Conformational studies on pyranoid sugar derivatives by N.M.R. spectroscopy. The conformational equilibria of some alkyl tri- O-acetyl- and tri- O-benzoyl-β- D-ribopyranosides in solution

Abstract

The conformational populations of tribenzoylated β- D-ribopyranosides ( 1– 4) having as aglycons the methyl, ethyl, isopropyl, and tert-butyl groups, rspectively, together with the corresponding triacetates ( 5– 8), were studied in acetone- d 6 by the n.m.r. spectroscopic method of averaging of spin coupling to determine the conformational effect of varying the aglycon in an otherwise fixed structure. The benzoylated methyl and ethyl glycosides ( 1 and 2) adopt the 1C( D) conformation to the extent of about 80%, whereas the acetylated analogs ( 5 and 6) have only about 60% of the 1C( D) form. In each series, the tert-butyl glycosides favor the 1C( D) form to a lesser extent, about 75% for the benzoate 4, and 55% for the acetate 8. The results are interpreted in terms of steric and polar effects. The isopropyl methyl groups in 3 and 7, and the ethyl methylene groups in 2 and 6 are magnetically nonequivalent as a result of their dissymmetric environment and not as a consequence of hindered rotation.