A Disaccharide Rearrangement Catalyzed by Molybdate Anion in Aqueous Solution
SCHEME 1
Qingquan Wu,† Qingfeng Pan,‡ Shikai Zhao,† Heidi Imker,‡ and Anthony S. Serianni*,‡ Omicron Biochemicals, Inc., 115 South Hill Street, South Bend, Indiana 46617, and Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556-5670
[email protected] ReceiVed December 5, 2006
Reaction of a galactosylated 2-C-(hydroxymethyl)-tetrofuranose with paramolybdate ion-exchange resin in aqueous solution at 67 °C gave an equililibrium mixture containing the reactant aldofuranose (42%) and a 2-ketopentofuranose galactosylated at O1 (58%). Observation of this stereospecific rearrangement supports prior arguments that substituents at C2 of the branched-chain aldofuranose reactant are located in a sterically accessible pocket of the putative dimolybdatesaccharide reactive complex during epimerization. This rearrangement provides a new and convenient route to 2-ketosugars glycosylated at the exocyclic C1 position. Carbohydrate complexes with cations and anions in aqueous solution have been studied extensively by Angyal and others1,2 and form the basis of convenient chromatographic methods to separate and purify saccharides.3-5 Ion binding strength is determined partly by the relative configuration of hydroxyl ligands on the saccharide. For example, complexation of the divalent Ca2+ ion is strongest for aldopyranosyl rings containing contiguous ax-eq-ax arrangements, whereas the presence of cis1,2-OH groups is important for cation binding to aldofuranosyl † ‡
Omicron Biochemicals, Inc. University of Notre Dame.
(1) Angyal, S. J. Sugar-Cation Complexes. In Carbohydrates in Solution; Advances in Chemistry Series, Vol. 117; American Chemical Society: Washington, DC, 1973; pp 106-120. (2) Baucke, E.; Behrends, R.; Fuchs, K.; Hagen, R.; Kaatze, U. J. Chem. Phys. 2004, 120, 8118-8124. (3) Angyal, S. J.; Bethell, G. S.; Beveridge, R. Carbohydr. Res. 1979, 73, 9-18. (4) Caruel, H.; Rigal, L.; Gaset, A. J. Chromatogr. 1991, 558, 89-104. (5) Angyal, S. J. Aust. J. Chem. 2002, 55, 1-2.
rings. Cation binding to sugars is relatively weak, however, and the biological implications of these interactions remain unclear. Anions such as borate, tungstate, arsenate, and molybdate bind saccharides strongly, but interestingly, the former three produce stable, unreactive complexes, whereas the latter forms a highly reactive complex that promotes an unusual, stereospecific rearrangement of the carbon skeleton.6 For example, the addition of catalytic amounts of resin-bound molybdate7 to an aqueous solution of D-[1-13C]erythrose 1 (pH 4.2) at 90 °C yields an equilibrium mixture of 1 (∼35%) and D-[2-13C]threose 2 (∼65%) after ∼3 h (Scheme 1). C2-Epimerization occurs with C1-C2 transposition and appears to proceed via an acyclic dimolybdate-saccharide intermediate.6 Recent work has shown that the aldehydic proton of 1 can be replaced by CH2OH, CH3, or CH2OPh substituents without compromising the viability of the transformation.8 2-C-(Methyl)-D-erythrose 3 and 1-deoxyD-xylulose 4 are thus interconverted by molybdate anion in aqueous solution, with the latter highly favored at equilibrium (Scheme 1). Given the tolerance for simple functional groups at C2 of branched-chain aldotetroses, we examined whether the rearrangement would proceed with a glycosyl residue present at this position. Methyl 2-C-(hydroxymethyl)-2,3-O-isopropylidene-D-erythrofuranoside 58 was prepared and condensed with 2,3,4,6-tetra-O-acetyl-R-D-galactopyranosyl bromide 6. Deprotection afforded β-D-galactopyranosyl-(1f2C)-2-C-(hydroxymethyl)-D-erythrose (7) (Scheme 2). The 1H NMR spectrum of 7 (Figure S1, Supporting Information) contained two singlets at 5.234 and 5.243 ppm, which were assigned to the furanose anomeric protons H1 (Table 1). Signal integration gave a 42/58 ratio, with the downfield H1 signal more intense. Two additional anomeric 1H signals at 4.436 and 4.403 ppm appeared as doublets with 3JH1′,H2′ ) 7.8 Hz. These signals were assigned to H1′ of the Gal residue. The relatively large 3JH1′,H2′ values show that the linkage has the β-configuration. Anomeric carbon signals were observed at 106.31, 105.95, 104.16, and 99.98 ppm (Figure S2, Table 1). The former two correlated with the Gal H1′ signals, and the latter two with the anomeric furanose signals (i.e., 104.16/5.234 and 99.98/5.243 (6) Hayes, M. L.; Pennings, N. J.; Serianni, A. S.; Barker, R. J. Am. Chem. Soc. 1982, 104, 6764-6769. (7) Clark, E. L.; Hayes, M. L.; Barker, R. Carbohydr. Res. 1986, 153, 263-270. (8) Zhao, S.; Petrus, L.; Serianni, A. S. Org. Lett. 2001, 3, 3819-3822.
10.1021/jo062494+ CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/20/2007
J. Org. Chem. 2007, 72, 3081-3084
3081
SCHEME 2
ppm). Because 3JH1,H2 were unavailable to assign furanoseanomeric configuration, two empirical rules were used to make the assignments. The syn-upfield rule9 was used to interpret the chemical shift differences between the H4R and H4S multiplets (Scheme 2), since prior work has shown10 that the R-erythro configuration yields a smaller difference (∼0.1 ppm) than the β-erythro (∼0.4 ppm). The cis-O1,O2 rule11 predicts that furanose anomeric carbon chemical shifts are more shielded in rings having O1 and O2 cis (e.g., in 7R). On the basis of these rules and HMQC data (Figure S3) to confirm the H4R/H4S 1H and 13C Chemical Shifts and 1H-1H Spin-Couplings in 7r and 7β in 2H2O
TABLE 1.
chemical shift (ppm)a nucleus
R-furanose β-furanose (7R) (7β)
H1 H2a H2b H3
5.243 4.175d 3.792d 4.161
5.234 3.946d 3.659d 4.385
H4R
4.092
4.260
H4S
3.846
3.703
H1′ H2′
4.403 3.549
4.436 3.559
H3′
3.646
3.657
H4′ H5′
3.907 3.681d
3.913 3.693d
H6′
3.781d
3.786d
H6′′
3.736d
3.740d
C1 C2a C2b C3 C4 C1′ C2′ C3′ C4′ C5′ C6′
99.98 80.51 73.92 72.77 73.20 105.95 73.50 75.29 71.34 77.91 63.75
signal assignments, the predominant furanose anomer was assigned to the R-configuration (7R). Furanose anomeric equilibrium in 7 contrasts with that found for D-erythrose (8) in which the β-furanose (8β) predominates in aqueous solution (Rf/βf ) 0.40 at 40 °C).12 The R-furanose is more favored in 2-C-(hydroxymethyl)-D-erythrose (9)13 with an Rf/βf ratio of 1.3, similar to that observed for 7 (1.4). Presumably the bulky CH2OR group at C2, being cis to O1, destabilizes the β-configuration in 7β and 9β. All proton and carbon chemical shifts in the Gal residue of 7 are influenced by furanose anomeric configuration, and a separate set of signals was observed for each anomer (Figures S1 and S2, Table 1).
JHH (Hz)b R-furanose (7R)
β-furanose (7β)
brc brc 2J 2J ) -11.0 H2a,H2b H2a,H2b ) -10.9 2J 2J H2a,H2b ) -11.1 H2a,H2b ) -10.8 3J H3,H4S ) 5.5 3J H3,H4R ) 6.4 3J 3J H3,H4R ) 6.4 H3,H4R ) 7.6 2J 2J H4R,H4S ) -9.5 H4R,H4S ) -8.9 3J 3J H3,H4S ) 5.4 H3,H4S ) 7.0 2J 2 JH4R,H4S ) -8.9 H4R,H4S ) -9.5 3J 3J H1′,H2′ ) 7.8 H1′,H2′ ) 7.8 3J 3J H1′,H2′ ) 7.8 H1′,H2′ ) 7.8 3J 3J H2′,H3′ ) 10.0 H2′,H3′ ) 10.0 3J 3J H3′,H4′ ) 3.5 H2′,H3′ ) 10.0 3J H2′,H3′ ) 10.0 3J H4′,H5′ ) 1.1 3J H5′,H6′′ ) 4.2 3J H5′,H6′ ) 8.0 3J H5′,H6′ ) 8.0 2J H6′,H6′′ ) -11.8 3J H5′,H6′′ ) 4.2 2J H6′,H6′′ ) -11.8
3J H4′,H5′ ) 1.0 3J H5′,H6′′ ) 4.2 3J H5′,H6′ ) 8.1 3J H5′,H6′ ) 8.1 2J H6′,H6′′ ) -11.7 3J H5′,H6′′ ) 4.2 2J H6′,H6′′ ) -11.7
104.16 82.63 73.80 73.44 72.77 106.31 73.63 75.32 71.37 77.89 63.79
a (0.001 ppm. b (0.1 Hz; redundant couplings are shown to illustrate internal consistency. c Anomeric signals are broadened as a result of the presence of long-range JHH. d Signal assignments to each anomer may be reversed.
3082 J. Org. Chem., Vol. 72, No. 8, 2007
JHH values in the 2-C-(hydroxymethyl)-D-erythrose moieties of 7R and 7β (Table 1) differ substantially from those observed in R- (8R) and β-D-erythrofuranoses (8β).10,14 The differences presumably result from a greater preference of the exocyclic CH2OH substituent of 7 to assume a quasi-equatorial or near quasi-equatorial orientation. For example,3JH3,H4S is larger in 7 than in 8 in both anomers, suggesting that H3 and H4S have a greater propensity to be trans in 7. Furanose conformations near E2/3E orient H3 and H4S approximately trans and the exocyclic CH2OH group quasi-equatorial. This preference appears reduced in 7R compared to 7β, however, presumably because O1 is quasi-equatorial in E2/3E conformations of 7R, which is an unfavorable orientation based on stereoelectronic considerations (anomeric effect).10 Intra-ring JHH values in 9β are similar to those observed in 7β,15 suggesting that glycosylation does not appreciably affect (9) Anteunis, M.; Danneels, D. Org. Magn. Reson. 1975, 7, 345-348. (10) Serianni, A. S.; Barker, R. J. Org. Chem. 1984, 49, 3292-3300. (11) Ritchie, R. G. S.; Cyr, N.; Korsch, B.; Koch, H. J.; Perlin, A. S. Can. J. Chem. 1975, 53, 1424-1433. (12) Serianni, A. S.; Pierce, J.; Huang, S.-G.; Barker, R. J. Am. Chem. Soc. 1982, 104, 4037-4044. (13) 13C chemical shifts for 9R/β (2H2O) were as follows: 101.63 (C1β), 97.15 (C1R), 80.58 (C2β), 78.76 (C2R), 70.99 (C4R), 70.62 (C3β), 70.29 (C4β), 69.95 (C3R), 63.32 (C2′R), 62.53 (C2′β). 1H chemical shifts for 9 (2H2O) were as follows: 5.206 (H1β), 5.133 (H1R), 4.304 (H3β), 4.241 (H4Rβ), ∼4.08 (H3/H4RR), ∼3.85 (H4SR), 3.798 (H2aβ), 3.697 (H4Sβ), 3.680 (H2bβ), 3.558 (H2a/bR). (14) 3JH3,H4R, 3JH3,H4S, and 2JH4R,H4S values in 8R and 8β are 5.1, 3.0, and -10.1 Hz and 4.9, 3.5, and -9.8 Hz, respectively. (15) In 9β, 3JH1,H2 ) br, 3JH3,H4R ) 7.4 Hz, 3JH3,H4S ) 6.6 Hz, and 2J H4R,H4S ) -8.9 Hz.
1H and 13C Chemical Shifts and 1H-1H Spin-Couplings in 10r and 10β in 2H2O
TABLE 2.
chemical shift (ppm)a nucleus
R-furanose β-furanose (10R) (10β)
H1a H1b H3 H4
4.039 3.829 4.116 4.320
3.988 3.767 4.121 4.391
H5R
4.269
4.213
H5S
3.921
3.681
H1′ H2′
4.466 3.584
4.457 3.576
H3′
3.672
3.672
H4′ H5′ H6′
∼3.94 ∼3.72 ∼3.81
∼3.94 ∼3.72 ∼3.81
H6′′
∼3.76
∼3.76
C1 C2 C3 C4 C5 C1′ C2′ C3′ C4′ C5′ C6′
72.57 107.76 83.14 78.36 74.85 105.84 73.44 75.25 71.30 77.82 63.64
73.88 104.70 79.66 77.53 72.57 105.90 73.38 75.22 71.26 77.85 63.64
SCHEME 3
JHH (Hz)b R-furanose (10R)
β-furanose (10β)
2J H1a,H1b ) -10.8 2J H1a,H1b ) -10.8 3J H3,H4 ) 2.6 3J H4,H5R ) 6.1 3J H4,H5S ) 4.1 3J H5R,H4 ) 6.0 2J H5R,H5S ) -9.8 3J H5S,H4 ) 4.1 2J H5R,H5S ) -9.7 3J H1′,H2′ ) 7.8 3J H1′,H2′ ) 7.9 3J H2′,H3′ ) 10.0 3J H3′,H4′ ) 3.4 3J H2′,H3′ ) 9.9 3J H4′,H5′ ) ∼1.1
2J H1a,H1b ) -11.0 2J H1a,H1b ) -10.9 3J H3,H4 ) 5.4 3J H4,H5R ) 6.4 3J H4,H5S ) 5.2 3J H5R,H4 ) 6.4 2J H5R,H5S ) -9.6 3J H5S,H4 ) ∼5.2 2J H5R,H5S ) ∼-9.3 3J H1′,H2′ ) 7.8 3J H1′,H2′ ) 7.8 3J H2′,H3′ ) 10.0 3J H3′,H4′ ) 3.4 3J H2′,H3′ ) 9.9 3J H4′,H5′ ) 1.1
3J H5′,H6′ ) ∼8.0 2J H6′,H6′′ ) ∼-11.8 3J H5′,H6′′ ) ∼4.2 2J H6′,H6′′ ) ∼-11.7
3J H5′,H6′ ) ∼8.0 2J H6′,H6′′ ) ∼-11.8 3J H5′,H6′′ ) ∼4.2 2J H6′,H6′′ ) ∼-11.7
a (0.001 ppm. b (0.1 Hz; redundant couplings are shown to illustrate internal consistency.
furanose conformation, at least in the β-anomer. However,2JH2a,H2b in 9β is -12.1 Hz compared to -10.9 Hz in 7β, which may reflect different rotameric distributions about the C2b-O1′ bond.16,17 Reaction of 7 with paramolybdate and formate resins at 67 °C for 2 h gave a mixture containing 7 (∼42%) and a new rearrangement product (∼58%), as determined by HPLC assay (Figure S4). Chromatography of the reaction mixture gave a pure product whose 1H NMR spectrum was devoid of furanose anomeric proton signals. A complete analysis of 1H and 13C spectra (Figures S5-S7, Table 2) yielded results consistent with β-D-galactopyranosyl-(1′f1)-D-threo-2-pentulose (10) (Scheme 2), which contains a D-threo-2-pentulofuranose ring. Aqueous solutions of D-[2-13C]threo-2-pentulose (11) contain both R- and β-furanose forms (11R/11β ) 0.29) and substantial amounts of acyclic keto form (∼20%), but no acyclic keto hydrate was detected.18 Similar behavior is observed for the 2-ketopentofuranose ring of 10 where the Rf/βf ratio is ∼0.32, but a significantly smaller percentage of acyclic keto form (∼1%) was observed, based on the detection of a weak 13C signal at ∼213 ppm (Figure S6) and integration of the H3 signals (Figure S5A,B). (16) Stenutz, R.; Carmichael, I.; Widmalm, G.; Serianni, A. S. J. Org. Chem. 2002, 67, 949-958. (17) A similar comparison between 7R and 9R was not made because the 600 MHz 1H NMR spectrum of 9R was non-first-order and not readily interpreted. (18) Vuorinen, T.; Serianni, A. S. Carbohydr. Res. 1990, 209, 13-31.
1H-1H
couplings in the 2-ketofuranose ring of 10 and in 11 are very similar (Table S1), suggesting that ring conformation is unaffected by glycosylation on the exocyclic C1 hydroxymethyl group. As observed in 7 and 9, 2JH1,H1′ decreased by ∼1
Hz in 10 relative to 11, which may indicate different conformational preferences about the C1-C2 bond in the two compounds.16 1H and 13C Chemical shifts are very similar in 10 and 11, except for H1/H1′ and C1, which are shifted downfield by ∼0.2 and ∼10 ppm, respectively, in the disaccharide. The largest changes in 1H and 13C chemical shifts thus occur for nuclei closest to the site of glycosylation, as expected. The observation that molybdate-catalyzed rearrangement of 2-C-(hydroxymethyl)-substituted D-erythroses occurs with monoglycosyl functionality appended to the CH2OH group supports prior arguments that the C2 substituents are located in a sterically accessible pocket of the putative dimolybdatesaccharide reactive complex during the reaction.8 Presumably, substitution with di- or oligosaccharides would also be tolerated. The rearrangement produces a 1,1′-linkage between an aldose and 2-ketosugar that is not readily accessible via enzymic or chemical routes. Few studies on the preparation of these linkages have been reported, focusing mainly on R-D-glucopyranosyl rings linked to the C1 hydroxymethyl group of D-fructose,19 D-tagatose,20 and L-sorbose.21 Potential applications of this chemistry can be realized as shown in Scheme 3. Reaction of a β-galactosylated 2-C-(hydroxymethyl)-D-lyxofuranose 12 with molybdate is expected to yield 13. Disaccharide 13 may be viewed as a structural analog of β-D-galactopyranosyl-(1f6)3-deoxy-β-D-glucopyranose 14 in which the “gluco” ring bears an axial OH group at “C5” instead of an “H5” and “C4” and “O5” have been exchanged. The conformation of the pseudo[1f6]-glycosidic linkage in 13 is expected to differ from that in 14, although conformation about this type of linkage remains poorly understood. Analogs such as 13 may serve as model systems in studies of glycoside bond conformation or as potential enzyme inhibitors. (19) Munir, M.; Schneider, B.; Schiweck, H. Carbohydr. Res. 1987, 164, 477-485. (20) Eder, B.; Lundt, I.; Stu¨tz, A. E.; Wrodnigg, T. M. J. Carbohydr. Chem. 2001, 20, 647-657. (21) Chiba, S.; Shimomura, T. Agric. Biol. Chem. 1971, 35, 1363-1370.
J. Org. Chem, Vol. 72, No. 8, 2007 3083
Experimental Section β-D-Galactopyranosyl-(1f2C)-2-C-(hydroxymethyl)-D-erythrose (7). A mixture of methyl 2-C-(hydroxymethyl)-2,3-O-isopropylidene-D-erythrofuranoside 5 (1.5 g, 7.3 mmol),8 2,3,4,6-tetraO-acetyl-R-D-galactopyranosyl bromide 6 (4.5 g, 11.0 mmol), and molecular sieves (4 Å, 5 g) in 200 mL of anhydrous nitromethane and dichloromethane (1:1) was stirred under N2 for 10 min. Mercury(II) cyanide (2.8 g, 11.0 mmol) was then added, and the reaction mixture was stirred with the exclusion of moisture at 50 °C for 4 h. Additional portions of 6 and catalyst (0.5 mmol each) were added, and the reaction mixture was stirred for another 2 h at 50 °C. After cooling, the reaction mixture was diluted with dichloromethane and filtered, and the filtrate was concentrated to a syrup. The syrup was chromatographed on a silica gel column (ethyl acetate/hexane 1:1), giving 2′,3′,4′,6′-tetra-O-acetyl-β-D-galactopyranosyl-(1′f2C)-2-C-(hydroxymethyl)-1-methyl-2,3-O-isopropylidene-D-erythrofuranoside as a white foam (3.3 g, 85%). To a solution of the acetylated erythrofuranoside (2.1 g, 3.93 mmol) in methanol (40 mL) was added 1 M sodium methoxide until the solution remained alkaline (pH test paper). The mixture was stirred at room temperature for 1 h, neutralized with Dowex H+ resin, filtered, and concentrated. Purification of the residue was performed on silica gel (dichloromethane/methanol 5:1) to afford β-D-galactopyranosyl-(1′f2C)-2-C-(hydroxymethyl)-1-methyl-2,3O-isopropylidene-D-erythrofuranoside as a yellow foam (1.3 g, 91%). The galactosylated isopropylidene erythrofuranoside (200 mg, 0.55 mmol) was dissolved in 8 mL of distilled water, 400 mg Dowex H+ resin (wet) was added, and the suspension was heated at 80 °C for 4 h. After the reaction was complete (TLC), the suspension was filtered, and the filtrate was concentrated. Column chromatography of the residue on silica gel (dichloromethane/ methanol 4:1) gave β-D-galactopyranosyl-(1f2C)-2-C-(hydroxymethyl)-D-erythrose (7) as a white foam (140 mg, 82%). C11H21O10; FAB+ in glycerol, m/z 313 (M + H)+; HRMS calcd 313.1135, obsd 313.1123.
3084 J. Org. Chem., Vol. 72, No. 8, 2007
β-D-Galactopyranosyl-(1′f1)-D-threo-2-pentulose (10). β-DGalactopyranosyl-(1f2C)-2-C-(hydroxymethyl)-D-erythrose (7) (78 mg, 0.25 mmol) was dissolved in 1.0 mL of H2O. To this solution were added paramolybdate resin7 (25 mg) and formate resin7 (10 mg). The suspension was heated with stirring in an oil bath for 2 h at 67 °C, and the reaction mixture assayed by HPLC every 20 min (see Figure S4 for chromatographic details). After 2 h, HPLC analysis indicated that the ratio of 7:10 was ∼1:1.4. The reaction mixture was cooled, the resin was removed by filtration, and the filtrate was concentrated in Vacuo at 30 °C. The residue was purified by chromatography on a Bio-Gel P2 column eluted with distilled water. Compound 10 eluted as a single component as determined by NMR. Fractions containing 10 were pooled and concentrated in Vacuo to give a clear, colorless syrup (37.4 mg, 0.12 mmol, 48%). C11H21O10; FAB+ in glycerol, m/z 295 (M - H2O + H)+; HRMS calcd 295.1029, obsd 295.1014.
Acknowledgment. This work was supported by a grant from Omicron Biochemicals, Inc. of South Bend, IN. Supporting Information Available: Figure S1 showing the 600 MHz 1H NMR spectrum of 7 with signal assignments; Figure S2 showing the 150 MHz 13C{1H} NMR spectrum of 7 with signal assignments; Figure S3 showing the 1H-13C HMQC 2D spectrum of 7 with assigned correlations; Figure S4 showing the HPLC assay of reaction mixture for the conversion of 7 to 10; Figure S5 showing the 600 MHz 1H NMR spectrum of 10 with signal assignments; Figure S6 showing the 150 MHz 13C{1H} NMR spectrum of 10 with signal assignments; Figure S7 showing the 1H-13C HMQC and HMQC-TOCSY spectra of 10 with assigned correlations; Table S1 giving 1H and 13C chemical shifts and JHH for 11. This material is available free of charge via the Internet at http://pubs.acs.org. JO062494+
