Conformational studies on pyranoid sugar derivatives by n.m.r. spectroscopy. The conformational equilibria of the 1,2- trans peracetylated aldopentopyranosyl benzoates and perbenzoylated aldopentopyranosyl acetates in solution

Abstract

2,3,4-Tri- O-acetyl- d-aldopentopyranosyl benzoates having the β- ribo ( 1), α- arabino ( 2), β- xylo ( 3), and α- lyxo ( 4) configurations, and 2,3,4-tri- O-benzoyl- d-aldopentopyranosyl acetates having the β- ribo ( 5), α- arabino ( 6), β- xylo ( 7), and α- lyxo ( 8) configurations have been prepared, and their conformations studied in acetone- d 6 or chloroform- d by n.m.r. spectroscopy at room temperature. Comparison of the results with similar data for the eight d-aldopentopyranose tetraacetates and tetrabenzoates indicates that the axial-directing effects of the OAc and OBz groups at C-1 are not independent of the total stereochemistry, nor are they independent of the nature of the acyloxy groups at C-2, 3, and 4. A concept of attractive interactions between syn-diaxial benzoyloxy groups provides a rationalization of the quantitative trends observed. The signal field-position of an acetyl group at O-1 is not subject to a strong upfield shift by the presence of benzoyloxy groups at C-2,3, and 4, nor does a 1- O-benzoyl group cause specific shielding of acetoxy groups at C-2, 3, and 4.