Newer developments in the conformational analysis of carbohydrates

Abstract

ABSTRACT·The lecture is mainly concerned with research from the author's laboratory on substitutional and solvation effects on conformational equilibria as estimated from NMR and optical rotation data. Reference is made to the use of empirical rules of rotation for estimating conformational preferences and to how x-ray crystallographic data on the torsional angles defined by vicinal atoms may become of importance to both the interpretation of rotation and NMR data. The anomeric and reverse anomeric effects on glycoside conformation are considered and reference is made to solvation effects on the interaction between oxygen atoms in gaueheandin syn-axial relationships. Effects of solvation on the orientation of the hydroxymethyl group of hexopyranoses and on intramolecular hydrogen bonding are presented.Remarkable advances were made in the past fifteen years in observing the conformational properties of carbohydrate structures. However, we are only beginning to probe the effects of the solvent in establishing the conformational preferences observed. The real value of conformational analysis will only be realized with a thorough understanding ofhow the carbohydrate solute interacts with the molecules in its environment and how this is related to conformational preferences as well as to how such interactions affect the energy requirements for changes in the environment of the carbohydrate. Answers to these questions are obviously of basic importance towards the eventual understanding ofthe enzyme-catalysed transformations of the carbohydrates, the antigenic properties of carbohydrate structures, the structure-activity relationships for carbohydrate antibiotics, etc. This lecture will be mainly concerned with our efforts to make contributions to this aspect of conformational analysis. Basic to our approach is the synthesis of model structures for which there should exist a substantial population of more than one conformation. Thus, solvation effects on the conformational equilibria for such compounds can be expected to be sufficiently large, to allow their observation, particularly through changes in nuclear magnetic resonance spectra and changes in optical rotation.As seen in Figure 1, this objective was realized by the preparation of methyl 3-deoxy-ß-L-erythro-pentopyranoside 1 . lt could be expected that in non-basie media, such as chloroform, an intramolecular hydrogen bond 527 P.A.C.-27/4---B

Synthesis and Reactivity of α‐Haloglycine Esters: Hyperconjugation in Action

“…Indeed, this peculiar large torsional angle in 4c matches the abnormally large vicinal coupling constant in CDCl 3 ( J NH‐H α = 10.2 Hz), thus supporting the argument that the low‐lying conformation in the crystal lattice reflects closely the major conformer of 4c in solution (Figure B) , . Similarly, the J NH‐H α coupling constants for 4a , 4b , and 4c are abnormally large in both apolar and polar protic solvents CDCl 3 and CD 3 CN ( J > 9.2 Hz), suggesting that the three α‐haloglycines experience a comparable solvent exposure and have a similar gauche ‐conformation .…”

3,7‐Dioxa‐1‐azabicyclo[3.3.0]octanes Substituted at the C‐5 Position − From Local to Global Stereochemistry

“…[18,19] The first, but unsuccessful, attempt to provide evidence for the mobility of 5-substituted structures of types A and B (Scheme 1) by means of dynamic 1 H NMR spectroscopy was reported by Crabb in 1973. [16] Our later developments in the field, starting from ab initio calculations, successive NOE difference experiments and NMR spectroscopy at low temperature, established some essential features to consider when analysing the stereochemistry of these systems: [20,21] (i) The absence of the pyramidal inversion of the bridged nitrogen atom, together with the cis relationship between its lone pair and the ligand at C-5, results in a stable cisfused double 1,3-oxazolidine system…”

Synthesis and Biological Evaluation of the Novel Growth Inhibitor Streptol Glucoside, Isolated from an Obligate Plant Symbiont

“…As the OBn group at the C-2 position of the sugar does not participate as a neighboring group, the more thermodynamically stable αproduct is formed, due to the anomeric effect. [23] Performing the reaction under our previously optimized conditions (AgOTf, 2,6di-tert-butyl-4-methylpyridine, CH2Cl2) at lower reaction temperature (-78°C), gave the product in good yield but with an α/β ratio of only 5.4/1. Changing the reaction solvent to Et2O, pleasingly delivered the desired glucosidic product 17 in good yield with improved selectivity (α/β=9.2/1).…”