Diels-Alder reaction structural challenges

A challenge of a different kind was encountered in the internal vinylation of various vinyl triflates and bromides as depicted in Eq. (11.13) [27]. The electron-rich structures obtained from the reactions were of interest for further use in Diels-Alder reactions, but the risk of degrading the products in the hot reaction medium posed a problem and a prudent choice of energy input was imperative. It turned out that single-mode microwave heating for 5 min at the very low power of 5 W was sufficient to yield 64% of the product with excellent regioselectivity. Measurements with a fluor-optic probe revealed an unexpectedly high temperature of 76 °C [27]. 

The Stereoselectivity of Diels-Alder Reactions. One of the most challenging stereochemical findings is Alder s endo rule for Diels-Alder reactions. The favoured transition structure 6.180 has the electron-withdrawing substituents in the more hindered environment, under the diene unit, giving the kinetically more favourable but thermodynamically less favourable adduct 6.181. Heating eventually equilibrates the adducts in favour of the exo adduct 6.182, by a retro-cycloaddition re-addition pathway. 

In further sections we shall often refer to the Diels-Alder reaction as the most versatile and popular synthetic method. It is not an exaggeration to claim that, in the absence of this reliable tool, synthetic chemists would never have dared to pursue the syntheses of many challenging structures which eventually yielded to their efforts. 

---