RESUMEN

We herein reported a new type of S-o-(p-methoxyphenylethynyl)benzyl donor for a highly efficient glycosylation method. The donor was activated by 10% Tf2O and underwent glycosylation with various acceptors to provide the corresponding glycosides in excellent yield. Furthermore, two repetitive fragments of Polyporus umbellatus polysaccharides (PUPs), isolated from traditional Chinese medicine "Polyporus umbellatus", were prepared by combining the "single-catalyst one-pot" and "latent-active" strategies for the first time for future clear studies on the structure-activity relationship of PUPs.

Asunto(s)

Polyporus , Glicosilación , Glicósidos , Polisacáridos

RESUMEN

The effects of water on radical-radical reactions are of great importance for the elucidation of the atmospheric oxidation process of free radicals. In the present work, the HO2 + HO2 reactions with (H2O)n (n = 1-3) have been investigated using quantum chemical methods and canonical variational transition state theory with small curvature tunneling. We have explored both one-step and stepwise mechanisms, in particular the stepwise mechanism initiated by ring enlargement. The calculated results have revealed that the stepwise mechanism is the dominant one in the HO2 + HO2 reaction that is catalyzed by one water molecule. This is because its pseudo-first-order rate constant (kRWM1') is 3 orders of magnitude larger than that of the corresponding one-step mechanism. Additionally, the value of kRWM1' at 298 K has been found to be 4.3 times larger than that of the rate constant of the HO2 + HO2 reaction (kR1) without catalysts, which is in good agreement with the experimental findings. The calculated results also showed that the stepwise mechanism is still dominant in the (H2O)2 catalyzed reaction due to its higher pseudo-first-order rate constant, which is 3 orders of magnitude larger than that of the corresponding one-step mechanism. On the other hand, the one-step process is much faster than the stepwise mechanism by a factor of 105-106 in the (H2O)3 catalyzed reaction. However, the pseudo-first-order rate constants for the (H2O)2 and (H2O)3-catalyzed reactions are lower than that of the H2O-catalyzed reaction by 3-4 orders of magnitude, which indicates that the water monomer is the most efficient one among all the catalysts of (H2O)n (n = 1-3). The present results have provided a definitive example that water and water clusters have important influences on atmospheric reactions.

RESUMEN

To study the catalytic effects of (H2O)n (n = 1-3), the mechanisms of the reaction HO2 + HO →3O2 + H2O without and with (H2O)n (n = 1-3) have been investigated theoretically at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory, coupled with rate constant calculations using the conventional transition state theory. Our results show that upon incorporation of (H2O)n (n = 1-3) into the channel of H2O + 3O2 formation, two different reactions, i.e. HO + HO2(H2O)n (n = 1-3) and HO2 + HO(H2O)n (n = 1-3), have been observed, and these two reactions are competitive with each other. The catalytic effects of (H2O)n (n = 1-3) mainly arise from the contribution of a single water vapor molecule; this is because the effective rate constants with water are respectively larger by 2-3 and 3-4 orders of magnitude than those of the reactions with (H2O)2 and (H2O)3. Furthermore, the catalytic effects of the water monomer mainly arise from the H2OHO2 + HO reaction, and the enhancement factor of this reaction is obvious within the temperature range of 240.0-425.0 K, with the branching ratio (k'(RW)/ktot) of 17.27-80.77%. Overall, the present results provide a new example of how water and water clusters catalyze gas phase reactions under atmospheric conditions.