Pyranoid systems

Conformational analysis of pentopyranosyl acetates, benzoates, and halides has been reviewed.  

The statistical distribution of the conformational orientations of the hydroxymethyl and esterified hydroxy-groups in glucopyranoses and galactopyranoses in the Ci conformation were shown to be g/g (60%) and g/t (40%) for the former, and gjt (58%), tig (34%), and g/g (8%) for the latter. The reasons for the unfavourable tjg conformation for glucopyranose and gjg conformation for galactopyranose were discussed in relation to steric and electronic interactions. Two-dimensional J spectroscopy has been applied to the 270 MHz spectra of (1) and 2) The influence of 0-alkyl groups on the chemical shift of the 

The conformational analysis of the peracetyl 1,5-anhydro-hex-l-enitol system has been carried out using n.m.r. at 250 MHz all were shown to adopt the (d) conformation except the lyxo isomer, which adopted the (d) conformation. The conformations of all eight 1,6-anhydro-D-aldohexopyranoses and their triacetates have been shown by n.m.r. to be the expected flattened C4 chair with the flattening being more pronounced when the C-3 substituent is cis to the anhydro-bridge. The same group has studied the complete series of 1,6 2,3- and 1,6 3,4-dianhydro-D-hexopyranoses in a similar manner all were shown to exist as or half-chairs. A -(2,3,4-Tri-0-acetyl-/3-D-lyxo-pyranosyl)-p-nitroaniline exists in the conformation, whereas the a-anomer adopts the conformation. A similar situation has been described for the corresponding D-ribo compounds. The non-acetylated iV-(a-D-ribopyranosyl)-p-nitroaniline is in conformational equilibrium between 4 and Ci chairs. 

data for 6-deoxy-6-fluoro-ot- and - D-glucopyranose obtained at 300 MHz in DjO has been reported with a discussion of chemical shifts and coupling constant values. N.m.r. data on the unsaturated sugar derivatives (5) and (6), as well as on some fructofuranose derivatives useful as intermediates in a new synthesis of sucrose, have been published. The complete elucidation of the H n.m.r. spectra of the 2,3-unsaturated glycosyl phosphonates of type (7) 

The anomeric configurations of the known carboxylates (4) and (5) have been determined by and H n.m.r.  

The pyranoid conformations of the amino-deoxyoctoses (6) and (7) have been calculated using modified Karplus equations. Enediol anion formation in inososes has been studied by u.v. and n.m.r. spectroscopy. The results suggested that the interpretation of n.m.r. data by Dufaye (Appl. Polym. Symp., 1976, 28, 955) was erroneous and that only small concentrations of enediolate ions are formed. 

Pinter, J. Kovics, and A. Messmer, Acta Chem. Acad. Sci. Hung., 1980,105, 231. 

The conformations adopted by the Ca complex of iV-glycolylneuraminic acid are referred to in Chapter 16. 

A -Ray analysis of the crystal structure of methyl 3-acetamido-2,3,6-trideoxy-3-C,4-0-dimethyl-P-L-arahino-hexopyranoside (383) has shown that the stereochemistry previously assigned to C-3 of this compound, and, consequently, to C-3 of the branched nitro-sugar evernitrose (a component of the everninomicins), is in error. It seems that caution must be exercised in assigning the stereochemistry at tertiary centres in such compounds as (383) solely on the basis of the chemical shift of the acetamido methyl group. 

The chemical shifts of the geminal protons at C-2 of methyl 3,4-di-O-benzoyl-2,6-dideoxy-p-L-/yxo-hexopyranoside (638) provide another exception to the 

Increment rules that predict the chemical shifts of protons in the H n.m.r. spectra of aldohexopyranoses to within 0.09p.p.m. have been presented. 300 MHz H n.m.r. spectroscopy has shown that both a- and p-D-ribopyranose exist as a mixture of C4 and C, conformations in deuterium oxide, while P-d-ribo- and P-D-xylo-pyranosylamines prefer the C, conformation. The con- 

de Bruyn, M. Anteunis, M. Claeyssens, and E. Saman, Bull. Soc. chim. beiges, 1976,85. 605 (Chem 

Analyses of the C and n.m.r. spectra of 2-substituted methylenecyclo-hexanes (e.g. 2-methoxy-l-methylenecyclohexane) and 3-substituted cyclohexenes have demonstrated that a double bond stabilizes the axial or pseudoaxial con-former, respectively, when an electronegative substituent is present at the allylic position. Stabilization of the (pseudo)axial conformer by the double bond in both series is adequately explained in terms of double bond-no bond resonance [e.0. (641) - (642)]. This effect is enhanced by a methoxy-group on the double bond, as in 2-methoxy-l-(methoxymethylene)cyclohexane. 

Equations for the calculation of the number of allowable conformational states for O-methylhexopyranosides have been developed. Coupling constants for in glycals have been calculated, using an FP-SCF-INDO 

Calculated rotamer states for the acetoxymethyl group at C-5 of gluco-and galacto-pyranosyl rings have been compared with those obtained from A -ray crystallography and found to be in good agreement. Molecular orbital calculations on dimethoxymethane, a model for the anomeric centre of methyl pyranosides, have produced results consistent with the anomeric and the exn-anomeric effects. The results were compared with evidence obtained by neutron diffraction. Dynamic H n.m.r. methods applied to the 

A range of glycenose derivatives incorporating a terminal 2,2-difluorovinyl group, whose preparation is mentioned in Chapter 12, have been studied by C and n.m.r. and by m.s.  

Chiroptical parameters for esters of L-altruronic acid in various solvents have been shown to reflect the conformational mobility and rotation of the chromophoric group about the C-5-C-6 bond. The conformational equilibria are sensitive to the anomeric configuration, and to the pattern of O-methyla-tion in methylated derivatives. In the permethylated methyl altruronates the a-anomer exists predominantly in the Ci conformation (4) while the j8-anomer prefers the C4 conformation (5), thus reducing the syn-axial interactions involving the glycosidic methoxyl in each case.  

The conformational analysis of a range of 1,2-O-isopropylidene-cx-D-gluco-and -allo-furanose derivatives (6) has been reported, using n.m.r., and the 3,S-phenylboronate of the giuco isomer was also examined. Rotamers about the C-5-C-6 bond were distinguished. The n.m.r. spectra of a- and /S-anomers of phenyl, thiophenyl, and 1-naphthyl 2,3,5-tri-O-acetyl-D-ribo-furanosides have shown that the preferred conformation is Cz-endo. Changing 

The internal energies of each of the sixteen possible Ci conformations of the aldopentopyranoses (i.e. the ol- and j8-anomers of both the d- and L-forms) have been derived from Monte-Carlo calculations that take into account contributions made by van der Waals attractive and repulsive forces, polar interactions, and 

In evaluating the stereochemical outcome of the free-radical addition of sodium hydrogen sulphite to several methyl hex-5-enopyranosides, the agreement between the experimental and calculated values was improved if an allowance of 0.45 kcal mol was made for a 1,3-interaction between an axial hydroxy-group and the ring-oxygen atom e.g. in the formation of methyl 6-deoxy-a-D-allo- and -jS-L-talo-hexopyranoside 6-sulphonic acids).  

Three papers have reported the results of studies with simple tetrahydropyrans. 2-Hydroxytetrahydropyran has been shown to adopt a conformation placing the hydroxy-group in an axial orientation, whereas the amino-group in related 2-aminotetrahydropyrans favours an equatorial orientation. Both 3-chloro-and 3-bromo-tetrahydropyrans show a preference for the conformation in which the halogeno-group is equatorial. The conformational equilibria of the nucleoside analogues (442) have also been studied the proportion of the 

Cantara, J. Sygusch, and T. Cyr, Chem. Phys, Aqueous and Gaseous Solutions, Proc. Symp.. 1975, 59 Chem. Abs., 1975, 83, 97 744v). 

Computer-refined parameters for a number of JV-hexopyranosylimidazoles and their tetra-O-acetyl derivatives revealed that some of these compounds exist as mixtures of Ci and C4 conformers in solution the possible contributions of steric and reverse anomeric (polar) effects were discussed. Mixtures of conformers - including boat conformers - were also indicated for several D-glucopyranosylpyridinium salts from the size of the proton-proton coupling constants.  

Paulsen has discussed the influence of jy/z-l,3-diaxial interactions and the anomeric effect on the conformations of various pyranoid derivatives. High proportions of the C4 forms, indicating that there are strong syn-d ax a interactions in the Ci forms, were observed for 2,4-diazido-2,4,6-trideoxy-a-D-idopyranoside and the corresponding 2,4-diamino dihydrochloride derivative. Strong 5y -diaxial interactions in the usual chair forms of l,6-anhydro-2,4- 

de Bruyn, M. Anteunis, P. J. Garegg, and T. Norberg, Acta Chem. Scand., 1976, B30, 

Angular dependences have been found for the H- Hg and C- Hg coupling constants in methyl 3,4,6-tri-C -acetyl-2-chIoromercuri-2-deoxy- -D-glucopyranoside (504) and the corresponding a-D-mannopyranoside derivative (505). The H- Hg couplings appear to conform to a Karplus relationship, 

---

This conformational change has also been detected in a pento-pyranoid system. Thus, although l-0-acetyl-3-benzamido-2,4-di-0-benzoyl-3-deoxy-3-C-(ethoxycarbonyl)-/3-D-ribopyranose (22) adopts the 1C(d) conformation in chloroform solution, the corresponding nucleoside derivative 23 exists preponderantly in the CJ(d) conformation. 

It is interesting that pyranoid systems having a cross-oriented, dienol ether linkage proved to be stable under the same conditions. Such a... 

Similarly, the high reactivity of the exocyclic, enolacetal double bond in pyranoid systems was proved by the following reaction of 225, which undergoes fast addition of alcohols in the presence of hydrogen chloride at —16°. The C-5 epimeric mixture of methyl 2,3,4-tri-0-acetyl-6-deoxy-S-C-methoxy-a-D-glucopyranoside (226) and methyl 2,3,4-tri-O-acetyl-6-deoxy-5-C-methoxy-/ -L-idopyranoside (227) was isolated. 

The fact that the glycosidic linkage in the 5,6-unsaturated pyranoid system, as in 234, is easy to cleave to the deoxydicarbonyl sugar (235) led to a simple preparative method for the synthesis of 6-deoxyglycos-uloses, for example, 6-deoxy-D-orabmo-hexofuranos-5-ulose (236) by way of 235. Compound 236 is the carbohydrate moiety of the antibiotic hygromycin A. ... 

In a novel route to 3 -deoxy-2 -methylenethymidine 85 (Scheme 7), the sulfone 84 was prepared from a-D-isosaccharinolactone reductive elimination of 84, in a manner previously described for pyranoid systems (Vol.26, p.l48) then gave an intermediate which underwent [3,3]-sigmatropic rearrangement as indicated. ... 

---