A New Look at the Diels-Alder Transition State

Abstract

The chronology of π-orbital mixing in the Diels-Alder cycloaddition of butadiene and ethylene differs for the two main interactions, the HOMO -LUMO and HOMO -LUMO interactions. The C2-C3 π bond of the cycloadduct is already well developed at the level of the transition state, which is reminiscent of a late transition state.

Radical Stabilization Algorithm as a Predictive Tool for Novel and Reported Noncanonical Thiele’s Acid Analogues

“…4b From a synthetic planning perspective, we are encouraged to note that simple radical stabilization appears to be valuable in predicting the outcomes of complex reactions. But we leave it for others to debate whether or not these results necessarily imply the existence of diradical (or diradicaloid) character in the Diels-Alder transition state, or whether 'radical stabilization' merely functions as a convenient proxy for some other property (e.g., unanticipated FMO coefficient contributions, subdominant orbital interactions, 21 paralocalization energy, 22 etc.) that may be important in governing the reaction outcome.…”

An Enyne Metathesis/(4 + 2)- Dimerization Route to (±)-Differolide

“…For example, the parent 2-carbomethoxybutadiene ( i ) dimerizes at room temperature to give ii (mikanecic acid dimethyl ester), exclusively . This adduct arises from reaction of the more electron deficient dienophilic π-bond in i [as does the minor adduct 11 in the dimerization of 2-vinylbutenolide ( 2 )] and results in para-selectivity (in contrast to the meta-orientation of the carbonyl groups observed in the minor isomer 11 ).…”

Synthesis of Illudinine from Dimedone

“…One of the major challenges posed by electron-deficient dienes is their tendency to dimerize, oligomerize, or otherwise decompose faster than they react with unstrained alkyne dienophiles. The dimerization pathway is suppressed in vinylarenes, and it is not unreasonable to anticipate a differential impact of aromaticity on other competing reaction pathways.…”