Synthesis of d- erythro-2-pentulose and d- threo-2-pentulose and analysis of the 13C- and 1H-n.m.r. spectra of the 1- 13C- and 2- 13C-substituted sugars

Abstract

The pentuloses d- erythro-2-pentulose ( 1) and d- threo-2-pentulose ( 2) and their 1- 13C- and 2- 13C-substituted derivatives were prepared by hydrogenating the corresponding isotopically normal and 13C-substituted d-pentos-2-uloses with a Pd-carbon catalyst. The threo isomer and its labeled derivatives were alternatively prepared from isotopically normal and 13C-substituted d-xyloses with immobilized d-xylose ( d-glucose) isomerase (E.C. 5.3.1.5). The equilibrium compositions of 1 and 2 (furanose anomers and acyclic keto forms) in 2H 2O were determined from 13C-n.m.r. spectra (75 MHz) of the 2- 13C-labeled derivatives. The conformational properties of the cyclic and acyclic forms in 2H 2O were assessed with the use of 1H 1H, 13C 1H, and 13C 13C spin-coupling constants obtained from 1H-n.m.r. (620 MHz) and 13C-n.m.r. (75 MHz) spectra. Compared with the structurally related aldotetrofuranoses the 2-pentulofuranoses more strongly prefer conformations in which the anomeric hydroxyl group is oriented quasi-axially. The strongly dipolarized carbonyl group in the acyclic keto forms of 1 and 2 appears to stabilize chain conformations having O-1 and O-3 eclipsed with the carbonyl oxygen.