Conformation of acyclic derivatives of sugars Part VIII. Conformations of the 2,3,4,5-tetra- O-acetylpentose dimethyl acetals in solution

Abstract

The conformations of the peracetylated aldopentose dimethyl acetals having the d- ribo ( 1), l- arabino ( 2), d- xylo ( 3), and d- lyxo ( 4) configurations have been studied in chloroform- d solution by n.m.r. spectroscopy at 100 and 220 MHz. The arabino derivative 2 adopts a planar, zigzag conformation almost exclusively, whereas the other three examples adopt conformations in which parallel 1,3-interactions between substituents, that would be present in the extended forms, are relieved. In the lyxo derivative 4, rotation about C-1—C-2 from the extended form is involved. The ribo derivative 1 tends to adopt a sickle conformation through rotation about C-2—C-3 to bring C-1 out of the approximate plane of the other atoms in the chain; the xylo derivative behaves similarly, but with rotation about C-3—C-4 to bring C-5 out of the plane. The derivatives 1–4 exhibit mass-spectral fragmentation pathways very similar to those of the peracetylated aldose diethyl dithioacetals.