Abstract
In Diels–Alder reactions, 2H-pyran-2-ones as dienes can yield a large variety of cycloadducts with up to four contiguous carbon stereogenic centers. Some of the potentially most useful, however difficult to prepare due to their low thermal stability, are the primary CO2-containing oxabicyclo[2.2.2]octenes, which could be formed as eight distinctive isomers (two sets of regioisomers, each of these composed of four different stereoisomers). A high-pressure synthesis of such products was recently described in a few cases where vinyl-moiety-containing dienophiles were used as synthetic equivalents of acetylene. However, structures of the primary products have been so far only rarely investigated in detail. Herein, we present seven novel single-crystal X-ray diffraction structures of such cycloadducts of both stereoisomeric forms, i.e., endo and exo. Additionally, we present a single-crystal structure of a rare case of a cyclohexadiene system stable at room temperature, obtained as a secondary product upon the retro-hetero-Diels–Alder elimination of CO2 under thermal conditions (microwave irradiation), during this elimination the symmetry is increased and out of eight initially possible isomers of the reactant, this number in the product is decreased to four. In oxabicyclo[2.2.2]octene compounds, centrosymmetric hydrogen bonding was found to be the predominant motif and diverse supramolecular patterns were observed due to rich variety of C–H⋯O and C–H⋯π interactions.
Highlights
The Diels–Alder reaction, mechanistically classified as a [4+2] cycloaddition belonging to the larger group of pericyclic reactions, is one of the crucial synthetic tools for the construction of novel
Microwave irradiation as a way to facilitate organic reactions has during the last decades found some surprisingly widespread applications, as recently vividly described by Kappe [48], including acceleration of Diels–Alder reactions [2,49], Kabachnik–Fields reaction [50] and many others
The cycloadditions arecycloadditions sluggish, at best, ambientat conditions, conditions, we found that the only possibility to obtain products was to avoid thermal we found that the only possibility to obtain products 3 was to avoid thermal activation of the reaction activation of the reaction with and instead use activation withwhich high pressure which is known to and instead use activation high pressure
Summary
The Diels–Alder reaction, mechanistically classified as a [4+2] cycloaddition belonging to the larger group of pericyclic reactions, is one of the crucial synthetic tools for the construction of novelC–C bonds [1,2], gaining importance in all its varieties, including (organo)catalytic and enantioselective versions [3,4]. The Diels–Alder reaction, mechanistically classified as a [4+2] cycloaddition belonging to the larger group of pericyclic reactions, is one of the crucial synthetic tools for the construction of novel. Cycloadditions are highly versatile reactions as a broad range of starting dienes and dienophiles (alkenes or alkynes) can be used. They are often highly stereoselective, providing up to four contiguous carbon stereogenic centers and opening access to other stereogenic centers as well, such as chiral phosphorus [5], being of paramount importance when a decrease in symmetry (i.e., increase in asymmetry) is desired or necessary as often is the case for the construction of scaffolds of many natural and related compounds. Accepted Alder endo rule predicts the formation accepted Alder endo rule predicts the formation of the endo product as the of thecycloadditions
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