(3) 2.43(3) 2.39( 3) 2.53(3) 5.8(6) 3.8(5) 4.8(6) 3.5(4) 2.8(4) 2.4(4) 3.4(4) 4.0(5)

a Equivalent isotropic thermal parameters are given for nonhydrogen atoms. The definition of this quantity is B,,= (8rr2/3)ZiZj Uij;ll’%*ai-aj.

3. Results and discussion

The ‘H NMR study at 400 and 500 MHz in D20 and Me,SO-ds has been carried out with the aim of the complete assignment of the chemical shifts and 3Jn, coupling constants that were not available from a previous study [ 121 at 100 MHz, probably because of the numerous overlapping resonances within a narrow range of chemical shifts and couplings of similar magnitude. The ‘H-IH COSY spectrum in Me,SO-d, is illustrated in Fig. 1 together with the 500-MHz ‘H NMR spectrum. The ‘H-13C correlation spectrum in D,O is shown in Fig. 2, and the proton and carbon chemical shift assignments are listed in Tables 3 and 4, respectively.

The ‘H NMR spectra showed a typical well-resolved pattern of a sugar furanose ring. It also showed the presence of the two anomeric forms, (Y and p, in D,O as well as in Me,SO- &. Integration of the peaks for the anomeric protons revealed that the ratios of LX/~ anomers were 34:66 in D,O and 72:28 in Me,SO-d,. This clearly demonstrates that in Me,SO, a solvent that inhibits mutarotation, when the restrications of the crystal lattice are removed, the fraction of the Q anomer is considerably higher than at equilibrium. Due to residual water in the solvent, ring opening occurs, thereby allowing a conformational equilibration to take place which leads to the KP ratio similar to that in aqueous solution.

The chemical shifts of the anomeric protons were used to tentatively assign the H-l resonances of the (Y and j3 anomers since their ‘H-IH coupling constants were nearly of equal magnitudes. Thus, the extreme downfield proton signal in D,O spectrum at 6 5.31

MA. ShaEaby et al. / Carbohydrate Research 265 (1994) 197-206 201

OH

0 H-I(6) 0 -0 H-3(a) a*

0 0

0 0

0 m

0 a l”“l”“i”“l”“l”“I”

6.5 6.0 5.5 5.0 4.5 4.0

5 (Pm) Fig. 1. ‘H-‘H COSY spectrum of D-mannnrono-y-lactone (1) in Me,SO-d,. The 500-MHz ‘H NMR spectrum is shown along the F2 axis.

was assigned to the p anomer and that at 6 5.15 to the Q anomer, whereas the proton signal in Me,SO-d, spectrum at 6 4.96 was assigned to the cy anomer and that at S 5.13 to the /3 anomer. This assignments were confirmed by NOE experiments in the Me,SO-d, solution, which showed an NOE between HO-1 at 6 6.68 and H-2 at 6 3.90, thus, indicating a syn relationship, whereas no NOE was observed upon saturation of the HO-l signal at 6 6.53 with the corresponding H-2 signal at 6 3.98 (see Table 3). Based on the assignment of the 'H resonances, the ring 13C resonances were assigned unambiguously by ‘H-13C correlation spectra (see Table 4).

The study of the molecular structure of the title compound 1 revealed that the molecule exists in the crystalline state in the p form (Fig. 3). It has a bicyclic structure, consisting

4.5

5.0

6 @pm)

5.5

6.0

6.5

202 MA. Shalaby et al. / CarbohydrateResearch 265 (1994) 197-206

100.0 90.0 6(ppm) 80.0

Fig. 2. lH-13C correlation spectrum of D-mannurono-y-lactone (1) in DZO measured at 400 MHz. The ‘H and 13C NMR spectra are shown along the F1 andFz axes, respectively.

of fused furanose and five-membered lactone rings with a dihedral angle of 102.68( 6)’ between their best planes; 0-4-C-l-C-2-C-3-C-4 for the furanose ring and 0-3-C-3-C- 4-C-5-C-6 for the lactone ring. The furanoid ring adopts an intermediate conformation lying between the :T and ‘E conformations with phase angle (P) and pseudorotation

Table 3 ‘H NMR spectral data for D-mannurono-y-lactone (1) measured in DZO at 400 MHz and in Me,SO-d, at 500 MHZ

Solvent Isomeric composition

Chemical shifts ( S values) a Coupling constants (Hz)

H-l H-2 H-3 H-4 H-5 3J1.z ‘Jz.J 3J~,., 3Ja,s

D,O j3 anomer 66% 5.31d 4.23t 5.01t 4.73dd 4.6d 4.7 4.7 4.7 6.5 cy anomer 34% 5.15d 4.16dd 4.98dd 4.85dd 4.64d 5.5 4.4 3.2 5.0

Me,SO-d, b (Y anomer 72% 4.96dd 3.90ddd 4.72dd 4.59dd 4.43dd 5.1 4.4 3.1 4.8 p anomer 28% 5.13t 3.98dddd 4.74dd 4Sldd 4.38dd 4.3 4.8 4.4 6.5

a Signal multiplicities: d, doublet; t, triplet, b Chemical shifts and coupling constants of hydroxy-group protons: HO-l, 6.68d, Jo.,,,,.,, 6.2; HO-2, 5.52d, JH.~~,H.~, 6.4; HO-5, 5.86d, JH_5,0H_5, 7.7 for the 1y anomer and HO-l, 6.53dd, JN_I,oH_l, 4.0, JK.Z,oH_l, 0.8; HO-2,4.66d, .&oH_~, 9.3; HO-5,5.26d, & OH_5, 8.1 for the /3 anomer.

MA. Shalaby et al. / Carbohydrate Research 26.5 (1994) 197-206 203

Table 4 13C NMR chemical shifts (ppm) for D-mannurono-y-lactone (1) in D20 (100 MHz) and in MezSO-dh ( 125

MHz)

Solvent

DzO

Me,SO-&

Isomers

fi anomer IY anomer ff anomer p anomer

C-l c-2 c-3 c-4 c-5 C-6

98.03 74.36 82.06 77.77 72.09 180.16 103.73 79.09 82.81 78.73 72.31 179.85 101.82 77.14 79.44 75.89 69.85 175.85

95.31 72.56 78.17 74.95 69.38 175.39

amplitude [ 131 (7,) of - 44.23” and 37.9”, respectively. The lactone ring adopts the envelope E5 conformation slightly distorted towards 6Ts, with a phase angle (P) of - 22.3” and a pseudorotation amplitude (7,) of 18.4”. Displacement of the atoms from the least- squares plane suggest the E, conformation. Four atoms, C-4, C-3,0-3, and C-6 are coplanar within an observed value of 0.005 A. The fifth atom, C-5, is displaced by 0.29 A from this plane in the exe direction with respect to the dihedral angle between the rings. The lactone group is not planar, having a carbon atom, C-3, 0.254(2) A out of the plane C-5, O-3, C- 6, O-6, which is planar within 0.005 (2) A. Thus, in spite of the fact that the lactone group is not planar, the C-3-0-3-C-6-0-6 torsion angle being 170.5(2)“, the lactone ring adopts an envelope conformation, which is presumably due to the strain generated from its fusion to the furanose ring.

The bond lengths, bond angles and selected torsion angles in the molecule are listed in Table 5. The C-C distances are normal, ranging from 1.517(2) to 1.528(2) A and are in good agreement with the values reported for other related structures [ 14-161. The C-OH bonds are 1.397(2), 1.406(2) and 1.400(2) A, with no significant difference from the mean value of 1.401( 1)w. The C=O bond has a length of 1.193(2)& which is within the range of expected values for the carbonyl group. The ring angles at the carbon atoms of the lactone range from 102.2 (1)’ to 106.7( 1)” with the exception of that at the carbonyl atom, C-6, which is larger, 109.8(l)“, as would be expected from the sp2 hybridization of its carbon atom.

Fig. 3. Molecular structure and atomic numbering of D-mannurono-y-lactone (1). Nonhydrogen atoms are represented with 40% ellipsoids and hydrogen atoms with circles of arbitrary radius.

204 MA Shalahy et al. / Carbohydrate Research 265 (1994) 197-206

Table 5 Bond lengths, bond angles, and selected torsion angles in P-D-mannurono-y-lactone (1)

Atoms Length (A)

O-l-c-l 1.397( 2) o-2-c-2 1.406(2) o-3-c-3 1.454(2) O-3-C-6 1.346(2) 0-4-C-l 1.425(2) o-4-C-4 1.428(2) o-5-C-5 1.400( 2) Atoms Angle (deg) C-3-0-3-c-6 111.5( 1) c-1-0-4-C-4 109.0( 1) 0-1-C-1-0-4 111.0(l) o-l-c-l-C-2 107.6( 1) o-4-C-l-c-2 103.8(l) 0-2&c-2-c-1 115.4( 1) 0-2-C-2-c-3 114.1( 1) Atoms Angle (deg) c-6-0-S-C-3-c-4 -0.9(2) C-1-0-4-c-4-C-3 - 16.9( 2) o-3-C-3-c-4-C-5 - 10.7(2) C-4-C-5X-GO-3 - 1X.8(2)

Atoms Length (A)

C-2-H-2 C-3-H-3 C-4-H-4 C-5-H-S O-6-C-6 C-l-C-2 C-2-c-3 Atoms C-l-C-2-C-3 O-3-C-3-C-2 o-SC-3-C-4 C-2-C-3X-4 o-4-C-4-c-3 c-3-c-4-c-5 o-5-C-5-C-4 Atoms

0.93(2) 0.95(2) 0.98(2) 0.94( 2) 1.193(2) 1.517(3) 1.523( 2)

Angle (deg) 102.2( 1) 108.9( 1) 106.7( 1) 104.1(l) 106.5( 1) 104.5(l) 115.2( 1)

Angle (dcg) c-3-o-3-C-&C-S 12.6( 2) O-4-C-l-C-2-C-3 -37.7(2) C-2-C-3-C-4-O-4 - 7.7(2)

Atoms Length (A)

c-3-c-4 c-4-c-5 C-5-C-6 C-1-H-1 O-l-HO-l 0-2-HO-2 O-S-HO-5 Atoms 0-5-c-5-c-4 O-5-C-5-C-6 C-4-C-5-c-6 0-3-C-6-0-6 o-3-G&C-5 o-6-c-6-C-5

Atoms -bzle (%I C-4-0-4-C-l-C-2 34.7( 2) C-l-C-2-C-3-c-4 27.2(2) C-3-C-4-C-5-c-6 17.1(2)

1.528( 2) 1.523(2) 1.517(2) l.OO( 2) 0.81(Z) 0.X7(2) 0.91(2)

Angle (de@ 115.2( 1) 114.3( 1) 104.0(l) 121.6( 2) 109.8( 1) 128.5(2)

In the crystal, the molecules are associated by an intermolecular hydrogen-bonding network, in which each molecule participates in six hydrogen bonds. The hydrogen-bond interactions are illustrated in Fig. 4, and the relevant hydrogen-xygen and oxygen-oxygen distances are listed in Table 6, together with angles about the H atoms. Two of the three hydroxyl groups, HO-2 and HO-5, participate both as intermolecular hydrogen-bond donors and acceptors, while the third hydroxyl group, HO-l, acts as donor only. Only the inter- molecular hydrogen bond involving O-l is near linearity. This is a result of the fact that O- 2 and O-5 donate to multiple acceptors. In addition to the intermolecular hydrogen bonds, HO-2 and HO-5 groups are each involved in two intramolecular contacts that meet the hydrogen bonding criteria O-H.**0 angle > 90” and H-*+0 distance < 3.0 A (see Table 6).

The values of the coupling constants and torsion angles of the vicinal protons obtained by X-ray analysis have been compared with those obtained by NMR spectroscopy through the Altona modification of the Karplus equation [ 171 (Table 7). In view of these results, one can conclude that the conformation in solution is different from that found in the crystal. This may be attributed to the intramolecular hydrogen bonds that fixes the conformation of 1 in the crystal, whereas in solution, these hydrogen bonds are to the external water mole- cules. This could certainly influence the orientations of the C-2-O-2 and C-5-O-S groups, which in turn have an effect on the conformation. The solution data for 1 indicated that the dienvelope conformation 2E:E, to be the favored conformation of 1 in solution.

To the best of our knowledge, when this work was started, the only structures reported for hexouronic lactones (Cambridge Structural Database, April 1993 release, class 45) were those of P-D-glucurono-y-lactone [ 1X] and some of its derivatives [ 19,201. A comparison

MA Shuluby et al. / Carbohydrate Research 265 (1994) 197-206 205

Fig. 4. Intermolecular hydrogen bonding scheme in the crystal of D-mannurono-y-lactone (1). For clarity, hydrogen atoms not involved in hydrogen bonding are omitted.

Table 6 Geometry of the hydrogen bonds in D-mannurono-y-la&one (1)

Number o..-0 (A) H.+-0 (A) O-H (A> O-H...0 (deg)

(i) Intermolecular 1. 0-l-H(O-1),--O-6” 2. 0-5-H(O-5) .-.0-Y 3. 0-Z-H(O-2)...0-Z6 (ii) Intramolecular 1.0~Z-H(O-2)...0-1 2.0-2-H(O-2)...03 3. 0-5-H( O-5) .-O-l 4. 0-5-H( O-5) . ..0-4

2816(Z) 2.02(2) 0.X1(2) 165(2) 2.783(2) 2.08(2) 0.91(2) 134(2) 2.792(2) 2.08(2) 0.87(2) 139(2)

2.727(2) 2.30(2) 0.87(2) llO(2) 2.718( 2) 2.46(2) 0.87(2) 113(2) 3.341(2) 2.72(2) 0.91(2) 126(2) 2.718( 2) 2.22(2) 0.91(2) 114(Z)

Symmetry operations: a - 0.5-z,-x,0.5 + y; b = 1 -z, x - 0.5, 0.5 -Y.

Table 7 A comparison of torsion angles and 3J mr coupling constant values (in parenthesis) between D-mannurono-y_ lactone vicinal protons derived from the ‘H NMR experiment ( DzO) and crystal structure

&de (deg) ‘H NMR X-ray

H-l-C-l-C-2-H-2 -36 [4.7] - 34(2) [5.2J H-2-C-2-C-3-C-3 36 [4.7] 41(2) [4.9] H-3-C-3-C-4-H-4 -36 [4.7] - lO(2) [7.2] H-4-C4-C-5-H-5 27 [6.5] 27(2) [6.7]

206 M.A. Shalaby et al. / Carbohydrate Research 265 (1994) 197-206

of the 3D structure of P-D-glucurono-y-lactone with the title compound 1 showed that in both compounds, the conformation of the fused furanose and la&one rings, as well as their bond lengths and angles, agreed very well. However, the two compounds differ in their hydrogen-bonding schemes and crystal packing. This can be attributed to the differences in configuration of the hydroxyl groups bearing carbon atoms. In P-D-glucurono-y-lactone, of the three hydroxyl groups that might be expected to form hydrogen bonds, one participates in two (one as donor and one as acceptor), the second in one (as donor only), and the third in none.

Acknowledgements

We are indebted to Professor Hassan S. El Khadem of The American University, Wash- ington D.C. for supplying the sample of D-mannurono-y-la&one. This work was supported, in part, by grant no. DK 40401 from the National Institutes of Health.

References

[l] M.A. Shalaby, F.R. Fronczek, and E.S. Younathan, Carbohydr. Rex., 264 (1994) 181-190. [2] M.A. Shalaby, F.R. Fronczek, and ES. Younathan, Carbohydr. Rex., 264 (1994) 191-198. [3] N. Eiichi, H. Anzai, N. Uchida, Nippon Srrisan Gakkaishi, 53(7) (1989) 1215-1219; F. Haugen and B.

Largen, Polym. Prepr. (Am. Chem. Sm., D~L’. Polym. Chem.), 30(2) (1989) 363-364. [4] W.L. Nelson and L.H. Cretcher, J. Am Chem. Sot., 54 (1932) 3409-3412. [5] H.S. Isbell and H.L. Frush, J. Res. Natl. Bur. Stand., 37 (1946) l-7. [6] D. Heslop and F. Smith, J. Chem. Sot., ( 1944) 574-576. [7] P. Main, S.J. Fiske, SE. Hull, L. Lessinger, G. Germain, J.P. Declercq, and M.M. Woolfson, MULTAN-80,

A System of Computer Programs for the Automatic Solution of Crystal Structures from X-ray Diffraction Data, Universities of York, UK and Louvain, Belgium.

[S] C.K. Fair, MolEN, An Interactive Structure Solution Procedure, Enraf-Nonius, Delft, The Netherlands, 1990.

[9] D.T. Cromer and J.T. Waber, International Tables for X-Ray Crystallography, Vol. IV, Table 2. 2B, The Kynoch Press, Birmingham, England, 1974 (current distributor, Kluwer Academic, Dordrecht).

[ 101 D.T. Cromer, International Tubbs for X-Ray Crystallography, Vol. IV, Table 2.3.1, The Kynoch Press, Birmingham, England, 1974 (current distributor, Kluwer Academic Publishers, Dordrecht) .

[ll] C.K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 1976.

[ 121 J-F. Kennedy, SM. Robertson, and M. Stacey, Carbohydr. Res., 57 ( 1977) 205-214. [ 131 C. Altona and M. Sundaralingam, J. Am. Chem. Sot., 94 (1972) 8205-8212. [ 14] G.A. Jeffrey and A.D. French, Chem. Sot. Spec. Publ., 6 ( 1978) 183-223. [ 151 F.H. Allen, Acta Crystullogr. Sect. B, 42 ( 1986) 515-522. [ 161 F.H. Allen, 0. Kennard, D.G. Watson, L. Brammer, A.G. Orpen, and R. Taylor, J. Chem. Sot., Perkin Trans.

2, (1987) SlS19. [ 171 C.A.G. Hasnoot, F.A.A.M. De Leeuw, and C. Altona, Tetrahedron, 36 (1980) 2783-2792. [ 181 S.H. Kim, G.A. Jeffrey, R.D. Rosenstein, and P.W.R. Corfield, Acta Crystallogr., 22 (1967) 733-743. [ 191 Z.R. Tors, B.K. Prodic, L. Golic, and S. Tomic, Carbohydr. Res., 152 ( 1986) 21-32. [20] B. Sheldrick, W. Mackie, and D. Akrigg, Acta Crystallogr. Sect. C, 39 (1983) 1257-12.59.