Part II. Using NOE to differentiate between the two diastereoisomeric glycosides heterodendrin and epi-heterodendrin.

Philips Dannie
Rayat Sundeep
Sen Subhabrata

Reference:
Lankhorst, P. P.; Smeets, J. W. H., Haasnoot, C. A. G. Carbohydrate Research 1995, 269,17-27.

Introduction

In this paper the researchers have described the application of NOE and molecular mechanics to distinguish between the two diastereomeric glycosides heterodendrin and epi-heterodendrin.

The proton NMR of the two diastereomers differ mainly in the chemical shift of the H2 in the side chains. However it did not
gave any stereospecific information. Nevertheless the NOE experiment on a time average basis helped in discriminating between the two diastereomers. This was further confirmed by molecular mechanics.

Questions:

The structure of two diastereoisomeric glycosides heterodendrin and epiheterodendrin are shown in the following picture.

Identification of components and relationships

1) Assign each diastereomer to the proton spectra shown.

Seeking Clarification

2) Does your assignment of the 1H NMR agree with the structure and what are the major differences in the 1H NMR of the two diastereomers?

Identification of components and relationships

3) What do the NOED spectra of the two diastereoisomers tell you?

Reasoning using quantitative data

4) Explain the NOED spectra for both the isomers? What information do you get about the distances between H-1 and H-2' in the two diastereoisomers? Support your answer with proper justification.

Seeking Clarification

5) Will the deuteration at 1 and 2' positions in the two diastereoisomers affect the NOED?