Accelerated Dimerization of α,β-Unsaturated D-Xylo-Hexofurano-5-ulose Derivatives through Asynchronous Hetero-Diels-Alder Reaction.

While some hetero-Diels-Alder (HDA) reactions are accelerated by either carbonyl or phosphate groups attached directly to the heterodiene moiety, the alkyl or aryl groups, on the other hand, have minimal influence. However, in this article, we demonstrate that aryl groups have a significant effect on the spontaneous dimerization reaction of α,β-unsaturated D-xylo-hexofurano-5-ulose derivatives to their respective pyrano adducts via intermolecular HDA reaction. Experimental and computational studies provide strong evidence that dimerization follows the Woodward-Katz two-stage mechanism reaction (asynchronous process), from which the aryl/aryl π-stacking interaction is mainly responsible for the rate-determining step (RDS) and electrostatic interaction for the second bond formation. Since the latter interaction is highly affected by dipolar moment, 5-ulose derivative having a strong electron-withdrawing group (R = CN; μ = 14.3 D) is spontaneously dimerized more than 15 times faster than 5-ulose that possesses an electron-donating group (R = OMe; μ = 2.1 D).