Diels–Alder reaction of α-tropolone and electron-deficient dienophiles prompted by Et3N or silica gel: a new synthetic method of highly functionalized homobarrelenone derivatives

Ambimodal Transition States in Diels–Alder Cycloadditions of Tropolone and Tropolonate with N‐Methylmaleimide**

“…Thus,t he thermodynamically instability of the cycloaddition products in presence of strong base was identified to be Angewandte Chemie responsible for the failure experiment with sodium methoxide.O ur calculations predict that only weak bases are effective due to ar elatively favorable cycloaddition barrier that allows for the reaction to proceed at room temperature. This result is in good agreement with the reactions in the presence of triethylamine, [5,6] or acinchonine catalyst, [7] which leads to mixtures of endo-a nd exo-adducts observed experimentally (Figure 1). In such cases,s tereoselectivity depends primarily on the strong association of the amine with the hydroxyl of TS structures.More importantly,our calculations predict astrong kinetic preference for formation of the endoadduct 3.Since the rearrangement is ahigher energy process than the formation of 3,w ec an expect at hermodynamic equilibration process after 3 by increasing the reaction temperature,w hich favors the conversion from 3 to thermodynamically favored exo-adduct 4.…”

“…1.1 ratio with prolonged reaction time. [5] Thetrend observed from these calculation data shows that the strong association of the amine with the hydroxyl of TS structure greatly influences the stereoselectivity of the reaction.…”

Dimethyl Acetylenedicarboxylate: A Versatile Tool in Organic Synthesis

“…The Diels-Alder reaction of α-tropolone 194 with DMAD, promoted by triethylamine or silica gel, yielded adduct 195 as reported by Okamura et al (Scheme 55) 61. …”

Ambimodal Transition States in Diels–Alder Cycloadditions of Tropolone and Tropolonate with N‐Methylmaleimide**

“…Tr opolone,a lthough generally thought to possess some aromaticity, [1][2][3][4] undergoes cycloadditions with the potent dienophile, N-methylmaleimide.W hile systematic studies of the thermal cycloaddition have not been reported, the reaction in the presence of triethylamine, [5,6] or ac inchonine catalyst, [6] leads to mixtures of endo-and exo-adducts (Figure 1). Ther eaction does not occur at all with the stronger base,s odium methoxide.I tw as proposed that the reaction occurs by competing exo-and endo-Diels-Alder reactions.…”