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Studies with radioactive sugars
56
STUDIES WITH FtADIOACTIVE SUGARS PARTIII1. THE MECFIANISM OF THE ANOMERISATION OF ElHYL a- ANDp-D-XYLOPYRANCBIDE R. J.
FERRER, L.
Department (Great
R.
HATTON, AND W. G. OVEREND
of Chemistry,
Birkbeck
College.
University
of London,
Malet
Street, London,
W. C. 1
Brifain)
(Received
March lst, 1968)
ABSTRACT Parallel anomerisations of the ethyl D-xylopyranosides in radioactive ethanol and of the radioactive glycosides in unlabelled ethanol have allowed the conclusion to be drawn that the reactions proceed largely by way of a cyclic, ionic mechanism. In principle, these procedures permit a quantitative assessment of the relative significance of this mechanism and a process involving an acyclic ion. The displacement reactions at C-l occurs predominantly with inversion of configuration, but, particularly with the B-D anomer, significant proportions of isomers having retained anomeric configuration are initially formed. INTRODUCTION Although the overall pattern of the alcoholysis of several sugars has been elucidated by the combined application of chromatographic and sensitive analytical procedures2*3, certain mechanistic details of the contributing reactions have still to be resolved. Recently, the fist direct evidence has been obtained4 to show that the final step in the methanolysis of D-glucose, i.e., the anomerisation of the D-glucopyranosides, occurs by a cyclic ion mechanism (Scheme 1; Route A; R = R” = Me, R’ = CH20H), rather than by a process involving an acyclic ion (Route B).
OH
Scheme Carbohyd.
Res.,
8
(1968) 56-60
1
R J. FERRER,
60
L. R. HATTON,
W. G. OVERFiND
Anomerisation reactions: radioactive glycosides in inactive ethanol (E,). - The labelled glycosides (20.0 mg, 1.0 pcurie) were heated under reflux with ethanol (1 .Oml, containing hydrogen chloride, 0.20%) for 24 h. Samples (0.05 ml) were withdrawn, neutralised with Deacidite FF resin (OH-; 3 mg), and applied to the paper chromatograms without special care being taken to ensure quantitative transfer. Inactive g2ycoside.s in radioactive ethanol (E2)_ - The pyranosides (20.0 mg) were heated under reflux in ethanol-I-‘4C [LO ml, 153 I.tcuries(calculated to give glycosides having the same tial specific activity as those employed in E,), containing hydrogen chloride (O-20%)] for 24 h. Samples were withdrawn, and treated as in E,, but great care was taken to ensure quantitative transfer to the chromatograms. In the lirst experiment (E,),
the percentages of anomerised products were
readily determined by direct comparison of the activities of the two components of each mixture. In the second (&), however, this could not be done, and, instead, the measured activities were compared with the total activities (averages of four determinations) measured in the 24-h (equilibrated) samples, in which it could be assumedthat complete incorporation of labelled ethanol had occurred. (This assumption implies that anomerisation has Route A, Scheme 1, available, and is clearly valid.) The results are given in the Tables. ACELNOWLEDGhfENT
The authors thank the Governors of Birkbeck College for the award of a grant to one of them (L. R. H.). REFERENCES 1 2 3 4 5 6
Part II: R. J. FERRIER. L. R. HATTON, AND W. G. OVEREND, Carbuhy~. Res., 6 (1968) 87. C. T. BEHOP AND F. P. COOPER, Can. J. Chem., 40 (1962) 224; 41 (1963) 2743. R. J. FERRIER AND L. R. HATTON, Curbohyd. Res., 6 (1968) 75. B. CAPON AND D. THACKER, J. Chem. Sot. (B), (1967) 1010. C. A. VERNON, Proc. Roy. Sot. (London), 167B (1967) 389. B. E. C. BANKS, Y. MEINWALD, A. J. RHIND-TUX, 1. SHEFX-,AND C. A. VERNON, J. Chem. Sot, (1961) 3240. 7 J. T. EDWARD, Chem. Ind. (London), (1955) 1102. 8 W. G. OVEREND, C. W. REES, AND J. S. SEQUIRA, J. Chem. Sot., (1962) 3429. Carbohyd. Res., 8 (1968) 56-60
STUDIES WITH FtADIOACTIVE SUGARS PARTIII1. THE MECFIANISM OF THE ANOMERISATION OF ElHYL a- ANDp-D-XYLOPYRANCBIDE R. J.
FERRER, L.
Department (Great
R.
HATTON, AND W. G. OVEREND
of Chemistry,
Birkbeck
College.
University
of London,
Malet
Street, London,
W. C. 1
Brifain)
(Received
March lst, 1968)
ABSTRACT Parallel anomerisations of the ethyl D-xylopyranosides in radioactive ethanol and of the radioactive glycosides in unlabelled ethanol have allowed the conclusion to be drawn that the reactions proceed largely by way of a cyclic, ionic mechanism. In principle, these procedures permit a quantitative assessment of the relative significance of this mechanism and a process involving an acyclic ion. The displacement reactions at C-l occurs predominantly with inversion of configuration, but, particularly with the B-D anomer, significant proportions of isomers having retained anomeric configuration are initially formed. INTRODUCTION Although the overall pattern of the alcoholysis of several sugars has been elucidated by the combined application of chromatographic and sensitive analytical procedures2*3, certain mechanistic details of the contributing reactions have still to be resolved. Recently, the fist direct evidence has been obtained4 to show that the final step in the methanolysis of D-glucose, i.e., the anomerisation of the D-glucopyranosides, occurs by a cyclic ion mechanism (Scheme 1; Route A; R = R” = Me, R’ = CH20H), rather than by a process involving an acyclic ion (Route B).
OH
Scheme Carbohyd.
Res.,
8
(1968) 56-60
1
R J. FERRER,
60
L. R. HATTON,
W. G. OVERFiND
Anomerisation reactions: radioactive glycosides in inactive ethanol (E,). - The labelled glycosides (20.0 mg, 1.0 pcurie) were heated under reflux with ethanol (1 .Oml, containing hydrogen chloride, 0.20%) for 24 h. Samples (0.05 ml) were withdrawn, neutralised with Deacidite FF resin (OH-; 3 mg), and applied to the paper chromatograms without special care being taken to ensure quantitative transfer. Inactive g2ycoside.s in radioactive ethanol (E2)_ - The pyranosides (20.0 mg) were heated under reflux in ethanol-I-‘4C [LO ml, 153 I.tcuries(calculated to give glycosides having the same tial specific activity as those employed in E,), containing hydrogen chloride (O-20%)] for 24 h. Samples were withdrawn, and treated as in E,, but great care was taken to ensure quantitative transfer to the chromatograms. In the lirst experiment (E,),
the percentages of anomerised products were
readily determined by direct comparison of the activities of the two components of each mixture. In the second (&), however, this could not be done, and, instead, the measured activities were compared with the total activities (averages of four determinations) measured in the 24-h (equilibrated) samples, in which it could be assumedthat complete incorporation of labelled ethanol had occurred. (This assumption implies that anomerisation has Route A, Scheme 1, available, and is clearly valid.) The results are given in the Tables. ACELNOWLEDGhfENT
The authors thank the Governors of Birkbeck College for the award of a grant to one of them (L. R. H.). REFERENCES 1 2 3 4 5 6
Part II: R. J. FERRIER. L. R. HATTON, AND W. G. OVEREND, Carbuhy~. Res., 6 (1968) 87. C. T. BEHOP AND F. P. COOPER, Can. J. Chem., 40 (1962) 224; 41 (1963) 2743. R. J. FERRIER AND L. R. HATTON, Curbohyd. Res., 6 (1968) 75. B. CAPON AND D. THACKER, J. Chem. Sot. (B), (1967) 1010. C. A. VERNON, Proc. Roy. Sot. (London), 167B (1967) 389. B. E. C. BANKS, Y. MEINWALD, A. J. RHIND-TUX, 1. SHEFX-,AND C. A. VERNON, J. Chem. Sot, (1961) 3240. 7 J. T. EDWARD, Chem. Ind. (London), (1955) 1102. 8 W. G. OVEREND, C. W. REES, AND J. S. SEQUIRA, J. Chem. Sot., (1962) 3429. Carbohyd. Res., 8 (1968) 56-60