Support for a Dioxyallyl Cation in the Mechanism Leading to (-)-Levoglucosenone

Levoglucosenone (LGO) is the major product formed when cellulose is pyrolyzed in the presence of acid at temperatures between 170 and 350 degrees C. The current intense interest in biomass conversion has led to a number of reports on its preparation; however, there is still uncertainty on the mechanism leading to LGO. We propose a new mechanism which involves a C2-C1 hydride shift followed by intramolecular trapping of a dioxyallyl cation. The reaction has been modeled using DFT calculations from the known LGO precursors levoglucosan and 1,4:3,6-dianhydro-alpha-D-glucopyranose to a common intermediate with calculated barriers of 10.6 and 13.5 kcal.mol(-1), respectively. A discussion of the literature on the formation of LGO from late pathway intermediates is also provided.