Influence of hydroxyl group configuration on pyrolytic formation of 1,6-anhydrohexopyranoses from various hexoses: An experimental and theoretical study

1,6-Anhydrohexopyranose, which is a major pyrolysis product of cellulose and other carbohydrates, has a unique C-1(4) chair conformer. Thus, during pyrolysis, the molecule must undergo a conformational transformation from the C-4(1) to the C-1(4) geometry, and this interconversion is expected to be influenced by the sugar's hydroxyl group configuration. In this article, the influences of the C2-, C3-, and C4-OH configuration on the 1,6-anhydrohexopyranose yield were evaluated under vacuum pyrolysis conditions for five hexoses (glucose, mannose, allose, galactose, and gulose). Furthermore, the results are discussed within the context of theoretical results that indicate the relative stability of two types of the conformers of glycopyranosyl oxocarbenium ions with C6-pseudo-equatorial and C6-pseudo-axial orientations, which were calculated with density functional theory (DFT). The latter conformation would lead to the formation of the 1,6-anhydrohexopyranose with a C-1(4) conformation. The theoretical results were in good agreement with the experimental yields; C2-OH (both axial and equatorial) and C3-OH (equatorial) enhance 1,6-anhydrohexopyranose formation, while C4-OH (axial) inhibits the formation (axial/equatorial: in C-4(1) form). These results also support the heterolysis mechanism for this transformation, and such conformational change was suggested to be more effective in the evaporation process from liquid to gaseous substance. (c) 2012 Elsevier B.V. All rights reserved.